Veterinary Clinical Pharmacology - Protein synthesis Inhibitors PDF

INHIBITORS OF VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 MODULE 5 Introduction to Chemotherapy Page | 1 PROTEIN M...

INHIBITORS OF VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 MODULE 5 Introduction to Chemotherapy Page | 1 PROTEIN Module 1 Introduction SYNTHESIS Overview In this module, you will learn the drugs that act on the ribosomes namely, aminoglycosides and aminocyclitols, tetracyclines, phenicols, lincosamides and macrolides. The mechanism of action of each drug was discussed for you to understand how they exert their antibacterial effects. Their spectrum of activity, clinical indications, pharmacokinetics, drug interactions and adverse reactions were also elaborated in sections. I. Learning Objectives Upon completion of this module, students will be able to: 1. Describe the chemical structure of protein synthesis inhibitors. 2. Explain the exact mechanism of action of protein synthesis inhibitors. 3. Identify the clinical indications of protein synthesis inhibitors. 4. Determine the pharmacokinetic properties of protein synthesis inhibitors. 5. Recognize the drug interactions and adverse reactions associated with the use of protein synthesis inhibitors. II. Learning Activities A. AMINOGLYCOSIDES Aminoglycosides are important antibiotics in the treatment of Gram-negative infections. These compounds are produced from strains of Streptomyces, Micromonospora and Bacillus species. Chemistry The aminoglycosides contain aminocyclitol group, with amino sugars attached to aminocyclitol ring in glycosidic linkages. The amino group contributes to basic nature of this class of antibiotic, and the hydroxyl group on sugars moieties to high aqueous solubility and poor lipid solubility. Differences in the substitutions on the basic ring structures within the various aminoglycosides account for the relatively minor differences in antimicrobial spectra, patterns of resistance, and toxicities. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 2 Mechanism of Action Aminoglycosides inhibit protein biosynthesis by irreversibly binding to one or more receptor proteins on the 30S subunit of the bacterial ribosome and thereby interfering with several mechanisms in the mRNA translation process. o They interfere with proper attachment of messenger RNA to the ribosome (30s subunit), causing misreading of genetic code, and decreased or abnormal protein synthesis. o Because of irreversible binding, significant postantibiotic effects can be observed. Figure 1. Effect of aminoglycosides on protein synthesis Aminoglycosides penetrate by bacteria by an energy-dependent step that is oxygen-linked. This uptake is inhibited by an anaerobic or acidic environment and by calcium or magnesium which competes with the transport system. o Anaerobic bacteria and induced mutants are generally resistant because they lack appropriate transport systems. With low oxygen tension, as in hypoxic tissues, transfer into bacteria is diminished. o Passive movement of aminoglycosides across bacterial cell membranes is facilitated by an alkaline pH; a low pH may increase membrane resistance more than 100-fold. o Aminoglycosides are bound to, and inactivated by, cellular debris and nucleic acid material that is released by decaying white blood cells. o Divalent cations (eg, calcium and magnesium) located in the LPS, cell wall, or membrane can interfere with transport into bacteria because they can combine with the specific anionic sites and exclude the cationic aminoglycosides. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 3 Classes of Aminoglycosides, Therapeutic Uses and Spectrum of Activity Aminoglycosides are still considered to be important drugs of choice for treating serious aerobic gram-negative infections in veterinary medicine, although newer and less toxic antimicrobials (i.e., third-generation cephalosporins and fluoroquinolones) have replaced the use of aminoglycosides for some bacterial infections. Because of their efficacy against P. aeruginosa, aminoglycosides might be considered higher-tier drugs. Selected staphylococci are susceptible; however, treatment should be based on synergistic effects via combination with other antimicrobials (e.g., beta-lactam antimicrobials). With such combination treatment, low doses of aminoglycosides are generally used. Netilmicin, sisomicin, and dibekacin are newer aminoglycoside compounds but there are no reports of their use in veterinary medicine. Table 1. Spectrum of activity of the classes of aminoglycosides Aminoglycosides Spectrum of Activity and Therapeutic Indication Narrow-spectrum Aminoglycosides Streptomycin Gram-negative bacilli are still susceptible, including strains of Dihydrostreptomycin Actinomyces bovis, Pasteurella spp., E. coli, Salmonella spp., Campylobacter fetus, Leptospira spp., and Brucella spp. Mycobacterium tuberculosis is also sensitive to streptomycin. Broad- and Expanded-spectrum Aminoglycosides Neomycin Broad spectrum Used orally for the treatment of enteric infections and topically for treating skin, ear, and eye infections. Gentamicin Extended spectrum that includes Pseudomonas aeruginosa Also with activity against Proteus, Staphylococcus, and Corynebacterium spp., as well as Gram (–) aerobes. They are used in all species for the treatment of susceptible infections of the skin, respiratory tract, ear, eye, urinary tract, and septicemia. Kanamycin Similar to gentamicin except it is not effective against Pseudomonas spp. Used as an oral preparation combined with bismuth subcarbonate and aluminum magnesium silicate for the treatment of bacterial enteritis in dogs and for symptomatic relief of the associated diarrhea. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 4 Amikacin Has greater activity against gram-negative bacteria compared to other drugs in this group because it resists degradation by bacterial enzymes. Used for gentamicin-resistant gram (-) organism Tobramycin Similar to gentamicin but more active against some strains of E. coli and Pseudomonas aeruginosa Apramycin Used to control enteric gram-negative infections, particularly E coli in piglets. It also is active against Proteus, Klebsiella, Brachyspira, and Mycoplasma spp. Pharmacokinetics Absorption o Aminoglycosides are poorly absorbed (usually < 10%) from the healthy GI tract. ▪ However, permeability may be increased in the neonate and in the presence of enteritis and other pathological changes, allowing absorption to be much greater. o Significant absorption may also occur from large burns when applied topically. o In the circumstance of renal failure, toxic (trough) concentrations may accumulate. o Aminoglycosides are rapidly absorbed systemically from serosal surfaces of body cavities and from IM and SC injection sites. ▪ Short dosing intervals, including continuous infusions, are contraindicated for all aminoglycosides. ▪ Once-daily treatment is indicated for safety considerations and to minimize adaptive resistance. Distribution o Aminoglycosides are distributed to the extracellular fluid and to transcellular fluids such as pleural and peritoneal fluids. o Penetration to CNS, prostate and ocular tissue is minimal. o Fetal tissue and amniotic fluid concentrations are very low in most species. o They tend to accumulate in the renal cortex and otic endolymph predisposing these tissues to toxicity. Excretion o Aminoglycosides are excreted unchanged in the urine via glomerular filtration. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 5 ▪ Excessive accumulation (mainly in the renal cortex) leads to a characteristic tubular cell necrosis. ▪ Glomerular filtration rates differ between species and are often lower in neonates, which may explain the greater sensitivity to aminoglycosides in newborn foals and puppies. ▪ The prolonged residues values in kidney severely limits the use of aminoglycosides in production animals to label use only. o Plasma half-lives are 1-3 hours for most species. Adverse Reactions Nephrotoxicity is caused by the damage of the membrane of proximal tubular cells (acute tubular necrosis with secondary interstitial damage that may result in renal failure) due to the binding of free amino groups of ionized aminoglycosides, resulting in loss of brush border enzymes, impaired absorption, proteinuria and decreased glomerular filtration rate. o Aminoglycosides are sequestered in lysosomes and interact with ribosomes, mitochondria, and other intracellular constituents to cause cell injury. o Several factors predispose to aminoglycoside nephrotoxicosis, including age (with young [especially the newborn foal] and old animals being sensitive), compromised renal function, total dose, duration of treatment, dehydration and hypovolemia, aciduria, acidosis, hypomagnesemia, severe sepsis or endotoxemia, concurrent administration of furosemide, and exposure to other potential nephrotoxins (eg, methoxyflurane, amphotericin B, cisplatinum, and perhaps some cephalosporins). o Enhanced nephrotoxicity may become evident with concurrent administration of aminoglycosides and other potentially nephroactive (eg, diuretics) or nephrotoxic (eg, NSAIDs) agents. o In renal insufficiency, generally the interval between doses is prolonged (rather than reducing the dose) to minimize toxicity while avoiding a negative impact on efficacy. The risk of toxicity is less in alkaline urine. o Acute tubular necrosis occurs more likely with prolonged therapy (6-7 days or longer). Tubular casts and increased protein in the urine are characteristic early signs of toxicity. o Gentamicin is the most nephrotoxic of the aminoglycosides. Ototoxicity results from progressive damage to cochlear sensory cells (leads to deafness) and vestibular cells (causes ataxia and incoordination especially in cats with prolonged exposures). o Binding or damage to mitochondria plays a prominent role in ototoxicity. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 6 o Factors increasing the risk of vestibular and cochlear damage are the same as for nephrotoxicity but also include preexisting acoustic or vestibular impairment and concurrent treatment with potentially ototoxic drugs. o Those with predominantly cochlear (auditory) toxicity are amikacin, kanamycin, and neomycin. o Those with predominantly vestibular toxicity are streptomycin and gentamicin. Neuromuscular blockade is a relatively rare adverse effect of aminoglycosides. It is caused by pre-junctional blockade of acetylcholine (Ach) release and decreased post-synaptic sensitivity to Ach. o The effect is more pronounced when aminoglycosides are used with other drugs that cause neuromuscular blockade and with gas anesthetics. o Neomycin, kanamycin, amikacin, gentamicin, and tobramycin are listed in order of most to least potent for these neuromuscular effects. o Muscle paralysis and apnea can be treated with neostigmine and calcium gluconate. Drug Interactions Aminoglycosides and penicillins in the same container inactivate each other, except penicillin G and streptomycin. o High concentrations of carbenicillin, ticarcillin, and piperacillin inactivate aminoglycosides because of direct interactions both in vitro and in vivo in the circumstance of renal failure. o Synergistic interactions that enhance antibacterial efficacy have been documented when aminoglycosides are administered with other antimicrobials, particularly beta-lactam antimicrobials. Gentamicin shows antagonism with chloramphenicol, tetracycline, and erythromycin. Aminoglycosides will potentiate the toxicity of nephrotoxic and neuromuscular blockade drugs. It binds to pus, calcium, and magnesium, and is neutralized by acid and 32x more active in alkaline urine. Bacterial Resistance Impaired transport across cell membrane. Because transport process is active and oxygen-dependent, anaerobic bacteria (Bacteroides and Clostridium) and facultative anaerobes (Enterobacteria and Staphylococcus) are more resistant when in an anaerobic environment. Enzymatic modification of aminoglycosides. This occurs in both gram-negative and gram-positive bacteria. More than 50 enzymes have been identified, with three Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 7 major types, each including several subclasses: acetylating enzymes (acetyltransferases), adenylating enzymes (nucleotidyltransferases), and phosphorylating enzymes (phosphotransferases). Reduced affinity of ribosomal binding site due to mutation of specific ribosomal proteins. Dose Rates Gentamicin 6-12 mg/kg/day, IM or SC Kanamycin 25-30 mg/kg/day, IM or SC Streptomycin/ 15-25 mg/kg/day, IM or SC Dihydrostreptomycin Neomycin 15 mg/kg , PO 2x daily Amikacin 15-22 mg/kg/day IM or SC B. SPECTINOMYCIN Spectinomycin is produced by Streptomyces spectabilis. It is a non- aminoglycoside aminocyclitol, and is, therefore, structurally related to aminoglycosides. An important difference between spectinomycin and the aminoglycosides is in the adverse effects and spectrum of activity. Spectinomycin lacks the toxic effects of aminoglycoside antibiotics and can be used without concerns about kidney injury. Chemistry Compared to aminoglycosides, spectinomycin does not contain amino sugars or glycosidic bonds, but it has an aminocyclitol nucleus like the aminoglycosides. It is a weak organic base with two pKas (6.4 and 8.7). It has low lipid solubility, thus, it is highly water-soluble and relatively stable in solution. Mechanism of Action Spectinomycin inhibits protein synthesis via a 30S ribosomal target but does not cause misreading of the genetic code. Unlike aminoglycosides, it is bacteriostatic rather than bactericidal. Spectrum of Activity and Therapeutic Uses It is active against several strains of streptococci and a wide range of gram- negative bacteria, but not as active as the aminoglycosides. It also has good activity against Mycoplasma but none against anaerobes. Most Chlamydia spp are resistant. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 8 It is available in combination with lincomycin. It is used in dogs, cats, horse, swine, and poultry for the treatment of enteric and respiratory diseases. In cattle, it is used against Mannheimia (Pasteurella) haemolytica, Pasteurella multocida, and Histophilus somni (formerly Haemophilus somnus). Pharmacokinetics Absorption o Spectinomycin is administered orally or parenterally twice a day in all species. ▪ It is poorly absorbed from the GI tract but is rapidly absorbed after IM administration, with blood concentrations peaking within 1 hour. Distribution o Like the aminoglycosides, spectinomycin has a small volume of distribution. o Spectinomycin penetrates tissues rather poorly and distributes principally into extracellular fluid because of its poor lipid solubility. ▪ Tissue concentrations seldom exceed 25-50% of the serum concentrations. Excretion o Most of an oral dose is eliminated in the feces, but after an injection the primary route of elimination is the kidneys. ▪ It is excreted rapidly unchanged in the urine by glomerular filtration. o 75% of doses is cleared in 4 hours; the half-life is only 60 minutes in most cases. Adverse Reactions It is relatively non-toxic even at high doses. Neuromuscular blockade is the most significant toxicity but rare. It can potentiate other neuromuscular blocking agents. IM injections may cause pain. Bacterial Resistance Resistant mutants fail to bind aminocyclitols to the 30s ribosome. Plasmid-mediated resistance, which is manifested as the production of degrading enzymes, is less common. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 9 C. TETRACYCLINES Tetracycline antibiotics were the first “broad-spectrum antibiotics” known. These were isolated from Streptomyces species (S. rimosus and S. aureofaciens), and later expanded to include various semisynthetic products. Chemistry The tetracyclines are a group of four-ringed amphoteric compounds that differ by specific chemical substitutions at different points on the rings. They are yellowish, acidic substances that in aqueous solutions, form salts with both acid and bases. o The most common salt form is the hydrochloride, except for doxycycline, which is available as doxycycline hyclate or monohydrate. o Preparations for parenteral administration must be carefully formulated, often in propylene glycol or polyvinyl pyrrolidone with additional dispersing agents, to provide stable solutions. They characteristically fluoresce when exposed to ultraviolet light. They are stable as dry powders but not in aqueous solutions, particularly at high pH ranges. They are zwitterions and ionized at physiological pH values. They form poorly soluble chelates with bivalent and trivalent cations, particularly calcium, magnesium, aluminum, and iron. Mechanism of Action Figure 2. Inhibition of bacterial protein synthesis by tetracyclines Tetracyclines inhibit protein synthesis by reversibly binding to the 30s ribosomal subunit and preventing the attachment of aminoacyl-tRNA acceptor (A) site on the mRNA ribosomal complex. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 10 Tetracyclines enter microorganisms in part via diffusion and in part via an energy- dependent, carrier-mediated system responsible for the high concentrations achieved in susceptible bacteria. They block the addition of amino acids to the growing peptide chain; hence, they are bacteriostatic. They become bactericidal at high concentrations, as may be attained in urine, when organisms seem to lose the functional integrity of the cytoplasmic membranes. Classes of Tetracyclines, Therapeutic Uses and Spectrum of Activity Table 2. Classes of tetracyclines Tetracyclines Naturally occurring Short-acting 1. Oxytetracycline 1. Tetracycline 2. Chlortetracycline 2. Oxytetracycline 3. Demethylchlortetracycline 3. Chlortetracycline Semi-synthetic Intermediate-acting 1. Tetracycline 1. Demethylchlortetracycline 2. Rolitetracycline 2. Methacycline 3. Methacycline 4. Minocycline Long-acting 5. Doxycycline 1. Doxycycline 6. Lymecycline 2. Minocycline Glycylcyclines 1. Tigecycline All tetracyclines are about equally active and typically have about the same broad spectrum, which comprises both aerobic and anaerobic gram-positive and gram- negative bacteria, mycoplasmas, rickettsiae, chlamydiae, and even some protozoa (amoebae). o Tetracyclines generally are the drug of choice to treat rickettsiae and mycoplasma. Among the susceptible organisms are Wolbachia, a rickettsia- like intracellular endosymbiont of Dirofilaria immitis, and Neorickettsia spp. o Tetracyclines can also quite effective against intracellular pathogens, such as Lawsonia intracellularis and Rhodococcus equi. o Specific indications include infectious keratoconjunctivitis in cattle, chlamydiosis, heartwater, anaplasmosis, actinomycosis, actinobacillosis, Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 11 nocardiosis (especially minocycline), ehrlichiosis (especially doxycycline), elimination of Wolbachia (i.e., as part of the treatment of a heartworm- infected animal), eperythrozoonosis, and haemobartonellosis. Resistance is common among Enterococcus species and members of the family Enterobacteriaceae (Enterobacter spp., E. coli, Klebsiella spp., Proteus spp., Salmonella spp.). Also commonly resistant to the tetracyclines are those infections involving Mycobacterium spp., Proteus vulgaris, Pseudomonas aeruginosa, and Serratia spp. Even though there is general cross-resistance among tetracyclines, doxycycline and minocycline usually are more effective against staphylococci. Tetracyclines also have been used as immunomodulating drugs and antiinflammatory drugs. This use of tetracyclines has focused on treatment of osteoarthritis, vasculitis, and dermatitis. Pharmacokinetics Absorption o All tetracyclines are adequately absorbed (60-90%) in the upper small intestine except for chlortetracycline (35%). o Doxycycline and minocycline are more lipophilic and are more completely absorbed (90-100%). o Tetracyclines can easily chelate to polyvalent cations, which decreases the absorption several-fold. ▪ Gastrointestinal absorption can be impaired by sodium bicarbonate, aluminum hydroxide, magnesium hydroxide, iron, calcium salts, and (except for the lipid-soluble tetracyclines doxycycline and minocycline) milk and milk products; hence, should be avoided 3 hours before and after oral administration. o Tetracycline may be absorbed after IM or SC injection but can be very painful and damaging to tissues. They are also absorbed following intrauterine or intra-mammary infusion. Distribution o Tetracyclines have wide distribution in the body, except CNS. ▪ They reach high concentrations in the kidneys, liver, bile, lungs, spleen, and bone. ▪ Lower concentrations are found in serosal fluids, synovia, CSF, ascitic fluid, prostatic fluid, and vitreous humor. o Minocycline and doxycycline, being more lipophilic penetrate brain, ocular tissues, spinal fluid, and prostate better than other tetracyclines, such as oxytetracycline or chlortetracycline. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 12 o Tetracyclines are commonly reported to concentrate intracellularly, and doxycycline has a higher affinity for intracellular accumulation than other tetracyclines. o There is also a good penetration in sinuses, mucosa and milk. They cross the placenta and enter the fetal circulation and amniotic fluid. o Because tetracyclines tend to chelate calcium ions (less so for doxycycline), they are deposited irreversibly in the growing bones and in dentin and enamel of unerupted teeth of young animals, or even the fetus if transplacental passage occurs Excretion o Doxycycline and minocycline may be more extensively biotransformed than other tetracyclines (up to 40% of a given dose). o Renal excretion is the major route of elimination of most tetracyclines, but small amounts are excreted into feces via bile or diffusion from blood into the intestine. Alkaline urine increases excretion. o Doxycycline is unique in that intestinal excretion is the major route of elimination (75%). o Tetracyclines are also eliminated in milk, saliva and tears. Adverse Reactions Tetracyclines (except doxycycline and minocycline) are potentially nephrotoxic and should be avoided if renal function is impaired. o The administration of expired tetracycline products may lead to acute tubular nephrosis. o They should not be used with methoxyflurane anesthesia because this combination may result to acute renal failure. Permanent staining of unerupted teeth may occur in young animals due to the formation of a tetracycline-calcium phosphate complex in enamel and dentine. At extremely high concentrations, the healing processes in fractured bones is impaired. Rapid IV injection of a tetracycline can produce hypotension and collapse. o This acute depression of cardiovascular function appears to be related to the ability of tetracycline to chelate ionized Ca. o This effect can be avoided via slow infusion of the drug (>5 minutes) or pretreatment with calcium gluconate administered IV. o Both rapid IV boluses and slow constant-rate infusions of doxycycline have been associated with tachycardia, arrhythmia, systemic arterial hypertension, collapse, and death in horses and therefore should not be used in this species. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 13 o Rapid IV administration in cow can precipitate signs of milk fever (hypocalcemia). Superinfection of fungi, yeast, or resistant bacteria may occur in the gastrointestinal tract with prolonged administration. Oral therapeutic doses may disrupt ruminal microflora in adult ruminants or colonic microflora in horses. Elimination of the gut flora in monogastric animals decreases the synthesis and availability of the B vitamins and vitamin K from the large intestine. Tetracyclines can have neuromuscular blocking effects and therefore are contraindicated in the treatment of neuromuscular junction diseases, such as botulism. Antianabolic effects are seen at high doses because of binding to mitochondrial ribosomes. This may result in an elevated blood urea nitrogen (BUN) especially with preexisting renal disease. Tetracyclines are capable of inhibiting leukocyte chemotaxis and phagocytosis when present in high concentrations at sites of infection. The use of immunosuppressive drugs such as glucocorticoids impairs immunocompetence even further. Dry-pilling of doxycycline in tablet or capsule form has been associated with esophageal erosion and strictures in cats; administration should be followed by approximately 5 mL of fluid. Phototoxicity and hepatotoxicity are rare side effects in animals. Bacterial Resistance Decreased penetration of drug into previously susceptible organisms is due to impaired uptake into bacteria and the much more common plasmid- or transposon- mediated acquisition of active efflux pump. Another mechanism is the production of a protective protein that acts by either preventing binding, dislodging the bound drug, or altering the negative impact of binding on ribosomal function. Among the tetracyclines, tigecycline is characterized by less resistance due to efflux or ribosomal protection. Resistance to one tetracycline will generally produce cross-resistance to the others in the group. Dose Rates Tetracycline Dog/cat: 7 mg/kg IM, IV q 12h; 20 mg/kg oral q8h Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 14 Oxytetracycline Dog/cat: 7 mg/kg IM, IV q12h Cattle/sheep/pig: 5-10 mg/kg IM, IV q 24h Calf/foal, Lamb/piglets: 10-20 mg/kg oral q8-12h Horse: 5 mg/kg IV Doxycycline Dog: 5-10 mg/kg PO q24h; 5 mg/kg IV q24h D. PHENICOLS Chloramphenicol is produced from Streptomyces venezuelae, a soil organism isolated from Venezuela. Chloramphenicol use in food animals is now banned in the Philippines because it is known to cause aplastic anemia in humans. Chemistry Chloramphenicol is an unusual natural compound because it contains dichloracetate and nitrobenzene moieties as part of its structure. It is used either as the freebase or in ester form: neutral-tasting palmitate for oral use and water-soluble sodium succinate for parenteral use. It is highly lipid soluble, only slightly soluble in water and very stable. Thiamphenicol is a derivative of chloramphenicol wherein the nitrophenol group is replaced by a methyl sulfonyl group. Florfenicol is a derivative of thiamphenicol which also contains a fluorine molecule. Loss of the dichloroacetyl group altogether results in loss of biological activity. Mechanism of Action Nascent polypeptide chain 50s aa mRNA 30 s Block by chloramphenicol Figure 3. Inhibition of bacterial protein synthesis by chloramphenicol Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 15 Chloramphenicol and its derivatives inhibit protein synthesis by 50S subunit of the 70S ribosome. This action prevents the binding of the amino acid-containing end of the tRNA to the active site of peptidyltransferase. Because peptide-bond formation is inhibited, peptides cannot elongate. The effect is usually bacteriostatic but, at high concentrations, chloramphenicol may be bactericidal for some species. Chloramphenicol affects mammalian protein synthesis to some degree, especially mitochondrial protein synthesis. Mammalian mitochondrial ribosomes have a strong resemblance to bacterial ribosomes (both are 70S), with the mitochondria of the bone marrow especially susceptible. Spectrum of Activity and Therapeutic Use Chloramphenicol has a broad spectrum of activity similar to that of tetracyclines. o Chloramphenicol has been used to treat infections caused by Staphylococcus spp., streptococci, Brucella spp., Pasteurella spp., E. coli, Proteus spp., Salmonella spp., Bacillus anthracis, Arcanobacterium pyogenes, Erysipelothrix rhusiopathiae, and Klebsiella pneumoniae. o Escherichia coli, Proteus spp., and Salmonella spp. may be susceptible, but resistance can occur with many gram-negative bacteria, especially the Enterobacteraceae. Pseudomonas aeruginosa is resistant. Several anaerobes such as Bacteroides fragilis, as well as Rickettsia and Chlamydia spp, are susceptible to phenicols. o Chloramphenicol is notable for its anaerobic spectrum. Chloramphenicol has been used to treat bacterial infections of the respiratory tract, infections of the CNS (encephalitis, meningitis), and conjunctivitis, panophthalmitis, endophthalmitis, and diseases of the cornea. Florfenicol is approved for use only in cattle for the treatment of bovine respiratory disease (BRD) caused by Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. Because problematic organisms such as methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus pseudintermedius (MRSP), and multidrug-resistant enterococci often retain susceptibility, chloramphenicol is regularly used in these cases. In humans, its use is restricted to typhoid fever or other severe infections against which no other antibacterial agents are effective. Chloramphenicol is not allowed for use in food-producing animals because the potential danger of residue-induced toxicity in humans. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 16 Pharmacokinetics Absorption o Chloramphenicol is rapidly absorbed after oral administration in monogastric animals, but absorption after IM or SC is unpredictable. o In adult ruminants, orally administered chloramphenicol is inactivated by rumen microorganisms. o The presence of food and intestinal protectants does not interfere with absorption of chloramphenicol, although drugs that depress GI motility do. o Florfenicol is rapidly absorbed after administration PO, although milk interferes with absorption. Distribution o Chloramphenicol has very wide distribution in the body due to high lipid solubility. o Chloramphenicol attains highest concentrations in the liver, bile and kidneys. o Chloramphenicol also reaches substantial concentrations (~50% of plasma concentrations) in many body fluids such as the CSF and aqueous humor. o Florfenicol also penetrates most body tissues, although penetration of CSF and aqueous humor is less than that of chloramphenicol. o Chloramphenicol can also cross the placental barrier in pregnant animals and can diffuse into the milk of nursing animals. o Florfenicol does penetrate the milk of lactating cows and residues persist for an extended duration. o The blood-prostate barrier is an exception to the extensive intracorporeal distribution of chloramphenicol, and concentrations in the inflamed prostate are low to nil. Elimination o Unlike many other antibacterial agents, chloramphenicol undergoes extensive hepatic metabolism. ▪ Free chloramphenicol is biotransformed primarily via glucuronide conjugation. ▪ In cats, a characteristic genetic deficiency in glucuronyl transferase activity leads to plasma half-lives that are often considerably longer than those in other species (eg, cats, 5.1 hours; ponies, 54 minutes). ▪ Very young animals frequently do not have full microsomal enzyme capabilities, and the plasma half-lives of chloramphenicol in the young (< 4 weeks old) of many species are often much longer than those of adults. Foals appear to be a notable exception to this generalization. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 17 ▪ Liver disease also prevents chloramphenicol from undergoing normal metabolic degradation, and active antimicrobial accumulates in the body. o The principal route of excretion is renal via glomerular filtration (5%–10%; free chloramphenicol and chloramphenicol sodium succinate dosage form), tubular secretion (90%–95%; glucuronide metabolite), or eliminated in the active, unchanged form (5%–15%). o Enterohepatic cycling is often pronounced, and prolongs blood concentrations to some degree in herbivores. Adverse Reactions and Toxicity Chloramphenicol may reduce appetite, and cause vomiting, weight loss, dehydration, and CNS depression. These effects are seen more frequently in cats than in dogs. In people, chloramphenicol (but not florfenicol) can produce two distinctive syndromes of bone marrow suppression. o One form is dose-dependent and reversible. It is characterized by nonregenerative anemia (with or without thrombocytopenia or leukopenia), increased serum iron, bone marrow hypocellularity, cytoplasmic vacuolization of blast cells and lymphocytes, and maturation arrest of erythroid and myeloid precursors. This is more likely to occur with florfenicol than chloramphenicol. o The second form of bone marrow suppression is an irreversible aplastic anemia that is not related to dose or duration. Peripheral blood showing pancytopenia may be associated with hypoplastic or aplastic bone marrow. Because tissue residues in production animals might induce aplastic anemia in people, use of chloramphenicol in production animals is prohibited. Due to this risk, humans should wear gloves and be properly educated on safe drug handling when administering chloramphenicol to animals. Chloramphenicol can delay wound healing in excessive topical application because of its inhibition of protein synthesis. Chloramphenicol can also suppress anamnestic response so animals should not be vaccinated while being treated with this antimicrobial. Drug Interactions Chloramphenicol interferes with the actions of many bactericidal drugs, such as the penicillins, cephalosporins, and aminoglycosides, and such combinations should not be used under most circumstances. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 18 Chloramphenicol should not be administered concurrently with other antibacterial agents that bind to the 50S ribosomal subunit (e.g., the macrolides and lincosamides). In combination with sulfamethoxypyridazine, chloramphenicol can cause hepatic damage. Chloramphenicol can substantially prolong the duration of action of several drugs such as pentobarbital, codeine, phenobarbital, xylazine, cyclophosphamide, phenytoin, NSAIDs, and coumarins if administered concurrently Chloramphenicol also delays the response of anemia to iron, folic acid, and vitamin B12. Bacterial Resistance Plasmid-mediated resistance is very important in Gram-negative bacteria. Different types of chloramphenicol acetyltransferase enzymes cause enzymatic inactivation by acetylation of the drug. o The fluorine atom of florfenicol prevents acetylation, thus enhancing the efficacy of this drug. Other mechanisms of resistance include efflux systems, inactivation by phosphotransferases, decreased bacterial cell wall permeability, altered binding capabilities at the 50S ribosomal subunit, and inactivation by nitroreductases. Resistance to chloramphenicol often develops together with resistance to tetracycline, erythromycin, streptomycin, ampicillin, and other antimicrobials because of multiple genes being carried on the same plasmid. Dose Rates Chloramphenicol Cat: 45-60 mg/kg PO, IV, IM q12h Dog: 45-60 mg/kg oral, IV. IM q6-8h Horse: 50 mg/kg PO q6-8h; 50 mg/kg IV q2-4h E. MACROLIDES The macrolide antimicrobials typically have 12–20 atoms of carbon in their large lactone ring structure. Erythromycin, the prototype of this class, is derived from Streptomyces erythreus isolated from the Philippine soil. Since erythromycin’s discovery, numerous other macrolides have been isolated or synthesized from it. Apart from erythromycin, the other most common agents used clinically in veterinary medicine are tilmicosin, azithromycin, gamithromycin, tylosin, tildipirosin, and tulathromycin. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 19 Chemistry The macrolide antimicrobials are colorless, crystalline substances. They are basic, lipid-soluble compounds consisting of a lactone ring to which various combinations of deoxy sugars are attached by glycosidic linkages. o They contain a dimethylamino group, which makes them basic. Although they are poorly water soluble, they do dissolve in more polar organic solvents. They are prepared as sulfate salts or as more stable esterified salts of tartrate, estolate, acetylates, estolates, lactobionate, succinates, propionates, and stearates. Mechanism of Action Figure 4. Inhibition of bacterial protein synthesis by macrolides antibiotics Macrolides interfere with protein synthesis by reversibly binding to the 50S subunit of the ribosome, similar to the phenicols. o They appear to bind at the donor site, thus preventing the translocation necessary to keep the peptide chain growing. o The effect is essentially confined to rapidly dividing bacteria and mycoplasmas. o Binding sites on the 50S ribosome overlap with binding sites of chloramphenicol and the lincosamides (especially clindamycin) and combination therapy should be avoided. Macrolides are regarded as being bacteriostatic but demonstrate bactericidal activity at high concentrations. Macrolides are considerably more active at higher pH ranges, and therefore have decreased activity in acidic environments such as abscesses. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 20 Classes of Macrolides, Therapeutic Uses and Spectrum of Activity In general, macrolides favors the gram-positive group and are not active against gram-negative bacteria. o Exceptions include tilmicosin, gamithromycin, and tulathromycin, for which the spectra are characterized as broader and include Mannheimia haemolytica and Pasteurella multocida, as well as some gram-negative bacteria (i.e., some strains of Pasteurella, Haemophilus, and Neisseria spp.). Most other gram-negative bacteria, such as those of the Enterobacteriaceae or Pseudomonas spp., are resistant. Helicobacter also is generally included in the spectrum. Azithromycin includes Bordetella in its spectrum. Macrolides are active against atypical mycobacteria, Mycobacterium, Mycoplasma, Chlamydia, and Rickettsia spp. but not against protozoa or fungi. General indications include upper respiratory tract infections, bronchopneumonia, bacterial enteritis, metritis, pyodermatitis, urinary tract infections, arthritis, and others. The macrolides appear to have immunomodulatory effects useful to treat respiratory infections. Table 3. Spectrum of activity of macrolide antibiotics Macrolides Spectrum of Activity 14-ring Group 1. Erythromycin Erythromycin is an alternate to penicillin for 2. Oleandomycin infections caused by Gram (+) aerobes and 3. Troleandomycin anaerobes. It is often chosen for the treatment of enteritis caused by Campylobacter jejuni and Rhodococcus equi. 15-ring Group (Azalides) 1. Azithromycin Azithromycin is effective against 2. Gamithromycin Staphylococcus, Streptococcus, and Mycoplasma. It is used as an alternative for erythromycin for R. equi pneumonia in foals. Gamithromycin is used in the treatment of bovine respiratory diseases associated with M. haemolytica, P. multocida, and Histophilus somni. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 21 16-ring Group 1. Spiramycin Tylosin is used in the treatment of local and 2. Josamycin systemic infections caused by Mycoplasma and 3. Tylosin Gram (+) bacteria. It is also added to feed as a 4. Tilmicosin growth promotant in cattle, sheep, and swine. 5. Tildipirosin Tylosin is used in dogs and cats for the treatment of chronic colitis. As with gamithromycin, tilmicosin and tildipirosin are also used in the treatment of bovine respiratory diseases. In swine, tilmicosin phosphate is added to feed or water for control of swine respiratory disease. Triamilide 1. Tulathromycin Tulathromycin is used for the treatment of bovine and swine respiratory diseases. It is effective against Mannheimia, Mycoplasma, and Haemophilus; it is concentrated in leucocytes and lung tissue. Ketolides (also include tylosin and spiramycin) Pharmacokinetics Absorption o Macrolides are readily absorbed from the GI tract if not inactivated via gastric acid. ▪ Oral preparations are often enteric coated, or stable salts or esters (eg, stearate, lactobionate, glucoheptate, propionate, and ethylsuccinate) are used. o Absorption patterns may be erratic due to the presence of food. Absorption from the rumenoreticulum is usually delayed and unreliable. o Absorption after injection is rapid, but pain and swelling can develop at the injection sites. Distribution o Macrolides are widely distributed to all tissues except those of the CNS. o They tend to concentrate in the spleen, liver, kidneys, and particularly the lungs. They enter pleural and ascitic fluids and concentrate in the eye but do not distribute to the eye or the CSF. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 22 o Due to their nature as a weak base, macrolides concentrate in the bile, CSF, and milk due to ion trapping. o Therapeutic concentrations are attained in the prostate and prostatic fluid. They can cross the placenta. o They actually accumulate within many cells, including macrophages; WBCs will then facilitate distribution to the site of inflammation. This accumulation accounts in part for the long dosing interval that characterizes some macrolides (e.g., tilmicosin). o Up to 75% of the dose is bound to plasma proteins, and they bind to alpha1- acid glycoproteins rather than to albumin. Elimination o Metabolic inactivation of the macrolides is usually extensive. o Erythromycin and its degradation products interfere with cytochrome P450- mediated metabolism of a number of other drugs via cytochrome P450 inhibition. o Macrolide antimicrobials and their metabolites are excreted mainly in bile and often undergo enterohepatic cycling. o A small amount of unchanged drug is eliminated in the urine. Adverse Reactions Gastrointestinal disturbances are the most common reactions and are dose- related. o Erythromycin is a motilin receptor agonist that operates via both cholinergic and noncholinergic pathways and therefore may also induce vomiting and diarrhea, particularly when high doses are administered. o Adult horses are sensitive to macrolide-induced GI disturbances that can be serious and even fatal, such as antimicrobial-induced colitis. o Rabbits have been reported to develop potentially fatal typhlocolitis after macrolide treatment. o Oral administration of erythromycin has also been associated with severe diarrhea in calves. Edema of the rectal mucosa with mild anal prolapse may be seen in swine following IM administration of tylosin. Tilmicosin produces cardiovascular toxicity (tachycardia and decreased contractility) in species other than cattle by increasing myocardial Ca2+ concentrations. In foals treated with macrolides, drug-induced anhidrosis may develop, leading to severe hyperthermia. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 23 o Erythromycin is most frequently associated with drug-induced anhidrosis, followed by azithromycin and clarithromycin. o Drug-induced anhidrosis is reversible and may take up to 10 days after cessation of treatment to resolve. There can be pain and swelling after IM injection, and thrombo-phlebitis after IV. Hypersensitivity reactions including anaphylaxis can occur but are rare. Hepatotoxicity and cholestatic hepatitis occur occasionally in humans, primarily with estolate esters. Drug Interactions Macrolide antimicrobials probably should not be used with chloramphenicol or the lincosamides because they may compete for the same 50S ribosomal subunit binding site. Erythromycin is both a substrate and an inhibitor for the cytochrome P450 enzymes, which is the enzyme system that is most often involved in drug metabolism. o As an inhibitor of the cytochrome P450 enzymes, it may inhibit metabolism of drugs such as theophylline, cyclosporine, digoxin, and warfarin. o Concentrations of these drugs may increase when animals receive erythromycin, resulting in a potentiation of the pharmacological effect or toxicity. Bacterial Resistance Resistance to macrolide antibiotics may be chromosomal or plasmid-mediated. Lack of cell wall permeability renders most gram-negative organisms inherently resistant to macrolides. Resistance to macrolides in gram-positive organisms results from alterations in ribosomal structure (target site methylation or mutation) and loss of macrolide affinity. o Posttranslational methylation results in cross-resistance to lincosamides and streptogramins (macrolide-lincosamide-streptogramin B, or MLSB, resistance). Less frequently, active efflux, or enzymatic inactivation by resistant bacteria may occur. Dose Rates Erythromycin Cattle: 8-15 mg/kg PO q12-24h Cat: 15 mg/kg oral q8h Foals: 25 mg/kg IM q8h Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 24 Tylosin Cattle: 10-20 mg/kg IM q12-24h Pig: 10 mg/kg IM q12-24h; 7-10 mg/kg oral q8h Cat: 10 mg/kg q12h F. LINCOSAMIDES Lincosamides are a group of monoglycoside antibiotics containing an amino acid- like side chain. In veterinary medicine, lincomycin and clindamycin are the most frequently used in this class, and pirlimycin is approved as an intramammary infusion in cattle. Lincomycin is the antibiotic produced by Streptococcus lincolnensis var. lincolnensis from cultures of soil that originated in Lincoln, Nebraska. Chemistry Lincosamides are derivatives of an amino acid and a sulfur-containing octose. They are highly lipid-soluble, weak organic bases (pKa 7.6). They are more stable in salt forms (hydrochlorides and phosphates). Lincomycin and clindamycin are structurally similar. o Lincomycin has a hydroxyl moiety at the 7 position of the molecule, and clindamycin contains a chlorine at this position, making clindamycin a more active molecule against bacteria than its parent molecule, lincomycin, and better absorbed orally. Mechanism of Action Lincosamides bind exclusively to the 50S subunit of bacterial ribosomes and suppress protein synthesis via inhibition of peptidyl transferases. o Lincosamides, despite not being structurally related, use the same ribosomal binding sites as macrolides, streptogramins, and phenicols. They are bacteriostatic. Activity is enhanced at an alkaline pH. Spectrum of Activity and Therapeutic Use The lincosamides, like the macrolides, are used primarily to treat gram-positive infections in cases where there is resistance or intolerance to penicillins. Many gram-positive cocci, except for enterococci, and Mycoplasma are inhibited by lincosamides; however, most gram-negative organisms are resistant. Lincomycin has a limited spectrum against aerobic pathogens but a fairly broad spectrum against anaerobes. Clindamycin has greater antibacterial activity than lincomycin especially against anaerobes. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 25 Bacteroides spp. and other anaerobes are usually susceptible but Clostridium difficile strains appear to be regularly resistant. o Clindamycin is synergistic with metronidazole against B. fragilis Clindamycin is less effective toward ureaplasmas. Clindamycin also has activity against some protozoa, including Toxoplasma gondii, but usually in combination with other antimicrobials. It is approved for use in cats and dogs for treatment of infected wounds, abscesses, and dental infections. It is frequently used in the treatment of postsurgical orthopedic infections. Pharmacokinetics Absorption o Lincomycin is incompletely absorbed from the GI tract in nonherbivorous species. o Clindamycin is better absorbed from the gastrointestinal tract than lincomycin, yielding higher plasma concentrations. o Absorption of lincomycin (but not clindamycin) may be affected by the ingestion of food. o Absorption from IM injection sites is good. o Clindamycin palmitate is administered PO, and clindamycin phosphate IM. Distribution o Generally, they are highly protein bound. o Lincosamides are highly lipid soluble, leading to wide distribution in many fluids and tissues, including bone. o Notable concentrations are not attained in the CSF even when the meninges are inflamed. ▪ Poor CSF concentrations are due to the high degree of plasma- protein binding and rapid elimination kinetics. o Lincosamides diffuse across the placenta in many species. o Due to the basic nature of lincosamides, accumulation occurs in acidic tissues such as abscesses, the prostate, or the udder, which can result in prolonged milk residues. o Clindamycin also accumulates in polymorphonuclear WBCs and alveolar macrophages. o Clindamycin is able to penetrate glycocalyx, such as that associated with dental tartar. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 26 Elimination o Lincosamides are metabolized by the hepatic microsomal enzymes into sulfoxide and other metabolites. o The bile is the major excretion route. ▪ Aside from the bile, unchanged lincosamides and several metabolites may also be excreted in urine. Adverse Reactions and Toxicity GI disturbances may occur in all species. o These adverse effects are relatively infrequent in dogs and cats but are very serious in herbivores. Dogs are relatively resistant to GI disturbance. o Clindamycin-induced pseudomembranous enterocolitis (due to toxigenic C. difficile) or lincosamide-induced disruption of GI flora is a serious adverse reaction in a number of species and can be lethal. o Lincosamides are contraindicated for use in horses, guinea pigs, hamsters, rabbits, chinchillas, and ruminants. Hypersensitivity reactions occasionally are seen. Lincosamides should not be used in neonates because of their limited ability to metabolize drugs. Administration of clindamycin without food or water has resulted in esophagitis as well as esophageal ulceration and stricture in cats. Lincosamides exhibit peripheral neuromuscular blockade and cardiodepressive effects and therefore should not be given with anesthetics or via rapid IV bolus. o Lincosamides have additive neuromuscular effects with anesthetic agents and skeletal muscle relaxants. Intramuscular injections may lead to injection site reactions and pain. Bacterial Resistance Resistance to lincosamides appears slowly, perhaps as a result of chromosomal mutation. Altered drug binding by bacterial ribosomes is the usual form of resistance. o This is most often caused by methylation of the 23S rRNA, which is the same mechanism that is most common for macrolide antibiotics. Cross-resistance occurs with macrolides and streptogramin group B, due to similarities in their mechanism of action, and is classified as macrolide- lincosamide-streptogramin B, or MLSB, resistance. Cross-resistance may also include ketolides (ie, telithromycin) and is referred to as MLSK resistance. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 5 Introduction to Chemotherapy Page | 27 Dose Rates Lincomycin Cattle: 10 mg/kg IM q12h Pig: 10 mg/kg IM q12h 7 mg/kg oral in feed Dog: 20 mg/kg IM q12h Cat: 12 mg/kg IM q12h Clindamycin: Cat: 10 mg/kg IM oral q12h III. Study Questions 1. Describe the differences of protein synthesis inhibitors in terms of chemical structures. 2. Why are aminoglycosides bactericidal? 3. Enumerate the aminoglycosides with anti-pseudomonal activity. 4. What are the bacteriostatic protein synthesis inhibitors? 5. Enumerate the clinical indications of tetracyclines. 6. How does doxycycline and minocycline differ from the rest of the tetracyclines? 7. Why is chloramphenicol banned for use in food animals? 8. How does the other phenicols differ from chloramphenicol? 9. What is the spectrum of activity of macrolides and lincosamides? 10. What protein synthesis inhibitors are primarily excreted in the bile? 11. Why macrolides, lincosamides and phenicols should not be used together? 12. List down the adverse reactions associated with protein synthesis inhibitors. References AIELLO, S.E. and M.A. MOSES. 2016. The Merck Veterinary Manual. 11th Ed. Merck and Co., Inc. Whitehouse Station, NJ, USA. GIGUERE, S. J.F. PRESCOTT and P.M. DOWLING. 2013. Antimicrobial Therapy in Veterinary Medicine. 5th Ed. John Wiley & Sons, Inc. Ames, Iowa, USA. LANGSTON, V. 1989. Factors to consider in the selection of antimicrobial drugs for therapy. The Compendium Vol 11 (3), 355-364. MCEWEN, S. A., & FEDORKA‐CRAY, P. J. 2002. Antimicrobial Use and Resistance in Animals. Clinical Infectious Diseases, 34(s3), S93–S106. doi:10.1086/340246 RIVIERE, J.E. and M.G. PAPICH. 2018. Veterinary Pharmacology and Therapeutics. 10th Ed. John Wiley & Sons, Inc. Hoboken, NJ, USA. UNIVERSITY OF MINNESOTA ANTIMICROBIAL RESISTANCE LEARNING SITE. 2022. Pharmacology. https://amrls.umn.edu/pharmacology#anti/ Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University  NUCLEIC ACID VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 MODULE 6 Introduction to Chemotherapy Page | 1 SYNTHESIS Module 1 INHIBITORS Introduction Overview In this module, you will learn the drugs that act on nucleic acids namely, fluoroquinolones, rifamycins, nitroimidazoles, and nitrofurans. The mechanism of action of each drug class was discussed for you to understand how they exert their antibacterial effects. Their spectrum of activity, clinical indications, pharmacokinetics, drug interactions and adverse reactions were also elaborated in sections. I. Learning Objectives Upon completion of this module, students will be able to: 1. Describe the chemical structure of nucleic acid synthesis inhibitors. 2. Explain the exact mechanism of action of nucleic acid synthesis inhibitors. 3. Identify the clinical indications of nucleic acid synthesis inhibitors. 4. Determine the pharmacokinetic properties of nucleic acid synthesis inhibitors. 5. Recognize the drug interactions and adverse reactions associated with the use of nucleic acid synthesis inhibitors. II. Learning Activities A. FLUOROQUINOLONES The original quinolone drugs (nalidixic, oxolinic and pipimedic acids) have bacterial activity restricted to Gram-negative aerobes and were administered orally for urinary tract infections in monogastric animals. Their use was limited by the development of resistance and their toxicity. Many broad-spectrum antimicrobial agents have been produced by modification of the various 4-quinolone ring structures. Fluoroquinolones are the latest and most used quinolones in veterinary medicine. Chemistry The fluoroquinolones consist of a carboxyl group, fluorine atom and piperazine ring attached to a quinoline ring. The quinolones are amphoteric and, with a few exceptions, generally have poor water solubility at pH 6–8. They appear to act as weak bases in that they are much less effective in acidic than in nonacidic urine pH. Various modifications have produced compounds with differing physical, chemical, pharmacokinetic, and antimicrobial properties. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 2 Figure 1. Typical structure of fluoroquinolones Mechanism of Action Figure 2. Inhibition of topoisomerase activity by (fluoro)quinolones The quinolones inhibit bacterial enzyme topoisomerases, including topoisomerase II (otherwise known as DNA gyrase) and topoisomerase IV. o Two-stranded DNA is tightly coiled in the cell and must be separated for transcription and translation. o To facilitate coiling, winding, and unwinding, the enzyme DNA gyrase allows the strands to be cut and reconnected. o Inhibition of topoisomerases reduces supercoiling, resulting in disruption of the spatial arrangement of DNA, and decreases DNA repair. This also results in degradation of chromosomal DNA at the replicating fork. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 3 Mammalian topoisomerase enzymes fundamentally differ from bacterial gyrase and are not susceptible to quinolone inhibition. The quinolones are usually bactericidal. o Susceptible organisms lose viability within 20 minutes following exposure to optimal concentrations of the newer fluoroquinolones. Quinolones are associated with a postantimicrobial effect in a number of bacteria, principally gram-negative (e.g., Escherichia coli, Klebsiella pneumoniae, and P. aeruginosa). The effect generally lasts 4–8 hours after exposure. Classes of Quinolones, Therapeutic Uses and Spectrum of Activity Table 1. Classes of quinolones Quinolones First Generation Third Generation 1. Nalidixic acid 1. Danofloxacin 2. Oxolinic acid 2. Enrofloxacin 3. Marbofloxacin 4. Pefloxacin 5. Pradofloxacin Second Generation Fourth Generation 1. Ciprofloxacin 1. Moxifloxacin 2. Enoxacin 2. Gatifloxacin 3. Flumequine 3. Trovafloxacin 4. Norfloxacin 4. Gemifloxacin 5. Ofloxacin 5. Garenoxacin The fluoroquinolones are active against a wide range of gram-negative organisms and several gram-positive aerobes. o Escherichia coli, Klebsiella spp., Proteus spp., Salmonella spp., and Enterobacter spp. are usually susceptible. o Ciprofloxacin has the greatest activity against Pseudomonas spp. o Staphylococcus aureus, Staphylococcus pseudintermedius, and other Staphylococcus species usually are susceptible. The fluoroquinolones are active against intracellular pathogens, including Brucella spp. Quinolones also have substantial activity against Mycoplasma, Rickettsia, and Chlamydia spp. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 4 Obligate anaerobes tend to be resistant to most quinolones, as are most enterococci (formerly group D Streptococcus spp., Enterococcus faecalis, and Enterococcus faecium). Nocardia and atypical mycobacteria may also be susceptible. The newer third- and fourth-generation fluorinated quinolones are often characterized by an effective anaerobic spectrum. Pharmacokinetics Absorption o Oral absorption of the fluoroquinolones is rapid. ▪ In dogs, cats, and pigs, oral absorption of fluoroquinolones approaches 100%, but in large animals, the extent of absorption has been less. o The presence of food may delay absorption in monogastric animals, which may impact efficacy. o Additionally, the use of antacids that contain divalent cations such as calcium or magnesium decreases fluoroquinolone bioavailability via chelation. o Absorption into the blood after IM or SC delivery is also rapid. ▪ Intramuscular bioavailability of the quinolones is nearly 100%; however, it should be noted that IM administration may be irritating to tissues. Distribution o With few exceptions, the quinolones penetrate all tissues well and quickly due to their high lipid solubility. o Particularly high concentrations are found in organs of elimination (kidneys, liver, and bile). o Concentrations found in prostatic fluid, bone, ocular fluid, endometrium, and CSF are also quite notable. o Most quinolones also cross the placental barrier. o The apparent volume of distribution of most quinolones is large. o Fluorinated quinolones as a group accumulate in phagocytic WBCs. Elimination o Some quinolones are eliminated unchanged (e.g., ofloxacin), some are partially metabolized (e.g., ciprofloxacin and enrofloxacin), and a few are completely degraded. o Metabolites are sometimes active; enrofloxacin is de-ethylated to form ciprofloxacin. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 5 o Renal excretion is the major route of elimination for most quinolones. ▪ Both glomerular filtration and tubular secretion are involved. ▪ In renal failure, clearance is impaired, and reductions in dose rates are essential. o Biliary excretion of parent drug, as well as conjugates, is an important route of elimination in some cases (e.g., ciprofloxacin, marbofloxacin, difloxacin, pefloxacin, and nalidixic acid). o Quinolones are excreted in the milk of lactating animals, often at high concentrations that persist for an extended time interval. Adverse Reactions Although adverse effects with the older quinolones (nalidixic and oxolinic acids) were relatively common, the newer ones seem to be well tolerated. High prolonged dosages in growing dogs and foals have produced cartilaginous erosions leading to permanent lameness. o Quinolones are contraindicated in the first 8 months of life for small dog breeds and 18 months of life for large breeds. o The mechanism for cartilage damage is via magnesium chelation, which decreases cell-matrix interaction in the chondrocytes, leading to radical damage, apoptosis, and tissue damage. Retinal degeneration may occur acutely in cats, with the risk greatest for enrofloxacin at doses of 5 mg/kg or higher. o Retinal damage occurs due to changes in the ABCG2 transporter leading to accumulation of photoreactive fluoroquinolones in the retina. Once the retina is exposed to light, these photoreactive fluoroquinolones lead to the generation of reactive oxygen species, which attack cellular lipid membranes then causing cause tissue damage, retinal degeneration, and blindness. o Pradofloxacin may be the least retinotoxic, followed by marbofloxacin and orbifloxacin. Quinolones tend to be neurotoxic, and convulsions can occur at high doses due to gamma-aminobutyric acid (GABA) receptor antagonism. o Rapid IV administration of high doses of enrofloxacin in horses results in transient neurologic clinical signs that include excitability and seizure-like activity. Vomiting and diarrhea may develop with fluoroquinolones. Both ciprofloxacin and moxifloxacin have been associated with potentially fatal antimicrobial-induced colitis. An emerging toxicity associated with fluoroquinolones is mitotoxicity (ie, damage to mitochondrial topoisomerase or other mitochondrial structures). Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 6 Drug Interactions Antacids or other drugs containing multivalent cations and sucralfate appear to interfere with the GI absorption of the quinolones. Combinations with other antibiotics neither antagonize nor enhance the microbiological effects of fluoroquinolones but nitrofurantoin impairs the efficacy of quinolones if used concurrently for urinary tract infections. Quinolones inhibit the biotransformation of methylxanthines, with theophylline being the most clinically relevant but also including caffeine and theobromine. o This inhibition leads to increased serum concentrations of methylxanthines that can result in CNS and cardiac toxicity. Bacterial Resistance Chromosomal mutational resistance to the original fluoroquinolones was considered to be low in frequency, and plasmid-mediated resistance nonexistent. Resistance to newer drugs (eg, gemifloxacin, trovafloxacin, gatifloxacin, and pradofloxacin) may be slower to emerge because of larger side chains that facilitate binding to either DNA gyrase or topoisomerase IV. Generally, there is cross-resistance among the fluoroquinolones. Alteration of topoisomerases. Resistance mechanisms in gram-negative bacteria more commonly target DNA gyrase while the primary target of resistance mechanisms in gram-positive organisms tends to be topoisomerase IV, followed by changes in DNA gyrase. Decreased permeability of the drug and increased membrane expression of efflux transporters. The combined effect of increased efflux pumps and decreased porins act in concert to decrease intracellular concentrations. Dose Rates Norfloxacin Dog/Cat: 22 mg/kg q12h oral (dog not < 8 months ) Enrofloxacin Dog/Cat: 2.5 mg /kg q 12 oral or IM, SC Swine: 2.5-5 mg /kg q 24h oral IM Poultry: 50 ppm in water oral; 0.5 mg/bird SID, IM Ciprofloxacin Dog/ Cat: 5-8 mg/kg q 12 oral Flumequine Poultry: 12.5 mg/kg in drinking water Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 7 B. RIFAMYCINS Rifamycins are a group of structurally similar complex of macrocyclic, high- molecular-weight antibiotics produced from Streptomyces mediterranei obtained from the soil of the pine forests of southern France. The semisynthetic derivatives of natural rifamycins include rifamycin, rifampin [rifampicin], and rifamide. Mechanism of Action Rifamycins are bactericidal antibiotics that bind to the subunits of sensitive DNA- dependent RNA polymerase. o This binding results in inactive enzymes and inhibition of RNA synthesis by preventing chain initiation. o This inhibition can also occur in mammalian cells, but much higher concentrations are needed. Spectrum of Activity and Therapeutic Use For the treatment of foals with Rhodococcus equi infection, rifampin has been combined with macrolide antibiotics – erythromycin, azithromycin, or clarithromycin most commonly. Susceptible organisms of interest to veterinarians include Staphylococcus species (including methicillin-resistant strains), Streptococcus spp., including Streptococcus zooepidemicus, Rhodococcus equi, Corynebacterium pseudotuberculosis, and most strains of Bacteroides spp., Clostridium spp., Neisseria spp., and Listeria spp. Organisms known to be resistant to rifampin are Pseudomonas aeruginosa, E. coli, Enterobacter spp., Klebsiella pneumoniae, Proteus spp., and Salmonella. Rifampin has also been used to treat Mycobacterium tuberculosis in elephants (10 mg/kg per day). The most common use in ruminants is for treatment of Mycobacterium paratuberculosis in cattle and sheep. It may cause remission of the infection, but does not eradicate the organism. There is synergism with amphotericin B against some fungi such as Saccharomyces cerevisiae, Histoplasma capsulatum, several species of Aspergillus and Blastomyces dermatitidis. Pharmacokinetics Absorption o Rifampin is readily but incompletely absorbed from the GI tract. o Rifampin absorption is highest in an acidic environment, although feeding has decreased oral absorption in foals and ruminants. o Because rifamycins penetrate tissues and cells to a substantial degree, they are particularly effective against intracellular organisms. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 8 Distribution o It is widely distributed in body tissues and fluids because of its high lipid solubility. o Despite high protein binding, high concentrations of the drug are found in the lungs, pulmonary epithelial lining fluid, liver, bile, and urine Elimination o Rifampin is biotransformed to several metabolites, some of which are active, and is primarily excreted in bile (used for cholangitis in people) and to a lesser degree in urine. o Enterohepatic cycling of the parent drug and its main metabolite (desacetylrifampin) commonly occurs. o Use of rifampin may affect the elimination of other hepatically metabolized drugs (e.g., barbiturates). Adverse Reactions and Toxicity Adverse effects have been associated with high doses and include liver injury and gastrointestinal disturbances. o Hepatotoxicity may occur in animals with preexisting liver disease. Rifampin may produce red-orange colored urine, sweat, and saliva but this is not harmful. Rifampin is teratogenic in laboratory animals, so its use in pregnant animals should be restricted. C. NITROFURANS Nitrofurans are comprised of several synthetic compounds derived from 5- nitrofuran. The 5-nitro group is required for antimicrobial activity. Over 3,500 nitrofurans have been synthesized to date, with only a handful being useful in animal chemotherapy. Sample compounds used in veterinary medicine include Nitrofurazone and Nitrofurantoin. Furazolidone is banned from use in food-producing animals. Mechanism of Action The nitrofurans serve as substrates of bacterial reductase enzymes. The nitrofurans are reduced by bacteria to reactive intermediates that inhibit nucleic acid synthesis. o They produce DNA fragmentation and may also block mRNA translation. The nitrofurans appear to inhibit a number of microbial enzyme systems, including those involved in carbohydrate metabolism. They are broad spectrum and bacteriostatic. Noraine College Paray Science of Veterinary Medina, and DVSM, Medicine MSc, PhD Marvin Central Bryan Luzon Segundo Salinas, DVM, MSc State University Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 6 Introduction to Chemotherapy Page | 9 Classes of Nitrofurans, Therapeutic Uses and Spectrum of Activity Table 2. Nitrofurans and their spectrum of activity and clinical uses Nitrofurans Spectrum of Activity Clinical Indication Nitrofurantoin E. coli, Staphylococcus Nitrofurantoin is occasionally aureus, Streptococcus used in the treatment of lower pyogenes, Aerobacter urinary tract infections in dogs aerogenes are susceptible. and cats. Proteus spp., Pseudomonas aeruginosa and Streptococcus faecalis are usually resistant. Nitrofurazone It is active against both Nitrofurazone is commonly used Gram-positive and Gram- as a topical antimicrobial in negative bacteria.