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INTRODUCTION VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 MODULE 1 Introduction to Chemotherapy Page | 1...

INTRODUCTION VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 MODULE 1 Introduction to Chemotherapy Page | 1 TO CHEMOTHERAPY Module 1 Introduction Overview In this module, you will be introduced to chemotherapeutics and the different chemotherapeutic agents. Various factors that should be considered in using these drugs will also be discussed as they affect therapeutic outcomes. Occasionally, there are unfavorable effects of antimicrobial therapy, and they will be reviewed in the latter part of the module, along with the probable reasons for therapeutic failure. I. Learning Objectives Upon completion of this module, students will be able to: 1. Explain the concept of chemotherapy. 2. Differentiate the different chemotherapeutic agents. 3. Describe the roles of antimicrobial agents in farm and companion animal practice. 4. Discuss the various considerations for the proper selection of antimicrobial agents. 5. Recognize the requisites for a successful antimicrobial therapy as well as the possible reasons for therapeutic failure and other potential unfavorable therapeutic outcomes. II. Learning Activities Chemotherapy is a branch of pharmacology dealing with drugs that selectively inhibit or destroy specific agents of disease such as bacteria, viruses, fungi, protozoa, worms, arachnids and insects, and neoplastic cells. Chemotherapeutic agent (or anti-infective or antimicrobial agent) is a drug used for chemotherapy. They have a selective toxicity, a property of a substance of being more harmful to certain living organism but not to others. Examples: Antibacterial – against bacteria Antiviral – against viruses Antiprotozoal- against protozoa Antifungal (antimycotic) – against fungi Anthelmintics- against parasitic worms Antineoplastic (anticancer)- against neoplastic cells Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 2 Antimicrobial is a natural or synthetic drug that acts against microorganism of one or more kinds Antibiotic is an antimicrobial agent naturally produced by various species of microorganisms (bacteria, fungi, and actinomycetes) which inhibits the growth of another microorganism. Principles of Antimicrobial Therapy Uses of Antimicrobial Agents Table 1. Types of antimicrobials use in food animals. Type of Purpose Route or Administration Diseased Animals Antimicrobial vehicle of to individuals Use administation or groups* Therapeutic Therapy Injection, Individual or Diseased individuals; feed, water group in groups, may include some animals that are not diseased or are subclinical Metaphylactic Disease Injection Group Some prophylaxis, (feedlot therapy calves), feed, water Prophylactic Disease Feed Group None evident, prevention although some animals may be subclinical Subtherapeutic Growth Feed Group None promotion Feed Feed Group None efficiency Feed Group None Disease prophylaxis *Food animals are usually grouped by pen, flock, pond, barn, or other aggregate. 1. Therapeutic use refers to the treatment of clinically ill animals, as well as to the use of antibiotics to prevent and control disease. 2. Non-therapeutic uses fall into two main categories: a. Use of antimicrobial drugs in animals for growth promotion Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 3 b. Use of antimicrobial drugs in animals to improve feed efficiency Prophylaxis refers to the use of antimicrobials in animals that are not exhibiting signs of an infection but are at high risk of acquiring it. Metaphylaxis refers to their use in animals that are vulnerable during an infectious disease outbreak due to exposure to disease agents or extremely unfavorable host or environmental conditions. Subtherapeutic Uses The mechanism by which antibiotics at subtherapeutic concentrations enhance growth remains unclear. Potential modes of action include: 1. Suppression of normal intestinal bacteria, leading to increased nutrient availability to the animal 2. Decrease of harmful metabolites produced by intestinal bacteria 3. Thinning of intestinal wall resulting in increased absorption of dietary nutrients 4. Anti-inflammatory effects that are independent of the gut microbiota 5. Inhibition of endemic subclinical disease Chemotherapeutic Triangle and Other Considerations Figure 1. Chemotherapeutic Triangle An effective antimicrobial therapy requires the knowledge on the complex interrelationships factors among the host animal (patient), the pathogen (disease-causing organism) and the drug (chemotherapeutic agent). Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 4 Ideally, the pathogen should be identified to determine the available options for therapy. How it affects the host and how the later responds to the infection are equally important factors in choosing the type of antimicrobial agent. The effectiveness of the antimicrobial agent depends on its pharmacologic action as well as pharmacokinetic properties. The right dose and route are essential to maintain the therapeutic levels of drug in the animal patient, though drug toxicity, allergic reaction and/or biological alteration can prevent their usage. Other factors such as public health safety and economic value of the animal and the drug may also determine the use of some antimicrobial agents. The emergence of antimicrobial resistance has led to the use of alternative drugs. Figure 2. Factors to consider prior to chemotherapy apart from the components of the chemotherapeutic triangle Requirements for Successful Antimicrobial Therapy 1. Correct clinical and microbiologic diagnosis a. Successful chemotherapy usually requires specific diagnosis, even though a reasonable preliminary diagnosis is often all that is possible initially. b. Examination of direct smears is helpful in establishing the general type of pathogens involved in the infection. Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 5 c. Treatment should be aimed at a specific pathogen whenever possible. However, mixed infections do occur and pose problems in therapy. d. Conclusive microbial diagnosis oftentimes is difficult. The treatment may be based on past clinical experience, that is, a clinician may diagnose an infection by an “educated guess”. 2. Appropriate choice of chemotherapeutic agent a. The infecting organism should be sensitive to the antimicrobial agent. Antimicrobial sensitivity test provides a sound foundation from which to proceed with the selection of appropriate antimicrobial drugs. b. Sensitivity test takes time and may not be too important in emergency cases. It is not always advantageous to wait for the result of sensitivity test before starting an antimicrobial therapy. c. Pseudo-sensitivity may also occur. This refers to the susceptibility of an organism to antimicrobial agent in vitro which cannot be demonstrated in vivo. This may be due to local factors that affect the pharmacokinetics of the agent. d. In selecting appropriate antimicrobial drug, it is best to consider the following: i. Probable microorganism involved in infection ii. Result of the sensitivity test (if possible) iii. Pathogenicity of the organism involved iv. Presence and nature of pathological lesions v. Duration of the infection vi. Pharmacokinetics (physiological disposition) of the drugs vii. Potential drug toxicity viii. Organ dysfunction in the host ix. Drug interactions x. Cost of treatment (especially in treating food animals) 3. Appropriate dosage regimen must be given a. The right dose, route and frequency of administration will achieve adequate therapeutic levels at the site of infection for a sufficient period without causing serious side effects. b. The drug must be available at the site of infection in a concentration above the minimum inhibitory concentration (MIC). i. Minimum inhibitory concentration (MIC) – the lowest dilution of the drug that prevents visible growth of microorganism. Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 6 ii. Breakpoint of antimicrobial agent – is the plasma concentration above which the host is likely to experience toxicity and is dependent on the pharmacokinetics of the antibiotic in the host. 4. Supportive therapy and ancillary treatment a. Fluid therapy b. Warmth c. Rest d. Good nutrition Essentials for a Successful Antimicrobial Therapy 1. Drug must come in contact with the microorganism. 2. Drug must be present in high enough concentrations. 3. Drug must be present for a sufficient length of time. 4. The microorganism must be sensitive to the drug. Reasons for Therapeutic Failure of Antimicrobial Agent 1. Wrong diagnosis was made. 2. Organism is not susceptible to the antimicrobial agents. 3. Mixed infection was present. 4. Combination of incompatible antimicrobial agent resulting to decreased antimicrobial effects. 5. Superinfection by a resistant opportunistic pathogen. 6. Re-infection by a resistant opportunistic pathogen, original or other pathogenic microorganism. 7. In surgical infections, drainage was inadequate or foreign body was present. 8. Reduction of effectiveness of antimicrobial agent due to some changes like hypoxia, acidosis, accumulation of tissue debris, inflammation, tissue destruction, abscessation, etc. 9. Defense mechanism of the patient were jeopardized by disease and concurrent therapy. 10. Inappropriate route of administration or incorrect dosage side effect 11. Expired or substandard products were used. 12. Selected agent had to be withdrawn because of adverse side effects. 13. Supportive therapy and nursing case were inadequate. 14. Nutritional deficits and predisposing management factors were not corrected. Possible Effects of Antimicrobial Therapy 1. No effect 2. Superinfection- new infection arising from the use of broad-spectrum antimicrobial 3. Induction of antimicrobial resistance to drugs 4. Toxic effects or adverse reactions with other drugs or with nutrients 5. Allergy Noraine College Paray Science of Veterinary Medina, and DVSM, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Module 1 Introduction to Chemotherapy Page | 7 6. Drug residues in animal tissue III. Study Questions 1. What is the difference of an antimicrobial from an antibiotic? 2. Give examples of specific antimicrobial agents. 3. What are the specific uses of antimicrobial agents in the livestock and poultry industry? 4. What are the factors to consider before choosing an antimicrobial agent? 5. What are the potential outcomes of antimicrobial use? References 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, MSc, PhD Medicine Marvin Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University  INTRODUCTION TO VPHAR 4205 –Veterinary Clinical Pharmacology Module 2 MODULE 2 Introduction to Chemotherapy Page | 1 ANTIBACTERIAL Module 1 Introduction AGENTS Overview In this module, you will know the different classifications of antibacterial agents as well as the microbial sources of the various antibiotics. Likewise, you will be introduced to antimicrobial resistance, including its types, mechanisms and management. I. Learning Objectives Upon completion of this module, students will be able to: 1. Classify the different antibacterial agents based of spectrum, effect and mechanism of action. 2. Identify the various sources of antibiotics. 3. Define antimicrobial resistance. 4. Explain the types and mechanisms of bacterial resistance. 5. Enumerate the approaches to minimize the risk of emerging resistance following the “3Ds”. 6. Discuss the interactions of antibacterial drug combinations. II. Learning Activities ANTIBACTERIALS are specific agents used against bacterial pathogens. However, their efficacy may not be limited to these organisms. CLASSIFICATION OF ANTIBACTERIAL AGENTS 1. Spectrum of activity a. Narrow spectrum– drugs that act specifically on the gram-positive family, OR specifically on the gram-negative family of bacteria b. Broad spectrum – those that act on both gram-positive AND gram- negative bacteria However, this distinction is not always absolute, as some agents may be primarily active against Gram-positive bacteria but will also inhibit some Gram- negatives. 2. Effect on bacteria a. Bactericidal- kills bacteria at the minimum inhibitory concentration. 1. These agents are more prone to causing superinfection because they may kill normal bacterial flora, which normally inhibits pathogens. Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 2 b. Bacteriostatic- inhibit bacterial growth and replication. 1. The normal defense mechanism of the patient must participate in eliminating the infectious bacteria. 2. Bacteriostatic agents may antagonize the action of some bactericidal agents. These definitions are not absolute; bacteriostatic drugs may kill some susceptible bacterial species, and bactericidal drugs may only inhibit growth of some susceptible bacterial species. Bacteriostatic agents become bactericidal when very high doses are given or when high concentrations are attained in certain body compartments such as the urinary tract, but toxic effects on the patient may appear. 3. Mechanism of action a. Inhibitors of cell wall synthesis b. Disruptors of cell membrane c. Inhibitors of protein synthesis d. Inhibitors of nucleic acid functions e. Inhibitors of folate cofactor synthesis Figure 1. Bacterial targets employed in the classification of antibacterials according to the mechanism of action Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 3 Table 1. Classification of antibacterial agents Mechanism of Action Spectrum of activity Effect on Pathogen A. Inhibitors of Cell wall Synthesis Penicillin Gram (+) cocci and bacilli Bactericidal Cephalosporins Broad spectrum Bactericidal Bacitracin Gram (+) cocci and bacilli Bactericidal Vancomycin Gram (+) cocci and bacilli Bactericidal B. Impairment of Cell Membrane Polymyxin B Gram (-) bacilli Bactericidal Colistin Gram (-) bacilli Bactericidal Tyrothricin Narrow Spectrum Bactericidal Gramicidin Narrow Spectrum Bactericidal Amphotericin B Broad spectrum Bactericidal Nystatin Broad spectrum Bactericidal C. Inhibition of Protein Synthesis Aminoglycosides Streptomycin Gram (-) bacilli Bactericidal Dihydrostreptomycin Gram (-) bacilli Bactericidal Neomycin Broad spectrum Bactericidal Gentamicin Broad spectrum Bactericidal Kanamycin Broad spectrum Bactericidal Apramycin Gram (-) bacilli Bactericidal Spectinomycin Gram (-) bacilli Bacteriostatic Tetracycline Broad spectrum Bacteriostatic Chloramphenicol Broad spectrum Bacteriostatic Macrolide Tylosin Gram (+) cocci and bacilli Bacteriostatic Erythromycin Gram (+) cocci and bacilli Bacteriostatic Spiramycin Gram (+) cocci and bacilli Bacteriostatic Oleandomycin Gram (+) cocci and bacilli Bacteriostatic Carbomycin Gram (+) cocci and bacilli Bacteriostatic Lincosamide Lincomycin Gram (+) cocci and bacilli Bacteriostatic Clindamycin Gram (+) cocci and bacilli Bacteriostatic Tiamulin Gram (+) Bacteriostatic Virginiamycin Gram (+) Bacteriostatic D. Inhibitors of Nucleic Acid Quinolones Broad spectrum Bactericidal Enrofloxacin Bactericidal Oxolinic acid Bactericidal Nalidixic acid Bacteriostatic Novobiocin Gram (+) spectrum Bacteriostatic Rifamycin Broad spectrum Bactericidal Nitrofurans Broad spectrum Bacteriostatic Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 4 Nitrofurazone Furazolidone Furaltadone E. Inhibitors of Folate Cofactor Synthesis Sulfonamides Bacteriostatic Sulfonamide-Trimethoprim Broad spectrum Bactericidal Trimethoprim Bacteriostatic Table 2. Review on the classification of bacterial pathogens Gram Positive Organisms Gram Negative Organisms Cocci Bacilli Cocci Bacilli Enterococcus Actinomyces Branamella Parvobartonella Staphylococcus Bacillus Neiserria Bartonella Streptococcus Clostridium Bordetella Corynebacterium Brucella Erysipelothrix Campylobacter Listeria Francisella Nocardia Haemophilus Mycobacterium Pasteurella Mycoplasma * Coliform Rhodococcus Escherichia Streptomyces Klebsiella Salmonella *(pleomorphic) Shigella Proteus Pseudomonas Vibrio Yersinia Anaerobes Bacteroides Fusobacterium Spirochetes Borrelia Leptospira Treponema Table 3. Microbial sources of antibiotics Sources of Antibiotics Apramycin S. tenebrarius Bacitracin Bacillus subtilis Cephalosporins Cephalosporium acremonium Chloramphenicol S. Venezuela Chlortetracycline Streptomyces aureofaciens Erythromycin S. erythreus Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 5 Gentamicin Micromonaspora purpurea Kanamycin S. kanamyceticus Lincomycin S. lincolnensis Neomycin/Tylosin S. fradiae Novobiocin S. niveus Oleandromycin S. antibioticus Oxytetracycline Streptomyces rimosus Penicillin Penicillium notatum; P. chrysogenum Polymyxin B B. polymyxa Spectinomycin S. flavopersicus Streptomycin Streptomyces griseus Thiostrepton A. aureus Tyrothricin Bacillus brevis Virginiamycin S. virginiae Bacterial Resistance to Antibiotics Antibiotic resistance occurs when bacteria evolve to evade the effect of antibiotics through multiple different mechanisms. Type of Resistance 1. Intrinsic resistance is the inherited property of the bacteria based on lack of either antimicrobial target sites or accessibility to them. i. It is natural to all the members of a specific bacterial taxonomic group, such as a bacterial genus, species, or subspecies. Example: Gram negative bacteria are resistant to penicillin because the latter attack only the cell wall which gram-negative organism only has few. 2. Acquired resistance results from the alteration of the genetic makeup of an organism, making it resistant to chemotherapeutic drugs. i. Genes for resistance can be acquired either spontaneously due to mutations or through sharing of genetic material via plasmids or transposons. ii. It can be manifested as resistance to a single agent, to some but not all agents within a class of antimicrobial agents, to an entire class of antimicrobial agents, or even to agents of several different classes. iii. Because of the changing patterns of sensitivity of many microorganisms to drugs, antibiotic susceptibility testing is a requirement in the treatment of infection. Mutation – is a random event where the bacterial chromosomal mutation may confer resistance with a widespread use of a particular drug, which suppresses the susceptible bacteria, hence, the resistant bacteria predominate. Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 6 i. Mutations occur continuously but at relatively low frequency in bacteria, thus leading to the occasional random emergence of resistant mutants. ii. Examples include mutated penicillin-binding proteins that confer methicillin resistance to staphylococci, mutations in DNA gyrases that confer resistance to fluoroquinolones, or mutations in ribosomal subunits that confer resistance to various ribosomal inhibitors. iii. Mutations also generally cause single-drug resistance. iv. However, for the majority of clinical isolates, antimicrobial resistance results from acquisition of extrachromosomal resistance genes. Acquisition of genetic material that confers resistance is possible through all of the main routes by which bacteria acquire any genetic material: Figure 2. The three mechanisms of horizontal transfer of genetic material between bacteria. 1. Transduction – transference of drug-resistant gene by a bacteriophage. i. It is particularly important in the transfer of antibacterial resistance among strains of Staphylococcus aureus. 2. Transformation - involves the incorporation into the bacteria of DNA encoding for drug resistance from the environment after its secretion or release by resistant organisms. 3. Conjugation – is a type of reproduction in which genetic material is transferred from cell to cell via pilus, that is encoded by a resistance transfer factor on a plasmid. i. Resistance factors (R factors) from plasmid DNA and/or chromosomal DNA may encode for resistance to multiple drugs and may be rapidly transferred to the bacterial population. This is termed Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 7 infectious drug resistance or transferable drug resistance and has been observed clinically in enteric infections with Salmonella spp., Shigella spp., and Escherichia coli. Plasmids and transposons can also rapidly disseminate resistance genes. Transfer via plasmids is the most recognized mechanism of shared resistance. i. Plasmids are extrachromosomal self-replicating genetic elements that are not essential to survival but that typically carry genes that impart some selective advantage(s) to their host bacterium, such as antimicrobial resistance genes. ii. Transposons (“jumping genes”) are genetic elements that can move from one location on the chromosome to another; the transposase genes required for such movement are located within the transposon itself. Mechanisms of Bacterial Resistance 1. Enzymatic inactivation – organism may produce enzymes via constitutive or inducible processes that inactivate the drug. Examples of antibiotic-inactivating enzymes include: a. β-lactamases (“penicillinases”) that hydrolytically inactivate the β-lactam ring of penicillins, cephalosporins, and related drugs b. acetyltransferases that transfer an acetyl group to the antibiotic, inactivating chloramphenicol or aminoglycosides c. esterases that hydrolyze the lactone ring of macrolides. 2. Decreased cell wall permeability- this will limit the uptake of drug by the organism. Reduced permeability can be due to either lack of permeability of the outer membrane (e.g., down-regulation of porins in Gram-negatives) or of the cell membrane (e.g., lack of aminoglycoside active transport under anaerobic conditions). 3. Active transport- this will increase the release of drug from the organism. Presence of an efflux pump can limit levels of a drug in an organism, as seen with tetracyclines. 4. Alteration of drug receptor – or binding site may result in reduced drug affinity. For example, resistance to β-lactam antibiotics involves alterations in one or more of the major bacterial penicillin-binding proteins, resulting in decreased binding of the antibiotic to its target. 5. Development of alternative metabolic or synthetic pathways- this results to the by-pass or repair of the effects of antimicrobial agents. Increased synthesis of a key metabolic intermediate that would thus require higher concentrations of the drug (e.g., para-aminobenzoic acid in sulfonamide resistance). Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 8 Figure 3. Common mechanisms of antibiotic resistance Factors that Contribute to the Development and Spread of Antibiotic Resistance Antimicrobial use in animals apparently contributes to the selection and spread of resistance among populations of bacteria in animals. 1. Overuse and misuse a. Frequent exposure to the same class of antibiotic b. Prior use of a less-effective drug of the same antibiotic class 2. Subtherapeutic dosing a. Inadequate levels of antibiotic at the site of infection 3. Noncompliance with the recommended course of treatment a. Too short treatment time. Ideal is 5-7 days. 4. Poor infection control, hygiene and sanitation a. Elevated number of microorganisms b. Spread of resistant organisms by means of infected carrier animals, contaminated feedstuffs, wildlife vectors, or on humans wearing pathogen- contaminated clothing c. Fecal waste disposal 5. Absence of new antibiotics being discovered Management to Prevent Bacterial Resistance 1. Decontamination- attentiveness to hygiene in the hospital and the home environment to decrease the spread of potentially resistant microbes 2. De-escalation- includes avoiding inappropriate or unnecessary systemic antimicrobial use. 3. Designing a dosing regimen- requires willingness to modify the routine recommended dosing regimens needed for individual patients. Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 9 a. The pathogen(s) should be identified and characterized, including antimicrobial susceptibility, so that the drug matches the organism as closely as possible, narrowing the spectrum of drug used. b. Once drugs to which the isolates are susceptible are identified, one that is more likely to penetrate the infected tissue should be chosen. i. Debris (eg, inflammatory materials, necrotic tissue, foreign bodies) and biofilm or reduced blood flow and hypoxia contribute to antimicrobial failure. ii. Intracellular organisms are able to avoid detrimental effects by phagocytic cells. iii. Dosing regimens in at-risk animals should be designed to kill. Bactericidal drugs should be chosen whenever possible. Combination Therapy. Treatment with antimicrobial combinations may be necessary in certain cases. The administration of 2 or more agents may be beneficial for the following reasons: a. Treatment of mixed bacterial infections b. Therapy of severe infections in which the specific cause is unknown c. Enhancement of antibacterial activity in the treatment of specific infections d. Prevention of the emergence of resistant microorganisms e. Reduction of toxicity in rationally combined drugs Guidelines for Combination Therapy a. Generally, only antibiotics with similar efficacy should be combined. b. Agents should act at different sites to avoid competing on the same enzyme and substrate. Possible Results of Combining Antimicrobials a. Additive / indifferent- if the combined effects of the drugs equal the sum of their independent activities measured separately. b. Synergistic- if the combined effects are significantly greater than the independent effects i. sequential inhibition of successive steps in metabolism (trimethoprim-sulfonamide) ii. sequential inhibition of cell wall synthesis (mecillinam- ampicillin) iii. facilitation of drug entry of one antibiotic by another (beta- lactam - aminoglycoside) iv. inhibition of inactivating enzymes (ampicillin - clavulanic acid) v. prevention of emergence of resistant populations (erythromycin - rifampin) Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University VPHAR 4205 –Veterinary Clinical Pharmacology Introduction to Chemotherapy Module 2 Page | 10 c. Antagonistic- if the combined effects are significantly less than their independent effects. The effects of several bactericidal antibiotics are substantially impaired by simultaneous use of drugs that impair microbial growth or “bacteriostatic” drugs (eg, most ribosomal inhibitors). i. inhibition of bactericidal activity such as treatment of meningitis in which a bacteriostatic drug prevents the bactericidal activity of another ii. competition for drug-binding sites such as macrolide- chloramphenicol combinations (of uncertain clinical significance) iii. inhibition of cell permeability mechanisms such as chloramphenicol- aminoglycoside combinations (of uncertain clinical significance) iv. induction of beta-lactamases by beta-lactam drugs such as imipenem and cefoxitin combined with older beta-lactam drugs that are beta-lactamase unstable. III. Study Questions 1. Enumerate at least 10 antibacterials and explain their mechanism of action. 2. What are the considerations in using bacteriostatic and bactericidal agents? 3. What are the types of antimicrobial resistance? 4. How does a bacteria become resistant to an antibacterial agent? 5. Give example of combination therapy and its advantage. References 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, DVSM, MSc, PhD and Medicine Central Marvin Luzon State University Bryan Segundo Salinas, DVM, MSc Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University  INHIBITORS OF VPHAR 4205 –Veterinary Clinical Pharmacology Module 3 MODULE 3 Introduction to Chemotherapy Page | 1 CELL Module 1WALL SYNTHESIS Introduction Overview In this module, you will learn the drugs that act on bacterial cell wall namely, penicillins, cephalosporins, bacitracin, vancomycin, fosfomycin, monobactams, and carbapenems. 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 beta-lactam antibiotics and other related drugs. 2. Explain the exact mechanism of action of the cell wall synthesis inhibitors. 3. Classify penicillins and cephalosporins based on their spectrum of activity and susceptibility to β-lactamases. 4. Identify the clinical indications of cell wall synthesis inhibitors. 5. Determine the pharmacokinetic properties of cell wall synthesis inhibitors. 6. Recognize the drug interactions and adverse reactions associated with the use of cell wall synthesis inhibitors. II. Learning Activities A. PENICILLINS Penicillin was the first antibiotic available for clinical use. It was discovered by Alexander Fleming who observed that a Penicillium notatum mold contaminating a Petri dish culture of staphylococci colonies was surrounded by a clear zone free of growth. Since then, more than 40 penicillins have been identified. Some occur naturally; others are biosynthesized (semisynthetic). 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 3 Introduction to Chemotherapy Page | 2 Chemistry Figure 1. Core structure of penicillins Penicillin is comprised of a thiazolidine ring (1), beta-lactam ring (2) and attached to amide chain (4) and carboxyl chain, site of salt formation (5). Cleavage of the β-lactam ring destroys antibiotic activity. Amidase cleavage of the amide bond side chain yields the 6-amino-penicillanic acid (6-APA) nucleus used in producing semisynthetic penicillins. The carboxyl group attached to the thiazolidone ring is the site of salt formation (sodium, potassium, procaine, etc.) which stabilizes the penicillins and affects solubility and absorption rates. Cleavage by a β-lactamase enzyme (penicillinase) produces penicilloic acid derivatives that are inactive but may act as the antigenic determinants for penicillin hypersensitivity. Mechanism of Action Bactericidal effect is due to the prevention of cell wall synthesis, thereby disrupting bacterial cell wall integrity. Growing bacteria and metabolically active bacterial cells are most susceptible to this effect. Cell wall synthesis involves (a) formation of precursors N-acetylglucosamine (NAGA) and N-acetylmuramic acid (NAMA); (b) formation of peptidoglycan chain; and (c) cross-linking of the peptidoglycan chain by the transpeptidase enzyme. Penicillins bind to and inhibit the transpeptidase enzyme involved in the cross- linking of the bacterial cell wall, the third and final step in cell-wall synthesis. The weakened cell wall ruptures, resulting in lysis and cell death. Penicillins also inhibit other peptidases (penicillin-binding proteins, PBPs) involved in cell wall synthesis and block the inhibition of autolysins. 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 3 Introduction to Chemotherapy Page | 3 Figure 2. Comparison of the structure and composition of Gram (+) and Gram (-) cell walls Classes of Penicillin, Therapeutic Uses and Spectrum of Activity The penicillins are primarily effective against Gram(+) aerobes and anaerobes. The broad-spectrum, semisynthetic penicillins are also effective against some Gram(–) pathogens. The penicillins are commonly used to treat or prevent local and systemic infections caused by susceptible bacteria. o Because of their synergistic interaction with other antimicrobials, they are often used as part of combination therapy. o Penicillins also are used topically in the eye and ear as well as on the skin; intramammary administration is common for treatment or prevention of bovine mastitis. o Amoxicillin with or without clavulanic acid is among the first-choice antimicrobials for treatment of canine or feline urinary tract infections. Penicillins are divided into subclasses based on chemical structure (eg, penicillins, monobactams, and carbapenems), spectrum (narrow, broad, or extended), source (natural, semisynthetic, or synthetic), and susceptibility to β-lactamase destruction. 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 3 Introduction to Chemotherapy Page | 4 Table 1. Spectrum of activity of the classes of penicillin Types of Penicillins Spectrum of Activity Natural Penicillins (Narrow-spectrum β-Lactamase–sensitive Penicillins) 1. Penicillin G (Benzathine, Streptococci, penicillin-sensitive Procaine, Na and K salts) Staphylococci, Trueperella (Arcanobacterium) 2. Penicillin V (Na and K salts) pyogenes, Clostridium spp., Erysipelothrix rhusiopathiae, Actinomyces bovis, Leptospira canicola, Bacillus anthracis, Fusiformis nodosus, and Nocardia spp. With a few exceptions (Haemophilus and Neisseria spp. and strains of Bacteroides other than B. fragilis), they are inactive against gram- negative organisms at usual concentrations. Broad-spectrum β-Lactamase–sensitive Penicillins Aminopenicillins Staphylococcus, Streptococcus, Trueperella, 1. Ampicillin Clostridium, Escherichia, Klebsiella, Shigella, 2. Amoxicillin Salmonella, Proteus, and Pasteurella Ampicillin precursors Aminopenicillins are inactive against 1. Hetacillin Pseudomonas, Bacteroides fragilis, and 2. Pivampicillin penicillinase-producing Staphylococcus spp. 3. Talampicillin Mecillinam Broad-spectrum β-Lactamase–sensitive Penicillins with Extended Spectra Carboxypenicillins Active against many strains of 1. Carbenicillin (its acid-stable Enterobacteriaceae and some strains of indanyl ester) Pseudomonas. 2. Ticarcillin Some activity against gram-positive aerobic Ureido-penicillins and anaerobic bacteria but have no 1. Azlocillin advantages against these organisms 2. Mezlocillin compared to penicillin G and aminopenicillins. Piperazine penicillins More active against Bacteroides fragilis than 1. Piperacillin are other available penicillins. Carbenicillin and ticarcillin are active against some strains of Some strains of E. coli, Morganella morganii, Proteus spp., and Salmonella. Mezlocillin and piperacillin are active against several Shigella and Proteus strains, and some Citrobacter and Enterobacter 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 3 Introduction to Chemotherapy Page | 5 Narrow-spectrum β-Lactamase–resistant Penicillins Isoxazolyl penicillins These antistaphylocccal penicillins are active 1. Oxacillin against many penicillinase-producing 2. Cloxacillin Staphylococcus spp., especially S. aureus and 3. Dicloxacillin S. epidermidis. 4. Flucloxacillin They are not as active against many gram- Methicillin positive bacteria as penicillin G and are inactive Nafcillin against almost all gram-negative bacteria. Temocillin β-Lactamase Inhibitors They are a specific class of drugs with little antibacterial effects of their own, but bind to the β-lactamase enzyme that is produced by gram-negative or gram-positive bacteria. o They have structures that resemble the β-lactam antibiotics. They are always combined with another active drug of the β-lactam class. Examples include: o Amoxicillin–clavulanic acid o Sulbactam–ampicillin o Ticarcillin–clavulanic acid o Piperacillin–tazobactam o The new β-lactamase inhibitor combinations with no record of use in veterinary medicine are Ceftazidime–avibactam and ceftolozane– tazobactam. All β-lactamase inhibitors are not equal with respect to potency and ability to bind β-lactamase enzymes. o Compared to clavulanate, sulbactam is less active against β- lactamase of Staphylococcus, Bacteroides, and some E. coli. Pharmacokinetics Absorption o Acid- stable penicillins (Penicillin V, Ampicillin, Amoxicillin, Hetacillin, Oxacillin, Cloxacillin and indanyl salt of Carbenicillin) are well absorbed orally, although food impairs the absorption of ampicillin. o Penicillin G, Methicillin, Ticarcillin are acid-labile hence poorly absorbed in the gut. o The long-acting formulations (procaine- or benzathine–penicillin) are given IM or SC (never IV). Procaine Penicillin G are slowly absorbed from IM injection and provide therapeutic levels for 24 hours with single dose. 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 3 Introduction to Chemotherapy Page | 6 Benzathine penicillin is even more slowly absorbed over 48-72 hours. Na or K PCN G may be administered IV or IM q4-6h. Distribution o Penicillins are reversibly and loosely bound to plasma proteins. o Penicillins are widely distributed in body fluids and tissues. Potentially therapeutic concentrations of the various penicillins are generally found in the liver, bile, kidneys, intestines, muscle, and lungs, but only very low concentrations are found in poorly perfused areas such as the cornea, bronchial secretions, cartilage, and bone. o The diethylamino salt of penicillin G produces particularly high concentrations in pulmonary tissue. o Inflammation allows entry of penicillin to blood-brain, placental, mammary, and prostatic barriers. Excretion o Penicillins are generally excreted unchanged by renal mechanism, especially by glomerular filtration (~20%) and active tubular secretion (~80%). Tubular secretion is inhibited by a competitor substance (weak organic acid) such as probenecid to prolong effective concentrations in the body. o Sufficient concentration of penicillin in urine is needed to affect urinary tract infection caused by Gram-negative bacteria. o Some are metabolized by the liver to penicilloic acid derivatives, which may act as antigenic determinants in penicillin hypersensitivity. o Clearance is considerably lower in neonates than in adults. o Penicillins are also eliminated in milk, although often only in trace amounts in the normal udder, and may persist for up to 90 hr. Penicillin residues in milk also have been found after intrauterine infusion. Adverse Reactions Allergic reactions ranging from delayed hypersensitivity skin reaction to acute anaphylaxis may occur. o Hypersensitivity reactions to penicillin as a hapten reflects, in part, formation of penicillinoic acid. o Hypersensitivity (particularly in cattle) includes skin reactions, angioedema, drug fever, serum sickness, vasculitis, eosinophilia, and anaphylaxis. CNS toxicity occurs with intrathecally administered penicillin G. Guinea pigs, chinchillas, birds, snakes, and turtles are sensitive to procaine penicillin. 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 3 Introduction to Chemotherapy Page | 7 Hyperkalemia and cardiac arrhythmias may result from IV administration of potassium penicillin in all species. The sodium salt of penicillin G may also contribute to the sodium load in congestive heart failure. Superinfections are most likely to occur with broad-spectrum penicillins. o Clostridium bacterial intestinal overgrowth from oral administration is a risk in guinea pigs, hamsters, gerbils, and rabbits. Penicillins are contraindicated in these species for this reason. Nephrotoxicity have been reported with the use of methicillin in humans but may also occur in animals. Drug Interactions β-lactams in general interact chemically with the aminoglycosides and should not be mixed in vitro. Combination with chloramphenicol/ tetracycline/ any bacteriostatic antibiotic may reduce the efficacy of penicillin. Aspirin/phenylbutazone and sulfonamide can displace penicillin at plasma binding sites. Gut-active penicillins potentiate the action of anticoagulants by depressing vitamin K production by gut flora. Absorption of ampicillin is impaired by the presence of food. Bacterial Resistance All penicillins are ineffective toward cell wall–deficient microorganisms such as Mycoplasma or Chlamydia spp. Inactivation by bacterial production of beta-lactamases is the most common mechanism of resistance. o β-Lactamases are produced by both gram-positive (Staphylococcus aureus, S. epidermidis, S. pseudointermedius but generally not enterococci) and gram-negative organisms. o Gram-negative bacteria capable of resistance as a result of β-lactamase production include Escherichia, Haemophilus, Klebsiella, Pasteurella, Proteus, Pseudomonas, and Salmonella spp; resistance may take longer to develop in some of these strains. A loss or decrease in affinity of crucial penicillin-binding proteins (PBPs) can lead to a significant increase in resistance to β-lactams. o Changes in PBP-2 of Staphylococcus spp. render the organism resistant to all β-lactams. Methicillin resistance in Staphylococcus spp. reflects acquisition of the mec gene, which results in a mutation in PBP-2. 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 3 Introduction to Chemotherapy Page | 8 Dose Rates Table 2. Recommended dosages for penicillins Penicillin Species Dose (per kg) Route Interval (hr) Penicillin G (Na) Dogs/Cats 22,000-55,000 IU IM, IV, SQ 6-8 Horse 20,000-60,000 IU IM, IV 6-8 Benzathine PCN Dogs/Cats 40,000-50,000 IU IM 120 Horse 50,000 IU IM 48 Ampicillin Dog/cats 10-20 mg IV, SQ 6-8 22-33 mg PO 8 Horse 10-20 mg IV SQ 8 Swine 6-8 mg SQ, IM 8 Amoxicillin Dog/cats 10-22 mg PO 8 Horse 20-30 mg IM, PO 6-12 Cattle 6-1 mg IM, SC 12-24 Amoxicillin + Dogs 12.5-25 mg PO 8-12 Clavulanate Cats 62.5 mg PO 8-12 B. CEPHALOSPORINS Cephalosporins were isolated in 1945 from Cephalosporium acremonium, a fungus isolated from raw sewage from the sea in Sardinia. There are now over 30 cephalosporin antibiotics on the market (most on the human pharmaceutical market), but newer ones have been introduced to veterinary medicine. Chemistry Like the penicillins, cephalosporins also contain a β-lactam ring. o Cephalosporins are composed of a six-member dihydrothiazine ring while penicillins are composed of a five-member thiazolidine ring. The active nucleus of most cephalosporins is 7-aminocephalosporanic acid. Modifications of the 7-aminocephalosporanic acid nucleus and substitutions on the sidechains by semisynthetic means have produced differences among cephalosporins in antibacterial spectra, β-lactamase sensitivities, and pharmacokinetics. o Cephamycins (a subclass of cephalosporins) differ from other cephalosporins in that they contain a 7-alpha-methoxy group, which imparts resistance to extended-spectrum β-lactamases. They are weak acids and are administered as the sodium salt, monohydrate, or free base. 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 3 Introduction to Chemotherapy Page | 9 Mechanism of Action Cephalosporins also inhibit the 3rd stage of cell wall synthesis by inhibiting the transpeptidase enzyme. Similar to other β-lactam antibiotics, the cephalosporins bind to PBPs. They are usually bactericidal and most often bind the PBP-2 and PBP-3. Classification of Cephalosporins, Therapeutic Uses and Spectrum of Activity The cephalosporins are broadly classified into first-, second-, third-, and fourth- generation cephalosporins. This classification is largely based on activity against gram-negative bacteria and susceptibility to β-lactamase. o There is also a new group active against methicillin-resistant staphylococci that has been called a fifth generation (e.g., ceftobiprole and ceftaroline) but there is no record of their use in veterinary medicine. Table 3. Spectrum of activity of the cephalosporin generations Classification Spectrum of Activity st 1 Generation Cephalosporins Oral These drugs are usually quite active against 1. Cefadroxil many gram-positive bacteria but are only 2. Cephalexin moderately active against gram-negative 3. Cephaglycine organisms. 4. Cephradine Susceptible gram-negative bacteria include Parenteral Escherichia coli and Proteus, Klebsiella, 1. Cephaloridine Salmonella, Shigella, and Enterobacter spp. 2. Cephapirin They are ineffective against enterococci. 3. Cephalothin Although generally less susceptible to β- 4. Cefazolin lactamase destruction than penicillins, they 5. Cephalozine are susceptible to cephalosporinases. They are not as effective against anaerobes as are the penicillins. First-generation cephalosporins have proved useful, particularly for infections involving Staphylococcus spp (eg, oral cephalexin for dermatitis) and for surgical prophylaxis (eg, cefazolin). Cephalexin should be anticipated to be ineffective against E. coli. Cephapirin benzathine is used for dry-cow therapy, and cephapirin sodium is used to treat mastitis. 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 3 Introduction to Chemotherapy Page | 10 2nd Generation Cephalosporins Oral Broader spectrum than the first generation, 1. Cefachlor generally active against both gram-positive 2. Cefprozil and gram-negative bacteria. Parenteral They are relatively resistant to β-lactamases 1. Cefuroxime compared with first-generation drugs. 2. Cefoxitin (a cephamycin) They are ineffective against enterococci, 3. Cephoxazole Pseudomonas aeruginosa (with the exception 4. Cefamandole of cefoxitin), Actinobacter spp, and many 5. Cefotiam obligate anaerobes (again, cefoxitin is an 6. Cefmetazole exception). 7. Cefonicid 8. Cefotetan 9. Ceforanide rd 3 Generation Cephalosporins Oral This group has more activity against gram- 1. Ceftriaxone negative bacteria than the earlier generations 2. Ceftizoxime of cephalosporins. 3. Cefmenoxime In general, they are less active against gram- 4. Cefpodoxime positive cocci, but there is considerable 5. Cefixime variability in the activity against staphylococci 6. Cefdinir and streptococci among this group. Parenteral Ceftazidime has the greatest activity against 1. Cefotaxime Pseudomonas aeruginosa. 2. Cefsulodine Ceftiofur has been specifically approved for 3. Cefoperazone use in cattle with bronchopneumonia, 4. Ceftazidime especially if caused by Mannheimia 5. Moxalactam (not a true haemolytica or Pasteurella multocida cephalosporin In dogs and cats, cefovecin is approved for 6. Cefovecin treatment of skin and soft-tissue infections. 7. Ceftiofur th 4 Generation Cephalosporins 1. Cefquinome Third- and fourth-generation cephalosporins 2. Cefepime were designed to be increasingly resistant to β-lactamases. Pharmacokinetics Absorption o Most cephalosporins are unstable in gastric acid and must be given parenterally. 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 3 Introduction to Chemotherapy Page | 11 o Cephalexin, cefadroxil, cefachlor, and cefixime are acid stable and are well absorbed orally. Distribution o Cephalosporins distribute well to the extracellular fluid, reaching therapeutic levels in most tissues and fluids, including the pericardial fluid, pleural fluid and infected bones. o Poor penetration into the CSF, even in inflammation, is a notable feature of the standard cephalosporins. Cephalosporins are substrates for P- glycoprotein efflux from the CNS. o Cefuroxime, moxalactam, cefotaxime and ceftizoxime penetrate into the CSF in sufficient concentration to treat meningitis. Elimination o Several cephalosporins (such as cephalothin, cephapirin, ceftiofur, cephacetrile, and cefotaxime) are actively deacetylated, primarily in the liver but also in other tissues. o Ceftiofur is transformed almost completely to the metabolite desfuroylceftiofur, which is responsible for its antibacterial efficacy. o Most cephalosporins are excreted unchanged in the urine by glomerular filtration and tubular secretion. Tubular secretion predominates, although glomerular filtration is important in some cases (cephalexin and cefazolin). Adverse Reactions Side effects are rare and cephalosporins are considered to be among the safest antimicrobials in use. Hypersensitivity and allergic reactions are most common. It can be an immediate reaction, anaphylaxis, bronchospasms or urticaria. Cephalosporins may have cross-reactivity with penicillin hypersensitivity. Renal toxicity that manifests as renal tubular necrosis may develop with prolonged administration. Other toxicities include gastrointestinal disturbances (nausea, vomiting, and diarrhea), local irritation and pain from IM injection and thrombophlebitis from IV injection. Bacterial Resistance Bacterial β-lactamase production may confer resistance, although cephalosporins tend to retain efficacy in contrast to the penicillins. 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 3 Introduction to Chemotherapy Page | 12 Dose Rates Table 4. Recommended dosages for penicillins Drug Species Dose (per kg) Route Interval (hr) Cefadroxil Dogs/cats 22 mg PO 8-12 Horse 25 mg PO 4 Cephalexin Dogs/cats 22 mg PO 8 Horses 22-33 mg PO 6 Cefoxitin Dogs/cats 30 mg IV 8 Cefotaxime Dogs/cats 25-50 mg IV, IM SC 8 Ceftiofur Cattle 1 mg IM 24 C. BACITRACIN Bacitracins are branched, cyclic, decapeptide antibiotics. Bacitracin A is the most active of the group and the main component of the commercial bacitracin preparations used either topically or PO. It was first isolated from a strain of Bacillus subtilis from a contaminated compound fracture of a girl named Tracy. Mechanism of Action Bacitracin kills bacteria by inhibiting the second step of bacterial cell wall synthesis. o It inhibits peptidoglycan synthesis by nonspecifically blocking phosphorylase reactions. It also interferes with cell membrane function and inhibit protein synthesis. Bactericidal activity requires the presence of divalent cations, such as zinc. Spectrum of Activity and Therapeutic Uses Bacteria susceptible to bacitracin include Gram-positive cocci and bacilli: staphylococci including β-lactamase producers and group A streptococci, Clostridium, Actinomyces and Fusobacterium. Spirochetes are susceptible. Since it is not absorbed orally, it is used for gut sterilization before gastrointestinal surgery. It is also added to swine and poultry rations for the prevention and treatment of clostridial enteritis and as a growth promotant. It is also used for topical application in superficial infection of the skin and mucous membrane and is often combined with polymixin B or neomycin for this purpose. Pharmacokinetics Bacitracin is not absorbed from GI tract when given orally. When used topically, bacitracin is nonirritating and rarely induces allergic reactions. 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 3 Introduction to Chemotherapy Page | 13 Adverse Reactions Systemic administration has resulted in a high incidence kidney injury (albuminuria, cylindruria, azotemia), in addition to pain, induration, and petechiae at the site of injection. Dose Rates Zinc bacitracin Calves/Lambs/pigs: 5-50 g/ton of feed (up to 16 weeks of age) 5-20 g/ton of feed (16-24 weeks of age); 5-80 mg/ton of feed (by addition to milk replacer Layer hens: 15-100 g/ton of feed Broiler chickens/turkeys: 5-50 g/ton of feed (up to 4 weeks of age); 5-20 g/ton of feed (5-26 weeks of age) D. GLYCOPEPTIDES Vancomycin, a high-molecular weight glycopeptide, is a fermentation product of Streptomyces orientalis. There are new drugs related to vancomycin that have been added to human treatment, but their use of these has not been reported in animals. These drugs include dalbavancin, oritavancin, and telavancin. Mechanism of Action Vancomycin blocks the second step of bacterial cell wall synthesis by inhibiting the transfer of the glycopeptide chain from the phospholipid to the acceptor site during bacterial cell wall synthesis. It produces a rapid bactericidal effect in dividing bacteria. Spectrum of Activity and Therapeutic Use Vancomycin is highly active against gram-positive cocci (in particular, Staphylococcus spp. and streptococci), enterococci (Enterococcus faecium and E. faecalis), as well as Neisseria spp. It also is active against gram-positive anaerobic cocci (but not anaerobic gram- negative bacteria). Vancomycin is a reserve antibiotic used intravenously for methicillin-resistant staphylococcal infections of bone and soft tissue in dogs and cats. Pharmacokinetics It is distributed to the ECF and transcellular fluid. 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 3 Introduction to Chemotherapy Page | 14 It is excreted unchanged by glomerular filtration. In renal insufficiency, striking accumulations may develop. Vancomycin is poorly absorbed orally and this route is not used except to treat intestinal infections. IM administration is painful and irritating. Adverse Reactions Hypersensitivity reactions are seen infrequently. Risk of kidney injury is greater with high doses and longer exposure. o Vancomycin toxicity acts as an oxidative stressor in the renal proximal tubule and can produce interstitial nephritis. The incidence of nephrotoxicity and ototoxicity may be partially caused by the common practice of simultaneously administering vancomycin with aminoglycosides. Rapid IV administration of vancomycin is associated induced flushing of the skin, pruritus, tachycardia, and other signs attributed to histamine release. Dose Rates Vancomycin 20 mg/kg at 12 h interval diluted in as least 200 ml of 5% dextrose solution 5-10 mg/kg PO BID E. FOSFOMYCIN Fosfomycin is a phosphonic acid that contains a carbon-phosphorus bond. It is a natural antibiotic produced by Streptomyces fradiae. Mechanism of Action Fosfomycin is a phosphoenolpyruvate analogue that irreversibly inhibits phosphoenolpyruvate transferase, an enzyme that catalyzes the first step of peptidoglycan synthesis of microbial cell walls. It is bactericidal when present at the site of infection at therapeutic concentrations. Spectrum of Activity and Therapeutic Use Its in vitro spectrum is broad, with potential efficacy toward isolates expressing multidrug resistance, including Escherichia coli and methicillin-resistant staphylococci. Fosfomycin has been added to the World Health Organization's list of critically important drugs. Accordingly, its use should be reserved, along with other critically important drugs, to situations in which lower-tier drugs are no longer appropriate. 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 3 Introduction to Chemotherapy Page | 15 Adverse Reactions Adverse effects of fosfomycin appear to be limited to diarrhea. F. MONOBACTAMS Monobactams like aztreonem have a β-lactam ring but the adjacent thiazolidine ring has been replaced. Mechanism of Action Aztreonem binds to penicillin binding proteins present in Gram (–) aerobic bacteria and disrupt cell wall synthesis. It is stable to most β-lactamases. Spectrum of Activity and Therapeutic Use Aztreonem is used in humans to replace aminoglycosides, which are more toxic when used with macrolides and lincosamides. It may be used as a reserve antibiotic in veterinary medicine to treat severe Gram (–) infections. Pharmacokinetics When given parenterally, aztreonem has a similar distribution to penicillin G. Penetration of CSF is good. It is excreted by the kidneys. Adverse Reactions Hypersensitivity reactions may occur but cross-allergy with penicillins or cephalosporins has not been observed. G. CARBAPENEMS Carbapenems have become valuable antibiotics because of a broad spectrum that includes many bacteria resistant to other drugs. Carbapenems (also called penems) include imipenem, doripenem, ertapenem, and meropenem. Chemistry Carbapenems are β-lactams with a structure similar to penicillin but the thiazolidine is replaced by a methyl group. 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 3 Introduction to Chemotherapy Page | 16 Mechanism of Action Similar to other β-lactam antimicrobial drugs but the carbapenems bind to more penicillin-binding proteins. They are more bactericidal than other β-lactam antibiotics against gram-negative bacteria because they affect PBP-1 and PBP-2 and produce postantibiotic effects (PAE) that are not seen with other β-lactams. The rapid bactericidal activity is less likely to induce release of endotoxin in patients from gram-negative sepsis during treatment. Spectrum of Activity and Therapeutic Use The carbapenems are used to treat very serious infections like peritonitis associated with ruptured GI tract or intestinal spillage during surgery. They are effective against Gram(+) and Gram(–) aerobic and anaerobic bacteria including Pseudomonas and Enterobacteriaceae. Carbapenems are not active against methicillin-resistant staphylococci or resistant strains of Enterococcus faecium. Pharmacokinetics Oral administration is not possible because of acid hydrolysis and poor absorption. Imipenem undergoes extensive metabolism by the kidney dehydropeptidase (DHP-1) in the brush border of the proximal tubule. The metabolite is nephrotoxic and exhibits antimicrobial action in the urine. o Imepenem is used with a DHP-1 inhibitor, cilastatin, to decrease toxicity and increase elimination t 1/2. Meropenem is a more recent derivative that is more DHP-1 stable that does not need cilastatin to inhibit kidney metabolism. Adverse Reactions Side effects may include anorexia, vomiting, and diarrhea; CNS toxicity including seizures and tremors; and hypersensitivity reactions including pruritis, fever, and rarely, anaphylaxis. III. Study Questions 1. What is/are the similarities and differences between the cell wall synthesis inhibitors in terms of mechanism of action? 2. What are the natural sources of cell wall synthesis inhibitor antibiotics? 3. Why penicillin cannot be combined with bacteriostatic agents? 4. How are penicillins and cephalosporins classified? 5. What are the clinical indications of bacitracin, vancomycin, fosfomycin, monobactams and carbapenems? 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 3 Introduction to Chemotherapy Page | 17 6. Describe the pharmacokinetic properties of cell wall synthesis inhibitors. 7. What are adverse reactions associated with cell wall 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  LABILIZERS OF VPHAR 4205 –Veterinary Clinical Pharmacology Module 4 MODULE 4 Introduction to Chemotherapy Page | 1 THE Module 1 CELL Introduction MEMBRANE Overview In this module, you will learn the antibacterial agents that act on the cell membrane. Their mechanism of action, clinical indications, pharmacokinetics, drug interactions and adverse reactions were also discussed. I. Learning Objectives Upon completion of this module, students will be able to: 1. Identify antibiotics that perturb cell membrane function. 2. Explain the mechanism of action of polymyxins. 3. Describe the clinical indications of polymyxins. 4. Determine the pharmacokinetic properties, drug interactions and potential adverse effects of polymyxins. II. Learning Activities Polymyxins Polymyxins are polypeptide antibiotics isolated from soil bacteria Bacillus polymyxa. They contain seven amino acids in a cyclic configuration. Several polymyxins have been isolated and have been named A, B, C, D, E, and M. Of these six antibiotics, polymyxin B and E (also called colistin from B. colistinus) in their sulfate salt forms are the only ones used clinically. Mechanism of Action Polymyxins interact with phospholipids in the bacterial cell membrane to produce a detergent-like effect and membrane disruption. They alter the osmotic pressure, selective permeability and regulatory properties of cell membrane resulting in cell death, giving polymyxins bactericidal properties. They may be combined with bacteriostatic drugs since their action does not require rapid multiplication of bacteria. They may also have anti-endotoxin and antipyretic effects. As basic (cationic) drugs, they may combine with endotoxin, which is anionic. Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine 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 4 Introduction to Chemotherapy Page | 2 Figure 1. Mechanism of action of polymyxins Spectrum of Activity and Therapeutic Uses The spectrum of activity of polymyxins is narrow, primarily against Gram-negative bacteria: Enterobacter, Klebsiella, Salmonella, Pasteurella, Bordetella, Shigella, Pseudomonas spp., and Escherichia coli. Proteus and Serratia spp. are resistant. They have a synergistic action with tetracyclines, chloramphenicol, sulfamethoxazole and carbenicillin against P. aeruginosa. Their major clinical application in veterinary medicine are oral treatment of E. coli and Salmonella diarrhea, and local treatment of Pseudomonas infection such as otitis externa and superficial lip infection. Pharmacokinetics Polymyxins are poorly absorbed from the gut and through the skin and mucous membrane but absorption is rapid following IM and SC administration. They distribute within the extracellular fluid only, but do not get to the CSF. They bind to kidney, liver, lung, heart and skeletal muscle. The polymixins undergo renal elimination mostly as degradation products. Drug Interactions Polymyxins act synergistically when combined with potentiated sulfonamides, tetracyclines, and some other antibacterials. They also reduce the activity of endotoxins in body fluids and may be beneficial in endotoxemia. Antibacterial activity is markedly decreased in the presence of pus, in tissues containing acidic phospholipids, divalent cations, unsaturated fatty acids, debris, purulent exudate, quaternary ammonium compounds, and in the presence of anionic detergents or other chemicals that antagonize cationic detergents. Noraine College Paray Science of Veterinary Medina, DVSM, MSc, PhD and Medicine 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 4 Introduction to Chemotherapy Page | 3 Adverse Reactions They are notably nephrotoxic and neurotoxic and, as such, systemic therapy at antimicrobial doses should be avoided. o Nephrotoxicity in the form of reduced tubular perfusion may decrease urine output. o Neurotoxicity such as central nervous depression, anorexia, dose- dependent curare-like paralysis which is additive with other drugs acting on the neuromuscular junction. Polymyxin B is a potent histamine releaser. Respiratory paralysis is usually due to rapid IV injection, too much peritoneal lavage, or a preexisting renal condition. No adverse effects have been reported when they are used topically or orally. Dose Rates Recommended dose rates for polymyxins vary considerably. A general guideline is 20,000 U/kg, PO, bid; 5,000 U/kg, IM, bid; 50,000–100,000 U by intramammary infusion; 100,000 U intrauterine in cattle. Polymyxin B Cow with severe coliform mastitis: 2.5 mg/kg IM Other animals: 2.5 mg/kg IV q12h; 5 mg/kg PO q12h Colistin sulfomethate 3 mg/kg IM q12h Colistin sulfate 5-10 mg/kg PO III. Study Questions 1. Identify the source of polymyxin antibiotics. 2. How does polymyxins differ from beta-lactam drugs in terms of mechanism of action? 3. What are the clinical indications of polymyxins? 4. Describe the pharmacokinetic properties of polymyxins. 5. Give possible adverse effect/s of oral or topical polymyxin use. 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