Antimicrobial Drugs PDF

Antimicrobial Drugs  Antimicrobial drugs used to treat infectious disease by interfering with growth of microorganisms Must be safe for host! – Ideally drugs will kill pathogens without harming the host Selective toxicity: attacks some cells but not others – this...

Antimicrobial Drugs  Antimicrobial drugs used to treat infectious disease by interfering with growth of microorganisms Must be safe for host! – Ideally drugs will kill pathogens without harming the host Selective toxicity: attacks some cells but not others – this refers to the drug’s ability to attack specific microbial cells while leaving other cells (including host cells) relatively unharmed Most of the drugs available are antibacterial, as opposed to anti-fungal, anti-protozoan or anti-helminthic. Even fewer are anti-viral…..  Definitions Antibiotic: substance that can inhibit the growth of a microorganism – Common for one species of microorganism to produce substances that inhibit growth of other microorganisms – Technically these substances are produced by microorganisms, but in practice synthetic drugs are also called antibiotics Antibiotics can be either: – Bacteriostatic agents: stop bacterial replication and prevent growth and the host immune system does the rest – Bactericidal agents: kill bacteria directly  Spectrum of Activity Range of microbes that an antimicrobial drug can affect: Narrow-spectrum: work against limited kinds of pathogens Broad-spectrum: affect wide range of Gram+ and Gram- – advantages: treat unknown infection and infections that could be caused by different organisms, such as bacterial meningitis – disadvantages: can destroy normal flora of host, leading to overgrowth of other species (superinfection) Examples: C. difficile diarrhea, C. albicans overgrowth, opportunistic growth of antibiotic resistant strains Semi-synthetics: Chemically altered antibiotics that are more effective than naturally occurring ones Synthetics: antimicrobials completely synthesized in a lab Modes of Action of Antimicrobial Drugs  Gram Negative Outer Membrane Limits Drugs “lipid bilayer” fatty acid tail hydrocarbons prevents passage of polar molecules (most drugs have polar properties) small, water-filled “pores” allow entry only by compounds that are soluble in water If drug can’t get to it’s specific target, it is completely useless! generally: small, hydrophilic drugs can enter Gram negative  Type 1: Inhibition of Cell Wall Synthesis cell wall is the major difference b/w prok and euk cells – prokaryotic cells have peptidoglycan interfere with synthesis of cell wall and therefore only affects actively growing cells – weakened cell wall, exposed plasma membrane, lysis  Penicillin “family” of over 50 chemically-related antibiotics common core structure: a β-lactam ring found in all of them – penicillins also called “β-lactam antibiotics” Penicillin V = prototype; 1st penicillin discovered different types of penicillin vary here β-lactam Ring Required for Penicillin Activity broken β-lactam ring β-lactamase some bacteria make β-lactamase which breaks β-lactam ring and therefore inactivates penicillin – β-lactamase also called “penicillinase” most common form of penicillin resistance  Natural vs Semisynthetic Penicillin Natural Penicillins – extracted directly from Penicillium cultures – narrow spectrum (G+); useful against most Staphylococci, Streptococci & spirochetes – often susceptible to β-lactamases – Pen G & Pen V are the most common natural penicillins Semisynthetic Penicillins – β-lactam core made by Penicillium – R-group is added in lab – Engineered for specific characteristics: can be designed to be more resistant to β-lactamase (methicillin) can be designed to have broader specificity: Gram+ & some Gram- (ampicillin, amoxicillin) Antibiotic resistance is a growing problem MRSA: methicillin-resistant Staphylococcus aureus penicillins usually interact with bacterial cell wall through penicillin binding proteins in peptidoglycan layer – MRSA posses a genetic mutation where bacteria doesn’t bind penicillin also produce beta-lactamase patients with this infection must be ISOLATED can usually be treated with vancomycin  Vancomycin named for the word “vanquish” glycopeptide antibiotic: completely different structure than penicillin naturally produced by a species of Streptomyces inhibits cell wall synthesis very narrow spectrum – mostly used to treat MRSA – recently strains of S. aureus and certain Enterococci sp. that are resistant to vancomycin have been discovered toxicity used to be a problem but improved manufacturing procedures have corrected this  Monobactams synthetic & semisynthetic antibiotics – potential advantage: not found in nature, therefore takes more time for pathogens to develop resistance structure is similar to penicillin but different enough that it is not sensitive to β-lactamase spectrum of activity: – Affects certain Gram negative bacteria (E. coli, H. influenzae, P. aeruginosa); effective in treating these infections in Cystic Fibrosis patients  Cephalosporins similar chemical structure to penicillin (β-lactam ring) examples: cephalothin, cefixime – comes from the fungus Cephalosporium inhibit cell wall synthesis in same way as penicillin, but tend to be more broad-spectrum than natural penicillin susceptible to a different group of β-lactamases  Mycobacteria have different cell walls M. tuberculosis, M. leprae – cause of tuberculosis, leprosy Cell walls contain mycolic acids (and a little peptidoglycan) Anti-mycobacterial antibiotics interfere with mycolic acid incorporation or synthesis – these drugs have minimal to no effect on other bacteria Isoniazid: inhibits mycolic acid synthesis Ethambutol: inhibits incorporation of mycolic acid – fairly weak on its own, so it is administered as part of a “cocktail” to prevent development of resistance – Dapsone tx Leprosy inhibits nucleic acid synthesis  Type 2: Inhibition of Protein Synthesis recall: ribosomes are the sites of protein synthesis eukaryotic and prokaryotic ribosomes are different – eukaryotic: 80S ribosomes (40S + 60S subunits) – prokaryotic: 70S ribosomes (30S + 50S subunits) Targeting 70S ribosomes directs action against bacteria – Problem: mitochondria have 70S ribosomes – some drugs in this group may have toxic effects on humans, but bacteria are sensitive to lower concentrations than human cells Protein synthesis inhibitors: drugs in this category all have slightly different modes of action with the same end result  Examples: Mechanisms of action of protein synthesis inhibitors Chloramphenicol: prevents peptide bond formation. [50S] Aminoglycosides: block initiation and cause misreading of mRNA. [30S] Tetracyclines: block attachment of tRNA to ribosome. [30S] Macrolides: prevent continuation of synthesis (translocation from A site to P site). [50S]  Chloramphenicol broad spectrum simple structure – small size allows it to diffuse into areas that are inaccessible to many other drugs inexpensive to manufacture – often used where low cost is essential down side: serious toxicity problems – suppression of bone marrow activity – aplastic anemia, potentially fatal – affects formation of blood cells – 1 in 40,000 users affected (normal: 1 in 500,000), so physicians advised not to use this drug when alternatives are available – Teratogenic in neonates: causes grey baby syndrome  Aminoglycosides among the first antibiotics found to have activity against Gram- bacteria bactericidal can be toxic, therefore use is declining – permanent damage to auditory nerve (Ototoxicity) and kidneys Current use: – Cystic fibrosis where lung infections with Pseudomonas aeruginosa are common (G-, difficult to treat) – tobramycin is delivered as an aerosol to control these infections  Tetracyclines Broad spectrum: effective against Gram+ and Gram-, intracellular bacteria – able to penetrate tissues & cells well Natural protein synthesis inhibitor, but semisynthetics have longer retention in body (doxycycline, minocycline) uses: – UTI, Mycoplasma, Chlamydia and Rickettsia infections – alternatives for syphilis & gonorrhea instead of penicillins problems: – suppress normal flora (broad spectrum) GI upsets leading to superinfections, often by C. albicans – may cause brown teeth discoloration in kids (Younger than 8yrs old) – may cause liver damage in pregnant women  Macrolides narrow spectrum (G+) – alternative to penicillin – too big to enter G- cells inhibit protein synthesis erythromycin oral administration – orange-flavored suspension often used to treat streptococcal and staphylococcal infections in children – useful to treat people who are allergic to β-lactams erythromycin azithromycin, clarithromycin – broader specificity, better tissue penetration – important for treatment of intracellular bacteria such as Chlamydia Type 3: Injury to Plasma Membrane change permeability of plasma membrane essential metabolites leave the cell probably not the best choice: eukaryotic plasma membrane is very similar to bacteria Example: polymyxin B – first drug active against gram(-) Pseudomonas – attaches to phospholipids, causes disruption – host toxicity: significant internally – used as a topical treatment for superficial infections – available in non-prescription antibiotic ointments Polysporin Type 4: Nucleic Acid Synthesis Inhibitors may interfere with replication or transcription may cause harm to human host – BUT useful drugs in this class are more harmful to the bacteria than the host – selective toxicity Rifamycins: most common is rifampin – inhibits mRNA synthesis, bactericidal – side effect: orange-red urine, feces, tears, sweat, saliva can penetrate tissues – therapeutic levels in CSF and abscess – useful for treatment of TB along with isoniazid & ethambutol tissue penetration required  Quinolones & Fluoroquinolones bactericidal, broad spectrum – specifically inhibit bacterial DNA replication quinolones: early drug (1960’s), limited use – only application: UTI fluoroquinolones: developed in 1980’s – norfloxacin, ciprofloxacin (Cipro) safe for adults, but not recommended for children, adolescents, pregnant women: – affect cartilage development new synthetic versions being developed that are broader spectrum, but adversely affects some drugs that control heart rhythm  Type 5: Drugs that inhibit metabolic pathways & enzymatic activity It is possible to block the activity of essential enzymes within a cell using specifically-designed drugs Competitive inhibition: drug with a very similar structure to the normal substrate can “block” the active site of an enzyme so the enzyme can’t carry out its normal function  Sulfonamides Sulfa drugs (among 1st synthetic antimicrobials) PABA (para aminobenzoic acid): required to make nucleic acids in pathogens, but not in humans most widely used today: – TMP-SMZ (trimethoprim & sulfamethoxazole): synergistic combo – broad spectrum – used for control of pneumonia caused by Pneumocystis carnii – effective in penetration of brain& CSF  Things to Consider in Combining Drugs Synergism: when 2 drugs used together are more effective than either one alone – penicillin (damage cell wall) + streptomycin (inhibit protein synthesis at ribosome) damage by penicillin allows entry by streptomycin Antagonism: when the activity of one drug works against activity of another when they are used together – penicillin (inhibits ACTIVE cell wall synthesis) + tetracycline (stops bacterial growth) bacteria that are not making a cell wall are not affected by penicillin  Antifungals (PART 2) fungal infections are increasing in frequency – opportunistic infections, immunosuppression, AIDS toxicity problem since fungi are eukaryotes Azoles: block fungal sterol synthesis – clotrimazole, miconazole (Monistat): topical treatment of athlete’s foot, yeast infection – Triazoles: less toxic, but still some liver damage Fluconazole, ketoconazole : treatment of systemic mycoses Polyenes: kills fungal cells via sterol recognition – Amphotericin B commonly used treatment for systemic mycoses – toxicity in kidney limits use  Antivirals & Antiviral Targets In the developed world many of the most serious infections are caused by viruses, but there are few antiviral drugs – Ideally, drugs that kill pathogens without harming the host but this is difficult when dealing with cellular hijackers 5. Release 1. Attachment 2. Penetration & Uncoating 4. Assembly 3. Synthesis of viral proteins & nucleic acid  Nucleoside Analogs block transcription of DNA can also affect host cells – virally infected cells are more active than host cells so these drugs have strongest effect on infected cells Acyclovir, ganciclovir: used to treat Herpes Simplex virus infections Zidovudine or azidothymidine (AZT): used to treat HIV – analog of thymidine, nucleoside analog reverse transcriptase inhibitor (NRTI) blocks viral reverse transcriptase  Nucleoside analogs: how do they work? the drug looks very similar to the natural molecule (mimic) virus mistakes acyclovir for deoxyguanosine and incorporates it into DNA chain – 3’-OH group is required for addition of next base in new DNA chain, nucleoside analogs are missing that – Once a nucleoside analog is incorporated into a DNA chain it is impossible to add another base (chain is prematurely terminated)  Drugs that inhibit viral attachment Also called attachment antagonists: – HIV: Celsentri binds to CCR5, preventing a GP120 interaction – referred to as a chemokine receptor antagonist or a CCR5 inhibitor Fuzeon binds to gp41, interferes with its ability to approximate the two membranes – referred to as a fusion inhibitor – Influenza: Relenza and Tamiflu are used to treat influenza by inhibiting the enzyme neuraminidase, which is required for attachment and detachment of the influenza virus to host cells  Interferons Interferon is a cellular protein released during an immune response to viral infection – prevents spread of virus Alpha-interferon is current drug of choice for chronic hepatitis (B and C) infections – lots of side effects – severe flu-like symptoms, bone-marrow suppression Imiquimod: new drug that stimulates interferon production – used to treat genital warts  Antihelminthic Drugs Niclosamide: tapeworm – inhibits aerobic ATP production – first choice for treatment of tapeworms acquired from sushi mebendazole: ascariasis, pinworms – interferes with the worm’s ability to absorb nutrients – few side effects  Testing is done to determine appropriate antimicrobial therapy different species and strains of bacteria are susceptible to different drugs susceptibility can change over time for the best treatment, susceptibility should be tested before treatment begins – not always possible – not always required: for some organisms identification is enough because susceptibility is predictable  Kirby Bauer Test: Disk Diffusion Method each disk contains different drug after incubation, measure inhibition of bacterial growth – cloudy: bacteria – clear: zone of inhibition diameter measured determine how sensitive bacteria is to the drug being tested – compared to standard table for each specific drug – simple, inexpensive test  MIC in Broth Dilution Test MIC = Minimum Inhibitory Concentration Lowest amount of drug that inhibits growth and reproduction Quantitative test for potency of antimicrobial agent  E-test combines MIC and disk diffusion method to estimate minimum antibiotic concentration that prevents visible bacterial growth strip contains gradient of antibiotic concentrations MIC read directly from scale printed on strip Mechanisms of resistance RESISTANCE TO ANTIMICROBIAL DRUGS  mechanisms by which microorganisms might exhibit resistance to drugs.  1. Microorganisms produce enzymes that destroy the active drug. Examples: Staphylococci resistant to penicillin G produce a β-lactamase that destroys the drug.  2. Microorganisms change their permeability to the drug (caused by an outer membrane change that impairs active transport into the cell.) RESISTANCE TO ANTIMICROBIAL DRUG  3. Microorganisms develop an altered structural target for the drug. E.g. Penicillin resistance in Streptococcus pneumoniae and enterococci is attributable to altered PBPs.  4. Microorganisms develop an altered metabolic pathway that bypasses the reaction inhibited by the drug.  5. Microorganisms develop an altered enzyme that can still perform its metabolic function but is much less affected by the drug RESISTANCE TO ANTIMICROBIAL DRUG  6. Microorganisms can develop efflux pumps that transport the antibiotics out of the cell. Many gram positive and especially gram- negative organisms have developed this mechanism for: tetracyclines (common), macrolides, fluoroquinolones, β-lactam agents.