Drugs, Microbes, Host The Elements of Chemotherapy

Transcription

1 Chapter 12 Drugs, Microbes, Host The Elements of Chemotherapy Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 12.1 Principles of Antimicrobial Therapy Administer a drug to an infected person that destroys the infective agent without harming the host s cells Antimicrobial drugs are produced naturally or synthetically 2

3 The Ideal Antimicrobial Drug 3

4 The rigins of Antimicrobial Drugs Antibiotics are common metabolic products of aerobic bacteria and fungi Bacteria in genera Streptomyces and Bacillus Molds in genera Penicillium and Cephalosporium By inhibiting the other microbes in the same habitat, antibiotic producers have less competition for nutrients and space Insight 12.1 Sir Alexander Fleming 4

5 Interactions Between Drugs and Microbe Antimicrobial drugs should be selectively toxic drugs should kill or inhibit microbial cells without simultaneously damaging host tissues As the characteristics of the infectious agent become more similar to the vertebrate host cell, complete selective toxicity becomes more difficult to achieve and more side effects are seen 5

6 Mechanisms of Drug Action Figure 12.2 Major targets of drugs in bacteria 1. Cell wall inhibitors Block synthesis and repair Penicillins Cephalosporins Vancomycin Bacitracin Monobactams/carbapenems Fosfomycin Cycloserine Isoniazid 2. Cell membrane Cause loss of selective permeability Polymyxins Ribosome Substrate Enzyme Product 4. Protein synthesis inhibitors acting on ribosomes Site of action 50S subunit Chloramphenicol Erythromycin Clindamycin Streptogramin (Synercid) Site of action 30S subunit Aminoglycosides Gentamicin Streptomycin Tetracyclines 3. DA/RA Inhibit replication and transcription Inhibit gyrase (unwinding enzyme) Quinolones (ciprofloxacin) Inhibit RA polymerase Rifampin mra DA Both 30S and 50S Blocks initiation of protein Synthesis Linezolid (Zyvox) 5. Metabolic pathways and products Block pathways and inhibit Metabolism Sulfonamides (sulfa drugs) Trimethoprim 6

7 The Spectrum of an Antimicrobic Drug Spectrum range of activity of a drug arrow-spectrum: effective on a small range of microbes Target a specific cell component that is found only in certain microbes Broad-spectrum: greatest range of activity Target cell components common to most pathogens (ribosomes) 7

8 12.2 Survey of Major Antimicrobial Drug Groups Antimicrobial Drugs That Affect the Bacterial Cell Wall Most bacterial cell walls contain peptidoglycan Penicillins and cephalosporins block synthesis of peptidoglycan, causing the cell wall to lyse Active on young, growing cells Penicillins that do not penetrate the outer membrane and are less effective against gram-negative bacteria Broad spectrum penicillins and cephalosporins can cross the cell walls of gram-negative bacteria 8

9 Figure 12.3 Activity of Penicillin Peptidoglycan (cell wall) Exposure to cephalosporin or penicillin (a) ormal untreated cells. Left and center views are Staphylococcus cells; right view is the molecular structure of peptidoglycan, with chains of AM (-acetyl muramic acid) and AG (-acetyl glucosamine) linked together by cross bridges (green). Janice Carr/CDC 10,000 Weak points lacking peptidoglycan Membrane bulges out as water diffuse into cell. Membrane breaks. Cell lyses. (b) Effects of treatment with penicillin. Left side depicts the stages in the breakdown of a Staphylococcus cell exposed to penicillin. Center view shows actual cells undergoing lysis and cell death. Right is a molecular view of how penicillin blocks the formation of the cross bridges, which removes the support that holds the cell wall together. CRI/Photo Researchers, Inc. 10,000 9

10 Antibacterial Drugs that Act on the Cell Wall Beta-lactam antimicrobials - all contain a highly reactive 3 carbon, 1 nitrogen ring Primary mode of action is to interfere with cell wall synthesis Greater than ½ of all antimicrobic drugs are betalactams Penicillins and cephalosporins most prominent beta-lactams 10

11 Figure 12.7 Penicillin and Its Relatives Large diverse group of compounds Could be synthesized in the laboratory More economical to obtain natural penicillin through microbial fermentation and modify it to semi-synthetic forms All consist of 3 parts: Thiazolidine ring Beta-lactam ring Variable side chain dictating microbial activity (R) R Group afcillin S Ticarcillin CH Cloxacillin C C Ca Cl Carbenicillin CH C C Ca Betalactam ucleus (Aminopenicillanic acid) S S S S Thiazolidine CH CH CH CH

12 Subgroups and Uses of Penicillins Penicillins G and V most important natural forms Penicillin is the drug of choice for gram-positive cocci (streptococci) and some gram-negative bacteria (meningococci and syphilis spirochete) Semisynthetic penicillins ampicillin, carbenicillin, and amoxicillin have broader spectra Gramnegative infections Penicillinase-resistant methicillin, nafcillin, cloxacillin Primary problems allergies and resistant strains of bacteria 12

13 Cephalosporins Account for one-third of all antibiotics administered Synthetically altered betalactam structure Relatively broadspectrum, resistant to most penicillinases, and cause fewer allergic reactions Some are given orally; many must be administered parenterally Generic names have root cef, ceph, or kef Figure 12.8 The structure of cephalosporins H H 2 H 2 R Group 1 Basic ucleus R Group 2 S S S CH 2 CH 2 CH C Ca C S or CH CH 2 C CH 2 S CH 2 CH 2 CH 2 S CH 2 *ew improved versions of drugs are referred to as new generations. 7 2nd 3rd 4th Cephalothin (first generation*) Cefotiam (second generation) Moxalactam (third generation) Cefepime (fourth generation) 13

14 on Beta-lactam Cell Wall Inhibitors Vancomycin narrow-spectrum, most effective in treatment of Staphylococcal infections in cases of penicillin and methicillin resistance or if patient is allergic to penicillin; toxic and hard to administer; restricted use Bacitracin narrow-spectrum produced by a strain of Bacillus subtilis; used topically in ointment Isoniazid (IH) works by interfering with mycolic acid synthesis; used to treat infections with Mycobacterium tuberculosis 14

15 Antimicrobial Drugs That Disrupt Cell Membrane Function A cell with a damaged membrane dies from disruption in metabolism or lysis These drugs have specificity for a particular microbial group, based on differences in types of lipids in their cell membranes Polymyxins interact with phospholipids and cause leakage, particularly in gram-negative bacteria Amphotericin B and nystatin form complexes with sterols on fungal membranes which causes leakage 15

16 Figure 12.4 Activity of Polymyxins Polymyxin B Cell membrane A detergent that causes openings in the cell membrane 16

17 Antibiotics That Damage Bacterial Cell Membranes Polymixins, narrow-spectrum peptide antibiotics with a unique fatty acid component Treat drug resistant Pseudomonas aeruginosa and severe UTI 17

18 Drugs That Act on DA or RA May block synthesis of nucleotides, inhibit replication, or stop transcription Chloroquine binds and cross-links the double helix; quinolones inhibit DA helicases Antiviral drugs that are analogs of purines and pyrimidines insert in viral nucleic acid, preventing replication 18

19 Drugs that Act on DA or RA Fluoroquinolones work by binding to DA gyrase and topoisomerase IV Broad spectrum effectiveness Concerns have arisen regarding the overuse of quinoline drugs CDC is recommending careful monitoring of their use to prevent ciprofloxacinresistant bacteria 19

20 Drugs That Interfere with Protein Synthesis Ribosomes of eukaryotes differ in size and structure from prokaryotes; antimicrobics usually have a selective action against prokaryotes; can also damage the eukaryotic mitochondria Aminoglycosides (streptomycin, gentamycin) insert on sites on the 30S subunit and cause misreading of mra Tetracyclines block attachment of tra on the A acceptor site and stop further synthesis 20

21 Figure 12.5 Targets on the 70S Ribosome aa 50S aa mra is misread, protein is incorrect Aminoglycosides 30S mra 50S Chloramphenicol aa 30S aa Formation of peptide bonds is blocked mra xazolidinones Prevent initiation and block ribosome assembly Tetracyclines 50S 30S tra is blocked, no protein is synthesized mra Erythromycin aa 50S aa Ribosome is prevented from translocating 21 30S mra

22 Drugs That Interfere with Protein Synthesis Aminoglycosides composed of one or more amino sugars and an aminocyclitol (6C) ring; binds ribosomal subunit - Figure 12.9 The structure of an aminoglycoside H 2 H C H H H H H H C H 2 6-carbon ring CH H H CH 2 H 2 sugars H 22 H

23 Drugs That Interfere with Protein Synthesis Aminoglycosides Products of various species of soil actinomycetes in genera Streptomyces and Micromonospora Broad-spectrum, inhibit protein synthesis, especially useful against aerobic gram-negative rods and certain gram-positive bacteria Streptomycin: bubonic plague, tularemia, TB Gentamicin: less toxic, used against gram-negative rods ewer tobramycin and amikacin gram-negative bacteria 23

24 Tetracycline Antibiotics Broad-spectrum, block protein synthesis by binding ribosomes Treatment for STDs, Rocky Mountain spotted fever, Lyme disease, typhus, acne, and protozoa Generic tetracycline is low in cost but limited by its side effects Figure Structure of a broad spectrum antibiotic R R R R H H CCH 2 H (a) Tetracyclines H 24

25 Chloramphenicol Potent broad-spectrum drug with unique nitrobenzene structure Blocks peptide bond formation and protein synthesis Entirely synthesized through chemical processes Very toxic, restricted uses, can cause irreversible damage to bone marrow Typhoid fever, brain abscesses, rickettsial, and chlamydial infections C CH Cl 2 Figure Structure of a broad spectrum antibiotic 2 H C H C CH 2 H (b) Chloramphenicol H H 25

26 Macrolides and Related Antibiotics Erythromycin large lactone ring with sugars; attaches to ribosomal 50s subunit Broad-spectrum, fairly low toxicity Taken orally for Mycoplasma pneumonia, legionellosis, Chlamydia, pertussis, diphtheria and as a prophylactic prior to intestinal surgery For penicillin-resistant gonococci, syphilis, acne ewer semi-synthetic macrolides clarithromycin, azithromycin Figure Structure of a broad spectrum antibiotic H H (c) Erythromycin Lactone ring CH 2 H Sugar Sugar H H ( ) 2 26

27 Drugs that Block Metabolic Pathways Sulfonamides and trimethoprim block enzymes required for tetrahydrofolate synthesis needed for DA and RA synthesis Figure 12.6 Competitive inhibition as a mode of action Sulfonamides inhibit enzyme PABA Dihydropteroic acid Dihydrofolic acid Trimethoprim inhibits enzyme Tetrahydrofolic acid (folic acid) Folic acid is coenzyme required to synthesize Purines Pyrimidines Amino acids nly certain bacteria perform this step PABA (substrate) (a) The metabolic pathway needed to synthesize tetrahydrofolic acid (THFA) contains two enzymes that are chemotherapeutic targets. Under normal circumstances, PABA acts as a substrate for the enzyme pteridine synthetase, the product of which is needed for the eventual production of folic acid. Enzyme Sulfa drug (inhibitor) Enzyme Sulfa drug (inhibitor) Enzyme Higher levels of sulfa drug more likely to bind to enzyme 27

28 Drugs that Block Metabolic Pathways Structural differences Figure 12.6 Competitive inhibition as a mode of action H H S H C H H H H Sulfanilamide (b) PABA Competitive inhibition drug competes with normal substrate for enzyme s active site Synergistic effect the effects of a combination of antibiotics are greater than the sum of the effects of the individual antibiotics 28

29 Drugs That Block Metabolic Pathways Most are synthetic; most important are sulfonamides, or sulfa drugs - first antimicrobic drugs arrow-spectrum; block the synthesis of folic acid by bacteria Sulfisoxazole: shigellosis, UTI, protozoan infections Silver sulfadiazine: burns, eye infections Trimethoprim: given in combination with sulfamethoxazole: UTI, PCP(Pneumocystis carinii pneumonia) ucleus H 2 S 2 H (a) (b) (c) R Group C 29

30 12.3 Drugs to Treat Fungal, Parasitic and Viral Infections Fungal cells are eukaryotic; a drug that is toxic to fungal cells also toxic to human cells Five antifungal drug groups: Macrolide polyene Amphotericin B mimic lipids, most versatile and effective, topical and systemic treatments ystatin topical treatment Figure an antifungal drug H H H H H H H (a) H 30

31 Antifungal Drugs Five antifungal drug groups: Griseofulvin: stubborn cases of dermatophyte infections, nephrotoxic Synthetic azoles: broad-spectrum; ketoconazole, clotrimazole, miconazole Flucytosine: analog of cytosine; cutaneous mycoses or in combination with amphotericin B for systemic mycoses Echinocandins: damage cell walls; capsofungin (b) Figure b. clotrimazole, inhibits fungal cell membrane C Cl (c) H 2 H F Figure c. flucytosine, inhibits DA and protein synthesis 31

32 Antiparasitic Chemotherapy Antimalarial drugs: quinine, chloroquinine, primaquine, mefloquine Antiprotozoan drugs: metronidazole (Flagyl), quinicrine, sulfonamides, tetracyclines Antihelminthic drugs: immobilize, disintegrate, or inhibit metabolism Mebendazole, thiabendazole: broad-spectrum, inhibit function of microtubules, interferes with glucose utilization and disables them Pyrantel, piperazine: paralyze muscles iclosamide: destroys scolex 32

33 Antiviral Chemotherapeutic Agents Selective toxicity is almost impossible due to obligate intracellular parasitic nature of viruses Block penetration into host cell Block replication, transcription, or translation of viral genetic material ucleotide analogs Acyclovir herpesviruses Ribavirin a guanine analog RSV, hemorrhagic fevers AZT thymine analog HIV Prevent maturation of viral particles Protease inhibitors : HIV 33

34 Drugs for Treating Influenza Amantadine, rimantidine restricted almost exclusively to influenza A viral infections; prevent fusion of virus with cell membrane Relenza and tamiflu slightly broader spectrum; blocks neuraminidase in influenza A and B I. Inhibition of Virus Entry or Release 1 2 Fuzeon Prevents binding of viral GP-41 receptors to cell receptor which blocks fusion of virus with cell Maravoc Covers up the cell receptors; HIV cannot adhere and is inert 2 HIV virus 1 Drug Table 12.6 Antiviral Drugs Cell receptors Host cell membrane o Fusion/no entry 3 4 Amantidine and relatives Block entry of influenza virus by interfering with fusion of virus with cell membrane (also release) Tamiflu, Relenza Stop the actions of influenza neuraminidase, required for entry of virus into cell 3 Influenza virus 4 4 Amantidine, Tamiflu ucleus Thought to interfere with influenza virus assembly or budding/release 34

35 Antiherpes Drugs Many antiviral agents mimic the structure of nucleotides and compete for sites on replicating DA Acyclovir (Zovirax), Valacyclovir (Valtrex), Famiciclovir (Famvir), Peniciclovir (Denavir) ral and topical treatments for oral and genital herpes, chickenpox, and shingles Table 12.6 Antiviral Drugs II. Inhibition of ucleic Acid Synthesis 5 Acyclovir, other cyclovirs Herpesvirus DA Polymerase Terminates DA replication in herpesviruses Virus DA 6 ucleoside analog reverse transcriptase (RT) inhibitors (zidovudine - retrovir) Drug Drug 5 Stop the action of reverse transcriptase in HIV, blocking viral DA production o Viral DA replication 7 on-nucleoside reverse transcriptase inhibitors (nevirapine) HIV virus HIV RA RT 6 o HIV DA Attach to HIV RT binding site, stopping its action 7 no reverse transcription 35

36 Drugs for Treating HIV Infections and AIDS Retrovirus offers 2 targets for chemotherapy: Interference with viral DA synthesis from viral RA using nucleoside reverse transcriptase inhibitors (nucleotide analogs) Interference with synthesis of DA using nonnucleoside reverse transcriptase inhibitors Azidothymidine (AZT) first drug aimed at treating AIDS, thymine analog 36

37 12.4 Interactions between Microbes and Drugs: The Acquisition of Drug Resistance Adaptive response in which microorganisms begin to tolerate an amount of drug that would ordinarily be inhibitory; due to genetic versatility or variation; intrinsic and acquired 37

38 How Does Drug Resistance Develop? Acquired resistance: Spontaneous mutations in critical chromosomal genes Acquisition of new genes or sets of genes via transfer from another species riginates from resistance factors (plasmids) encoded with drug resistance, transposons Figure Transfer of drug resistance DA fragment Dead bacterium Resistance gene Transformation Transfer of free DA Bacterium Receiving Resistance genes Plasmid donor Resistance gene Gene goes to plasmid or to chromosome Conjugation Plasmid transfer Plasmid Virus Transduction Transfer by viral delivery 38

39 Specific Mechanisms of Drug Resistance Figure Examples of mechanisms of drug resistance Drug inactivation Result R Drug S Active penicillin Cell surface of microbe CH 2. Decreased permeability Penicillinase ormal receptor R C H H Cell surface of microbe S CH Inactive penicillin 1. Inactivation of a drug like penicillin by penicillinase, an enzyme that cleaves a portion of the molecule and renders it inactive. 2. The receptor that transports the drug is altered, so that the drug cannot enter the cell. Different receptor 3 3. Activation of drug pumps Drug Cell surface of microbe Inactive Drug pump 4. Change in drug binding site Cell surface of microbe Active Drug pump 3. Specialized membrane proteins are activated and continually pump the drug out of the cell Binding site on target (ribosome) is altered so drug has no effect 5. Use of alternate metabolic pathway 5 Drug acts A B X C D Product C D 5. The drug has blocked the usual metabolic pathway (green), so the microbe circumvents it by using an alternate, unblocked pathway that achieves. the required outcome (red) 39

40 12.5 Interactions Between Drugs and Hosts Estimate that 5% of all persons taking antimicrobials will experience a serious adverse reaction to the drug side effects Major side effects: Direct damage to tissue due to toxicity of drug Allergic reactions Disruption in the balance of normal florasuperinfections possible Figure Drug induced side effects 40 Kenneth E. Greer/Visuals Unlimited

41 Figure Superinfections Infection (a) A primary infection in the throat is treated with an oral antibiotic Circulating drug Drug Superinfection (c) The primary infection is cured, but drug-resistant pathogens have survived and create an intestinal superinfection. (a) (b) (c) (b) The drug is carried to the intestine and is absorbed into the circulation. Intestine Intestine Intestine Potential pathogen resistant to drug but held in check by other microbes Drug destroys beneficial flora Pathogen overgrows ormal flora important to maintain intestinal balance 41

42 12.6 Considerations in Selecting an Antimicrobial Drug Identify the microorganism causing the infection Test the microorganism s susceptibility (sensitivity) to various drugs in vitro when indicated The overall medical condition of the patient 42

43 Testing for the Drug Susceptibility of Microorganisms Essential for groups of bacteria commonly showing resistance Kirby-Bauer disk diffusion test xytetracycline 30 µg (R < 17 mm; S 22 mm) Kirby-Bauer Disk Diffusion Test* Enrofloxacin 5 µg (R < 17 mm; S 22 mm) Gentamicin 10 µg (R < 17 mm; S 21 mm) CTX Figure Techniques for preparation and interpretation of disc diffusion tests. AMC 0mm ER T 30 G 10 CTX 30 C 30 AMP 10 Cefotaxime 30 µg Ampicillin 10 µg (R < 14 mm; S 23 mm) (R < 14 mm; S 22 mm) Chloramphenicol 30 µg (R < 21 mm; S 21 mm) ER = Zone of = Region of = Antibiotic carrier (disc) 5 imprinted with abbreviation Inhibition bacterial growth and concentration Disk DifusionT est (schematic). Example and evaluation of a sensitivity test, agar diffusion methoid R = resistant, I = intermediate, S = sensitive (a) If the test bacterium is sensitive to a drug, a zone of inhibition develops around its disk. The larger the size of this zone, the greater is the bacterium s sensitivity to the drug. The diameter of each zone is measured in millimeters and evaluated for susceptibility or resistance by means of a comparative standard (R, I, or S). Prof. Patrice Courvalin, MD, FRCR, Unite desagentsantibactcriens, Institut Pasteur, Paris, France (b) Result for gram negative Klebsiella pneumoniae, with zones of inhibition indicating sensitivity to cefotaxime (CTX) and ampicillin and clavulanic acid (AMC). bserve the expanded zone between these two drugs that indicates their enhanced action, or synergy, when combined. Kathy Park Talaro (c) Result for gram positive Staphylococcus aureus using gentamicin (GM, left) and aztreonam (ATM, right), Although the bacteria are sensitive to both drugs, the GM has a few colonies growing in the zone of inhibition, indicating some cells that have developed resistance to that concentration of the drug. 43