1. Ch 20 Antimicrobial Drugs

2. SLOs Compare penicillin, cephalosporin, and vancomycin
Briefly explain the modes of action of some antifungal drugs.
Explain the modes of action of some antiviral drugs.
Explain the modes of action of some antiprotozoan and antihelminthic drugs.
Describe the Kirby Bauer test.
Describe mechanisms of drug resistance.
Describe the history of chemotherapy.
Name the microbes that produce the most antibiotics.
Describe problems of chemo- therapy for viral, fungal, protozoan, and helminthic infections.
Define: Therapeutic index, antibiotic, activity spectrum, bacteriostatic, -cidal
Identify five modes of action of antimicrobial drugs.
Describe the mechanism of action of penicillin and the mechanism of resistance to penicillin.

3. Antimicrobial Drugs Chemotherapy: The use of drugs to treat a disease.
Antimicrobial drugs: Interfere with the growth of microbes within a host.
Antibiotic: Of biological origin. Produced by a microbe, inhibits other microbes.
Chemotherapeutic agent: synthetic chemicals
Today distinction blurred  many newer "antibiotics" are biological products that are
chemically modified or
chemically synthesized

4. The History of Chemotherapy
Paul Ehrlich and Sahachiro Hata developed Salvarsan (Arsphenamine) against syphilis in 1910: The concept of chemotherapy to treat microbial diseases was born.
Sulfa drugs (sulfanilamide) discovered in 1932  against Gram+ bacteria

5. The History of Chemotherapy cont.
1928: Fleming discovered penicillin
1940: Howard Florey and Ernst Chain performed first clinical trials of penicillin.
Fig 20.1

6. Features of Antimicrobial Drugs
Selective toxicity: Drug kills pathogens without damaging the host.
Therapeutic index: ratio between toxic dose and therapeutic dose – or ratio of LD50 to ED50 High therapeutic index  less toxic
Antimicrobial action – Bacteriostatic vs. bactericidal
Activity Spectrum – Broad-spectrum vs. narrow- spectrum
Tissue distribution, metabolism, and excretion – BBB; Unstable in acid; half-life duration
The ratio of the dose required to produce toxic or lethal effects to dose required to produce nonadverse or therapeutic response.

7. The Action of Antimicrobial Drugs
Foundation Fig 20.2

8. Inhibition of Protein Synthesis by Antibiotics
Figure 20.4

9. Antibacterial Antibiotics Inhibitors of Cell Wall Synthesis: Penicillin
Natural and semisynthetic penicilins contain β-lactam ring
Natural penicillins produced by Penicillium are effective against Gram + cocci and spirochetes
Semisynthetic penicillins: made in laboratory by adding different side chains onto β-lactam ring  penicillinase resistant and broader spectrum of activity

10. Retention of Penicillin G
Figure 20.7

11. Penicillin cont.
Penicillinase (β-lactamase): bacterial enzyme that destroys natural penicillins
Penicillinase resistant penicillins: methicilin replaced by oxacilin and nafcilin due to MRSA
Extended-spectrum penicilins: Ampicilin, amoxicilin; new: carboxypenicilins and ureidopenicillins (also good against P. aeruginosa)
Fig 20.8

12. Cephalosporins
Fungi of genus Cephalosporium  4 Generations of cephalosporins
First-generation: Narrow spectrum, gram-positive
Second-generation: Extended spectrum includes gram-negative
Third-generation: Includes pseudomonads; mostly injected, some oral.
Fourth-generation: Most extended spectrum
Their broad spectrum of activity and safety profile make the cephalosporins one of the most widely prescribed class of antimicrobials. The earlier generation cephalosporins are commonly used for community-acquired infections, while the later generation agents, with their better spectrum of activity against gram-negative bacteria make them useful for hospital-acquired infections or complicated community-acquired infections.
The Professor of Hygiene of the University of Cagliari, Giuseppe Brotzu ( ), wondered why typhoid fever was less virulent in his city than elsewhere. He had formulated various hypotheses, all in vain. One day, while he was passing through the neighbourhood of the bay of "Su Siccu" he saw some young people bathing in the sea , just in the waters where Cagliari sewer system debouches without contracting the disease. So he took a sample of the water to test its effect on a culture of salmonella typhi. Since it was wartime and there was no meat to make the broth for the culture medium, Brotzu used the placentas obtained from parturient women in the maternity ward, that he then boiled in order to obtain the medium. With the help of his assistant Antonio Spanedda ( ), he isolated a fungus from this water that produced an effective substance against Gram-negative bacteria- (the typhoid is in fact caused by Salmonella Typhi, which like all the enterobacteriaceae is Gram- negative) however, the fungal extract he had prepared was a raw compound which could not be produced on a larges scale. When Brotzu asked for funding from the Italian Committee for research it was not given to him for political reasons since he had previously adhered to Fascism, as did almost all Italian university professors. Moreover, Brotzu dedicated himself less to research because he got more involved in politics, first becoming Mayor of Cagliari and then President of the Region of Sardinia.
During fascist times, Brotzu had already been involved in the fight against malaria. After the war he was a consultant to the Rockefeller Foundation which, thanks to land reclamation and the massive use of DDT, succeeded in eradicating malaria (1950) by eliminating its carrier. Notwithstanding his great merits (for having discovered the cephalosporium he was proposed for the Nobel Prize) he is not usually mentioned in Italian texts, whereas, in contrast, he is named in pharmacological textbooks throughout the world. Brotzu received neither honour, nor money. In fact, he consigned several strains of cefalosporium fungus to an English sanitary official who had come to Sardinia in the anti-malaria campaign. In turn, this official gave the material to Edward Abraham ( ), a student of Fleming, who isolated the cephalosporin. The drug was then sold throughout the world by the pharmaceutical companies Glaxo and Lilly making a good profit for them.

13. Cephalosporins cont. Structure and mode of action resembles penicilins
More stable to bacterial - lactamases than penicilins
Broader spectrum  used against penicillin- resistant strains
Their broad spectrum of activity and safety profile make the cephalosporins one of the most widely prescribed class of antimicrobials. The earlier generation cephalosporins are commonly used for community-acquired infections, while the later generation agents, with their better spectrum of activity against gram-negative bacteria make them useful for hospital-acquired infections or complicated community-acquired infections.
The Professor of Hygiene of the University of Cagliari, Giuseppe Brotzu ( ), wondered why typhoid fever was less virulent in his city than elsewhere. He had formulated various hypotheses, all in vain. One day, while he was passing through the neighbourhood of the bay of "Su Siccu" he saw some young people bathing in the sea , just in the waters where Cagliari sewer system debouches without contracting the disease. So he took a sample of the water to test its effect on a culture of salmonella typhi. Since it was wartime and there was no meat to make the broth for the culture medium, Brotzu used the placentas obtained from parturient women in the maternity ward, that he then boiled in order to obtain the medium. With the help of his assistant Antonio Spanedda ( ), he isolated a fungus from this water that produced an effective substance against Gram-negative bacteria- (the typhoid is in fact caused by Salmonella Typhi, which like all the enterobacteriaceae is Gram- negative) however, the fungal extract he had prepared was a raw compound which could not be produced on a larges scale. When Brotzu asked for funding from the Italian Committee for research it was not given to him for political reasons since he had previously adhered to Fascism, as did almost all Italian university professors. Moreover, Brotzu dedicated himself less to research because he got more involved in politics, first becoming Mayor of Cagliari and then President of the Region of Sardinia.
During fascist times, Brotzu had already been involved in the fight against malaria. After the war he was a consultant to the Rockefeller Foundation which, thanks to land reclamation and the massive use of DDT, succeeded in eradicating malaria (1950) by eliminating its carrier. Notwithstanding his great merits (for having discovered the cephalosporium he was proposed for the Nobel Prize) he is not usually mentioned in Italian texts, whereas, in contrast, he is named in pharmacological textbooks throughout the world. Brotzu received neither honour, nor money. In fact, he consigned several strains of cefalosporium fungus to an English sanitary official who had come to Sardinia in the anti-malaria campaign. In turn, this official gave the material to Edward Abraham ( ), a student of Fleming, who isolated the cephalosporin. The drug was then sold throughout the world by the pharmaceutical companies Glaxo and Lilly making a good profit for them.

14. Vancomycin Glycopeptide from Streptomyces
Inhibition of cell wall synthesis
Used to kill MRSA
Emerging Vancomycin resistance: VRE and VRSA
Expensive!

15. Antifungal Drugs
Polyenes, such as nystatin and amphotericin B, for systemic fungal infections. Inhibition of ergosterol synthesis  fungicidal. Nephrotoxic
Griseofulvin from Penicillium. Systemic/oral. Binds to tubulin  For Tineae
Ergosterol in fungi membrane. animals have cholesterol.
Tinea capitis (ringworm of the scalp) Tinea corporis (ringworm of the body) Tinea pedis (athlete's foot) Tinea unguium (onychomycosis) Tinea cruris (ringworm of the thigh) Tinea barbae (barber's itch)

16. Antiviral Drugs Nucleoside analogs inhibit DNA synthesis
Acyclovir and newer derivatives: Selective inhibition of herpes virus replication. Acyclovir conversion to nucleotide analog only in virus infected cells  very little harm to uninfected cells!
Nucleoside analogs were among the first compounds shown to be effective against viral infections. The pioneering work of Elion and colleagues at Burroughs-Wellcome led to the development of acyclovir which is used extensively in the treatment of herpetic infections (1). Therefore, it is not surprising that the first four anti-HIV drugs to be approved, AZT, ddI, ddC and D4T ; were nucleoside analogs.
Ganciclovir (9-[(1,3-dihydroxy-2-propoxy)methyl]guanine) is a potent inhibitor of viruses of the herpes family, including cytomegalovirus (CMV), that are pathogenic for humans and animals. The primary mechanism of ganciclovir action against CMV is inhibition of the replication of viral DNA by ganciclovir-5'-triphosphate (ganciclovir-TP). This inhibition includes a selective and potent inhibition of the viral DNA polymerase. Ganciclovir is metabolized to the triphosphate form by primarily three cellular enzymes: (1) a deoxyguanosine kinase induced by CMV-infected cells; (2) guanylate kinase; and (3) phosphoglycerate kinase. Other nucleotide-metabolizing enzymes may be involved as well. The selective antiviral response associated with ganciclovir treatment is achieved because of the much weaker inhibition of cellular DNA polymerases by ganciclovir-TP. Activity and selectivity are also amplified by the accumulation of ganciclovir-TP in CMV-infected cells.
The compound of choice for the prophylaxis and treatment of HSV infections for many years, ACV selectively inhibits HSV DNA replication with low host-cell toxicity. Following uptake of ACV by virus-infected cells, phosphorylation of the drug to ACV-monophosphate (ACV-MP) occurs via HSV-encoded TK. Subsequent conversion to ACV-triphosphate (ACV-TP) is catalyzed by host cell enzymes. ACV-TP preferentially inhibits viral DNA polymerase over cellular DNA polymerases. Of all the human herpesviruses (HHV), HSV-2 and HSV-1 are the most susceptible to ACV, followed (in order of decreasing susceptibility) by VZV, Epstein-Barr virus (EBV), HHV-6, and human cytomegalovirus (CMV). ACV does not exhibit in vitro activity against viruses outside the herpesvirus family.[18-21]
Fig 20.16

17. Mechanism of Action of Acyclovir
Nucleoside = base bound to a ribose or deoxyribose sugar
Fig 20.16

18. Antiviral Drugs for Treating HIV/AIDS: HAART
NRTIs and NNRTIs
Protease Inhibitors
Fusion Inhibitors
Integrase Inhibitors
NRTIs: nucleoside analog reverse transcriptase inhibitor
The HIV protease enzyme cleaves polyproteins of the virus into essential functional protein products during the maturation process of the virion (Figure 1, green and red bars). This critical process occurs as each new virion buds forth from the membrane of an HIV-infected cell and continues after the immature virus is released from the cell. If the polyproteins are not cleaved, the virus fails to mature and is incapable of infecting a new cell.
HIV protease cleaves viral polypeptide into functional proteins
Protease inhibition  HIV cannot mature and noninfectious viruses are produced.

19. Antiprotozoan and Antihelminthic Drugs
Examples of Antiprotozoan:
Chloroquine: Malaria
Quinacrine: Giardia
Metronidazole (Flagyl): Vaginitis, anaerobic bacteria
Examples of Antihelminthic:
Niclosamide and praziquantel: Tapeworm
Mebendazole: broadspectrum antihelmintic
Ivermectin: nematodes, mites, lice . . .

20. Antibiotic Assays to Guide Chemotherapy
Agar Disk Diffusion Method determines susceptibility of an organism to a series of antibiotics: Kirby-Bauer test
More sophisticated methods available for clinical labs

21. Vertical evolution due to spontaneous mutation
Drug Resistance
Penicillin G resistance of S. aureus from 3% to > 90%
Multidrug-resistant S. aureus = MRSA or “super-bug”
Vancomycin-resistance 
Multi drug resistant TB = MDR-TB
Evolution of drug resistance:
Vertical evolution due to spontaneous mutation
Horizontal evolution due to gene transfer ??

22. Antibiotic Resistance
A variety of mutations can lead to antibiotic resistance
Mechanisms of antibiotic resistance
Enzymatic destruction of drug
Prevention of penetration of drug
Alteration of drug's target site
Rapid ejection of the drug
Resistance genes are often on plasmids or transposons that can be transferred between bacteria.

23. Resistance to Antibiotics
Fig 20.20

24. Antibiotic Resistance
Read Clinical Focus: Antibiotics in Animal Feed Linked to Human Disease (p. 577)
Antibiotic Resistance
Misuse of antibiotics selects for resistance mutants. Misuse includes
Using outdated or weakened antibiotics
Using antibiotics for the common cold and other inappropriate conditions
Using antibiotics in animal feed
Failing complete the prescribed regimen
Using someone else's leftover prescription
the end
ANIMATION Antibiotic Resistance: Origins of Resistance and Forms of Resistance