Ch. 9: Antimicrobial Chemotherapy

chemotherapy

anything given to a host to treat a disease

most chemotherapeutic agents are

antibiotics

synthetic antibiotics

comes from lab

ex: Salvarsan and prontosil (dye)

semi-synthetic antibiotic

modified true antibiotic

true antibiotic

(most) comes from living organism

Thomas & Bertheim

• early research in chemotherapy

• arsenical dyes

  • synthesized Atoxyl (→ blindness)

Atoxyl

arsenical that treats African sleeping sickness

• caused by protist

• Trypanosoma

Ehrlich and Hata

• dyes and selective toxicity in bacteria

• identified Salvarsan (abx)

  • syphillus

Salvarsan

treats Treponema pallidum (syphillis)

Domagk, Jacques and Trefouel

• developed Prontosil dye (streptococcal & staphylococcal infections)

  • led to discovery of sulfonamides/sulfa

Gerhard Domagk

tried growing prontosil in a petri dish (en vitro) but it showed no activity. he tried it on his sick daughter (en vivo) and found that it did work

• modified in living organism which made it work

• prontosil is a pro drug which is inactive outside the body

Penicillin

• first true antibiotic made by fungus penicillium

Ernest Duchesne

noticed fungi (penicillin) killing bacteria

Alexander Fleming

Staph petri dish with fungus contaminant - fungus (penicillin) is killing bacteria

couldnt isolate it

Florey, Chain, and Heatley

• received Nobel Prize in 1945

• isolated penicillin from penicillum; discovered therapeutic use

Waksman

• first to identify streptomycin

  • treated M. tuberculosis

Actinomycetes

grow with filament

Streptomyces

• bacteria; produces the most true antibiotics

why do bacteria kill other bacteria

competition

what antibiotics were discovered by 1953

chloramphenicol

terramycin

neomycin

tetracyclin

AI and antibiotics

2022:

• AI yielded potential drugs Abaucin against Acinetobacter baumannii (microbe from ESCAPE, one of the worst pathogens, NARROW spectrum) and other drug-resistant bacteria

2023:

• screened millions of compounds and tested 283 promising compounds

• several were effective against MRSA

Abaucin

• narrow spectrum

• works against Acinetobacter baumanii

selective toxicity

ability of an antimicrobial agent to kill or harm the microorganism cells without harming the cells of the host

therapeutic index

• ratio of toxic dose to therapeutic dose

• higher index: better drug

• drugs have a minimum concentration necessary to kill or inhibit growth of bacteria

  • aka how much drug you have to give to cause an outcome to microbe

broad vs narrow spectrum antibiotics

• narrow spectrum can only kill a few microbes

• obligately parasitic bacteria: atypical

• no antibiotics for viral infections

how is effectiveness of antimicrobial drugs expressed

• effect of an agent may vary

• expressed in 2 ways

  • minimal inhibitory concentration (MIC)

  • minimum lethal concentration (MLC)

minimal inhibitory concentration (MIC)

static

drug inhibits microbe from continuing replication

minimum lethal concentration (MLC)

cidal

• destruction of the cell, can kill microbe - doesn’t wait for immune system

• need this if immunocompromised

• lytic: breaks it apart

  • do not want to use bacteriolytic abx with gram-negatives because lipid A is an endotoxin

ways to determine level of antimicrobial activity

• dilution susceptibility tests for MIC

  • drug inhibits microbe from further replication

• disk diffusion test (Kirby Bauer)

  • zone of inhibition: clear area around the disc has no bacterial growth - sensitive to abx

    • bigger zone doesnt mean it’s better

• the Etest

  • strip test

main modes of action of antimicrobial drugs

• inhibitors of cell wall synthesis

• protein synthesis inhibitors

• metabolic antagonists

• nucleic acid synthesis inhibition

inhibitors of cell wall synthesis

• most specific to bacteria (peptidoglycan cell walls)

• no inhibitor of cell membrane because cell membrane between bacteria and eukaryotes are very similar

• Mycoplasma resistant to these abx

• antibiotics are most effective when bacteria are actively dividing (new cell walls being synthesized)

inhibitors of cell wall synthesis - PENICILLINS

• first identified true abx from fungus Penicillium - penicillin V and G

• semisynth alternatives (ampicillin)

• bactericidal (destroy)

• β-lactam ring - four sided ring

• 1-5% of adults in US are allergic

β-lactam ring

• recognized by penicillin binding proteins

• inhibits transpeptidation in bacteria from making peptide bonds

• β-lactamase/penicillinase (specifically penicillin)

what happens if the cell wall is destroyed

there is no protection against osmotic pressure, causing the cell to lyse

β-lactamase/penicillinase

enzymes that bacteria contain to break down the β-lactam ring and destroy penicillin

• penicillin is unable to block transpeptidation, allowing bacteria to survive

Penicillin V and G

against gram-positive only

• narrow spectrum

what spectrum are semisynthetic penicillins

broad

semisynthetic penicillins

Ampicillin

Carbenicillin

Piperacillin

Nafcillin

• made more soluble to be able to get to peptidoglycan

Ampicillin

works against Gram-negative H. influenzae, Salmonella spp. and S. dysenteriae

inhibitors of cell wall synthesis - CEPHALOSPORINS

• bactericidal, broad spectrum

• true antibiotic made by fungus (mold) Acremonium (previously Cephalosporium)

  • can be used for those allergic to penicillin

• β-lactam ring

• has resistant bacteria

resistant bacteria against Cephalosporins

N. gonorrhoeae and Enterobactericeae HAI

inhibitors of cell wall synthesis - CARBAPENEM and other β-lactam

• semi-synthetic

  • combination of penicillin and cephalosporin

• broad spectrum

• β-lactam

• type of penem = penicillin and cephalosporin hybrid drugs

penem

modified penicillins with
→ Sulfur replaced by carbon
→ A double bond added to the ring

• pentane → pentene

what antibiotics have β-lactam rings

Penicillin

cephalosporins

carbapenem

• inhibit transpeptidation from making peptide bonds → destroys bacteria

inhibitors of cell wall synthesis - VANCOMYCIN

• bacterialcidal, narrow-spectrum

  • gram-positive only: Staphylococcus, Clostridium, Bacillus, Streptococcus, Enterococcus

  • Clostridium + Bacillus: sporeformers

    • turns into spore when exposed to abx and will not get killed

• true antibiotic produced by bacterium Streptomyces orientalis

• inhibits transpeptidation by not allowing removal of terminal alanine

• treats antibiotic-resistant staphylococcal and enterococcal infections

  • previously considered “drug of last resort” against MRSA

  • vancomycin resistant enterococcus (VRE)

    • gram-positive

no beta-lactam ring

entero means?

intestines

protein synthesis inhibitors

• multitude of steps within protein synthesis

• generally associated w/ ribosomes or steps that lead to process of translation within ribosomes

  • side effects associated bc of mitochondria also being 70s

• many antibiotics bind specifically to bacterial ribosome

bacterial ribosome

small subunit: 30s

• 16s rRNA: aligns ribosome w/ shine-dalgarno on mRNA and decodes mRNA during translation

large subunit: 50s

• 23s rRNA: peptidyl transferase activity; forms peptide bonds between AA

70s ribosome

the subunits are made of rRNA or protein

30s subunit

• reads mRNA

• tRNA anticodon recognizes codon

  • 30s ensures correct codon-anticodon pairing

• drugs that target 30s:

  • aminoglycosides: mRNA misreading

  • tetracyclines: blocks tRNA from binding at A site

50s subunit

• tRNA - APE site

• tRNA brings AA to recognized codon and allows peptide bonds to form between 2 AA

eukaryotic ribosome

small subunit: 40s

large subunit: 60s

80s ribosome

steps that antibiotics inhibit in protein synthesis

• aminoacyl-tRNA binding

• peptide bond formation

• mRNA reading

• translocation

protein synthesis inhibitors - AMINOGLYCOSIDES

• bacteriocidal, broad-spectrum

• binds to 30s subunit of ribosome

• true antibiotics:

  • streptomycin, kanamycin, neomycin by bacteria Streptomyces spp.

  • Gentamicin by bacteria Micromonospora spp.

• all other abx are semisynthetic

• can be toxic and cause renal damage

Gentamicin

• made by bacteria Micromonospora spp.

• used for aerobic gram-negative Proteus, Escherichia, Klebsiella, and Serratia

protein synthesis inhibitors - TETRACYCLINES

• 4 rings

• bacteriostatic, broad spectrum

  • binds to 30s subunit of ribosome

• treats intracellular pathogens rickettsias, chlamydiae, and mycoplasmas

• true antibiotics, Oxytetracycline and chloretetracycline prod by Streptomyces spp.

  • others are semisynthetic (ex: doxycycline)

• side effects: black teeth (chelating of calcium) and liver toxicity

  • not Rx to pregnant women and young children (< 8 yrs)

protein synthesis inhibitors - MACROLIDES

• large bacteriostatic, broad spectrum; contains lactone ring

  • binds to 50s subunit

  • treats against gram-positive, mycoplasma spp., and some gram-negative

• erythromycin is a true abx (bacteria Saccharopolyspora), all others are semisynthetic

• Azithromycin - chlamydia (atypical) “Z-pack”

  • most common Rx along with amoxicillin

• used for pt’s allergic to penicillin and cephalosporin

protein synthesis inhibitors - LINCOSAMIDES

• large bacteriostatic, broad psectrum

  • binds to 50s subunit

  • treats against gram-positive cocci, some gram-neg anaerobes, and β-lactamase producers

• true abx produced by Streptomyces spp.

• disrupts colon’s microbiota, supporting growth of C. diff (gram-pos rod spore former)

• treats CA-MRSA

• clindamycin

problem with prescribing lincosamides

• disrupts colon’s microbiota, supporting growth of C. diff (gram-pos rod spore former)

• C. diff leads to pseudomembranous colitis → no absorption of nutrients, diarrhea, hard to kill

• Lincosamides are a last resort because CDI is problematic and requires a fecal transplant to treat

  • should be reserved for serious infx where less toxic antimicrobial agents are innappropriate

  • do not use in pt’s with nonbacterial infections

protein synthesis inhibitors - OXAZOLIDINONES

• bacteriostatic, broad spectrum

  • may be bactericidal for some bacteria

  • binds to 50s subunit (specifically 23s rRNA) → no 70S formation

• synthetic

• linezolid introduced in 21st century

• treats MRSA (methicillin), VRE (vancomycin), penicillin-resistant S. pneumoniae, and some gram-pos anaerobes

• very few side effects, only used IN hospitals

protein synthesis inhibitors - CHLORAMPHENICOL

• bacteriostatic, first broad-spectrum

  • binds to 50s

  • against most gram-pos, many gram negative anaerobes, and rickettsias

• true abx produced by streptomyces venezuelae

• toxic with numerous side effects

  • aplastic anemia, leukemia, neurotoxin reactions

  • grey baby syndrome

  • inhibits mitochondrial protein synthesis

  • not allowed in food-producing animals

metabolic antagonists

• binds to enzymes that we don’t have, inhibits pathways that aren’t used

• acts as antimetabolites bc they look like the real version

  • ex: PABA and SFA

    • both fighting to bind to active site, if there is more of SFA, it’s more likely to bind than PABA, though PABA still has a chance

    

  • enzyme is only looking for the left side and binds to SFA (doesn’t notice difference)

• structural analogs

• bacteriostatic, broad spectrum

• binding of substrate to enzyme isn’t covalent, can detach

folic acid is required for production of

DNA

metabolic antagonists - SULFONAMIDES/SULFA DRUGS

• bacteriostatic, broad spectrum

• synthetic

  • selectively toxic d/t competitive inhibition

  • works against some protozoa (unicell euk)

• inhibits making of folic acid → pathways with no function

  • folic acid is precursor for purines (A and G), methionine, and ATP

    • without folic acid: no DNA, RNA, proteins, or energy

• sulfonamides are a p-aminobenzoic acid (PABA) analog

  • inhibits dihydropteroate synthase (1st step for folic acid production)

metabolic antagonists - TRIMETHOPRIM

• bacteriostatic, broad spectrum

• synthetic

• inhibits dihydrofolate reductase (2nd step for folic acid production)

• when combined with sulfa drugs, it is more effective treatment, inhibiting the 1st and 2nd steps

  • synergism, works better together

  • forms Bactrim

• side effects: abd pain + photosensitivity

nucleic acid synthesis inhibition

• least specific with most side effects bc RNA and DNA polymerase look alike and are hard to target

• can inhibit transcription (RNA) and replication (DNA)

• variety of mechanisms

• most commonly

  • inhibit RNA polymerase (rifamycin)

  • inhibit topoisomerases (fluoroquinolone)

• not as selectively toxic as other abx bc bacteria and eukaryotes do not differ greatly in the way they synthesize nucleic acids

• small toxicity index

topoisomerases

unwind and separate DNA, prevents overcoiling and getting stuck

nucleic acid synthesis inhibition - FLUOROQUINOLONES

• bactericidal, broad spectrum

• synthetic

• highly effective against enteric bacteria

• inhibit bacterial DNA-gyrase and topoisomerase IV (type II topoisomerases)

• Norfloxacin first approved for humans in 1986

  • currently 9 approved for humans

• ciprofloxacin (Cipro)

  • tendonitis and tendon rupture

nucleic acid synthesis inhibition - RIFAMYCINS

• bactericidal, broad spectrum

• Rifampin is most used member (semisynthetic)

  • blocks transcription by binding to RNA polymerase (β-subunit)

• used:

  • in multidrug treatment of TB & other mycobacterial infections (leprae)

  • Can be used prior to disease - for prophylaxis of close contacts of patients with meningococcal meningitis or (bacteria) Haemophilus influenzae type B meningitis

• may cause red sweat or urine

• inhibit RNA polymerase

antiviral drugs

• drug development is slow because it is hard to identifiy since viruses use enzymes from our cells

• drugs currently used inhibit virus-specific enzymes and life cycle processes

difficult to treat viral infections w/ chemotherapeutic agents bc viruses use metabolic machinery of their hosts which limits potential points of attack

antiviral drugs for influenza

influenza is segmented RNA virus

• Tamiflu (oseltamivir)

  • neuraminidase inhibitor → no virion release → halted viral replication

  • Not a cure for influenza, but shortens course of illness

    • Inhibits virus from binding to cell and replicating

• Xofluza (marboxil baloxavir)

  • blocks transcription of DNA → RNA

  • 2018

there are viruses resistant to both drugs

antiviral drugs for Herpesviridae

• Herpesviridae are DNA viruses

  • use their own enzymes to phosphorylate nucleosides → nucleotides

• Analogs that inhibit virus replication

  • Acyclovir and vidarabine

HIV

• mutates quickly

• can’t be cured

• low viral load (hard to identify)

cocktail of multiple anti-HIV drugs targets the virus from multiple angles, inhibiting the angles from mutating

anti-HIV drugs

1. Nucleoside reverse transcriptase inhibitors (NRTIs) produce faulty DNA

   a. includes AZT

2. Nonnucleoside reverse transcriptase inhibitors (NNRTIs) prevent HIV DNA synthesis

3. Protease inhibitors (PIs) mimic peptide bond that is normally attacked by the protease 

4. Integrase inhibitors prevent HIV genome incorporation

5. Fusion inhibitors prevent HIV entry into cells

• Most successful are drug cocktails to curtail resistance (HAART)

  • Multiple drugs work together to inhibit 

• Preexposure prophylaxis (PrEP) – two NRTIs daily

  • vaccines

antiviral drugs for Hep C

• RNA virus (mutates faster than DNA because RNA polymerase more error prone than DNA viruses)

  • possible opportunistic infection of HIV

• cases of acute hep C have increased in US since 2010

• Interferon-α (cytokine) and ribavirin (antiviral) was first used

  • nucleotide analog

  • These drugs given together cause too many side effects (depression)

• Since 2014, combination drugs

  • sofosbuvir-velpatasvir (Epclusa)

  • sofosbuvir-ledipasvir (Harvoni)

antifungal drugs

• fewer effective agents

  • many have low therapeutic index and are toxic

  • d/t being eukaryotes, identifying something unique to them is difficult

• Easier to treat superficial mycoses than systemic

• Many are fungistatic

• Fungi prefer to grow in cooler temps

treating superficial mycoses

• nystatin (from Streptomyces) – used to control superficial candidiasis

• Thrush from Candida albicans

treating systemic mycoses

• harder to treat

• polyenes and azoles – block cell membrane synthesis (ergosterol - cell membrane of fungi)

• amphotericin B (from Streptomyces) – binds ergosterol in membranes, highly toxic

• 5-flucytosine – uracil analog, disrupts RNA function

• fluconazole – low side effects, used prophylactically for immunocompromised patients

• Echinocandins

  • inhibit β-1,3-D-glucan (building block of fungal cell wall) synthase

    • similar to peptidoglycan inhibitors

  • Some Candida and Aspergillus spp. have β-1,3-D-glucan cell wall

Pneumocystis jiroveci has cholesterol instead of ergosterol

• uses trimethoprim (sulfa drug) and sulfisoxazole instead

antiprotozoal drugs (eukaryotes)

mechanism not precisely known - maybe nucleic acid or some metabolic process

• Chloroquine - Plasmodium (malaria)

• Metronidazole

  • reduced to RNS (reactive nitrogen species) by Entamoeba (dysentery) and Trichomonas (vaginal infection)

• combination therapy of pyrimethamine, dapsone, sulfadiazine, folinic acid  – Toxoplasma gondii (abortion)

  • pyrimethamine and dapsone similar to trimethoprim

    • inhibit folic acid; purines; A/G; ATP

E. histolytica: dysentery

T. gondii: toxoplasmosis

P. falciparum: malaria

T. brucei: african sleeping sickness

Bacteria: T. pallidum: syphilis

drug effectiveness

need to know:

• how the drug arrives at the site of infection

• pathogen is susceptible

• drug level concentration exceeds pathogen’s MIC

  • cannot go lower than the MIC

drug resistance

• microbes in abscess or biofilms

• resistance mutants

mechanisms for drug resistance

intrinsic or acquired (potentially mutation)

mechanisms include:

• modify the abx target

  • enzymes are specific

• enzymes degrade or alter abx

  • β-lactamase

• decrease abx concentration inside cell

  • efflux pumps

• use an alternate pathway or increase target metabolite production

  • change how much metabolite is present; PABA and sulfa

• biofilm formation

• acquisition of resistance genes thru HGT

overcoming drug resistance

• give drug in appropriate concentrations

• give 2 or more at same time (cocktails: less likely for microbe mutation)

• use drugs only when necessary

possible future solutions:

• phages - can use in plants

  • lytic and lysogenic