Lecture 4: Ribosomal Inhibiting Antibacterial Agents

What are the five relevant tetracyclines and the new related drug?

oxytetracycline, chlortetracylcine, tetraclycline, minocycline, doxycycline, new drug = tigecycline

What is the MOA of tetracyclines?

inhibitors of the bacterial 30S ribosome, binding is reversible = bacteriostatic

What is the spectrum of tetracyclines?

• limited due to resistance, except for doxycycline

• Brucella, rickettsia, Chlamydia, and spirochetes, some Mycoplasma

• very good vs L-form bacteria (L-forms are resistant to B-lactams since L-forms lack PTDGs)

Why are L-forms not resistant to tetracyclines?

the tetracycline efflux pumps are often to thecell membrane and thus are lost when the outer “shell” is lost

What is the main mechanism for tetracycline resistance, but which drug is least

affected?

efflux is the main resistance mechanism, L-form bacteria are not resistant, widespread except for doxycycline and tigecycline

Describe two therapeutic effects of tetracyclines that are not related to antimicrobial activity.

• contracted tendons in foals → inhibit collagen contraction of myofibroblasts

• Lyme disease (can cause dysbiosis if used chronically)

What are the toxicities associated with tetracyclines?

• mildly nephrotoxic: not doxcycline or minocycline

• esophageal strictures following an oral dose: cats and doxycycline

• injection site irritant

• photosensitivity

• Oral tetracyclines will cause intestinal dysbiosis in the horse (disruption of the good bacteria in the intestinal microbiome)

• Chelated by Ca2+ and Mg2+ → poorly absorbed in the presence of milk, tetracyclines will deposit in bone and teeth = yellow teeth in young pets, minocycline leaves black deposits in thyroid

What are the special PK considerations of doxycycline?

• doxycycline is neuroprotective → inhibits apoptosis in neurons, helpful vs CNS infections

• doxycycline is a good choice (safe) if a nephropathy is present → not eliminated via the kidneys

How are tetracyclines eliminated via kidney?

1. Most drugs passively move from the afferent arteriole into the glomerular filtrate and then stay in the urinary filtrate

2. some drugs, such as doxy can move through tubule cells and enter a peritubular capillary

3. once in the peritubular capillary, the doxycycline will return to the systemic circulation

4. the majority of antibiotics do NOT re-enter the systemic circulation

5. kidney dz → drug retention → potential toxicity, but not for doxy

What are the clinical applications of tetracyclines (especially doxycycline)?

• non-responsive abscesses (L-form bacteria)

• Chlamydia infections

• used to treat contracted tendons in foals

• Lyme disease

• bovine respiratory treatment (oxytetracycline) (combined with the NSAID flunixin)

How are tetracycline included in the veterinary feed directive?

• oxytetracycline: no longer available as a feed additive for preventing (metaphylaxis) BRD

• chlortetracycline: still available for preventing liver abscesses in feedlot cattle but only by prescription

What is the mechanism of action for aminoglycosides and aminocyclitols (gentamicin, amikacin)?

AMGs inhibitors of 30S ribosomal subunit → covalently and irreversibly fuse to the 30S ribosome = bactericidal

AMC reversibly bind = bacteriostatic

What is the spectrum of AMGs and AMCs?

• Gm (-) aerobes and Gm (+) cocci

• useless against anaerobes because transport of these drugs into bacteria is oxygen-dependent

• streptomycin has activity vs Mycobacterium tuberculosis

• both tobramycin and amikaicn have activity vs. pseudomonas

What is the mechanism of resistance of aminoglycosides and aminocyclitols?

primarily via bacterial enzymes that modify and inactivate the drugs, some cross-resistance between AMGs and AMCs

Describe the nephrotoxicity of AMGs and AMCs.

• these drugs are cationic and have a tropism for binding to the anionic phospholipids on the renal tubule epithelial cell → absorbed into the renal tubule cell via lysosomes → drugs rupture the lysosomes, killing the tubule cell

• minimized my increased the dose interval (trough)

• accentuated by diuretics and calcium supplementation

What type of bacteria are universally insensitive to aminoglycosides?

anaerobes

What are the most and least nephrotoxic AMG/AMCs?

• neomycin is most nephrotoxic

• streptomycin and tobramycin are the least nephrotoxic

Describe the ototoxicity of AMGs.

• damages ear cells by binding to and harming the 12S ribosome present in mitochondria of ear cells

• cochlear (hearing) toxicity: most frequent with kanamycin, amikacin, and neomyin; mostly seen in dogs; can be irreversible

• vestibulopathies (balance): reversible, most frequent with streptomycin, genatmicin, and tobramycin; mostly observed in cats

What are the neuromuscular side effects of aminoglycosides?

neuromuscular blockade: inhibition of nAChR, mostly ocurring in conjuction with anaesthetic agents → worse in patients with myasthenia gravis → tx: calcium gluconate or neostigmine

Describe the physical incompatibility of aminoglycosides with other drugs.

• AMGs are polar and very reactive with other drugs so do not mix in an IV

• Most incompatible with beta-lactams, but clinically synergistic with beta-lactams → flush the IV line between administering these drugs

What is the post-antibiotic effect of aminoglycosides?

even momentary exposure can have effect on bacteria → an elevated short-term dose kills bacteria for long periods of time

What is the bioavailability of aminoglycosides?

polar drugs given injectable and Vd is low → tend to remain in the blood which is mostly water → therefore, AMGs rarely cross the BBB (lipid barrier)

What needs to be ruled out before AMGs can be used topically vs otitis?

need to make sure the ear drum is not patent because ototoxicity will ensue if the ear drum is ruptured

Why are AMGs useful against UTIs?

AMGs are already water soluble → liver does not need to metabolize for renal excretion → AMGs are lost into urinary filtrate in active bactericidal form → AMGs reach bladder in active state and kill bacteria

Why are the clinical applications of AMGs limited?

• resistance: exceptions are amikacin and apramycin

• toxicities: exception is plazomicin, a new drug with minimal toxicity

• AMGs are not available in pill form for dispensing (not tx continuity)

• fluoroquinolones have clinically supplanted AMGs

• extended withdrawl times in food animals; NO systemic extralable use

What are the clinical applications of gentamicin?

• topical applications in companion animals

• equine intrauterine vs metritis

• equine injectable vs Gm(-) sepsis

• swine injectable → colibacillosis

What are the clinical applications of amikacin?

• Gm (-) enteric bacteria seem incapable of mounting resistance to drug

• companion animals and equine IV limb → joint

• warranted when dictated by the antibiogram, but remember the toxicities

What aminoglycoside is used to treat cryptosporidium?

paromomycin

What is the only feed grade AMG available, though restricted?

neomycin

What drug class is used metaphylactically on most transported calves as they arrive at a feedlot to lower the morbidity of BRD?

macrolides

Name the human macrolides (n=3) that can be used in veterinary medicine.

erythromycin, clarithromycin, azithromycin

What is the MOA of macrolides?

reversible 50S inhibitors, bacteriostatic

For macrolides, be able to recognize the five veterinary-specific ones.

tulathryomycin, tilmicosin, tylosin, gamithromycin, tildipirosin

Which bacteria are typically resistant to macrolides?

Gm (-) aerobes

What is the spectrum of macrolides?

• all Gm (+) aerobes

• all anaerobes

• azithromycin has enhanced spectrum that includes Gm (-) aerobes, but does not include pseudomonas

• tylosin has good efficacy for mycoplasma, but long-acting BRD metaphylactic drugs are superior

Describe the primary mechanism for macrolide resistance and how this relates to other antibiotics.

MOA: methylation of ribosome which also confers cross-resistance to most other 50S inhibitors

Describe four side effects of erythromycin.

1. sterile abscesses (erythromycin and tylosin)

2. swine edema

3. rectal prolapses in swine

4. GI distress: motilinR agonist → hyperperistalsis

What are the side effect issues with tilmicosin?

• fatal IV dose is fairly low (5mg/kg) in cattle

• accidental injection in humans can be fatal (cardiotoxic): prolonged Q-T interval

What is the beneficial side effect of macrolides?

anti-inflammatory by inducing neutrophil apoptosis

Describe the favorable pharmacokinetics of tulathromycin and the two new drugs?

concentrates in the lungs and macrophages (weaponizes macrophages)

Why should dead cattle that had been treated with tulathromycin plus ketoprofen be rendered immediately?

ketoprofen is fatally nephrotoxic for avian scavengers

What are the clinical applications of tylosin?

• IBD in dogs or puppies w/ diarrhea after anthelmintic tx

• prevent epiphora (tear) staining in dogs by neutralizing the porphyrin in tears that causes staining (fed at lower dose than antibacterial dose)

• prevention of liver abscess in feedlot cattle (in feed)

• Mycoplasma in swine and poultry (in feed)

• Brachyspira in swine (in feed)

What are the clinical applications of clarithromycin and azithromycin?

C: vs equine Rhodococcus if combined with rifampin

A: vs feline respiratory disease

What are the 5 requirements/restrictions for tilmicosin as a feed additive to control BRD?

• at least 10% of cattle must actively have BRD

• only for 14 days of the first 45 days on feed

• must be fed in complete feed with no other feedstuffs available

• must be only macrolide that has been or is being used

• VFD order only good for 45 days

Name three specific reasons why florfenicol is superior to chloramphenicol (hint: toxicity and resistance)?

1. florfenicol is bactericidal, chloramphenicol is bacteriostatic

2. chloramphenicol causes aplastic anemia that can be fatal, but not florfenicol

3. Chloramphenicol is easier for bacteria to inactivate due to its 2 acetylation sites (florfenicol only has 1)

What is the MOA of phenicols?

50S inhibitors

What is the spectrum for phenicols?

• very broad and includes rickettsiae

• Norcardia and Mycoplasma are usually not susceptible

• florfenicol inhibits virulence gene expression in pasteurella

Why is chloramphenicol banned in food animals?

chloramphenicol binds and inhibits ribosomes in mitochondria in immature RBCs → death of all new RBCs = aplastic anemia → potential for human (and cat) toxicity

What are the phenicol resistance methods of bacteria?

• some bacteria have an enzyme that acetylates and inactivates the phenicols

• efflux mechanism

What are Herxheimer’s reactions?

massive release of toxic bacterial components especially endotoxin (LPS) = iatrogenic endotoxicosis → can occur with florenicol since it is bactericidal 

Describe the relationship between phenicols, neurologic disease, and B-lactams.

about 60% of plasma concentration of phenicols leak into the CSF → florfenicol is therefore a good choice vs meningitis → used with a beta-lactam vs CNS infections because beta-lactams are neuroprotective since they foster an egress of neurotoxic glutamate from brain by increasing the # of glutamate exporters at the BBB

What are the two major clinical indications for florfenicol?

• tx of BRD: long acting single injection of large and thick volume or single injection with an NSAID

• swine respiratory disease: two injections of in the water

What is the clinical application of chloramphenicol?

• tx of chlamydia in koalas

• vs chytridiomycosis in amphibians

What are the 7 long-acting tx options for BRD?

fix every almost dead cow please

florfenicol, enrofloxacin, ampicllin, danofloxacin, draxxinKP = tulathromycin + ketoprofen, ceftiofur, pradofloxacin

Clindamycin has a special “affinity” for which type of Gm+ bacteria?

toxoplasma

What is the MOA of lincosamides (clindamycin, lincomycin, pirlimycin)?

• reversible 50S inhibitors

• Bacteriostatic

What is the major side effect of clindamycin and how is it treated?

clindamycin causes pseudomembranous colitis → inhibits the good intestinal microbiome allowing C. difficle to flourish (intestinal dysbiosis) → alleviated by inclusion of metronidazole

How do bacteria become resistant to lincosamides?

primarily via methylation of ribosome which can confer cross-resistance to macrolides, oxazolidinones, pleuromutilins, and streptogramins (MOPLS), but not florfenicol

Why is pirlimycin to be used only in uncomplicated mastitis?

because it has a short milk withhold (36hrs)

Why is clindamycin a good choice to treat osteomyelitis?

about 20% of the plasma concentration of clindamycin will leak into bone and most osteomyelitic bacteria are anaerobes

What are the clinical applications of lincosamides?

• lincomycin is primarily a poultry and swine feed additive

• clindamycin: dental infections, osteomyelitis, anal sacculitis

• pirlimycin: Gm (+) mastitis, short milk withold (36hrs)

What is the MOA and spectrum of tiamulin?

reversible 50S inhibitor and thus bacteriostatic affecting Gm (+) aerobes and anaerobes

What is the main side effect of tiamulin?

urine metabolite causes dermatitis in pigs = useful indicator that pigs are overcrowded

Which 50S inhibitors are bactericidal?

• virginiamycin

• dalfopristin + quinupristin

What is a common resistance theme in the 50S inhibitors.

dual binding phenomenon called cooperativity

Describe the pharmacokinetic properties of streptogramins that (a) create a potential food safety hazard and (b) help to eliminate a “hiding place” for a superpathogen.

drugs concentrate in macrophages where Enterococcus hides

Why is virginiamycin banned for food animal use in Europe?

used as feed additive in poultry diets → drug present in intestinal contents → intestinal contents contaminate meat during processing

What are the clinical applicaitons of streptogramins?

• virginiamycin in horse feed to prevent laminitis

• prevent rumen acidosis in cattle