ID Pharmacotherapy - 02/24 2b Antibacterial Pharmacology 2

What are the proteins synthesis inhibitors, where do they work, and which are bactericidal?

• 30S

  • Aminoglycosides: bactericidal

  • Tetracyclines

• 50S

  • Macrolides

  • Lincosamides

  • Pleuromutilin

  • Oxazolidinones: bactericidal for strep only

  • Streptogramins: bactericidal together in MRSA

What is the protein synthesis process?

• tRNA w/ AA1 binds donor site of 50s

• tRNA w/ AA2 binds acceptor site of 50S

• Peptidyl transferase creates peptide bond btwn AA1 & AA2

• Ribosome moves on to next codon, acceptor site becomes ready to accept AA3

What is the SAR of aminoglycosides?

• Hexose w/ amino-sugars creating polycations (polar molecules) attracted to LPS(-)

• Less effective in acidic (PO isnt best route b/c of acidic stomach pH)

What is the MoA of aminoglycosides?

• Bind 30S subunit

• Block initiation of protein synthesis

• Block further translation and elicit premature termination

• Incorporation of incorrect AA leading to misfolded membrane proteins (enhance drug uptake) and hydroxy radicals

What are the effects of aminoglycosides?

• Concentration-dependent killing (peak >> MIC)

  • Longer dosing interval

• Post antibiotic effect

What is post-antibiotic effect and what causes it?

Aminoglycoside effect in which effect lingers after conc goes below MIC

• Misfolded proteins make the bacteria more sensitive to drug & other toxins

• Toxic hydroxyl radicals remain

What are the ADRs of aminoglycosides?

Nephro & ototoxicity: tubular epithelial damage & saturation at vestibular/cochlear components of ear

Concentration vs. Time-Dependent Killing

• Aminoglycosides, quinolones, metronidazole: high conc above MIC required so that drug can saturate inside bacteria, then by threshold effect forms a stable complex w/ the target

  • Clinical: longer dosing interval w/ higher doses

• Beta-Lactams: persistent conc above MIC for longer periods is required to inhibit target

  • Clinical: frequent dosing or extended/continuous infusion

What is the spectrum of aminoglycosides?

• Gram- b/c:

  • Negative LPS

  • Porin channels allow polar aminoglycosides in (dependent on electrochemical gradient)

What is the entry of aminoglycosides dependent on and what is the clinical significance of these?

• Porin channels

• Electrochemical gradient: requires O2, high-neutral pH, inhibited by divalent cations Ca²⁺/Mg²⁺

  • Less active against gram+

  • Less active against anaerobes

  • Purulent exudate may be acidic and require beta-lactam coadmin to increase delivery

What are the general characteristics of native tetracycline and the newer ones?

• Native is amphoteric

• Modified ones (doxy, mino) have increased absorption, long-action, and distribution difference

  • Long-acting due to inc ppb

• Multivalent cations chelate most members

• Dairy products reduce GI absorption

What is the MoA of tetracyclines?

Bind 30S to block tRNA docking at acceptor site

What is the ADME of tetracyclines?

• Good intestinal absorption unless polyvalent cations

• PPB status affects t_1/2; poor penetration

• Doxy, mino are hepatically metabolized and undergo entero-hepatic cycling

• Urinary and fecal excretion

What are the ADRs of tetracyclines?

• Toxicity: chelating by divalent cations (Ca²⁺/Mg²⁺) affects bone and teeth formation in children

• (Less by doxy)

What is tigecycline?

Glycylcyclines (improved tetracyclines to overcome resistance)

What is the MOA of tigecycline?

Bind 30S to block tRNA docking at acceptor site (similar to tetracyclines)

What is the spectrum of tigecycline?

• Broad: gram+, gram-, some anaerobes

• Tetracycline-resistant bacteria

What are the indications of tigecycline?

• Serious soft-tissue and intra-abdominal infections

• CRE (carbapenem-resistant Enterobacteriaceae)

What are the structures of macrolides?

Large lactone ring and sugars

• Erythromycin is the prototype drug

What is the MoA of macrolides?

Bind 50S to inhibit translocation of peptidyl RNA from acceptor to donor site and cause premature release of peptide

What are the macrolides?

• Native: erythromycin

• Modified: clarithromycin, azithromycin

• Semi-synthetic ketolides: telithromycin, cethromycin, solithromycin (Europe)

What are the advantages of modified macrolides?

• Good oral absorption

• Greater tissue distribution

• Increased cell penetration

• Longer t_1/2

What are the macrolide resistance mechanisms?

• Reduced entry or active efflux

• Modified 50S (binding target) due to mutation or methylation

• Esterases (hydrolyze macrolides)

What is the cross-resistance seen in macrolides?

• Complete resistance across macrolides

• Potential cross-resistance among clindamycin and streptogramins (MLS resistance)

What are the ADRs of macrolides?

• Cardio: QT prolongation & arrhythmia

• Cholestatic hepatitis w/ prolonged use

• CYP3A4-I → potentiates digoxin, corticosteroids, etc.

What is the MoA of lincosamides?

• Taken up by phagocytes to penetrate abscesses

• Bind 50S near macrolides and chloramphenicol binding site

What is the spectrum and uses of lincosamides?

Gram+: staph/strep-mediated soft tissue infections, abscesses

What is the cross resistance seen in lincosamides?

W/ macrolides and clindamycin due to methylase induction (MLS resistance)

What is the MoA of streptogramins?

• Streptogramins A and B bind to 50s, inhibit polypeptide elongation, and cause early termination

• Dalfopristin binds 50S to cause conformational change that promotes quinupristin binding to 50S

• Leads to elongation inhibition and early termination

• Developed for MRSA; later for VREF

What are streptogramins used for?

• SYNERCID

• Bactericidal against MRSA

• Bacteriostatic against vancomycin-resistant E. faecium (VREF)

What are the ADRs of streptogramins?

CYP3A4-I → prolongs t_1/2 of CYP3A4 substrates (i.e., simvastatin)

What is the cross resistance seen in streptogramins?

W/ macrolides and lincosamides due to methylase induction causing 50S modification

What is the MoA of oxazolidinones?

Bind 50S and inhibit INITIATION complex formation (ribosome-fMET-tRNA complex)

What is the significance of oxazolidinones?

• Not derived from nature

• Active against gram+ mostly; poor against most gram-

What are the the oxazolidinones and what are they used for?

• Linezolid

• Tedizolid

• Used for skin, soft tissue infections including MRSA

What is tedizolid phosphate?

• Newer generation oxazolidinone

• Prolonged t_1/2

• Less toxic

• Prodrug activated by phosphatase

What are the ADRs of oxazolidinones?

• Myelosuppression

• Mitochondrial toxicity

• MAO-I by linezolid causing serotonin syndrome

What are the mechs of resistance to oxazolidinones?

• rRNA mutations (the binding target)

• Ribosomal protection caused by methyltransferase Cfr mutation leading to meth

• RELATIVELY LOW

What are the nucleic acid synthesis inhibitors and how do they work?

• Fluoroquinolones: inhibit topo2 (DNA gyrase) and 4 to block replication

• Sulfa-trimethoprim: inhibit bacterial folate synthesis, impeding DNA synthesis

• Rifamycins: inhibit bacterial RNA polymerase to block RNA synthesis, transcription

What are the normal functions of topoisomerases?

• Topo2 (DNA gyrase): relax supercoiled DNA

• Topo4: decatenates DNA to allow separation of rings after replication

What is the spectrum of fluoroquinolones?

Broad: gram+, gram-, MT

• (synthetic derivatives of Nalidixic acid)

What is the mech of resistance to fluoroquinolones?

• OCCURS QUICKLY

• Specific proteins protect DNA gyrase

• Acetyltransferases modify and weaken some quinolones to allow replication to occur

• Leads to point mutations in DNA gyrase being inherited by daughter cells and cross-resistance to other quinolones

What is the process of folic acid synthesis and where do folate synthesis inhibitors have their effects?

• PABA + pteridine through DHP synthase form dihydropteroic acid (SULFONAMIDES)

• Dihydropteroic acid + glutamate through DHF synthase form dihydrofolic acid

• Dihydrofolic acid is reduced by DHF reductase + NAPDH to THF (TRIMETHOPRIM)

What are the MoAs of sulfonamides and trimethoprim?

• Sulfonamide: inhibit DHP synthase by mimicking PABA (bacteriostatic on its own)

• Trimethoprim inhibit DHR reductase

What is the spectrum of bactrim including examples?

• Broad: gram+ and gram-

• Includes Staph, E coli, Klebsiella, Enterobacter, Proteus

What is bactrim used for and why?

UTIs b/c effective against them and accumulates in prostatic fluid

What are the ADRs of bactrim?

• DUE TO SULFA

• Megaloblastic anemia, bleeding disorders (common in folate-deficient pts)

• Renal toxicity due to crystalluria (keep hydrated)

What are the mechs of resistance to bactrim?

• Alter enzyme to reduce sensitivity to inhibition

• Overproduction of PABA to compensate for inhibition

• Reduced cell permeability

What is the MoA of rifamycins?

Binds beta-subunit of bacterial RNA pol (rpoB) to block transcription

• treats M. tuberculosis

What are the mechs of resistance of rifamycins?

• Modified RNA pol (rpoB gene mutation)

• Efflux pumps

What is the MoA of metronidazole?

• Selective accumulation in anaerobic bacteria & protozoan parasites

• Anaerobic bacteria reduce metronidazole to generate toxic byproducts that bind DNA and inhibit growth

What are the uses of metronidazole?

• Anti-protozoal

• C. diff, anaerobic or mixed GI infections

What are the ADRs of metronidazole?

• GIT rxn

• DDI w/ alcohol

What is the MoA of polymyxins?

Bind to LPS and disrupt lipids in outer and inner membranes by acting as cationic detergent (inactivates LPS)

What is the spectrum of polymyxins?

Mostly gram- bacteria like E coli, klebsiella, salmonella due to LPS attracting cationic portions of proteins

What are the uses of polymyxins and why?

Mostly topical and last resort for resistant gram- b/c nephrotoxicity

What attenuates ADRs of polymyxins?

Cholesterol in eukaryotic membranes reduces ability to act like cationic detergent

What are the mechs of resistance of polymyxins?

• Altered LPS composition due to chromosomal changes

• Efflux pumps

How does antimicrobial resistance manifest?

• Dec sensitivity of the organism to the agent aka potency (EC50)

• Dec maximal effect of the agent aka efficacy (Emax)

What are the types of antimicrobial resistance mechanisms?

• Destruction/modification of antimicrobial

• Modification of antimicrobial target

• Altered influx/efflux of antimicrobials

How is antimicrobial resistance transferred?

• Vertically to daughter cells

• Horizontally through transformation (naked DNA uptake), transduction (bacteriophage infection), conjugation (R-plasmid)

What are examples of antimicrobial target modification leading to resistance?

• Modifying drug targets to reduce affinity as seen in:

  • Methylation of rRNA leading to MLS resistance

  • Mutation in PBPs leading to beta-lactam resistance

  • Altered DHFR leading to trimethoprim resistance

• Newly synthesized protection molecules as seen in:

  • TetM, TetO ribosomal protection proteins that displace tetracycline

  • Novel proteins binding and protecting DNA gyrase from quinolones

What are examples of antimicrobial influx/efflux alteration leading to resistance?

• Reduced aminoglycoside influx due to mutated porins and altered bacterial EC gradients

• Increased efflux of tetracyclines through mutant TetA efflux pump in resistant gram- bacteria

• Increased efflux of aminoglycosides by AcrD (multi-drug efflux transporter) in resistant E. coli

What are examples of antimicrobial destruction/modification leading to resistance?

• Beta-lactamases expression due to plasmid or chromosomal-mediated resistance

• Aminoglycoside acetylation or adenylation due to plasmid-mediated resistance

What occurs in the different horizontal resistance transfers?

• Transformation: naked DNA picked up

• Transduction: bacteriophage spreads resistance

• Conjugation: sex pili and plasmids used to spread resistance