Penicillin V and Penicillin G: Acid Stability and Side Effects

Penicillin Stability

• Penicillin V is stable in stomach acid, whereas penicillin G is not.
• Penicillin V has replaced penicillin G for oral therapy due to its acid stability.

Side Effects of Penicillins

• Hypersensitivity
• Diarrhea
• Nephritis (especially with methicillin)
• Neurotoxicity if given intrathecally
• Platelet dysfunction (carbenicillin and ticarcillin)
• Cationic toxicity

β-Lactamase Resistant Penicillins

• Examples: Flucloxacillin, methicillin, nafcillin, oxacillin, and dicloxacillin
• These agents have a very narrow antimicrobial spectrum and are used only against penicillinase-producing strains of staphylococci (Staph. aureus and Staph. epidermidis)

Methicillin-Resistant Staphylococcus aureus (MRSA)

• Resistance appears to result from the production of PBPs to which the penicillinase-resistant penicillins cannot bind.
• Vancomycin is the treatment of choice for MRSA.

Nafcillin, Oxacillin, and Dicloxacillin

• Nafcillin: usually administered IV, absorption from the GI tract is erratic and incomplete
• Oxacillin and dicloxacillin: similar in structure and pharmacokinetic properties, both are acid-stable, but only dicloxacillin is formulated for oral dosing

Broad-Spectrum Penicillins (Aminopenicillins)

• Examples: ampicillin and amoxicillin
• Have the same antimicrobial spectrum as penicillin G, plus increased activity against certain gram-negative bacilli
• Both are readily inactivated by beta-lactamases and hence are ineffective against most infections caused by Staph. aureus

Ampicillin

• Was the first broad-spectrum penicillin in clinical use
• Useful against infections caused by Enterococcus faecalis, Proteus mirabilis, E. coli, Salmonella, Shigella, and H. influenzae
• Common side effects: rash and diarrhea, both of which occur more frequently with ampicillin than with any other penicillin

Amoxicillin

• Similar to ampicillin in structure and actions
• More acid-stable than ampicillin, hence, when orally administered in equivalent doses, blood levels of amoxicillin are greater
• Produces less diarrhea than ampicillin, perhaps because less amoxicillin remains unabsorbed in the intestine

Extended-Spectrum Penicillin (Antipseudomonal Penicillin)

• Example: piperacillin
• Antimicrobial spectrum includes organisms that are susceptible to aminopenicillins plus Pseudomonas aeruginosa, Enterobacter species, Proteus (indole positive), Bacteroides fragilis, and many Klebsiella
• Piperacillin is susceptible to beta-lactamases and hence is ineffective against most strains of Staph. aureus

Cephalosporins

• First-generation cephalosporins: used primarily for infections with staphylococci and nonenterococcal streptococci
• Second-generation cephalosporins: have enhanced activity against gram-negative bacteria
• Third-generation cephalosporins: have a broad spectrum of antimicrobial activity, are considerably more active against gram-negative aerobes, and reach clinically effective concentrations in the CSF
• Fourth-generation cephalosporin: cefepime, highly resistant to beta-lactamases and has a very broad antibacterial spectrum
• Fifth-generation cephalosporin: ceftaroline, used to treat MRSA-associated infections

Polypeptide and Glycopeptide Antibacterial Agents

• Mechanism of action: inhibit the synthesis of cell walls and thereby promote bacterial lysis and death
• Examples: vancomycin, teicoplanin, and bacitracin

Imipenem

• Adverse effects: gastrointestinal effects (nausea, vomiting, diarrhea), superinfections with bacteria or fungi, and seizures
• Elimination half-life: about 1 hour
• Used to treat a wide range of infections, including those caused by gram-negative bacteria, gram-positive bacteria, and fungi

Monobactam

• Aztreonam: belongs to a class of beta-lactam antibiotics known as monobactams
• Mechanism of action: binds to PBP3, inhibiting bacterial cell wall synthesis and promoting cell lysis and death
• Antimicrobial spectrum: active only against gram-negative aerobic bacteria, including Neisseria species, H. influenzae, P. aeruginosa, and Enterobacteriaceae
• Not active against gram-positive bacteria and anaerobes

This quiz covers the difference between penicillin V and penicillin G, including their acid stability and side effects. It also touches on the different types of penicillins and their effects on the body.