“If this blog helped you out, don’t keep it to yourself—share the link on your socials!” 👍 “Like what you read? Spread the love and share this blog on your social media.” 👍 “Found this useful? Hit share and let your friends know too!” 👍 “If you enjoyed this post, please share the URL with your friends online.” 👍 “Sharing is caring—drop this link on your social media if it helped you.”

Tuesday, July 29, 2025

Phenoxymethylpenicillin


Generic Name: Phenoxymethylpenicillin
Also Known As: Penicillin V, Penicillin VK (U.S. naming)
Brand Names: Pen V, Veetids, Apo-Pen VK, Broxil, Betapen-VK, Phenoxymethylpenicillin Potassium


Drug Class: Beta-lactam antibiotic; Natural penicillin
Pharmaceutical Category: Antibacterial agent
Formulations: Oral tablets, oral solution (syrup or suspension)
Route of Administration: Oral only

1. Pharmacological Classification

Phenoxymethylpenicillin is a narrow-spectrum beta-lactam antibiotic, classified under natural penicillins. It is a derivative of penicillin G (benzylpenicillin) but modified with a phenoxymethyl group, making it acid-stable and thus suitable for oral administration, unlike penicillin G which requires parenteral delivery.

2. Mechanism of Action

Phenoxymethylpenicillin exerts bactericidal activity by inhibiting bacterial cell wall synthesis:

  • It binds to penicillin-binding proteins (PBPs), particularly transpeptidases, on the inner membrane of bacterial cell walls.

  • This blocks the cross-linking of peptidoglycan chains, a critical structural component of bacterial cell walls.

  • The inhibition causes cell wall weakening, osmotic instability, and ultimately bacterial lysis and death, especially in rapidly dividing Gram-positive bacteria.

Its effectiveness is time-dependent, meaning that bacterial killing is related to the duration that the drug concentration exceeds the MIC (minimum inhibitory concentration) rather than peak levels.

3. Spectrum of Activity

Phenoxymethylpenicillin has activity against many Gram-positive organisms and some Gram-negative cocci, specifically:

Susceptible organisms:

  • Streptococcus pyogenes (Group A β-hemolytic streptococci)

  • Streptococcus pneumoniae

  • Streptococcus viridans group

  • Neisseria meningitidis (limited)

  • Treponema pallidum (syphilis)

  • Corynebacterium diphtheriae

  • Listeria monocytogenes (limited)

  • Actinomyces israelii

  • Clostridium spp. (except Clostridium difficile)

Resistant organisms:

  • Most Staphylococcus aureus (due to beta-lactamase production)

  • Most Enterobacteriaceae

  • Pseudomonas aeruginosa

  • Mycoplasma, Chlamydia, Legionella (no cell wall)

4. Therapeutic Indications

Phenoxymethylpenicillin is primarily used in mild to moderate infections where susceptible organisms are known or strongly suspected:

A. Respiratory Tract Infections

  • Streptococcal pharyngitis and tonsillitis

  • Otitis media (Streptococcus spp.)

  • Sinusitis (limited efficacy; local resistance varies)

  • Pneumococcal pneumonia (only if susceptible)

B. Dental Infections

  • Dental abscesses or gingivitis

  • Prophylaxis against infective endocarditis in at-risk individuals undergoing dental procedures

C. Skin and Soft Tissue Infections

  • Erysipelas

  • Impetigo (limited use)

  • Cellulitis (if caused by Streptococcus spp.)

D. Rheumatic Fever

  • Long-term prophylaxis after rheumatic fever to prevent recurrence

E. Others

  • Diphtheria (adjunctive treatment and prophylaxis)

  • Syphilis (mild early cases, penicillin G preferred parenterally)

  • Actinomycosis

5. Dosage and Administration

Adults

  • Typical dose: 250–500 mg every 6 hours (q.i.d.)

  • Duration: 5–10 days depending on the infection

  • For rheumatic fever prophylaxis: 250 mg twice daily (long-term)

Children

  • Dosing based on weight or age:

    • 1–6 years: 125 mg every 6 hours

    • 6–12 years: 250 mg every 6 hours

  • For rheumatic fever prophylaxis: 125–250 mg twice daily

Administration Notes

  • Best taken on an empty stomach (1 hour before or 2 hours after meals)

  • Oral suspension should be refrigerated and shaken well before each use

6. Pharmacokinetics

  • Absorption: Well absorbed orally, but food may reduce absorption

  • Peak plasma concentration: 0.5–1 hour post-dose

  • Protein Binding: 60–70%

  • Distribution: Widely distributed in body fluids (except CNS unless meninges are inflamed)

  • Metabolism: Minimal hepatic metabolism

  • Elimination: Primarily renal excretion (90%) via glomerular filtration and tubular secretion

  • Half-life: 30–60 minutes in healthy individuals

7. Contraindications

  • Known hypersensitivity to penicillins or other beta-lactams

  • History of severe allergic reactions (e.g., anaphylaxis, Stevens-Johnson syndrome) to beta-lactams

  • Patients with phenylketonuria (PKU) (some formulations may contain aspartame)

8. Warnings and Precautions

  • Hypersensitivity reactions can be immediate (anaphylaxis) or delayed (rash, fever)

  • Superinfection with non-susceptible organisms (e.g., Candida, Clostridium difficile) may occur with prolonged use

  • Caution in renal impairment: Dose adjustment may be required

  • Not effective against beta-lactamase–producing organisms

  • Cross-sensitivity with cephalosporins (approx. 5–10% in those with penicillin allergy)

9. Adverse Effects

Common (≥1%)

  • Gastrointestinal:

    • Nausea

    • Vomiting

    • Diarrhea

    • Abdominal discomfort

  • Skin:

    • Rash

    • Urticaria

Less Common

  • Oral/vaginal candidiasis

  • Fever

  • Eosinophilia

  • Black hairy tongue

Serious (Rare)

  • Anaphylaxis

  • Stevens-Johnson syndrome (SJS)/Toxic epidermal necrolysis (TEN)

  • Angioedema

  • Clostridium difficile–associated diarrhea

  • Serum sickness-like reactions

  • Interstitial nephritis

  • Hemolytic anemia (immune-mediated)

10. Drug Interactions

Probenecid

  • Inhibits renal excretion of penicillin → increases plasma concentration

  • Sometimes used intentionally to enhance therapeutic effect

Tetracyclines

  • May antagonize the bactericidal action of phenoxymethylpenicillin (bacteriostatic interference)

Oral Contraceptives

  • Theoretical risk of reduced efficacy due to changes in gut flora; advise additional contraceptive measures

Methotrexate

  • Penicillins can reduce renal clearance of methotrexate → increased toxicity risk

11. Pregnancy and Lactation

Pregnancy

  • Category B (US); safe for use in pregnancy

  • Widely used in pregnant women for streptococcal infections

Breastfeeding

  • Present in breast milk in small amounts

  • Generally safe; monitor infant for GI disturbances or allergic reactions

12. Monitoring Parameters

  • Renal function in prolonged use or elderly patients

  • Watch for signs of allergy or hypersensitivity

  • Monitor treatment response (fever, CBC, CRP)

  • For long-term therapy: check for superinfection and monitor LFTs and CBC

13. Formulations and Brand Availability

Tablets

  • 250 mg, 500 mg (phenoxymethylpenicillin potassium or calcium salts)

Oral Suspension

  • 125 mg/5 mL, 250 mg/5 mL

  • Often flavored for pediatric use

  • Brands:

    • Pen VK (US)

    • Phenoxymethylpenicillin Oral Suspension BP (UK)

    • Broxil, Betapen-VK, Ospen

14. Clinical Pearls

  • First-line for Strep throat (Group A strep) to prevent rheumatic fever

  • Often preferred over amoxicillin in patients with higher risk of broad-spectrum resistance development

  • Less effective against organisms producing beta-lactamases (e.g., Staph aureus)

  • For dental infections, especially when penicillin-sensitive streptococci are suspected

  • Good tolerability profile in children and pregnancy



on
Labels:

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)