Cloxacillin
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cloxacillin staphylococcus clinical use
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Cloxacillin
Drug Class & Overview
Cloxacillin is a semisynthetic isoxazolyl penicillin belonging to the penicillinase-resistant (anti-staphylococcal) group of beta-lactam antibiotics. Its congeners include oxacillin and dicloxacillin. It is not available in the United States (where nafcillin and oxacillin are preferred) but is widely used in many other countries. It is not a substitute for penicillin G in infections amenable to that agent.
- Goodman & Gilman's, p. 1172
- Jawetz, Melnick & Adelberg's Medical Microbiology 28e, p. 404
Mechanism of Action
Cloxacillin acts like all penicillins: it inhibits bacterial cell wall synthesis by binding to and inactivating penicillin-binding proteins (PBPs), specifically transpeptidases involved in cross-linking peptidoglycan chains. This leads to cell lysis and death in actively dividing bacteria.
Beta-lactam resistance can fail when:
- Bacteria produce beta-lactamase (cleaves the beta-lactam ring) - cloxacillin resists this
- Bacteria express a low-affinity PBP2a (encoded by mecA gene) - this confers MRSA resistance to ALL beta-lactams including cloxacillin
Spectrum of Activity
Cloxacillin is a narrow-spectrum antibiotic, and its use is essentially confined to one indication:
| Organisms | Activity |
|---|---|
| Penicillinase-producing S. aureus (MSSA) | Primary indication - excellent activity |
| Coagulase-negative staphylococci (MSSE) | Active |
| Streptococci (not as potent as Pen G) | Some activity |
| MRSA / MRSE | No activity (mecA-mediated resistance) |
| Enterococci, Listeria | No activity |
| Gram-negative organisms | No activity |
Infection with beta-lactamase-producing staphylococci is the only indication for penicillinase-resistant penicillins. - Jawetz, p. 404
Pharmacokinetics
| Parameter | Details |
|---|---|
| Route | Oral (tablets/capsules); parenteral (IV/IM) |
| Oral absorption | Rapid but incomplete (30-80% bioavailability) |
| Effect of food | Food impairs absorption - take 1 hour before or 2 hours after meals |
| Time to peak | ~1 hour after oral dose |
| Protein binding | Very high (~90-95%), bound to plasma albumin |
| Distribution | Widely distributed; CSF penetration generally poor but adequate in meningitis |
| Metabolism | Hepatic degradation (significant) + renal excretion |
| Elimination | Dual: renal tubular secretion AND biliary excretion |
| Half-life (t½) | 30-60 minutes |
| Renal failure | No dose adjustment required (biliary route compensates) |
| Hemodialysis | Not significantly removed |
- Goodman & Gilman's, p. 1172; Katzung's Basic & Clinical Pharmacology 16e, p. 1242
Clinical Uses
-
Penicillinase-producing (MSSA) infections:
- Skin and soft tissue infections (cellulitis, wound infections, abscesses)
- Bone and joint infections (osteomyelitis, septic arthritis)
- Pneumonia due to S. aureus
- Staphylococcal endocarditis (serious infections require higher parenteral doses)
- Bacteremia
-
Mild staphylococcal infections: Oral cloxacillin is appropriate for outpatient management of mild-to-moderate MSSA infections.
-
Anti-staphylococcal prophylaxis/treatment in ENT surgery: Used in head and neck surgery to cover staphylococci and streptococci (Scott-Brown's Otorhinolaryngology).
-
Neonatal sepsis (severe disease): IM cloxacillin + gentamicin used when initial therapy fails (Park's Preventive Medicine).
Dosing (Typical)
| Indication | Dose |
|---|---|
| Mild-moderate infections (oral) | 250-500 mg every 6 hours (1 hour before meals) |
| Severe/serious infections (IV) | 1-2 g every 4-6 hours |
| Pediatric | 25-50 mg/kg/day divided every 6 hours (oral); up to 200 mg/kg/day IV for severe disease |
No dosage adjustment in renal failure. Dosage adjustment may be considered in severe hepatic impairment.
Adverse Effects
| Effect | Notes |
|---|---|
| Hypersensitivity | Rash, urticaria, anaphylaxis (cross-reactivity with all penicillins, partial cross-reaction with cephalosporins) |
| GI effects | Nausea, diarrhea, epigastric discomfort |
| Pill-induced esophagitis | Direct mucosal toxicity if tablet lodges in esophagus - listed among antibiotics causing pill-induced esophageal injury (Goldman-Cecil Medicine) |
| Hepatotoxicity | Cholestatic hepatitis (less common than with flucloxacillin; HLA-B*57:01 not strongly linked unlike flucloxacillin - odds ratio for liver injury much lower) |
| Neutropenia/leukopenia | Rare, especially with prolonged high-dose IV therapy |
| Interstitial nephritis | Rare, class effect |
Resistance
- MRSA (mecA gene): Produces PBP2a, a low-affinity penicillin-binding protein that binds poorly to all beta-lactams. Around 50% of S. aureus isolates in U.S. hospitals are MRSA. Use vancomycin or daptomycin for MRSA.
- MRSE: >75% of S. epidermidis isolates may be methicillin-resistant.
- Goodman & Gilman's, p. 1172
Key Comparisons Within Class
| Drug | Route | Absorption | Notable |
|---|---|---|---|
| Cloxacillin | Oral / IV | 30-80% | Available in most countries outside USA |
| Dicloxacillin | Oral | Better than cloxacillin | Most potent orally; USA |
| Oxacillin | IV/IM | Poor oral | USA, parenteral preferred |
| Nafcillin | IV | Poor oral | USA, hepatic clearance dominant |
Recent Evidence Note
A 2025 systematic review and meta-analysis (PMID 40349971) compared cefazolin vs. antistaphylococcal penicillins (including oxacillin/cloxacillin) for MSSA bacteremia. This is an active area of debate - some evidence suggests cefazolin may have comparable or superior outcomes with fewer adverse effects for MSSA bacteremia compared to antistaphylococcal penicillins like cloxacillin/nafcillin, though antistaphylococcal penicillins remain a standard of care in many guidelines.
Summary
Cloxacillin is a beta-lactamase-resistant, narrow-spectrum penicillin whose use is exclusively for MSSA infections. It is taken on an empty stomach for best absorption, requires no renal dose adjustment (dual renal/biliary clearance), and has excellent oral bioavailability for a penicillinase-resistant agent. It has no activity against MRSA, gram-negatives, or enterococci.
How does cloxacillin compare to nafcillin, oxacillin, and dicloxacillin?
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