cefalexin
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cephalexin clinical pharmacology
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Cefalexin (Cephalexin)
Overview
Cefalexin (INN) / cephalexin (USAN) is an oral first-generation cephalosporin antibiotic. It shares the core 7-aminocephalosporanic acid nucleus of all cephalosporins and works by inhibiting bacterial cell wall synthesis. Trade names include Keflex and several generics.
Mechanism of Action
Cefalexin, like all beta-lactams, inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), blocking transpeptidation of peptidoglycan cross-links. The result is a structurally deficient cell wall, leading to cell lysis and death. Notably, cephalosporins as a class do not bind to essential PBPs in Enterococcus spp., which is why the entire class lacks clinically useful activity against enterococci.
- Goodman & Gilman's Pharmacological Basis of Therapeutics, Ch. 58
Antibacterial Spectrum
Cefalexin is a narrow-spectrum (first-generation) agent:
| Target Organisms | Notes |
|---|---|
| Staphylococcus aureus (MSSA) | Active; somewhat less active vs. penicillinase-producing strains |
| Streptococcus spp. | Excellent activity |
| Some Proteus, E. coli, Klebsiella | Limited gram-negative coverage |
| MRSA, Enterococcus, Listeria | Not active |
| Atypical organisms (Legionella, Mycoplasma, Chlamydia) | Not active |
Katzung's Basic and Clinical Pharmacology, 16th ed.
Pharmacokinetics (ADME)
-
Route: Oral only (no parenteral form is marketed)
-
Dosing: Usually 0.5 g two to four times daily; pediatric doses adjusted by weight
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Absorption: Well absorbed orally; peak plasma concentrations are adequate to inhibit many gram-positive and gram-negative pathogens
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Metabolism: Not metabolized (excreted unchanged)
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Excretion: 70-100% excreted in urine via renal elimination (glomerular filtration and tubular secretion); dose reduction required in significant renal impairment
-
Goodman & Gilman's, p. 889
Key Clinical Indications
| Indication | Notes |
|---|---|
| Skin & soft tissue infections (non-purulent cellulitis, impetigo, wound infections) | First-line when MSSA/Streptococcus suspected; use with TMP-SMX if CA-MRSA possible |
| Urinary tract infections (uncomplicated cystitis) | Prophylaxis: 125-250 mg daily; acute treatment 500 mg QID |
| Streptococcal pharyngitis | Alternative to penicillin in penicillin-tolerant patients |
| Animal/bite wounds | Broad-spectrum cover for skin flora |
| Recurrent UTI prophylaxis in women | Cefalexin 250-500 mg daily is listed among preferred agents |
- Washington Manual of Medical Therapeutics; Comprehensive Clinical Nephrology, 7th ed.; ROSEN's Emergency Medicine
Adverse Effects
- GI disturbance - nausea, diarrhea (most common)
- Hypersensitivity reactions - rash, urticaria, anaphylaxis (uncommon); the most clinically important adverse effect class of beta-lactams
- Cross-reactivity with penicillins - historically overstated at ~10%, but true cross-reactivity is much lower (~1-2%), mainly in those with severe IgE-mediated penicillin allergy. Use with caution (or avoid without skin testing/desensitization) in patients with documented anaphylaxis, angioedema, or urticaria to penicillins
- C. difficile colitis - risk with any antibiotic; low but not zero
- Nephrotoxicity - rare at standard doses; relevant in overdose or in combination with nephrotoxic agents
- Harrison's Principles of Internal Medicine 22E (2025); Washington Manual
Pregnancy & Lactation
Cefalexin is FDA Pregnancy Category B (considered safe; widely used in pregnancy for UTI and skin infections). It is excreted in breast milk in small amounts - generally considered compatible with breastfeeding.
Special Considerations
- Renal impairment: Dose reduction required (excreted renally). In severe renal failure (CrCl <10 mL/min), extend dosing intervals or reduce dose.
- Acne (dermatology): Cefalexin kills P. acnes in vitro, but because it is hydrophilic it penetrates poorly into the pilosebaceous unit. Any benefit in acne is likely anti-inflammatory rather than antimicrobial. Its use in acne is discouraged due to risk of promoting staphylococcal resistance. - Fitzpatrick's Dermatology, Vol. 1
- Anticholinergic burden: Rated 0 (no anticholinergic activity) - relevant in polypharmacy for elderly patients. - Maudsley Prescribing Guidelines
Dosing Summary (Adults)
| Indication | Dose | Frequency |
|---|---|---|
| Mild-moderate skin/soft tissue infection | 500 mg | QID (4x daily) |
| Uncomplicated UTI | 500 mg | BID-QID |
| Streptococcal pharyngitis | 500 mg | BID x 10 days |
| UTI prophylaxis | 125-250 mg | Once daily |
Key Comparisons Within First-Generation Cephalosporins
| Drug | Route | Notes |
|---|---|---|
| Cefalexin | Oral | Standard oral first-gen |
| Cefadroxil | Oral | Longer half-life, once/twice daily dosing |
| Cefazolin | IV/IM only | Drug of choice for surgical prophylaxis; ~85% protein-bound |
Recent evidence note: A 2023 systematic review (PMID 37414282) examined cephalosporins for uncomplicated pyelonephritis - cefalexin's limited gram-negative spectrum means it is generally not preferred for upper tract UTI (pyelonephritis); broader agents are used in that context.
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