Keflex (Cephalexin) Drug Class
Keflex (cephalexin) is a first-generation cephalosporin antibiotic, which belongs to the broader beta-lactam antibiotic class. 1
Chemical Structure and Classification
Cephalexin is a semisynthetic cephalosporin antibiotic with the chemical structure 7-(D-α-amino-α-phenylacetamido)-3-methyl-3-cephem-4-carboxylic acid, monohydrate. Its molecular structure is related to other cephalosporin antibiotics, with key differences:
- Contains a D-phenylglycyl group at the 7-amino position
- Has an unsubstituted methyl group at the 3-position
- The cephalosporins differ from penicillins in the structure of their bicyclic ring system 1
Antimicrobial Spectrum
As a first-generation cephalosporin, cephalexin has:
- Excellent gram-positive coverage
- Limited gram-negative coverage 2
- Particularly effective against:
- Streptococci (including beta-hemolytic streptococci)
- Staphylococci (methicillin-susceptible)
- Some common gram-negative organisms 3
Clinical Applications
Cephalexin is commonly used for:
- Skin and soft tissue infections (first-line therapy) 4, 5
- Upper respiratory tract infections 6
- Urinary tract infections 7
- Mild diabetic foot infections without complicating factors 4
Pharmacokinetic Properties
- Absorption: Not absorbed from the stomach but totally and rapidly absorbed in the upper intestine
- Distribution: Rapidly distributed to tissues (except spinal fluid and aqueous humor)
- Metabolism: No measurable destruction or metabolism in the body
- Excretion: Rapidly cleared by the kidneys with 70-100% of the dose found in urine 6-8 hours after administration
- Dosage adjustment: Required in patients with creatinine clearance less than 30 ml/min 3, 8
Cross-Reactivity Considerations
Important considerations regarding cross-reactivity with penicillin allergies:
- Cephalexin shares an R1 side chain with amoxicillin and ampicillin, potentially increasing cross-reactivity risk in patients with penicillin allergies 4
- For patients with a history of anaphylaxis, angioedema, or urticaria with penicillins, cephalexin should not be administered due to possible cross-reactions 4
- Cross-reactivity between penicillins and cephalosporins was previously estimated at around 10%, but more recent data suggests it may be lower (2-4.8%) for newer cephalosporins 4
Resistance Patterns
- Resistance rates to cephalexin among viridans group streptococci can be high (reported at 96% in one study)
- First-generation cephalosporins generally have higher resistance rates compared to later generations 4
Dosing Considerations
- Adults: Typically 250-500 mg orally every 6 hours
- Children: May require higher doses per kilogram than adults due to greater body water turnover 3
- Renal impairment: Dose reduction proportional to reduced kidney function 3
Cephalexin remains an effective antibiotic for many common infections, particularly those caused by susceptible gram-positive organisms, with the advantage of oral administration and generally good tolerability.