Cephalexin Family: First-Generation Cephalosporins
Cephalexin belongs to the first-generation cephalosporin family, and other antibiotics in this same class include cefadroxil, cefazolin, cephalothin, and cefprozil. 1, 2
First-Generation Cephalosporins (Same Family as Cephalexin)
Oral Agents
- Cefadroxil: This is the most closely related oral first-generation cephalosporin to cephalexin, with identical MIC distributions (MIC50 of 2 μg/mL and MIC90 of 4 μg/mL against MSSA) 3
- Cefadroxil offers the advantage of less frequent dosing (500 mg twice daily) compared to cephalexin (500 mg 2-3 times daily) due to slower renal clearance 4
- Cephalexin and cefadroxil are both recommended by WHO as first-choice antibiotics for mild skin and soft tissue infections 5
Parenteral Agents
- Cefazolin: A first-generation parenteral cephalosporin with a unique R1 side chain that distinguishes it from other members of this class 1
- Cephalothin: Another parenteral first-generation agent with similar antimicrobial spectrum 3
Structural Relationship and Cross-Reactivity
Shared Chemical Structure
- All cephalosporins, including first-generation agents, share a bicyclic ring system (nucleus) that differs from penicillins in structure 2
- Cephalexin specifically has a D-phenylglycyl group at the 7-amino position and an unsubstituted methyl group at the 3-position 2
Important Cross-Reactivity Pattern
- Aminocephalosporins (cephalexin, cefadroxil, cefprozil, cefaclor) share the R1 side chain with aminopenicillins, resulting in a 16.45% cross-reactivity risk (95% CI: 11.07-23.75) in patients with confirmed penicillin allergy 1, 6
- In contrast, cefazolin has very low cross-reactivity with penicillins despite being a first-generation cephalosporin, with only 0.8% reaction rate (95% CI: 0.13%-4.1%) among confirmed penicillin-allergic patients 1
Clinical Spectrum Characteristics
Antimicrobial Activity Pattern
- First-generation cephalosporins are most potent against Gram-positive cocci (especially Staphylococcus aureus and Streptococcus species) but have limited Gram-negative coverage 7, 8
- They are bactericidal against most clinically important gram-positive cocci and some gram-negative bacilli 8
- None of the cephalosporins, including first-generation agents, have activity against enterococci 8
Common Clinical Applications
- Surgical prophylaxis and treatment of staphylococcal/streptococcal infections in penicillin-allergic patients 8
- Mild skin and soft tissue infections caused by gram-positive cocci 5, 6
- Upper respiratory tract infections (except those caused by Haemophilus influenzae, where failure rates reach 50%) 9
Important Caveat
Avoid using cephalexin or other aminocephalosporins (cefadroxil, cefprozil) in patients with a history of anaphylaxis, angioedema, or urticaria to penicillins or ampicillin due to the shared R1 side chain and significant cross-reactivity risk 6. In such cases, cefazolin would be a safer first-generation cephalosporin alternative if this class is required 1.