Cephalexin is a 1st-generation cephalosporin antibiotic. The cephalosporins are bactericidal and similar to the penicillins, act by inhibiting synthesis of the bacterial cell wall.
It has good activity against a wide spectrum of gram-positive bacteria including penicillinase-producing, but not methicillin-resistant staphylococci; enterococci are however, resistant. Its activity against gram-negative bacteria is modest.
Pharmacokinetics: Absorption: Cephalexin is acid stable and is readily absorbed after oral administration. Food may delay absorption.
Blood Concentration: After an oral dose of 500 mg, peak serum concentrations of about 20 mcg/mL are attained in 1 hr; after IM dose of 1 g, serum concentration of about 20 mcg/mL in 1 hr is obtained; after IV dose of 1 g, serum concentration of about 50 mcg/mL in 15 min is obtained. Peak concentrations are subject to wide individual variation.
Metabolism: Serum half-life is about 0.5-2 hrs in patients with normal renal function and about 20 hrs in patients with severe renal failure. In newborn, it is 2.5-5 hrs.
Distribution: Widely distributed throughout the body. Maximum concentrations being reached in the liver and kidney; cephalexin does not accumulate following multiple doses. It does not enter the cerebrospinal fluid unless the meninges are inflammed but it does cross the placenta and is secreted in the milk. Six percent (6%) to 15% of the dose is bound to plasma proteins.
Excretion: About ≥80% of a dose is excreted in the urine in the first 6 hrs by glomerular filtration and tubular secretion, urinary concentrations >1 mg/mL have been achieved after dose of 500 mg. Probenecid delays urinary excretions and has been reported to increase biliary excretion. Therapeutically effective concentrations may be found in the bile.