Clinical pharmacology of cephalexin administered by intravenous injection

Probenecid effects on cephalexin pharmacokinetics and pharmacodynamics in healthy volunteers

OBJECTIVES

We evaluated the effects of probenecid on the Pharmaco Kinetics (PK) and pharmacodynamics (PD) of oral cephalexin in healthy volunteers.

METHODS

Cephalexin 1000 mg was administered orally to 11 healthy volunteers following a standardized meal, with and without probenecid 500 mg orally, on two separate days one week apart. Total plasma concentrations of cephalexin and probenecid over a 12 h period were measured by liquid chromatography tandem mass spectrometry. Standard pharmacokinetic measures and contemporary PK/PD targets were compared.

RESULTS

Probenecid increased the mean (95% CI) cephalexin area under the concentration-time curve (AUC_0-∞) 1.73-fold (1.61-1.85, p < 0.0001), peak concentration 1.37-fold (1.16-1.58, p < 0.01), time to peak concentration 1.45-fold (1.1-1.8, p < 0.01)_, and half-life 1.33-fold (1.03-1.62, p < 0.05). The effects resulted in clinically meaningful increases in the probability of PK/PD target attainment (PTA). As an example, the PTA of total concentrations above the minimum inhibitory concentration required to inhibit methicillin-susceptible Staphylococcus aureus isolates (MIC ≤ 8 mg/L) for 70% of a 6 h dose interval approached 100% for cephalexin + probenecid while for cephalexin alone it was <15%.

CONCLUSIONS

Probenecid prolonged and flattened the plasma concentration-time curve, enhancing the probability of attaining PK/PD targets. Co-administration of probenecid may expand the clinical benefits of oral cephalexin.

Cephalosporin-probenecid drug interactions

The effect of concurrent probenecid administration on the pharmacokinetics of cephalosporin antibiotics varies with the available cephalosporins. Most cephalosporins are affected to some degree by concurrent probenecid administration, although ceforanide, ceftazidime, ceftriaxone and latamoxef (moxalactam) have no significant changes in pharmacokinetics. For those cephalosporins affected by probenecid, the predominant findings are impairment in renal clearance resulting in increased peak serum concentrations, an increased area under the concentration-time curve (AUC), and both delayed and prolonged recovery of the cephalosporin in the urine. The distribution of the cephalosporins is affected to varying degrees, with reports of increased penetration into ocular, central nervous system and blister fluids noted with some agents. The clinical relevance of the changes in cephalosporin distribution associated with probenecid administration has not been investigated. The dose and timing of probenecid administration appear to be major determinants in any possible interaction. Studies with ceftizoxime and cefoxitin suggest that larger probenecid doses result in greater changes in the pharmacokinetics of cephalosporins. Prolonged probenecid therapy before administration of a cephalosporin did not seem to be as relevant as the probenecid dosage in determining the magnitude of the interaction. Probenecid administration with or immediately before cephalosporin administration appears able to produce these documented changes in cephalosporin pharmacokinetics. The route of administration (oral versus parenteral) of either prolosporin pharmacokinetics. The route of administration (oral versus parenteral) of either probenecid or the cephalosporin does not appear to influence the characteristics of the interactions. The therapeutic efficacy of a combination of a cephalosporin with probenecid has been most thoroughly studied for single-dose treatment of gonorrhoea. The addition of probenecid to cephalosporin therapy results in sustained systemic concentrations adequate for eradication of Neisseria gonorrhoeae. Regimens involving either second or third generation cephalosporins demonstrate good success rates with single-dose therapy. However, the success of ceftriaxone administered alone for treatment of both penicillase-producing and non-penicillase-producing strains of N. gonorrhoeae suggests that the addition of probenecid is unnecessary. The use of probenecid, in combination with cephalosporins, to enhance the treatment of other venereal and systemic infections has preliminary, inconclusive support.