Sex differences in the intramuscular absorption and bioavailability of cephradine

Six male and six female volunteers each received a single intramuscular injection of cephradine, a new cephalosporin antibiotic, once weekly for 3 consecutive weeks. The drug was injected into the gluteus maximus, vastus lateralis, or deltoid muscle groups. Injection sites were rotated each week so that each subject received an injection into each muscle. Pharmacokinetic evaluation of serum concentrations and urinary excretion data indicated a sex difference with respect to the rate and extent of cephradine absorption from the three injection sites. Smaller areas under the curve and absorption rate constants were observed for females after injection into each muscle group. The most striking difference was observed when cephradine was injected into the gluteus maximus muscle, where the exponential function describing the alpha phase was observed to be 1.16 +/- 0.17 hr(-1) for females and 2.70 +/- 0.34 hr(-1) for males. Total area under the mean serum concentration-time curves, mean time to peak, and peak height parameters consistent with the slower rate of absorption and lesser bioavailability in females were observed. These results show that the vastus lateralis or deltoid muscle groups are preferable to the gluteus maximus as injection sites because of the more rapid rates of drug absorption from those muscles.