Effects of gender and phase of the menstrual cycle on the kinetics of ranitidine in healthy volunteers.

The present study was undertaken to determine if differences exist in the pharmacokinetic parameters of oral ranitidine caused by gender and stage of the menstrual cycle. The study was performed in two steps, in the first a pharmacokinetic study was performed on 10 men (average age 35.5 yrs) and 10 women (average age 34.7 yrs) during the follicular phase, and in the second the pharmacokinetic study was performed only on the same women in their luteal phase. Subjects received a tablet dose of 300 mg ranitidine, and blood samples were drawn at several times after its ingestion. Plasma ranitidine concentration was determined by high performance liquid chromatography. Comparison of the pharmacokinetic parameters of women and men revealed statistically significant differences both in distribution volume (Vd) with values of 2.0 and 6.3 l/kg, Area Under Curve (AUC) with values of 7312.15 and 11471.94 ng/ml/h, and clearance (CLt) with values of 0.65 and 0.59 l/kg/h, respectively. Several pharmacokinetic parameters in women were different in the follicular compared to the luteal phase; for example, Vd was 2.0 and 5.6 l/kg, AUC was 7312.15 and 5195.83 ng/ml/h, and CLt was 0.65 and 0.97 l/kg/h, in the respective phases. Moreover, the maximum concentration (Cmax) was 1086 ng/ml in the follicular vs. 864 ng/ml in the luteal phase. The first study detected differences between men and women in several pharmacokinetic parameters, mainly those indicative of drug availability, for example, Vd, AUC, and CLt. Comparison of data obtained in the follicular phase with those obtained in the luteal phase revealed differences in most pharmacokinetic parameters, which is seemingly indicative of the characteristic physiological changes associated with the luteal phase that largely affect the kinetics and availability of drugs such as ranitidine. Although it has been postulated that hormonal fluctuation within the menstrual cycle phase is the primary cause of documented gender differences in the pharmacokinetics and pharmacodynamics of drugs, further study of related factors is required to understand how gender and menstrual cycle rhythms affect the phannacokinetic process in their entirety.