A comparison of the pharmacokinetic properties of three estradiol esters

In order to assess the pharmacokinetics properties of estradiol cypionate, valerate and benzoate, the daily plasma levels of estradiol and estrone were analysed in groups of 10, 9 and 10 subjects, respectively, before and during 3 weeks after the intramuscular administration of a single dose of 5.0 mg in 1.0 ml arachis oil. In order to minimize the contribution of endogenous estrogens to the plasma levels, all subjects were receiving a combined oral contraceptive consisting of levonorgestrel (150 micrograms) and ethinyl estradiol (30 micrograms) for three months prior to the study and during the study period. The administration of estradiol cypionate gave significantly lower peak levels of estradiol and estrone than that of the valerate and benzoate. Peak plasma levels of estradiol and estrone were reached in approximately 4 days following the administration of estradiol cypionate and in a significantly shorter time (approximately 2 days) following the administration of both the valerate and benzoate. One hour after the injection of the esters, the average percentage increases in plasma estradiol and estrone levels were significantly higher in the valerate and benzoate groups compared to the subjects receiving estradiol cypionate. The average duration of elevated estrogen levels was shortest in the benzoate group (4-5 days) followed by the valerate (7-8 days) and cypionate (approximately 11 days). In none of the subjects studied were elevated estradiol and/or estrone levels encountered 2 weeks after the injection of the various esters. The data suggests that among the three esters studied, the valerate provides the most predictable pharmacokinetic behaviour.