Equivalent single-dose pharmacokinetics of two different dosing methods of prolonged-release fulvestrant ('Faslodex') in postmenopausal women with advanced breast cancer

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Abstract
To compare the pharmacokinetics of two different dosing methods of fulvestrant ('Faslodex'), an estrogen receptor antagonist with no known agonist activity, for the treatment of advanced breast cancer. Postmenopausal women with advanced breast cancer were randomly assigned to receive a single 5-ml intramuscular injection of 250 mg fulvestrant, or two 2.5-ml intramuscular injections with a total of 250 mg fulvestrant. Blood samples were taken for pharmacokinetic analysis up to 28 days after injection. Plasma concentrations of fulvestrant were measurable up to 28 days after both dosing methods. The concentration-time profiles were relatively shallow, spanning an approximate threefold range from 3 h after dosing to Cmin measured on day 28. Peak plasma concentrations (Cmax) of fulvestrant occurred between 1 and 11 days after dosing, with mean Cmax values of 6.0 and 6.2 ng/ml following one 5-ml injection and two 2.5-ml injections, respectively. The plasma concentration-time profiles were very similar in terms of duration and concentration, and overall exposure to fulvestrant was similar in both dosing groups (the ratio of the AUC0–28 of the single-injection group to that of the double-injection group was 1.01; 95% confidence interval 0.68–1.51). This study found no evidence of any pharmacokinetic difference between one 5-ml injection and two 2.5-ml injections. The two methods can be used interchangeably, depending on which is more convenient in any particular clinical setting.

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