Characterization of anastrozole effects, delivered by an intravaginal ring in cynomolgus monkeys.

STUDY QUESTION

Is it feasible to deliver anastrozole (ATZ), an aromatase inhibitor (AI), by a vaginal polymer-based drug delivery system in the cynomolgus monkey (Macaca fascicularis) to describe the pharmacokinetic profile?

SUMMARY ANSWER

The present study showed the effective release of ATZ into the systemic circulation from intravaginal rings in cynomolgus monkeys.

WHAT IS KNOWN ALREADY

ATZ is a marketed drug with well documented pharmacological and safety profiles for oral administration. Aromatase is the key enzyme catalyzing estrogen biosynthesis and is overexpressed in endometriotic lesions. AIs show therapeutic efficacy in endometriosis in exploratory clinical trials.

STUDY DESIGN, SIZE, DURATION: The pharmacokinetics of the in vivo release and the pharmacodynamic activity of ATZ released by intravaginal rings (IVR) were investigated in healthy cycling female cynomolgus monkeys in three different dose groups (n = 5) for one menstrual cycle.

PARTICIPANTS/MATERIALS, SETTING, METHODS: IVRs for the cynomolgus monkey, releasing three different doses of ATZ were designed and tested for in vitro/in vivo release for up to 42 days. For pharmacokinetic and pharmacodynamic evaluation, plasma samples were taken once daily from Day 1 to 3 and then every third day until menses occurred (17-42 days).

MAIN RESULTS AND THE ROLE OF CHANCE

ATZ was shown to be compatible with the IVR drug delivery system. An average in vivo release of 277 µg/day/animal of ATZ for one menstrual cycle was effective in causing a decrease of systemic estradiol (E₂) levels by ∼30% without inducing counter regulation such as the elevation of FSH or the formation of ovarian cysts.

LIMITATIONS, REASONS FOR CAUTION: The study was limited to three dose groups in which only the highest dose decreased the E₂ level. Hence, additional research with IVRs releasing higher amounts of ATZ is required to define the threshold for an ATZ-dependent ovarian stimulation in cynomolgus monkeys.

WIDER IMPLICATIONS OF THE FINDINGS

The release rate administered from IVRs is sufficient and in a range that supports feasibility of IVR administration of ATZ as a new approach for long-term therapy of estrogen-dependent diseases such as endometriosis in human.