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The pharmacokinetic and pharmacodynamic effects of a long-acting formulation of levonorgestrel microencapsulated in a biodegradable polymer poly(DL-lactide-CO-glycolide) was tested in baboons. The polymer microspheres provided continuous release of levonorgestrel for up to 6 months following a single intramuscular injection. The treatment inhibits ovarian function for 3-6 months, depending on the dose. The duration and pattern of levonorgestrel release varies according to the quality and size of the microspheres. The microsphere delivery system offers a promising new approach to developing a long-acting injectable contraceptive based on levonorgestrel.

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Biomedical engineering approaches used to develop controlled-release delivery systems for hormones are here reviewed regarding system design and therapeutic applications. The biomedical engineering approach uses a system of non-drug components to control the rate and duration of hormone delivery. The non-drug components vary from system to system, but generally include: a reservoir for the hormone; a barrier or regulator to contain the hormone within the reservoir and to control its release; an energy source to remove the hormone from the reservoir; and a pathway for egress of the hormone from the system. Controlled-release delivery systems for hormones discussed in this review include mechanical and osmotic pumps; intraocular, intravaginal and intrauterine platform devices; biodegradable and non-biodegradable subcutaneous implants; and small particulate systems including microcapsules, microspheres and liposomes. Examples of the therapeutic application of the various systems are given along with a discussion of design factors and pharmacological aspects relevant to their clinical use.