Controlled Release of Hormones by Pellet Implants

Administering exogenous hormones to experimental animals has been in practice for centuries, primarily to elucidate the physiological function of a particular endocrine system or a hormone. Like any other drug, the purpose of hormone administration is to maintain a therapeutic level of that hormone and this has been a challenge for various reasons. The administration route, hormone solubility, its metabolism, and half-life are some of the key factors to be considered when aiming to achieve therapeutic levels of hormones in vivo. This chapter discusses various hormone delivery modalities that are routinely used in experimental animals, with their practical advantages and disadvantages. Commonly used hormone administration routes include delivery via food and/or water, oral gavage, par nasal, intravenous injection, subcutaneous injection, osmotic pump implants, Silastic® tube implants, matrix-driven delivery hormone pellets, beeswax-hormone pellets, and these different administration routes results in different plasma hormone profiles. While delivery through some of the methods leads to toxic plasma concentrations other methods may lead to a decline even below the minimum effective dose soon after administration. However, the latter forms of hormone delivery such as subcutaneous implantable ones have reported achievement of sustained release of hormone in studies. While comparing the pros and cons of various implantable devices in hormone delivery, the matrix-based ready-to use commercially available hormone pellets would appear to be suitable for the purpose of controlled and sustained hormone delivery. However, one should be aware of the fact that hormone delivery via subcutaneous implants have their own limitations such as high cost and initial burst release. Therefore, caution needs to be exercised in the interpretation of data from some of the studies. An alternative newer approach has emerged in hormone delivery methods such as bioartificial endocrine tissues or cell-based/tissue-based hormone delivery that could be preferable based on the scope of the study.