Release rate of testosterone and estrogens from polydimethylsiloxane implants for extended periods in vivo compared with loss in vitro.

Release rates of testosterone, estrone, and estradiol placed in chambers made from polydimethylsiloxane (PDS) tubing (Dow Corning "Silastic," 3.35 mm ID x 4.65 mm OD) were studied in 14 freemartin cattle with minimal or non-detectable endogenous hormone secretion, and in 0.9% saline:methanol (1:1) baths shaken at 38 degree C. Eighty-seven implants, varying in length from 2 to 10 cm, were placed in 14 animals for 27 to 235 days. The average release rates +/- standard errors, in microgram/cm/day, were testosterone, 55.9 +/- 2.4, estrone, 12.6 +/- 1.8, and estradiol, 11.1 +/- 1.1. A relatively constant release rate was found over the period of time studied and sufficient steroid remained for potential release over periods exceeding 1 year. The dose of hormone delivered was sufficient to increase mounting activity in testosterone-treated animals and estrual activity in those receiving estrogens. Corresponding release rates in vitro for four 10-cm implants containing either testosterone, estrone, or estradiol were 94.3 +/- 1.9, 15.5 +/- 0.7, and 12.7 +/- 0.6 microgram/cm/day, respectively. The general magnitude of release rate in animals could be predicted from laboratory tests.