Method for localized and sustained administration of drugs to the vas deferens of rats

A simple method is described for constructing collars (15 mm long, 6.5 mm diameter with a central hole 2.8 mm diameter) consisting of powdered tyramine HCI dispersed in polydimethylsiloxane polymer (Silastic). Release rate was measured (1) by incubation at 37 degrees C in 100 ml of tris buffer at pH = 7.4 (in vitro) and (2) after fitting to the vasa deferentia of rats, by collection of their urine and analysis for p-hydroxyphenylacetic acid (in vivo). Collars containing 50% tyramine released their load over about 7 weeks both in vivo and in vitro. The rate of release was initially linear and later proportional to (time) 1/2, these kinetics being consistent with the matrix model. The useful life of these devices is about 3 weeks since this is the period of substantial daily release. The rate of release is greater when the proportion of tyramine in the tyramine/Silastic mixture is increased (useful range = 25-50%). Release is prolonge and further localized by partly coating the collar with a layer of drug-free Silastic.