A study of reversibly inactivated lipases for use in a morphine-triggered naltrexone delivery system

A key component of an implant that can be triggered by external morphine to release naltrexone is an inactivated enzyme that can be activated by morphine and which can then rapidly remove a protective coating surrounding a bioerodible polymer containing dispersed naltrexone. In this article we describe a lipase that has been conjugated with O3-carboxymethylmorphine, morphine-beta-3-glucuronide and O3-carboxypropylmorphine. The enzyme conjugate was then inactivated by complexation with affinity-purified goat polyclonal antimorphine antibodies. Antibody lipase interactions were measured by pH Stat and ELISA techniques. Affinity constants of the antibodies determined by radioimmunoassay using tritium-labeled morphine were 4.10 x 10(6), 3.18 x 10(6) and 3.38 x 10(7), respectively. While a concentration of 10(-5)M morphine was required to restore lipase activity, it is likely that a combination of correct morphine tether and correct affinity-purified antibody can increase sensitivity to the desired 10(-8)10(-9)M morphine level. Thus, a functioning device can almost certainly be constructed. However, it is unlikely that reactivation times of 1-2 h necessary for clinical usefulness in treatment of narcotic addiction can be achieved.