Main Study of Lofexidine and Methadone Pharmacodynamic Interaction in Methadone Maintained Patients

The primary objective of this study is to assess QTc (an interval of the heart rhythm) interaction effects between lofexidine and methadone. The secondary objectives of the study are to evaluate the safety and tolerability of lofexidine by evaluating and monitoring pharmacokinetics (amounts of drug in the blood), vital signs (heart rate and blood pressure) and adverse events (side effects) when co-administered with methadone; to describe effects on opiate withdrawal when lofexidine is introduced following a 50% methadone dose reduction, as required to elicit a withdrawal response; and to evaluate the QTc interaction effects of lofexidine compared with placebo. The investigators hypothesize that while both agents (lofexidine and methadone) are known to prolong the QTc interval, the combination of the drugs will not create an additive effect which creates a significant safety concern. The investigators further hypothesize that subjects will be able to tolerate the therapeutic dose of lofexidine (0.8 mg four times daily) when the methadone maintenance dose is lowered to elicit withdrawal.