Opioid drugs increase glial cell activation and consequent cytokine release. These changes in glial cell activation may be related to the abuse liability of opioid drugs including heroin and prescription opioids. Data supporting this hypothesis have demonstrated that glial cell attenuators decrease the positive rewarding aspect of opioids in laboratory animals. Ibudilast (MN-166, formerly AV411) is a compound that inhibits the activation of glia and thereby inhibits the release of cytokines. Recent preclinical studies demonstrate that while ibudilast increases the analgesic effects of opioids, it decreases the rewarding effects of such drugs. It has also been shown that ibudilast suppresses morphine-induced release of dopamine, a primary neurotransmitter involved in the rewarding and reinforcing effects of abused drugs. Additionally, we recently found that ibudilast decreases subjective symptoms of opioid withdrawal in opioid dependent humans during detoxification.

Therefore, the primary aim of this 6-7 week inpatient study is to investigate the ability of MN-166 to dose-dependently alter the reinforcing, analgesic, subjective, performance, and physiological effects of oxycodone, a commonly abused prescription opioid. A secondary aim is to verify the ability of the drug to decrease opioid withdrawal symptoms during the initial inpatient detoxification.

This inpatient study includes a detoxification and two 18-day study phases. Upon study initiation, participants are tapered with morphine before study phase 1 starts, when they are randomized to receive placebo or 50 mg MN-166 BID (po at 0800 and 2000 hr), and then switched and stabilized on the medication. Thereafter, participants will complete 6 laboratory sessions over 9-10 days.

Subsequently, during Phase 2, participants will cross over to the other study arm (Pbo to MN-166 or MN-166 to Pbo), stabilize, and complete again 6 laboratory sessions. Days 1-10 of the study include a morphine taper, while each of the two subsequent study phases consist of a 7-8-day medication switch and stabilization phase, followed by 6 laboratory sessions over the next 9-10 days (3 sample sessions and 3 choice sessions). During sample sessions, participants will receive one dose of oxycodone (0, 15, or 30 mg/70 kg, PO) that will be available during the choice session the following day. At least 72 hrs after the previous sample session, the second sample session will be completed, followed by a choice session the next day. And then at least 72 hrs after the second sample session, the third and final sample session will be completed, followed by the final choice session the next day. The analgesic, subjective, performance, and physiological effects of oxycodone will be measured. During the choice session, a drug versus money self-administration paradigm will be employed, and the progressive ratio that is completed for drug and/or money will be measured.

We hypothesize that MN-166 will dose-dependently decrease oxycodone self-administration and positive subjective responses while increasing the analgesic effects of the drug.

A secondary additional objective is to collect exploratory information on potential predictors of prescription opioid self-administration including genetic polymorphisms, neurocognitive functioning, and response to stress. Blood samples will be collected to measure various genetic markers hypothesized to contribute to opioid drug effects (e.g., OPRM1, OPRD1, OPRK1, PENK, PDYN, DRD2, CYP3A4, and CYP2D6 genes). Performance on neurocognitive tasks and physiological response to the Trier Social Stress Test will be assessed in all participants.