Antagonist-agonist combinations as therapies for heroin addiction: back to the future?

Combination of a Rapidly Penetrating Agonist and a Slowly Penetrating Antagonist Affords Agonist Action of Limited Duration at the Cellular Level

The capsaicin receptor TRPV1 (transient receptor potential vanilloid 1) has been an object of intense interest for pharmacological development on account of its critical role in nociception. In the course of structure activity analysis, it has become apparent that TRPV1 ligands may vary dramatically in the rates at which they interact with TRPV1, presumably reflecting differences in their abilities to penetrate into the cell. Using a fast penetrating agonist together with an excess of a slower penetrating antagonist, we find that we can induce an agonist response of limited duration and, moreover, the duration of the agonist response remains largely independent of the absolute dose of agonist, as long as the ratio of antagonist to agonist is held constant. This general approach for limiting agonist duration under conditions in which absolute agonist dose is variable should have more general applicability.

Considerations on the role of buprenorphine in recovery from heroin addiction from a UK perspective

The United Kingdom Drug Strategy emphasises recovery as a key focus in the treatment of drug dependence. A framework for recovery is defined in the Recovery-Orientated Drug Treatment report, written by an expert working group, and comprises four key phases: engagement and stabilisation, including the establishment of treatment goals; preparation for change, involving engagement in psychosocial and pharmacological interventions; active change, including detoxification and medical withdrawal; and completion, including interventions that strengthen community integration. A body of evidence supports the benefits of buprenorphine, a partial agonist at mu opioid receptors, in supporting individualised recovery based on this framework, specifically in relation to the potential for rapid stabilisation, flexibility to transition to other treatment options or achieve abstinence, effective blocking of on-top use of illicit drugs, the treatment of comorbidities through the minimisation of drug-drug interactions, and a good safety profile. In addition, the newer abuse-deterrent formulation of buprenorphine combined with the opioid antagonist naloxone is likely to strengthen recovery-orientated systems of care due to its potential to reduce misuse and diversion. Progress through the recovery journey and the ability to sustain recovery will depend on individual needs and goals and on the amount of recovery capital that individuals have developed.

A non-rewarding, non-aversive buprenorphine/naltrexone combination attenuates drug-primed reinstatement to cocaine and morphine in rats in a conditioned place preference paradigm

Concurrent use of cocaine and heroin is a major public health issue with no effective relapse prevention treatment currently available. To this purpose, a combination of buprenorphine and naltrexone, a mixed very-low efficacy mu-opioid receptor agonist/kappa-opioid receptor antagonist/nociceptin receptor agonist, was investigated. The tail-withdrawal and the conditioned place preference (CPP) assays in adult Sprague Dawley rats were used to show that naltrexone dose-dependently blocked the mu-opioid receptor agonism of buprenorphine. Furthermore, in the CPP assay, a combination of 0.3 mg/kg buprenorphine and 3.0 mg/kg naltrexone was aversive. A combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone was neither rewarding nor aversive, but still possessed mu-opioid receptor antagonist properties. In the CPP extinction and reinstatement method, a combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone completely blocked drug-primed reinstatement in cocaine-conditioned rats (conditioned with 3 mg/kg cocaine, drug prime was 3 mg/kg cocaine) and attenuated drug-primed reinstatement in morphine-conditioned rats (conditioned with 5 mg/kg morphine, drug prime was 1.25 mg/kg morphine). These data add to the growing evidence that a buprenorphine/naltrexone combination may be protective against relapse in a polydrug abuse situation.

Mixed κ/μ partial opioid agonists as potential treatments for cocaine dependence

Cocaine use activates the dopamine reward pathway, leading to the reinforcing effects of dopamine. There is no FDA-approved medication for treating cocaine dependence. Opioid agonists and antagonists have been approved for treating opioid and alcohol dependence. Agonists that activate the μ opioid receptor increase dopamine levels in the nucleus accumbens, while μ receptor antagonists decrease dopamine levels by blocking the effects of endogenous opioid peptides. Activation of the κ opioid receptor decreases dopamine levels and leads to dysphoria. In contrast, inhibition of the κ opioid receptor decreases dopamine levels in the nucleus accumbens. Antagonists acting at the κ receptor reduce stress-mediated behaviors and anxiety. Mixed partial μ/κ agonists have the potential of striking a balance between dopamine levels and attenuating relapse to cocaine. The pharmacological properties of mixed μ/κ opioid receptor agonists will be discussed and results from clinical and preclinical studies will be presented. Results from studies with some of the classical benzomorphans and morphinans will be presented as they lay the foundation for structure-activity relationships. Recent results with other partial opioid agonists, including buprenorphine derivatives and the mixed μ/κ peptide CJ-15,208, will be discussed. The behavioral effects of the mixed μ/κ MCL-741, an aminothiazolomorphinan, in attenuating cocaine-induced locomotor activity will be presented. While not a mixed μ/κ opioid, results obtained with GSK1521498, a μ receptor inverse agonist, will be discussed. Preclinical strategies and successes will lay the groundwork for the further development of mixed μ/κ opioid receptor agonists to treat cocaine dependence.