Conjugate addition ligands of opioid antagonists. Methacrylate esters and ethers of 6 alpha- and 6 beta-naltrexol

Role of active metabolites in the use of opioids

The opioid class of drugs, a large group, is mainly used for the treatment of acute and chronic persistent pain. All are eliminated from the body via metabolism involving principally CYP3A4 and the highly polymorphic CYP2D6, which markedly affects the drug's function, and by conjugation reactions mainly by UGT2B7. In many cases, the resultant metabolites have the same pharmacological activity as the parent opioid; however in many cases, plasma metabolite concentrations are too low to make a meaningful contribution to the overall clinical effects of the parent drug. These metabolites are invariably more water soluble and require renal clearance as an important overall elimination pathway. Such metabolites have the potential to accumulate in the elderly and in those with declining renal function with resultant accumulation to a much greater extent than the parent opioid. The best known example is the accumulation of morphine-6-glucuronide from morphine. Some opioids have active metabolites but at different target sites. These are norpethidine, a neurotoxic agent, and nordextropropoxyphene, a cardiotoxic agent. Clinicians need to be aware that many opioids have active metabolites that will become therapeutically important, for example in cases of altered pathology, drug interactions and genetic polymorphisms of drug-metabolizing enzymes. Thus, dose individualisation and the avoidance of adverse effects of opioids due to the accumulation of active metabolites or lack of formation of active metabolites are important considerations when opioids are used.

Synthesis and hydrolytic behavior of two novel tripartate codrugs of naltrexone and 6beta-naltrexol with hydroxybupropion as potential alcohol abuse and smoking cessation agents

A codrug approach for simultaneous treatment of alcohol abuse and tobacco dependence is considered as very desirable because of substantial evidence that smoking is increased significantly during drinking, and that smoking is regarded as a behavioral 'cue' for the urge to consume alcohol. The purpose of this study was to design and synthesize codrugs for simultaneous treatment of alcohol abuse and tobacco dependence. Two novel tripartate codrugs of naltrexone (NTX) and naltrexol (NTXOL) covalently linked to hydroxybupropion (BUPOH) were synthesized (25 and 26, respectively), and their hydrolytic cleavage to the parent drugs was determined. These codrugs were generally less crystalline when compared to NTX, or NTXOL, as indicated by their lower melting points, and were expected to be more lipid-soluble. Also, the calculated clogP values were found to be higher for the codrugs compared to those for NTX and NTXOL. The studies on the hydrolysis of the codrugs provided good evidence that they could be efficiently converted to the parent drugs in buffer at physiological pH. Thus, these codrugs are likely to be cleaved enzymatically in vivo to generate the parent drugs, and are considered to be potential candidates for simultaneous treatment of alcohol abuse and tobacco dependence.