Methadone: a review of its pharmacokinetic/pharmacodynamic properties.

During the past decades the use of methadone has been increased as a result of the interest of optimizing its therapeutics in opioid addicts, one of the groups with higher risk for AIDS infection. However standard dose of methadone are far from being the appropriate for relief pain or prevent withdrawal signs in maintenance programs in many patients. To achieve an optimal dose regimen for an individual, the knowledge of the relationship between the pharmacokinetics/pharmacodynamics (pk/pd) drug properties and the demographic and physiopathological characteristics of the subject is required. Unfortunately, there is a lack of studies dealing with the population pk/pd properties of methadone. In the current study, a review of the pk/pd properties of methadone is presented with the aim of understanding the sources of variability in response. This will help in the design of prospective pk/pd studies; in particular, individual data including sex, weight, alpha(1)-acid glycoprotein levels in plasma, concomitant medications, time after starting treatment with methadone and previous exposure to other opioids should be requested. In addition, designs for drug administration should allow the characterization of the plasma-versus-biophase distribution and the development of tolerance processes. Because methadone is usually administered as a racemic mixture, the use of enantioselective techniques to determine both enantiomers in plasma is also highly recommended.