In vitro studies of the effect of methadone on natural killer cell activity.

Heroin addicts are reported to have numerous immunological abnormalities. The purpose of this study was to determine whether methadone, a long-acting synthetic opioid agonist used in the maintenance treatment of heroin addiction as well as in the treatment of chronic pain, alters natural killer (NK) cell cytotoxicity activity. We incubated human peripheral mononuclear cells with a wide concentration range (1 x 10(-13) to 1 x 10(-3) M) of (-) methadone, the active enantiomer of methadone, or with (+) methadone, the inactive enantiomer, prior to and during an assay of NK activity. We found no effect of methadone at concentrations of 1 x 10(-13) to 1 x 10(-5) M, but higher concentrations (1 x 10(-4) to 1 x 10(-3) M) of either enantiomer of methadone caused a significant reduction in NK activity. We conclude that methadone concentrations that are normally achieved in the plasma during methadone maintenance treatment or during chronic pain management, and much higher concentrations, have no effect on NK activity in vitro.