After absorption, heroin is transformed into mono-acetyl-morphine and then into morphine. Morphine, in turn, is metabolized to morphine-3-glucuronide (M3G), an inactive compound, and morphine-6-glucuronide (M6G), a potent opioid agonist. Thus, changes in the rate of formation of M6G may alter the pharmacological consequences of a treatment with heroin or morphine. In this study, we investigate the effect of repeated exposures (10 daily i.p. injections) to heroin, morphine, cadmium (which has been previously shown to inhibit M3G formation in vitro), or heroin ϩ cadmium on morphine glucuronidation both in vivo and ex vivo (i.e., microsomal preparation obtained from rats treated in vivo). Repeated heroin (2.5, 5.0, and 10 mg/kg) increased plasma levels of M6G (which was undetectable in all other groups) and reduced those of M3G. Also, the microsomal preparations obtained from the liver of repeated heroin rats, when incubated with morphine, yielded significant amounts of M6G (which was undetectable in all other groups) and decreased levels of M3G relative to the control groups. These effects were reversible upon discontinuation of heroin administration. In contrast, repeated morphine (10, 20, and 40 mg/kg) only slightly reduced M3G formation at the dose of 40 mg/kg. Repeated cadmium (5, 15, and 45 g/kg) reduced the rate of M3G formation without inducing M6G synthesis. The effects of the repeated coadministration of heroin (10 mg/kg) and cadmium (15 g/kg) were virtually identical to those of repeated heroin alone. In summary, repeated exposure of rats to heroin can shift morphine glucuronidation toward the formation of the active metabolite M6G.

Chronic exposure to street heroin causes a relative increase in concentrations of the active metabolite, M6G. Since the pattern of M6G action seems closer to heroin than to morphine, the increased synthesis of M6G observed in IHU may prolong the narrow window of heroin effects.

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