Disposition and metabolism of ( 3 H)pseudomorphine in the rat.

[3H]pseudomorphine (3H-PM) was prepared by the oxidation of [3H]morphine. A method for the estimation of 3H-PM in biological materials was developed with a minima1 sensitivity of 2.5 ng/ml of biologic material. Following the subcutaneous injection of 3H-PM (10 mg/kg as free base) in rats, the levels of 3H-PM in brain and spinal cord over a 2-hr period ranged between 46 to 17.9 rig/g and 24 to 76.5 rig/g,, respectively. No detectable levels of 3H-PM were present at 4 hr in the CNS. Peak plasma levels occurred at 30 min (501 ng/ml) and then fell rapidly with no detectable amounts present at 4 hr. About 78.2 per cent of the administered 3H-PM was accounted for in the urine and feces after 48 hr. Conjugated pseudomorphine was not detected in biological material, and direct chromatography of urine provided evidence for the presence of only free 3H-PM. The existence of a polar metabolite, however, cannot be precluded. Studies on rat brain extracts following the administration of [jH]morphine (10 mg/kg as free base) provided no evidence for the formation of pseudomorphine as an oxidative metabolite of morphine in vivo. PSEUDOMORPHINE (2: 2’-bimorphine), as an oxidation product of morphine,lV3 is of special interest because of a possible involvement in the agonistic action of morphine or the development of tolerance and/or physical dependence following chronic administration of morphine. This view has not received wide acceptance,4s5 primarily because pseudomorphine has not been identified as a metabolite in vivo of morphine in biological materials, although Hosoya and Brody,6 utilizing rat tissue homogenates in the presence of cytochrome c, apparently demonstrated the oxidation of morphine to pseudomorphine in vitro. The known physicochemical characteristics of pseudomorphine,’ i.e. very limited solubility in saline, biological fluids and common organic solvents, difficulty of separation from proteins and formation of an insoluble salt with trichloroacetic acid, have interfered with the development of a suitable, sensitive analytical method for the extraction and estimation of this compound from biological materials. This communication describes the preparation, disposition and biotransformation of [3H]pseudomorphine (3H-PM) in the rat. The method described for the estimation of [3H]pseudomorphine from biological materials has been used to ascertain whether this compound is a metabolite of [3H]morphine in the CNS of rats. MATERIALS AND METHODS Preparation of tritium-labeled pseudomorphine (3H-Ph4). [3H]Pseudomorphine was prepared by the oxidation of [3H]morphine (50 mg) by a microscale adaptation of the * Present address: New York State NACC, Testing and Research Laboratory, Brooklyn, New York 11217.