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[14C]Streptozotocin was synthesized specifically labelled at three positions in the molecule. The biological activity ofsynthetic streptozotocin was characterised by studies in vivo ofits diabetogenic activity and its dose-response curves. After this characterization the excretion pattern of all three labelled forms of streptozotocin was studied. With [1-14C]-streptozotocin and [2'-14C]streptozotocin the injected radioactivity was excreted (approx.70% and 80% respectively) mainly in the urine, the greater part of the excretion occurring in the first 6h period; small amounts (approx. 9% and 8 % respectively) were found in the faeces. In contrast, with [3'-methyl-'4C]streptozotocin a much smaller proportion (approx.42 %) of the injected radioactivity was excreted in the urine, the major proportion appearing in the first 6h, whereas approx. 53 % of the injected radioactivity was retained in the carcasses. In whole-body radioautographic studies very rapid renal clearance and hepatic accumulation of the injected radioactivity was observed with all three labelled forms of the drug. There was some evidence for biliary and intestinal excretion. Major differences were apparent in the tissue-distribution studies, with each of the three labelled forms, particularly with [3'-methyl-14C]streptozotocin. There was no accumulation of [1-_4C]streptozotocin in the pancreas for the 6h period after administration. However, with [3'-methyl-14C]-streptozotocin (and also [2'-'4C]streptozotocin) there was evidence of some pancreatic accumulation after 2h. The results indicate that streptozotocin is subjected to considerable metabolic transformation and to rapid renal clearance. The implication of these suggestions is evaluated with particular reference to the diabetogenic action of streptozotocin.

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