Distribution of 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl free radical in normal and neoplastic animal tissues.

The free radical 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (TMPO) was injected i.p. in doses of 100 mg/kg bw into Syrian hamsters: untreated, partially hepatectomized and grafted with transplantable tumours as well as into mice with B16 melanotic melanoma or with an adenocarcinoma. One hour after application of the compound its content was determined spectrometrically in livers, kidneys, lungs and all tumours. The characteristic triplet signal of TMPO was registered in the hamster and mouse melanotic melanomas but not in all other animal tissues and tumours. This may be the result of an enzyme defect in the melanotic melanoma cells or of a retention of TMPO by melanin. It was also found that TMPO is metabolized predominantly in the livers of the animals.