Intracellular distribution of 57Co-bleomycin

Since the first promising results of Nouel et al. 1972, additional positive experience has been obtained with 57Co-Bleomycin (57Co-BLM) as a tumour-localizing agent. In this preclinical study, mice with transplanted osteosarcoma and lymphosarcoma were used and rats with transplanted rhabdomyosarcoma. 57CoCl2 served as a control substance. 57Co-BLM had concentrated in the tumours with a factor 2 to 10 as compared to the (normal) liver of the animals. No preferential concentration in the tumours was found when 57CoCl2 was used. The highest specific activity of 57Co-BLM (cpm/mg protein) was found in a fraction containing mitochondria and lysosomes. Evidence for a lysosomal localization of this diagnostic compound was obtained from experiments in which the mitochondrial-lysosomal fraction was treated with hypertonic media of different osmolarities. Conditions could be found in which many lysosomes burst while almost all mitochondrial were intact. From these experiments it appeared that the radioactivity in the particles obtained from animals injected wtih 57Co-BLM was released very rapidly. It is concluded that 57Co-BLM is preferentially localized in the heavy lysosomes sedimenting together with most of the mitochondria of the cell and that these structures are more fragile than the light lysosomes.