Further studies of transport and distribution of bleomycin in EAT cells using 57Co-bleomycin

Synergistic action of bleomycin and X-rays has been observed on rates of DNA synthesis of EAT cells in predominantly plateau phase suspensions where the drug dose (20μg/ml) is split into two halves, one delivered 30 mins before exposure to X-rays (2.5krad) and one immediately afterwards. An improved response to the split drug dose radiation schedule (2x1μg/ml) compared to bleomycin given as a single dose plus radiation (200rad) has been observed in the depression of growth rates of EAT cells in the exponential phase in culture. These findings are considered in relation to values obtained for the biological half-life of bleomycin incubated with cells and to the transport kinetics, intracellular distribution and dialysibility of 57Co-bleomycin within the cell. The results suggest that only a small proportion of intracellular bleomycin is bound to DNA and decreases after reaching a maximum, irrespective of the steady levels of 57Co-bleomycin in the cell as a whole. Most of the intracellular bleomycin appears to be associated with proteins, forming a non-dialysable complex which does not influence cell growth and will be unable to potentiate radiation damage. Bleomycin is believed to be transported into the cells by facilitated diffusion. Some evidence is presented to suggest that extracellular bleomycin may stimulate increased carrier availability. (author)