Pharmacokinetics, subcellular distribution, and covalent binding of [3H]bleomycin in hamsters after intratracheal...

Pharmacokinetics, subcellular distribution (SCD), and covalent binding of a single dose of 1 microCi of [S-methyl-3H]bleomycin ([3H]-BLM]) in combination with one unit of unlabeled bleomycin were studied in hamsters following intratracheal (IT) injection. The radioactivity decreased from the lung biexponentially with time. The apparent half-time of absorption for the alpha-phase was 1.1 and 17.9 hr for the beta-phase. The plasma disappearance curve of [3H]BLM fits to a two-compartmental model with the apparent half-life removal for the alpha-phase being 1.6 hr and for the beta-phase 116.9 hr. The radioactivity was detected in all studied tissues. The radioactivity from spleen, testicle, liver, fat, RBC, brain, adrenal, and kidney manifested only the alpha-phase of the disappearance curve, while the beta-phase was complicated by redistribution processes. Of the eight tissues, the spleen had the shortest (2.0 hr) and kidney the longest (12.1 hr), and the remaining tissues had half-lives which ranged from 4 to 10 hr. The SCD study revealed that 85 to 95% of the total radioactivity in the lung and liver homogenate was associated with the soluble fraction (SF) at 30 min after treatment, thereafter, the radioactivity from both tissues gradually decreased to 60% of the total at 24 hr. The SF of the lung homogenate had the highest specific radioactivity (SRA) of any of the fractions during the period between 0.5 and 6 hr. The SRA, however, decreased biexponentially and attained a value similar to that of the mitochondrial and microsomal fractions at 12 and 24 hr after treatment. In the case of liver, the SF had the highest, the nuclear the lowest, and mitochondrial and microsomal fractions the same level of SRA at 30 min. Thereafter, the SRA of all fractions were increased with time. A significant amount of radioactivity from [3H]BLM was covalently bound to lung, liver, and plasma proteins. The SF of the lung contained an increasing amount of radioactivity covalently bound after 1.5 hr of [3H]BLM injection and nearly all radioactivity measured in the plasma was covalently bound. It was concluded from the findings of this study that the presence of a major portion of [3H]BLM in the SF of the lung and its covalent linkage with the proteins of this fraction might initiate the complex sequence of events at the metabolic level necessary for the pneumotoxicity.