It is known that SN-38, a main metabolite of the novel camptothecin derivative CPT-11, shows a more potent anti-proliferative effect than CPT-11, in vitro. Therefore, it is presumed that SN-38 greatly influences the antitumor effect of CPT-11. In this study, we examined the generation of SN-38 from CPT-11 by plasma and tissue homogenates from the rat, dog, and man.
1. In the rat, the plasma, followed by the intestine, showed the highest activity in the generation of SN-38, and activity was also detected in the lung, liver, and kidney.
2. In the dog, the liver showed the highest sustained activity, but little activity was detected in the plasma and intestine.
3. In human subjects, almost the same activity was initially detected in the liver and intestine, but only the liver showed sustained activity. Plasma possessed marginal activity.
4. Comparison of the rat, the dog, and man showed remarkable species differences in the activity of the plasma, but not in that of the liver.
5. Among rat liver cell fractions, microsomes showed the highest activity, and the activity was also detected in the mitochondria. However, the activity of lysosomes was low.
6. The microsomal fraction of rat liver and tissue homogenates (liver, intestine, kidney, and lung) prepared from the rat showed almost equal Michaelis constants, rang from 20 to 26μM, for the generation of SN-38.
7. The activity of the human liver homogenate was inhibited by more than 80% by DFP (1 × 10^-4M), an inhibitor of serine esterases such as carboxylesterases and cholinesterases. However, 40% inhibition was induced by physostigmine (1 × 10^-4M), an inhibitor of cholinesterases, but none by PCMB (1 × 10^-4M), an inhibitor of arylesterases.