CPT binds topoisomerase I, trapping it as a reaction intermediate covalently bound to DNA.
Topoisomerase I trapping approached saturation at 5 10 μ M. Topoisomerase I trapping increased with time such that 10 μ M topotecan had led to the trapping of 54% of the intracellular topoisomerase I after 1 h.
Topoisomerase band depletion Western blots were performed to measure the topotecan dose relationship with topoisomerase I trapping.
The bioactive dose range (0.5 10 μ M) for 1-h topotecan exposures was defined by rapid drug delivery and topoisomerase I trapping.
Interestingly, topoisomerase I trapping was both topotecan dose- and time-dependent, while the intracellular topotecan concentration peaked within 10 12 min, and then declined.
In order to measure the pharmacokinetics of the actions of short exposures to topotecan in MCF-7 cells, the dose relation for the trapping of topoisomerase I, the intracellular drug concentration and the reversibility of topoisomerase I trapping were examined.
