Intermittent exposure of medulloblastoma cells to topotecan produces growth inhibition equivalent to continuous exposure.

Camptothecin analogues such as topotecan increase the number of covalent topoisomerase I-DNA complexes, which, in turn, have been proposed to initiate apoptosis. If induction of apoptosis by the camptothecins is, in fact, dependent on the formation of topoisomerase I-DNA complexes, then it would be of clinical relevance to identify schedules of exposure to the camptothecins that maximize the formation of these complexes but minimize the total amount of the drug administered. The time and dose dependence of topoisomerase I-DNA complex formation was determined by incubating Daoy pediatric medulloblastoma cells in vitro with topotecan at concentrations equivalent to those achievable in the plasma clinically (10, 50, or 200 nM) and measuring the number of complexes present in cells incubated for 15 min to 48 h with the drug. Regardless of the concentration of topotecan used, covalent topoisomerase I-DNA complexes were maximal within 15 min following addition of the lactone form of topotecan to the tissue culture medium. After 2 h of exposure to topotecan, complexes had decreased from maximum to approximately half of that value. Few, if any, complexes were detectable with topotecan incubations of 24-48 h. Growth inhibition studies showed that the IC50s of topotecan for the Daoy cell line (2.2 x 10(-9) M) and also for a second pediatric medulloblastoma cell line, SJ-Med3 (3.6 x 10(-9) M), exposed to topotecan 8 h daily for 5 days or continuous exposure were equivalent. The decrease in topoisomerase I-DNA complexes between 15 min and 1 h was consistent with a pH-dependent re-equilibration of topotecan to the less active hydroxyacid form of the drug. The decrease in complexes after a 2-48-h incubation with the drug was attributable neither to biological inactivation of topotecan as shown by sequential growth inhibition studies nor to a decrease in amount of topoisomerase I in the drug-treated cells. Indirect immunofluorescence labeling of topoisomerase I in Daoy cells incubated for 48 h with 10 nM topotecan showed a redistribution of nucleolar topoisomerase I. We are currently evaluating the antitumor effect of intermittent repetitive exposures to topotecan in mice bearing Daoy cells as a xenograft. The clinical utility of each effective schedule of exposure will depend on whether the therapeutic index of repetitive intermittent exposure to the drug is more or less favorable than the therapeutic index of continuous exposure.