Cytotoxicity (IC₅₀, μM) of CPT and cleavable complex formation in the presence or absence of PPNO.

Cytotoxicity studies were carried out as described in the Methods section. The concentration of PPNO was 100 μM, and cells were pretreated with PPNO for 6 h before the various concentrations of CPT were added and incubated for an additional 48 h. *** Significantly different (< 0.001) from CPT alone; significance was determined by a Student’s t-test (n = 3). Formation of the DNA-topo I-CPT cleavable complexes was assayed by an SDS-KCl precipitation assay as described in the Methods section. Cells were preincubated with 100 PPNO μM for 6 h before adding CPT for 1 h. # Fold increase compared to the control values, without CPT. Cytotoxicity (IC50, μM) of CPT and cleavable complex formation in the presence or absence of PPNO.