On the other hand, although camptothecin at low dose
(30 nM) had no effect on U937 cells, but when combined
with K. parviflora ethanolic extract (20 and 40 μg/ml),
an antagonism was observed (Figure 5). At high dose
(300 nM) camptothecin was able to increase percent U937
cells with decreased MTP, this result was not attenuated
by K. parviflora ethanolic extract (20 and 40 μg/ml).
Camptothecin, which induces an unusual type of DNA
damage by trapping cellular topoisomerase I on
chromosomal DNA in the form of drug-enzyme-DNA
cleavable complexes, inhibits DNA synthesis and
specifically kills S-phase cells. Cotreatment of L1210 cells
with aphidicolin, which is an inhibitor of replicative DNA
polymerases, completely abolishes camptothecin
cytotoxicity, suggesting the involvement of DNA
replication in camptothecin cytotoxicity