MODULATION IN KINETICS OF LACTONE RING HYDROLYSIS OF CAMPTOTHECINS UPON INTERACTION WITH TOPOISOMERASE-I CLEAVAGE SITES ON DNA

The kinetics of hydrolysis of the alpha-hydroxylactone ring of antican cer agents belonging to the camptothecin (CPT) series has been followe d using their fluorescence emission. Data obtained for CPT, CPT-11, an d SN-38, either in their free form or in the presence of DNA and/or to poisomerase I (top1), have been compared. DNA was modeled using three types of double-strand oligonucleotides corresponding to top1 cleavage site enhanced in the presence of the drug (olg1), top1 site independe nt of CPT (olg2), and nonspecific synthetic oligonucleotide containing only AT and no GC base pairs (olg3). Cleavage assays indicated the ab sence of top1-mediated cleavage on olg3, both in the presence and in t he absence of CPT. The kinetics data also showed ratio-dependent stabi lization of the lactone forms of CPTs when in the presence of an exces s of olg1 or olg2, but not of olg3. These observations correlate with the previously reported preferential binding of CPTs to guanines. Alth ough lactone hydrolysis was not perturbed by top1 alone, this enzyme h indered lactone stabilization by specific oligonucleotides. After addi tion of top1 to CPT-olg1 or CPT-olg2 complexes, the lactone ring of th e drug was destabilized. No lactone stabilization was observed when ol g1 was added to CPT-top1 complexes or when olg1-top1 complexes were ad ded to CPT.