SUNY at Albany
June 19-23, 2001
The Behavior of the DNA Topoisomerase I Inhibitor Topotecan in Aqueous Solution
An alkaloid camptothecin (CPT) is a classical inhibitor of DNA topoisomerase I (topo I). It is poorly soluble in water, and this makes rather difficult its physicochemical investigation. Therefore, topotecan (TPT, hycamtin, NSC609699), a water-soluble antitumor analog of CPT used in clinical practice, was chosen for our studies. The chemical structures of CPT and TPT are displayed in the figure. It was thought for a long time that CPT does not directly interact with DNA or topo I alone, but only binds to the DNA-topo I complex. However, recent results show that the molecules of the CPT family (TPT etc.) are able to interact directly with DNA (1, 2).
In view of the possible role of the aggregation state in the biological activity of CPT, the investigations of TPT aggregates in solution preceded studies of its interaction with DNA. Optical methods and quantum-chemical computations were used (3). Absorption spectra of TPT were obtained in the pH range from 0.5 to 11.5, and their pKa were determined. All charge states of TPT molecule were calculated for its lactone and carboxylic forms. The calculated absorption maxima coincided with the experimental ones. The D ring of the TPT molecule was found to be protonated (pKa ~ 3.6). The comparison of experimental and theoretical data has shown that the preferable TPT structure is the one with a proton on the oxygen at C16a rather than on N4 nitrogen.
At a concentration higher than 1 x 10(-5) M, TPT molecules start to form dimers. The TPT dimerization is accompanied by an increase in pKa of a hydroxyl of A ring from 6.5 ([TPT] - 10(-6) M) to 7.1 ([TPT] - 10(-4) M), suggesting that this group is involved in dimer stabilization, probably through formation of an intermolecular hydrogen bond with N1 of the B ring of another TPT molecule. Probable structures of TPT dimers were proposed. The TPT dimerization constant was estimated at (4.0 ± 0.7) x 10(3) M(-1).
Supported by RFBR grants # 01-04-48657,# 00-15-97834 and INTAS grant # 97-0522.References and Footnotes
S.A. Streltsov(1), S.L. Grokhovsky(1,2), I.A. Kudelina(3), V.A. Oleinikov(3) and A.L. Zhuze(1)
(1)Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 119991 Moscow, Russia