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In an effort to further extend the number of targets for development of antiretroviral agents, we have used an in vitro integrase assay to investigate a variety of MATERIALS AND METHODS Materials. HIV-1 integrase protein (3.5 pmol per reaction), produced via an Escherichia coli expression vector as described (13), was obtained from R. Craigie (Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases) and stored at -70°C in 1 M NaCl/20 mM Hepes, pH 7.6/1 mM EDTA/1 mM dithiothreitol/20% glycerol (wt/vol). Caffeic acid phenethyl ester (CAPE) was brought to our attention by Dezider Grunberger (Columbia University, New York), who supplied the compound. Doxorubicin, 5-iminodaunorubicin, mitoxantrone, ellipticine, ellipticinium, 9-aminoacridine, amsacrine, ditercalinium, ethidium, camptothecin, 9-aminocamptothecin, 10,11-methylenedioxycamptothecin, etoposide (VP-16), teniposide (VM-26), and quercetin were obtained through the Developmental Therapeutics Program, National Cancer Institute. Hydroxyrubicin and adriamycinone were obtained through Waldemar Priebe (M. D. Anderson Hospital, Houston). Naphthoquinone, 5,8-dihydroxy-1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone, and dihydroxyanthraquinone were purchased from Aldrich. Chloroquine, primaquine, quinacrine, and amodiaquine were obtained through Sigma. Hydroxychloroquine was from SterlingWinthrop Research Institute. Mefloquine was from Hoffmann-La Roche.Oligonucleotide Substrate. Oligonucleotides were obtained from Midland Certified Reagent (Midland, TX), and were HPLC-purified before use. The following complementary oligonucleotides were used as substrates:AE118: 5'-GTGTGGAAAATCTCTAGCAGT-3' and AE117: 5'-ACTGCTAGAGATTTTCCACAC-3' (2). 2399The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

DNA topoisomerase I (topl) is a ubiquitous nuclear enzyme. It is specifically inhibited by camptothecin, a natural product derived from the bark of the tree Camptotheca acuminata. Camptothecin and several of its derivatives are presently in clinical trial and exhibit remarkable anticancer activity. The present study is a further investigation of the molecular interactions between the drug and the enzyme-DNA complex. We utilized an alkylating camptothecin derivative, 7-chloromethyl-10,11-methylenedioxycamptothecin (7-ClMe-MDO-CPT), and compared its activity against calf thymus topl in a DNA oligonucleotide containing a single topl cleavage site with the activity of its nonalkylating analog, 7-ethyl-10,11-methylenedioxycamptothecin (7-Et-MDO-CPT). In the presence of topl, 7-CIMe-MDO-CPT produced a DNA fragment that migrated more slowly than the toplcleaved DNA fragment observed with 7-Et-MDO-CPT. Topl was unable to religate this fragment in the presence of high NaCI concentration or proteinase K at 50°C. This fragment was resistant to piperidine treatment and was also formed with an oligonucleotide containing a 7-deazaguanine at the 5' terminus of the topl-cleaved DNA (base +1). It was however cleaved by formic acid treatment followed by piperidine. These observations are consistent with alkylation of the +1 base (adenine or guanine) by 7-CIMe-MDO-CPT in the presence of topl covalent complexes and provide direct evidence that camptothecins inhibit topl by binding at the enzyme-DNA interface.Camptothecin derivatives are presently among the most promising anticancer agents (1, 2). They kill malignant cells by trapping DNA topoisomerase I (topl) catalytic intermediates, the cleavable complexes (3-6), Cleavable complexes are enzyme-linked DNA breaks (through the 3' DNA terminus; see Fig. 2D). They provide a swivel point for DNA relaxation (7,8). Under physiological conditions, cleavable complexes are transient and topl catalyzes the religation of the 5'-hydroxyl group of the broken DNA. Camptothecin inhibits the religation step of the top! catalytic reaction (9).The molecular interactions between camptothecins and topl-DNA complexes have not been established. Lack of structure for the catalytic region of eukaryotic topl has been a major limitation. Circumstantial evidence suggests that camptothecins bind to a stereospecific site in the toplcleavable complexes (4), probably in a ternary complex with the enzyme and the DNA (10). DNA sequence analysis of the topl cleavable complexes trapped by camptothecin led to the hypothesiE that the drug interacts with the base (preferentially guanine) at the 5' DNA terminus of the topl break (position + 1-see Fig. 2D) (11). Specific interaction of camptothecin with guanine residues of DNA can also be detected by photoactivation at 365 nm (12).The present study was aimed at elucidating the interactions of camptothecin in the ternary complex. We employed a camptothecin derivative with an alkylating group at position 7 ( Fig. 1) and with a 10,11-methylenedioxy substitution that...

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