A novel polypyrimidine antitumor agent FdUMP[10] induces thymineless death with topoisomerase I-DNA complexes.

FdUMP[10], a 10mer of 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP), the thymidylate synthase inhibitory metabolite of 5-fluorouracil (FU), is most closely correlated with the DNA topoisomerase I (Top1) inhibitor camptothecin in the National Cancer Institute COMPARE analysis, but not with FU. FdUMP[10] exhibits more potent antiproliferative activity than FdUMP or 5-fluoro-2'-deoxyuridine (FdU) and is markedly more active than FU. Camptothecin-resistant P388/CPT45 cells lacking Top1 are cross-resistant to FdUMP[10] as well as to FdUMP, FdU, and the thymidylate synthase inhibitor raltitrexed (Tomudex). FdUMP[10] induces DNA single-strand breaks and cellular Top1-DNA complexes. Such complexes are also observed in response to FdUMP, FdU, raltitrexed, and FU. The FdUMP[10]-induced Top1-DNA complexes are not inhibited by the caspase inhibitor z-VAD-fmk and form independently of apoptotic DNA fragmentation, indicating that they do not correspond to apoptotic Top1-DNA complexes. In biochemical assay, Top1 is directly trapped at uracil and FdU misincorporation sites. We propose that FdUMP[10] damages DNA by trapping Top1 at uracil and FdU misincorporation sites resulting from thymidylate synthase inhibition and thymine depletion.

DNA-protein cross-links and replication-dependent histone H2AX phosphorylation induced by aminoflavone (NSC 686288), a novel anticancer agent active against human breast cancer cells.

Aminoflavone (5-amino-2,3-fluorophenyl)-6,8-difluoro-7-methyl-4H-1-benzopyran-4-one) (NSC 686288) is a candidate for possible advancement to phase I clinical trial. Aminoflavone has a unique activity profile in the NCI 60 cell lines (COMPARE analysis; http://www.dtp.nci.nih.gov/docs/dtp_search.html), and exhibits potent cellular and animal antitumor activity. To elucidate the mechanism of action of aminoflavone, we studied DNA damage in MCF-7 cells. Aminoflavone induced DNA-protein cross-links (DPC) and DNA single-strand breaks (SSB). Aminoflavone induced high levels of DPC and much lower level of SSB than camptothecin, which induces equal levels of DPC and SSB due to the trapping topoisomerase I-DNA complexes. Accordingly, neither topoisomerase I nor topoisomerase II were detectable in the aminoflavone-induced DPC. Aminoflavone also induced dose- and time-dependent histone H2AX phosphorylation (gamma-H2AX). Gamma-H2AX foci occurred with DPC formation, and like DPC, persisted after aminoflavone removal. Aphidicolin prevented gamma-H2AX formation, suggesting that gamma-H2AX foci correspond to replication-associated DNA double-strand breaks. Accordingly, no gamma-H2AX foci were found in proliferating cell nuclear antigen-negative or in mitotic cells. Bromodeoxyuridine incorporation and fluorescence-activated cell sorting analyses showed DNA synthesis inhibition uniformly throughout the S phase after exposure to aminoflavone. Aminoflavone also induced RPA2 and p53 phosphorylation, and induced p21(Waf1/Cip1) and MDM2, demonstrating S-phase checkpoint activation. These studies suggest that aminoflavone produces replication-dependent DNA lesions and S-phase checkpoint activation following DPC formation. Gamma-H2AX may be a useful clinical marker for monitoring the efficacy of aminoflavone in tumor therapies.

A systematic study of nitrated indenoisoquinolines reveals a potent topoisomerase I inhibitor.

The biological activity of indenoisoquinoline topoisomerase I inhibitors is significantly enhanced by nitration of the isoquinoline ring. In the present study, nitrated analogues were synthesized with the indenone ring substituted with methoxy groups to further explore a previously identified structure-activity relationship between the nitrated isoquinoline ring and a methylenedioxy-substituted indenone ring. The results indicate that a single methoxy group at the 9-position of an indenoisoquinoline affords superior biological activity. Hypothetical binding models have been developed to rationalize these results, and they indicate that pi-stacking between the indenoisoquinolines and the DNA base pairs, as visualized by electrostatic complementarity, is important for the intercalation and biological activity of the indenoisoquinoline analogues. Collectively, the analysis of methoxy groups on the indenone ring also illustrates a strict steric requirement for substituents extending toward the nonscissile DNA backbone and emphasizes a need for planarity to afford potent biological activity.

Synthesis and evaluation of indenoisoquinoline topoisomerase I inhibitors substituted with nitrogen heterocycles.

In connection with an ongoing investigation of indenoisoquinoline topoisomerase I (Top1) inhibitors as potential therapeutic agents, the pharmacophore possessing di(methoxy) and methylenedioxy substituents was held constant, and new derivatives were synthesized with nitrogen heterocycles appended to the lactam side chain. Compounds were evaluated for Top1 inhibition and for cytotoxicity in the National Cancer Institute's human cancer cell screen. Some of the more potent derivatives were also screened for in vivo activity in a hollow fiber assay. The results of these studies indicate that lactam substituents possessing nitrogen heterocycles can provide highly cytotoxic compounds with potent Top1 inhibition. Molecular modeling of these compounds in complex with DNA and Top1 suggests that some of the lactam substituents are capable of interacting with the DNA base pairs above and below the site of intercalation and/or with Top1 amino acid residues, resulting in increased biological activity.

Synthesis and biological evaluation of bisindenoisoquinolines as topoisomerase I inhibitors.

The indenoisoquinolines represent a class of non-camptothecin topoisomerase I (Top1) inhibitors that exert cytotoxicity by trapping the covalent complex formed between DNA and Top1 during relaxation of DNA supercoils. As an ongoing evaluation of Top1 inhibition and anticancer activity, indenoisoquinolines were linked via their lactam side chains to provide polyamines end-capped with intercalating motifs. The resulting bisindenoisoquinolines were evaluated for cytotoxicity in the National Cancer Institute's human cancer cell screen and for Top1 inhibition. Preliminary findings suggested that the 2-3-2 and 3-3-3 linkers, referring to the number of carbons between nitrogen atoms, were optimal for both potent Top1 inhibition and cytotoxicity. Using optimized linkers, bisindenoisoquinolines were synthesized with nitro and methoxy substituents on the aromatic rings. The biological results for substituted compounds revealed a disagreement between the structure-activity relationships of monomeric indenoisoquinolines and bisindenoisoquinolines as Top1 inhibitors, but cytotoxicity was maintained for both series of compounds.

Camptothecin analogs with enhanced activity against human breast cancer cells. I. Correlation of potency with lipophilicity and persistence in the cleavage complex.

The effect of 7-alkyl substitutions on growth inhibition in seven Camptothecin (CPT) ring systems with various groups at the ten position was evaluated in three human breast cancer cell lines that model (1) hormone-sensitive (MCF-7/wt), (2) hormone insensitive (MDA-MB-231), or (3) alkylator-resistant (MCF-7/4-hc) forms of disease. To assess the impact of persistence of cleavage complexes on antiproliferative activity, a post-exposure recovery period in drug-free medium was incorporated into the growth inhibition assay. This modification produced on average a twofold reduction in the growth inhibition endpoint (the IC50), suggesting a greater apoptotic response. The results further revealed a three log range in potency from a mean IC50 of 2 nM (7-butyl-10,11-methylenedioxy-CPT) to 2.5 microM (7-bromomethyl-10-hydryoxy-CPT). Increasing 7-alkyl chain length in six of the ten-substituted CPTs enhanced potency, which was directly correlated with persistence of topoisomerase I-induced DNA cleavage complexes in 10-hydroxy, 10-methoxy, and 10,11-methylenedioxy substituted CPTs. Modeling of the binding mode of 7-butyl-10-amino-CPT revealed a direct hydrogen bond contact for the 10-amino to the side chain of Glu-356 of Core Subdomain I of top1 in addition to known contacts found for other camptothecins. More important, residues 350-356 and 425-431 of Core Subdomain I may provide induced fit stabilization to the lipophilic alkyl moiety at the seven position.

Anti-tumor drug candidate 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole induces single-strand breaks and DNA-protein cross-links in sensitive MCF-7 breast cancer cells.

PURPOSE: The fluorinated benzothiazole analogue 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203, NSC 703786) exhibits selective and potent anticancer activity, and its lysylamide prodrug (Phortress, NSC 710305) recently entered Phase I clinical trials in the United Kingdom. Only cancer cells sensitive to the anti-proliferative effects of 5F 203 deplete this drug candidate from nutrient media. 5F 203 induces cell cycle arrest, cytochrome P450 1A1 (CYP 1A1) mRNA and protein expression, and is metabolized into reactive electrophilic species that can covalently bind to DNA and form adducts in sensitive (i.e., MCF-7) but not in resistant (i.e., MDA-MB-435) breast cancer cells. METHODS: In this present study, we investigated additional anticancer effects of 5F 203 in MCF-7 cells. In addition, we sought to determine if cells deficient in the xeroderma pigmentosum D gene, a gene critical in DNA repair, would show greater sensitivity to the cytotoxic effects of 5F 203 than those complemented with XPD. RESULTS: Alkaline Elution revealed that 5F 203 induced single-strand breaks and DNA-protein cross-links in sensitive MCF-7 cells. In contrast, we detected no double-strand breaks or protein-associated strand breaks typically associated with topoisomerase I (top1) or topoisomerase II (top2) inhibition. In addition, 5F 203 was unable to trap top1- or top2-DNA cleavage complexes in MCF-7 cells. 5F 203 induced cell cycle arrest in MCF-7 cells following DNA damage after brief exposures. Cells deficient in the nucleotide excision repair xeroderma pigmentosum group D (XPD) gene displayed sensitivity to 5F 203 while cells complemented with XPD displayed resistance to 5F 203. CONCLUSION: These data suggest that the anti-cancer activity of 5F 203 depends upon targets other than top1 or top2 and on the ability of this benzothiazole to form single-strand breaks and DNA-protein cross-links in cancer cells.

Processing of nucleopeptides mimicking the topoisomerase I-DNA covalent complex by tyrosyl-DNA phosphodiesterase.

Tyrosyl-DNA phosphodiesterase-1 (Tdp1) is the only known enzyme to remove tyrosine from complexes in which the amino acid is linked to the 3'-end of DNA fragments. Such complexes can be produced following DNA processing by topoisomerase I, and recent studies in yeast have demonstrated the importance of TDP1 for cell survival following topoisomerase I-mediated DNA damage. In the present study, we used synthetic oligodeoxynucleotide-peptide conjugates (nucleopeptides) and recombinant yeast Tdp1 to investigate the molecular determinants for Tdp1 activity. We find that Tdp1 can process nucleopeptides with up to 13 amino acid residues but is poorly active with a 70 kDa fragment of topoisomerase I covalently linked to a suicide DNA substrate. Furthermore, Tdp1 was more effective with nucleopeptides with one to four amino acids than 15 amino acids. Tdp1 was also more effective with nucleopeptides containing 15 nt than with homolog nucleopeptides containing 4 nt. These results suggest that DNA binding contributes to the activity of Tdp1 and that Tdp1 would be most effective after topoisomerase I has been proteolyzed in vivo.

Altered serine/arginine-rich protein phosphorylation and exonic enhancer-dependent splicing in Mammalian cells lacking topoisomerase I.

DNA topoisomerase I (Topo I) specifically phosphorylates arginine-serine-rich (SR proteins) splicing factors and is potentially involved in pre-mRNA-splicing regulation. Using a Topo I-deficient murine B lymphoma-derived subclone (P388-45/C) selected for its resistance to high dosage of the antitumor drug camptothecin, we show that Topo I depletion results in the hypophosphorylation of SR proteins and impairs exonic splicing enhancer (ESE)-dependent but not constitutive splicing. The Affymetrix GeneChip system analysis revealed that several alternatively spliced genes, characterized by small exons and large introns, are down-regulated in Topo I-deficient cells. Given that ectopic expression of green fluorescent protein-Topo I fusion in Topo I-deficient cells restores both wild-type phosphorylation of SR proteins and ESE-dependent splicing, we conclude that Topo I-mediated phosphorylation plays a specific role in ESE-regulated splicing.

Inhibition of histone deacetylase increases cytotoxicity to anticancer drugs targeting DNA.

Several anticancer drugs target DNA or enzymes acting on the DNA. Because chromatin DNA is tightly compacted, accessibility to the drug target may reduce the efficiency of these anticancer drugs. We thus treated four human cancer cell lines and two normal epithelial cell lines with either trichostatin A (TSA) or SAHA, two histone deacetylase inhibitors, before exposing the cells to VP-16, ellipticine, camptothecin, doxorubicin, cisplatin, 5-fluorouracil, or cyclophosmamide. Pretreatment with TSA or SAHA increased the killing efficiency of VP-16, ellipticine, doxorubicin, and cisplatin. The magnitude of sensitization is cell type specific and is >10-fold for VP-16 in D54, a brain tumor cell line intrinsically resistant to topoisomerase II inhibitors. Topoisomerase II levels and activity were not affected by this treatment, but p53, p21, and Gadd45 protein levels were markedly induced. Moreover, pretreatment with TSA also increased VP-16-induced apoptosis in a p53-dependent and -independent manner. Treating the cells in the reverse order (anticancer drug first, followed by TSA or SAHA) had no more cytotoxic effect than the drug alone. These data suggest that loosening-up the chromatin structure by histone acetylation can increase the efficiency of several anticancer drugs targeting DNA. This may be advantageous for treating tumors intrinsically resistant to these drugs.

Differential induction of topoisomerase I-DNA cleavage complexes by the indenoisoquinoline MJ-III-65 (NSC 706744) and camptothecin: base sequence analysis and activity against camptothecin-resistant topoisomerases I.

Camptothecin (CPT) and its derivatives target mammalian DNA topoisomerase I (top1) and are among the most effective novel anticancer drugs. However, the activity of CPTs is limited by several factors, including drug inactivation by lactone ring opening, tumor drug resistance, and toxicity in patients. Novel top1 inhibitors are being searched to overcome such limitations and expand the anticancer spectrum of camptothecins. MJ-III-65 (NSC 706744) is among the most promising indenoisoquinolines to date. In this study, we show that MJ-III-65 enhances top1 cleavage complexes by both inhibiting their reversal (religation) more efficiently than CPT and by enhancing their formation. The top1 DNA cleavage complexes induced by MJ-III-65 exhibit a different distribution pattern compared with CPT and exhibit different base sequence preferences immediately around the top1 cleavage sites. Although CPTs have a preference for thymine at the (-1) position and guanine at the (+1) position of the top1-mediated DNA cleavage sites, MJ-III-65 can accommodate different base pairs at the (-1), (+1), or (+2) position, with a preference for a cytosine at the (-1) position on the scissile strand. Another difference with CPTs is the activity of MJ-III-65 against CPT-resistant top1 enzymes, implying that the amino acid residue interactions with top1 are different for MJ-III-65 and CPTs. As with CPT, MJ-III-65 is inactive against vaccinia top1. This study shows the specific molecular interactions of MJ-III-65 with top1 and demonstrates that MJ-III-65 is a potentially useful top1 inhibitor that enhances and traps top1 cleavage sites not sensitive to CPTs.

Association of XRCC1 and tyrosyl DNA phosphodiesterase (Tdp1) for the repair of topoisomerase I-mediated DNA lesions.

DNA topoisomerase I (Top1) is converted into a cellular poison by camptothecin (CPT) and various endogenous and exogenous DNA lesions. In this study, we used X-ray repair complementation group 1 (XRCC1)-deficient and XRCC1-complemented EM9 cells to investigate the mechanism by which XRCC1 affects the cellular responses to Top1 cleavage complexes induced by CPT. XRCC1 complementation enhanced survival to CPT-induced DNA lesions produced independently of DNA replication. CPT-induced comparable levels of Top1 cleavage complexes (single-strand break (SSB) and DNA-protein cross-links (DPC)) in both XRCC1-deficient and XRCC1-complemented cells. However, XRCC1-complemented cells repaired Top1-induced DNA breaks faster than XRCC1-deficient cells, and exhibited enhanced tyrosyl DNA phosphodiesterase (Tdp1) and polynucleotide kinase phosphatase (PNKP) activities. XRCC1 immunoprecipitates contained Tdp1 polypeptide, and both Tdp1 and PNKP activities, indicating a functional connection between the XRCC1 single-strand break repair pathway and the repair of Top1 covalent complexes by Tdp1 and PNKP.

Gemcitabine (2',2'-difluoro-2'-deoxycytidine), an antimetabolite that poisons topoisomerase I.

PURPOSE: Gemcitabine-containing regimens are among standard therapies for the treatment of advanced non-small cell lung,pancreatic, or bladder cancers. Gemcitabine is a nucleoside analogue and its cytotoxicity is correlated with incorporation into genomic DNA and concomitant inhibition of DNA synthesis. However, it is still unclear by which mechanism(s) gemcitabine incorporation leads to cell death. EXPERIMENTAL DESIGN: We used purified oligodeoxynucleotides to study the effects of gemcitabine incorporation on topoisomerase I (top1) activity and tested the role of top1 poisoning in gemcitabine-induced cytotoxicity in cancer cells. RESULTS: We found that top1-mediated DNA cleavage was enhanced when gemcitabine was incorporated immediately 3' from a top1 cleavage site on the nonscissile strand. This position-specific enhancement was attributable to an increased DNA cleavage by top1 and was likely to have resulted from a combination of gemcitabine-induced conformational and electrostatic effects. Gemcitabine also enhanced camptothecin-induced cleavage complexes. We also detected top1 cleavage complexes in human leukemia CEM cells treated with gemcitabine and a 5-fold resistance of P388/CPT45 top1-deficient cells to gemcitabine, indicating that poisoning of top1 can contribute to the antitumor activity of gemcitabine. CONCLUSIONS: The present results extend our recent finding that incorporation of 1-beta-D-arabinofuranosylcytosine into DNA can induce top1 cleavage complexes [P. Pourquier et al. Proc. Natl. Acad. Sci. USA, 97: 1885-1890, 2000]. The enhancement of camptothecin-induced top1 cleavage complexes may, at least in part, contribute to the synergistic or additive effects of gemcitabine in combination with topotecan and irinotecan in human breast or lung cancer cells.

In vitro evaluation of dimethane sulfonate analogues with potential alkylating activity and selective renal cell carcinoma cytotoxicity.

We identified five structurally related dimethane sulfonates with putative selective cytotoxicity in renal cancer cell lines. These compounds have a hydrophobic moiety linked to a predicted alkylating group. A COMPARE analysis with the National Cancer Institute Anticancer Drug Screen standard agent database found significant correlations between the IC50 of the test compounds and the IC50 of alkylating agents (e.g., r = 0.68, P < 0.00001 for chlorambucil). In this report, we examined whether these compounds had activities similar to those of conventional alkylating agents. In cytotoxicity studies, chlorambucil-resistant Walker rat carcinoma cells were 4- to 11-fold cross-resistant to the test compounds compared with 14-fold resistant to chlorambucil. To determine effects on cell cycle progression, renal cell carcinoma (RCC) line 109 was labeled with bromodeoxyuridine prior to drug treatment. Complete cell cycle arrest occurred in cells treated with an IC90 dose of NSC 268965. p53 protein levels increased as much as 5.7-fold in RCC line 109 and as much as 20.4-fold in breast cancer line MCF-7 following an 18-hour drug exposure. Finally, DNA-protein cross-links were found following a 6-hour pretreatment with all compounds. Thus, the dimethane sulfonate analogues have properties expected of some alkylating agents but, unlike conventional alkylating agents, appear to possess activity against RCC.

Characterization of BTBD1 and BTBD2, two similar BTB-domain-containing Kelch-like proteins that interact with Topoisomerase I.

BACKGROUND: Two-hybrid screening for proteins that interact with the core domain of human topoisomerase I identified two novel proteins, BTBD1 and BTBD2, which share 80% amino acid identities. RESULTS: The interactions were confirmed by co-precipitation assays demonstrating the physical interaction of BTBD1 and BTBD2 with 100 kDa topoisomerase I from HeLa cells. Deletion mapping using two-hybrid and GST-pulldown assays demonstrated that less than the C-terminal half of BTBD1 is sufficient for binding topoisomerase I. The topoisomerase I sequences sufficient to bind BTBD2 were mapped to residues 215 to 329. BTBD2 with an epitope tag localized to cytoplasmic bodies. Using truncated versions that direct BTBD2 and TOP1 to the same cellular compartment, either the nucleus or the cytoplasm, co-localization was demonstrated in co-transfected Hela cells. The supercoil relaxation and DNA cleavage activities of topoisomerase I in vitro were affected little or none by co-incubation with BTBD2. Northern analysis revealed only a single sized mRNA for each BTBD1 and BTBD2 in all human tissues tested. Characterization of BTBD2 mRNA revealed a 255 nucleotide 90% GC-rich region predicted to encode the N-terminus. BTBD1 and BTBD2 are widely if not ubiquitously expressed in human tissues, and have two paralogs as well as putative orthologs in C. elegans and D. melanogaster. CONCLUSIONS: BTBD1 and BTBD2 belong to a small family of uncharacterized proteins that appear to be specific to animals. Epitope-tagged BTBD2 localized to cytoplasmic bodies. The characterization of BTBD1 and BTBD2 and their interaction with TOP1 is underway.

Design, synthesis, and biological evaluation of indenoisoquinoline topoisomerase I inhibitors featuring polyamine side chains on the lactam nitrogen.

The indenoisoquinolines are a class of noncamptothecin topoisomerase I inhibitors that display significant cytotoxicity in human cancer cell cultures. They offer a number of potential advantages over the camptothecins, including greater chemical stability, formation of more persistent cleavage complexes, and induction of a unique pattern of DNA cleavage sites. Molecular modeling has suggested that substituents on the indenoisoquinoline lactam nitrogen would protrude out of the DNA duplex in the ternary cleavage complex through the major groove. This indicates that relatively large substituents in that location would be tolerated without compromising biological activity. As a strategy for increasing the potencies and potential therapeutic usefulness of the indenoisoquinolines, a series of compounds was synthesized containing polyamine side chains on the lactam nitrogen. The rationale for the synthesis of these compounds was that the positively charged ammonium cations would increase DNA affinity through electrostatic binding to the negatively charged DNA backbone, and the polyamines might also facilitate cellular uptake by utilization of polyamine transporters. The key step in the synthesis involved the condensation of Schiff bases, containing protected amine side chains, with substituted homophthalic anhydrides, to afford cis-3-aryl-4-carboxy-1-isoquinolones. These isoquinolones were then converted to indenoisoquinolines with thionyl chloride. Although monoamines were much more potent than the lead compound, no significant increase in potency was observed through incorporation of additional amino groups in the side chain. However, one of the monoamine analogues, which features a bis(2-hydroxyethyl)amino group in the side chain, proved to be one of the most cytotoxic indenoisoquinoline synthesized to date, with a GI50 mean-graph midpoint (MGM) of 0.07 microM in the NIH human cancer cell culture screen, and topoisomerase I inhibitory activity comparable to that of camptothecin.

Synthesis of new dihydroindeno[1,2-c]isoquinoline and indenoisoquinolinium chloride topoisomerase I inhibitors having high in vivo anticancer activity in the hollow fiber animal model.

A number of novel dihydroindenoisoquinolines and indenoisoquinolinium salts were synthesized and examined for cytotoxicity in cancer cell cultures and for inhibition of topoisomerase I (top1). The top1-mediated DNA cleavage patterns produced in the presence of several of the new analogues were also investigated, and a few of the more potent compounds were examined for activity in hollow fiber animal models. Very cytotoxic dihydroindenoisoquinoline and isoquinolinium salts were obtained with mean graph midpoints (MGMs) for growth inhibition in the low submicromolar range. Two of the new dihydroindenoisoquinolines were found to be weaker top1 inhibitors than the lead compound 1, while two of the indenoisoquinolinium salts were more potent. The top1-mediated DNA cleavage patterns of the indenoisoquinolines examined were found to be similar to each other but different from that of camptothecin. Several of the more potent indenoisoquinolines displayed promising anticancer activities in hollow fiber animal models.

Synthesis and anticancer activity of simplified indenoisoquinoline topoisomerase I inhibitors lacking substituents on the aromatic rings.

The indenoisoquinolines are a class of cytotoxic topoisomerase I inhibitors that offer certain advantages over the camptothecins, including the greater stabilities of the compounds themselves, as well as the greater stabilities of their drug-enzyme-DNA cleavage complexes. To investigate the possible biological roles of the di(methoxy) and methylenedioxy substituents present on the aromatic rings of the previously synthesized indenoisoquinoline topoisomerase I inhibitors, a series of compounds lacking these substituents was synthesized and tested for both cytotoxicity in cancer cell cultures and for enzyme inhibitory activity. The results indicate that the aromatic substituents make a small, but consistently observable contribution to the biological activity. Molecular models derived for the binding of the unsubstituted indenoisoquinolines in ternary complex with DNA and topoisomerase I indicate that the substituents on the lactam nitrogen project out of the major groove, and the carbonyl group is directed out of the minor groove, where it is involved in a hydrogen bonding interaction with the side chain guanidine group of Arg364. The DNA cleavage patterns observed in the presence of topoisomerase I and various indenoisoquinolines were similar, although significant differences were detected. There were also variations in the DNA cleavage pattern seen with camptothecin vs the indenoisoquinolines, which indicates that these two classes of topoisomerase I inhibitors are likely to target the cancer cell genome differently, resulting in different spectra of anticancer activity. The most cytotoxic of the presently synthesized indenoisoquinolines has a 4-amino-n-butyl group on the lactam nitrogen.

Design, synthesis, and biological evaluation of cytotoxic 11-alkenylindenoisoquinoline topoisomerase I inhibitors and indenoisoquinoline-camptothecin hybrids.

The indenoisoquinolines are a novel class of topoisomerase I (top1) inhibitors that are cytotoxic in cancer cell cultures and are therefore under development as potential anticancer agents. As inhibitors of the DNA religation reaction occurring after DNA cleavage by the enzyme, they are classified as top1 poisons, similar to the camptothecins. Two strategies were employed in order to further develop the structure-activity relationships of the indenoisoquinolines and enhance their therapeutic potential. The first strategy involved the synthesis of indenoisoquinoline-camptothecin hybrid molecules to take advantage of a proposed structural analogy between the indenoisoquinolines and camptothecin. The desired hybrids were synthesized by reaction of halogenated phthalides with a dihydropyrroloquinoline. The second strategy involved the attachment of various alkenyl substituents to the C-11 position of the indenoisoquinolines, which were assumed to project into the DNA minor groove. The required C-11-substituted indenoisoquinolines were synthesized by McMurry reactions of 11-ketoindenoisoquinolines with aldehydes, and the geometries of the resulting alkenes were established by nuclear Overhauser effect difference NMR spectroscopy. All of the new indenoisoquinolines were examined for cytotoxicity in human cancer cell cultures as well as for activity vs top1. Although the indenoisoquinoline-camptothecin hybrid molecules proved to be less cytotoxic and displayed less activity against top1, an analogue incorporating a 3'-aminoalkenyl substituent at the C-11 position of the indenoisoquinoline system was significantly more potent than the prototype indenoisoquinoline in both assays. These results indicate that C-11 aminoalkyl substituents that are assumed to project into the minor groove enhance the cytotoxicity and top1 inhibitory activity of the parent indenoisoquinoline system.

Cytotoxicity, DNA strand breakage and DNA-protein crosslinking by a novel transplatinum compound in human A2780 ovarian and MCF-7 breast carcinoma cells.

Cisplatin, cis-[PtCl2(NH3)2], is commonly utilized in various combination chemotherapy protocols for the treatment of both ovarian and breast cancer while the corresponding trans isomer is therapeutically inactive. This work describes efforts to elucidate the cellular mechanism of action of a novel trans-platinum compound, trans-(dichloroamminethiazole)platinum(II) (ATZ), which demonstrates antiproliferative and cytotoxic effects against both MCF-7 human breast and A2780 human ovarian carcinoma cells in culture. A2780 cells were approximately twofold more sensitive to ATZ than MCF-7 cells in both cell growth and clonogenic survival assays. Dye exclusion studies revealed a 50-70% loss in cell viability within the first 12 h of drug treatment in both cell lines. This initial wave of cell death was succeeded by a prolonged interval of growth arrest during which a small fraction of apoptotic cells was detected. Binding of ATZ to DNA, as estimated by atomic absorption spectroscopy, was similar for the two cell lines and was almost completely reversed 24 h after drug removal. ATZ also induced DNA strand breakage as well as DNA-protein crosslinking during the initial 12 h period when the bulk of cell death was evident. However, neither the extent of DNA strand breakage nor that of DNA protein crosslinking was sufficient to explain the different drug sensitivity in the two cell lines. At 24 and 48 h after exposure of MCF-7 cells to high concentrations of ATZ, the formation of DNA-topoisomerase I complexes is detected, coincident with a high degree of apoptosis. These studies suggest that ATZ has the capacity to interfere with topoisomerase I in the tumor cell, a function not evident in cis-platinum-based drugs.