Abstract C217: Genz‐644282, a novel non‐camptothecin topoisomerase I inhibitor for cancer treatment

Topoisomerase I (Top1), a well‐established anticancer target, is a nuclear enzyme producing a single‐strand DNA break to allow transcription. Camptothecin Top1 inhibitors are important drugs for the treatment of a variety of cancers including colon cancer, leukemias and ovarian cancer. Potent non‐camptothecin Top1 inhibitors were identified. One of these, Genz‐644282, binds tightly and selectively to the DNA:Top1 complex. The effects of Genz‐644282, topotecan and irinotecan/SN38 in cell‐based assays were compared. The colony formation assay is the gold‐standard for determination of the cytotoxicity of compounds toward bone marrow and malignant cells as it measures the survival/killing of cells capable of self‐renewal. Mouse and human bone marrow were subjected to colony forming unit ‐granulocyte/macrophage (CFU‐GM) assays over a 5‐log concentration range with continuous exposure to the compounds for 13–15 days. The IC 90 s for topotecan and SN38 for mouse CFU‐GM were 519 nM and 331 nM, respectively. For human CFU‐GM, the IC 90 s for topotecan and SN38 were 19 nM and 26 nM, resulting in mouse to human differentials of 28‐ and 13‐fold. Genz‐644282 was more potent, with IC 90 s of 8.4nM for mouse CFU‐GM and 1.2 nM for human CFU‐GM; thus only a 7‐fold differential between species. The cytotoxicities of topotecan, SN38 and Genz‐644282 were compared in the ability of seven human tumor cell lines of varied histologies to form colonies after a week of exposure to a 5‐log concentration range of each compound. Human bone marrow CFU‐GM was much more sensitive to topotecan than were RPMI‐8226 multiple myeloma, KBV‐1 cervical carcinoma or NCI‐H460 non‐small cell lung carcinoma cells. The IC 90 s for topotecan ranged between 11 and 257 nM. SN38 cytotoxicity to human bone marrow CFU‐GM and to several human tumor cell lines was very similar with IC 90 s in the cancer lines ranging between 2 and 55 nM. HT29 human colon cancer cells, however, were markedly sensitive to SN38 (IC 90 = 2 nM). Genz‐644282 was a more potent cytotoxic agent than topotecan or SN38 with IC 90 values ranging between 0.7 and 9.1 nM across the seven cell lines. MDA‐MB‐231 human breast carcinoma, HT29 colon carcinoma and RPMI‐8226 cells were most sensitive to Genz‐644282 with IC 90 s ranging between 0.7 and 7.0 nM. The growth inhibition produced by 72‐hr exposure to Genz‐644282 was examined in 26 established human tumor cell lines representing pancreatic carcinoma, melanoma, non‐small cell lung cancer, breast carcinoma, ovarian carcinoma and renal cell carcinoma using a Cell TiterGlo ATP content endpoint. The IC 90 s for Genz‐644282 ranged between 0.054 micromolar and 5 micromolar in this assay. The most sensitive tumor lines were 4 of 6 pancreatic cancer lines and the least sensitive lines included 3 of 4 melanoma lines. Colony formation data were analyzed by non‐linear regression using SAS version 8.2. Based upon these findings and other data, Genz‐644282 was selected as a development candidate. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C217.