Effects of drug efflux proteins and topoisomerase I mutations on the camptothecin analogue gimatecan.

Clinically relevant resistance to the currently approved camptothecins, irinotecan and topotecan, is poorly understood but may involve increased expression of ATP-dependent drug transporters such as ABCG2 (breast cancer resistant protein, BCRP). Gimatecan (ST1481) is a lipophilic 7-substituted camptothecin derivative that exhibits potent anti-tumor activity in a variety of preclinical cancer models and is under investigation in the clinic. Previous studies reported that gimatecan cytotoxicity was not affected by expression of ABCG2. To confirm and extend this finding, we assessed the cytotoxicity of gimatecan in pairs of isogenic cell lines consisting of transfectants expressing either ABCG2 (including wild-type, R482T, or R482G mutants), ABCB1 (P-glycoprotein), ABCC1 (MRP1), ABCC2 (MRP2), or ABCC4 (MRP4). Expression of wild-type or mutant ABCG2 in human cell lines conferred resistance to topotecan but not to gimatecan. Similarly, intracellular accumulation of gimatecan was unaffected by expression of wild-type ABCG2. Furthermore, expression of P-glycoprotein or MRP2 did not alter gimatecan cytotoxicity. Whereas expression of MRP1 had a minor effect on gimatecan cytotoxicity, expression of ABCC4 was found to significantly reduce the anti-proliferative effects of this drug. Cells containing resistance-conferring mutations in topoisomerase I were also resistant to gimatecan. These results suggest that gimatecan may be more effective than irinotecan or topotecan in cancers that express ABCG2, but not in cancers that express high levels of ABCC4 or contain certain topoisomerase I (TOP1) mutations.

In vitro transport of gimatecan (7-t-butoxyiminomethylcamptothecin) by breast cancer resistance protein, P-glycoprotein, and multidrug resistance protein 2.

Lipophilic camptothecin derivatives are considered to have negligible affinity for breast cancer resistance protein (BCRP; ABCG2). Gimatecan, a new orally available 7-t-butoxyiminomethyl-substituted lipophilic camptothecin derivative, has been previously reported to be not a substrate for BCRP. Using a panel of in vitro models, we tested whether gimatecan is a substrate for BCRP as well as for P-glycoprotein (MDR1) or multidrug resistance protein 2 (MRP2; ABCC2), ATP-binding cassette drug efflux transporters involved in anticancer drug resistance, and able to affect the pharmacokinetics of substrate drugs. Cell survival, drug transport, accumulation, and efflux were studied in IGROV1 and (human BCRP overexpressing) T8 cells, Madin-Darby canine kidney II (MDCKII-WT, MDCKII-Bcrp1, MDCKII-MDR1, and MDCKII-MRP2), and LLCPK (LLCPK-WT and LLCPK-MDR1) cells. Competition with methotrexate uptake was studied in Sf9-BCRP membrane vesicles. In vitro, expression of BCRP resulted in 8- to 10-fold resistance to gimatecan. In Transwell experiments, gimatecan was transported by Bcrp1 and transport was inhibited by the BCRP/P-glycoprotein inhibitors elacridar and pantoprazole. Efflux of gimatecan from MDCKII-Bcrp1 cells was faster than in WT cells. In Sf9-BCRP membrane vesicles, gimatecan significantly inhibited BCRP-mediated transport of methotrexate. In contrast, gimatecan was not transported by MDR1 or MRP2. Gimatecan is transported by BCRP/Bcrp1 in vitro, although to a lesser extent than the camptothecin analogue topotecan. Implications of BCRP expression in the gut for the oral development of gimatecan and the interaction between gimatecan and other BCRP substrate drugs and/or inhibitors warrant further clinical investigation.

Depletion of methionine aminopeptidase 2 does not alter cell response to fumagillin or bengamides.

Inhibition of endothelial cell growth by fumagillin has been assumed to be mediated by inhibition of the molecular target methionine aminopeptidase 2 (MetAp2). New data show that depletion of MetAp2 by siRNA does not inhibit endothelial cell growth. Moreover, MetAp2-depleted endothelial cells remain responsive to inhibition by either fumagillin or a newly identified MetAp2 enzyme inhibitor. These data suggest that MetAp2 function is not required for endothelial cell proliferation.

Selective growth inhibition of tumor cells by a novel histone deacetylase inhibitor, NVP-LAQ824.

We have synthesized a histone deacetylase inhibitor, NVP-LAQ824, a cinnamic hydroxamic acid, that inhibited in vitro enzymatic activities and transcriptionally activated the p21 promoter in reporter gene assays. NVP-LAQ824 selectively inhibited growth of cancer cell lines at submicromolar levels after 48-72 h of exposure, whereas higher concentrations and longer exposure times were required to retard the growth of normal dermal human fibroblasts. Flow cytometry studies revealed that both tumor and normal cells arrested in the G(2)-M phase of the cell cycle after compound treatment. However, an increased sub-G(1) population at 48 h (reminiscent of apoptotic cells) was observed only in the cancer cell line. Annexin V staining data supported our hypothesis that NVP-LAQ824 induced apoptosis in tumor and transformed cells but not in normal cells. Western blotting experiments showed an increased histone H3 and H4 acetylation level in NVP-LAQ824-treated cancer cells, suggesting that the likely in vivo target of NVP-LAQ824 was histone deacetylase(s). Finally, NVP-LAQ824 exhibited antitumor effects in a xenograft animal model. Together, our data indicated that the activity of NVP-LAQ824 was consistent with its intended mechanism of action. This novel histone deacetylase inhibitor is currently in clinical trials as an anticancer agent.

Proteomics-based target identification: bengamides as a new class of methionine aminopeptidase inhibitors.

LAF389 is a synthetic analogue of bengamides, a class of marine natural products that produce inhibitory effects on tumor growth in vitro and in vivo. A proteomics-based approach has been used to identify signaling pathways affected by bengamides. LAF389 treatment of cells resulted in altered mobility of a subset of proteins on two-dimensional gel electrophoresis. Detailed analysis of one of the proteins, 14-3-3gamma, showed that bengamide treatment resulted in retention of the amino-terminal methionine, suggesting that bengamides directly or indirectly inhibited methionine aminopeptidases (MetAps). Both known MetAps are inhibited by LAF389. Short interfering RNA suppression of MetAp2 also altered amino-terminal processing of 14-3-3gamma. A high resolution structure of human MetAp2 co-crystallized with a bengamide shows that the compound binds in a manner that mimics peptide substrates. Additionally, the structure reveals that three key hydroxyl groups on the inhibitor coordinate the di-cobalt center in the enzyme active site.

N-hydroxy-3-phenyl-2-propenamides as novel inhibitors of human histone deacetylase with in vivo antitumor activity: discovery of (2E)-N-hydroxy-3-[4-[[(2-hydroxyethyl)[2-(1H-indol-3-yl)ethyl]amino]methyl]phenyl]-2-propenamide (NVP-LAQ824).

A series of N-hydroxy-3-phenyl-2-propenamides were prepared as novel inhibitors of human histone deacetylase (HDAC). These compounds were potent enzyme inhibitors, having IC(50)s < 400 nM in a partially purified enzyme assay. However, potency in cell growth inhibition assays ranged over 2 orders of magnitude in two human carcinoma cell lines. Selected compounds having cellular IC(50) < 750 nM were tested for maximum tolerated dose (MTD) and for efficacy in the HCT116 human colon tumor xenograft assay. Four compounds having an MTD > or = 100 mg/kg were selected for dose-response studies in the HCT116 xenograft model. One compound, 9 (NVP-LAQ824), had significant dose-related activity in the HCT116 colon and A549 lung tumor models, high MTD, and low gross toxicity. On the basis, in part, of these properties, 9 has entered human clinical trials in 2002.