BNP1350, 7‐[(2‐trimethylsilyl)ethyl]‐20(S)‐camptothecin, is a novel semi‐synthetic, highly lipophilic, silicon‐containing camptothecin and an inhibitor of topoisomerase I. It has been supercomputer engineered for superior oral bioavailability, superior lactone stability, broad anti‐tumor activity, increased potency and insensitivity to Pgp/MRP/LRP drug resistance. We determined the efficacy of BNP1350 in experimental human colon cancer and compared its anti‐tumor effects with those of CPT‐11/SN‐38. We also determined a possible influence of Pgp, MRP and LRP on the efficacy of BNP1350. The in vitro anti‐proliferative capacity of the compounds using various exposure times was assessed in five colon cancer cell lines and indicated that BNP1350 was similarly effective or slightly more potent than SN‐38. Four cell lines of other origin with sublines expressing Pgp, MRP and/or LRP showed that BNP1350 was significantly more effective than SN‐38 (p < 0.05) and that the activity of BNP1350 was not reduced in multidrug‐resistant cells. For in vivo experiments, BNP1350 was given 1.0 mg/kg i.p. or 1.5 mg/kg p.o. daily × 5 and CPT‐11 20 mg/kg i.p. daily × 5 being equitoxic schedules in nude mice bearing s.c. human tumor xenografts. The schedules were studied in colon cancer xenografts COLO320, COLO205 or WiDr as well as in two Pgp‐positive xenografts 2780AD and BRO/mdr1.1 and the parental Pgp‐negative A2780 ovarian cancer xenografts and BRO melanoma xenografts. Growth inhibition of >50% was obtained for BNP1350 given i.p. in six out of the seven xenografts studied. BNP1350 was similarly effective when given i.p. or p.o. CPT‐11 was as effective as BNP1350, except in BRO and BRO/mdr1.1 xenografts. Pgp expression in xenografts in vivo confirmed that there was no negative influence on the efficacy of BNP1350. In conclusion, BNP1350 shows a broad spectrum of activity in experimental human tumors and is a suitable candidate for oral treatment of cancer. Int. J. Cancer 88:260–266, 2000. © 2000 Wiley‐Liss, Inc.

The novel camptothecin derivative BNP1350 (7-[2-trimethylsilyl)ethyl]-20(S)-camptothecin), also known as Karenitecin, has been developed for superior oral bioavailability and increased lactone stability. In our study, we describe the antiproliferative effects of BNP1350, SN-38 and topotecan in 4 human ovarian cancer cell lines. BNP1350 was found to be slightly more potent than SN-38 (p<0.01) and was considerably more potent than topotecan (p<0.01). We extended these studies to well-established human ovarian cancer xenografts in which we compared the growth inhibition induced by BNP1350 with that of topotecan given in equitoxic schedules. The growth inhibition in all 3 xenografts induced by BNP1350 was >75%, which was significantly better than that resulting from topotecan (p<0.05). We then selected BNP1350-resistant variants of the A2780 human ovarian cancer cell line, 2780K4 (resistance factor: 41) and 2780K32 (resistance factor: 90), to analyze possible resistance mechanisms. These variants exhibited cross-resistance against all camptothecins tested. In comparison with 2780K4 cells, 2780K32 cells were relatively more resistant against SN-38, topotecan, DX-8951f and BNP1350. In addition, 2780K32 cells were highly cross-resistant against mitoxantrone. In both 2780K4 and 2780K32, the amount of topoisomerase I was not changed but the catalytic activity was reduced. Furthermore, 2780K32 cells clearly overexpressed the breast cancer resistance protein (BCRP), as demonstrated for both the gene and the protein. In contrast to topotecan, BNP1350 proved not to be a good substrate for BCRP. Overall, we conclude that BNP1350 offers advantages over topotecan expressed by high efficacy in experimental human ovarian cancer and poor affinity for BCRP.

DX-8951f (exatecan mesylate), a new water-soluble derivative of camptothecin, is currently being evaluated in phase II clinical trials. Resistance may be acquired when treating cancer patients with DX-8951f. Therefore, we selected a subline of the human ovarian cancer cell line A2780 for resistance against DX-8951f to investigate possible mechanisms of resistance. This DX-8951f-resistant subline, designated 2780DX8 (resistance factor=9.3), displayed a typical cross-resistance pattern including compounds, such as topotecan (resistance factor =34), SN-38 (resistance factor =47), mitoxantrone (resistance factor =59) and doxorubicin (resistance factor =2.9), which have previously been associated with the expression of breast cancer resistance protein. 2780DX8 cells did not show changes in the topoisomerase I gene, in topoisomerase I protein levels or catalytic activity. Overexpression of breast cancer resistance protein could be detected, both at the mRNA and protein level, while staining for Pgp, MRP1, or LRP was negative. GF120918, an inhibitor of breast cancer resistance protein, was able to reverse the DX-8951f-induced resistance in 2780DX8 cells. In vivo experiments in well-established 2780DX8 human tumour xenografts demonstrated that the growth inhibition induced by CPT-11 was more affected by breast cancer resistance protein expression than that of DX-8951f. These data indicate for the first time that DX-8951f is able to induce breast cancer resistance protein as a mechanism of resistance. Breast cancer resistance protein, however, results in only minor reduction of antitumour activity of DX-8951f which is an advantage over topotecan and CPT-11/SN-38.

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