The interaction of cis-diamminedichloroplatinum (cisplatin) and α-difluoromethylornithine (DFMO) has been previously shown by us to be roughly additive in enhancing the growth-inhibitory effects of cisplatin and by another group of investigators to be antagonistic. Since two different schedules of administration were used, we sought to investigate systematically the role of schedule dependence in the interaction of cisplatin and DFMO in a panel of pancreatic adenocarcinoma cell lines (PANC-1, of human origin, and WD PaCa and PD PaCa, both of hamster origin). Dose-effect relationships of single drug alone and in combination were analyzed by the median-effect principle and by the combination indices for the quantitation of synergism or antagonism with the aid of a microcomputer. Pre-cisplatin administration of DFMO for 2 or 5 to 6 days at concentrations of 50 or 100 µg/ml (0.21 or 0.42 mm) was found to antagonize the effects of cisplatin to various degrees in the cell lines. In contrast, whenever post-cisplatin DFMO was administered, marked enhancement, which was synergistic in most instances, of cisplatin's inhibition of colony formation was found. Thus, the interaction of cisplatin and DFMO is felt to be schedule dependent with deleterious effects found only when DFMO is administered prior to and not following cisplatin. Furthermore, the combination shows promise as an approach to overcoming drug resistance in pancreatic cancer.

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Supported by Grants CA34170, CA27569, and CA18856 awarded by the National Cancer Institute, Department of Health and Human Services, the Medical Research Service of the Veterans Administration, and the Elsa E. Pardee Foundation. Presented in part at the annual meeting of the American Association for Cancer Research, Toronto, May 1984 (28).

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