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Synergistic Inhibition of Polyamine Synthesis and Growth by Difluoromethylornithine plus Methylthioadenosine in Methylthioadenosine Phosphorylase-deficient Murine Lymphoma Cells¹

Department of Basic and Clinical Research, Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, La Jolla, California 92037

The antiproliferative effects of the ornithine decarboxylase inhibitor -difluoromethylornithine (DFMO) are limited by the inability of the compound to deplete completely cellular polyamine pools. 5'-Deoxy-5'-methylthioadenosine (MeSAdo), the purine end product of the polyamine biosynthetic pathway, is an inhibitor of spermine and spermidine synthesis. Furthermore, a substantial number of human tumors are deficient in MeSAdo phosphorylase, and cannot degrade MeSAdo. It therefore seemed possible that DFMO and MeSAdo could interact synergistically to inhibit polyamine synthesis in MeSAdo phosphorylase-deficient malignant cells. To test this hypothesis, we have analyzed the effects of DFMO, in combination with MeSAdo, on polyamine synthesis and growth in a MeSAdo phosphorylase-deficient murine lymphoma cell line (R1.1-H), and a MeSAdo resistant mutant (R1.1-H3). Cultivation of the R1.1-H3 cells in medium containing 250 µM DFMO and 500 µM MeSAdo caused profound depletion of putrescine, spermidine, and spermine, and the accumulation of both decarboxylated S-adenosylmethionine and its acetylated derivative to levels that exceeded by nearly 3-fold the total cellular content of S-adenosylmethionine. Similarly, DFMO sensitized the lymphoma cells to the growth inhibitory effects of MeSAdo. Supplementation of the medium with putrescine, spermidine, or spermine partially protected R1.1-H3 cells from the DFMO-MeSAdo drug combination. It is conceivable that MeSAdo, or related nucleosides, may potentiate the cytostatic effects of DFMO toward MeSAdo phosphorylase-deficient tumors.

¹ Supported in part by Grants GM-23200 and CA-35058. This is publication number 4528BCR from the Research Institute of Scripps Clinic, La Jolla, CA 92037.

² Present address: Department of Pediatrics, Kyoto University Hospital, Shogoin, Sakyo-Ku, Kyoto 606, Japan.

³ To whom requests for reprints should be addressed, at Dept. of Basic and Clinical Research (BCR4), Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, La Jolla, CA 92037.

Received 9/ 4/86. Revised 12/ 2/86. Accepted 12/31/86.

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