title: Unexpected synergy between N-phosphonacetyl-L-aspartate and cytidine against human tumor cells.
abstract: Cytidine, a non-toxic endogenous nucleoside, was found unexpectedly to augment the cytotoxicity of a pyrimidine antimetabolite N-phosphonacetyl-L-aspartate (PALA) in human ovarian carcinoma cells. The PALA/cytidine synergy is confirmed here in other human tumor cells (T242 melanoma, HL60 promyelocytic leukemia and SKOV3 ovarian carcinoma) in the cytidine concentration range of 1-10 micromolar. The synergy was not observed in Chinese hamster ovary (CHO) cells. Exogenous uridine (5-50 microM) completely reversed the PALA/cytidine cytotoxicity in a concentration-dependent manner. Measurements of cellular ribonucleotide levels revealed that the PALA treated cells had reduced UTP and CTP pools (10% and 40% of control respectively); and the PALA/cytidine treated cells had elevated CTP and GTP levels while their UTP levels remained at 10% of control. Deoxyribonucleotide levels were unremarkable except for a slight elevation of dCTP in the PALA/cytidine treated cells. Uridine competitively inhibited radioactive cytidine transport into 2008 cells, which may explain its ability to antagonize the PALA/cytidine synergy. These results suggest that the ribonucleotide biosynthetic mechanism is the primary cellular target for PALA/cytidine activity, and that the ratio of ribonucleotides to each other is an important determinant of tumor cell viability. The use of non-cytotoxic nucleosides to augment the activity of antimetabolites may have clinical relevance in cancer therapy.
authors.affiliation: Department of Physiology and Pharmacology, School of Veterinary Medicine, Purdue University, West Lafayette, Indiana 47907.
authors.forename: T C
authors.initials: TC
authors.lastname: Chan
authors.forename: S B
authors.initials: SB
authors.lastname: Howell
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000008:administration & dosage; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D001224:Aspartic Acid / Q000008:administration & dosage / Q000031:analogs & derivatives; D003562:Cytidine / Q000008:administration & dosage; D003854:Deoxyribonucleotides / Q000378:metabolism; D004354:Drug Screening Assays, Antitumor; D004357:Drug Synergism; D006801:Humans; D010746:Phosphonoacetic Acid / Q000008:administration & dosage / Q000031:analogs & derivatives; D012265:Ribonucleotides / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
chemical_list: D000964:Antimetabolites, Antineoplastic; D003854:Deoxyribonucleotides; D012265:Ribonucleotides; D001224:Aspartic Acid; D003562:Cytidine; C013195:sparfosic acid; D010746:Phosphonoacetic Acid