IN-VIVO ANTITUMOR-ACTIVITY OF 5-FLUOROCYTOSINE ON HUMAN COLORECTAL-CARCINOMA CELLS GENETICALLY-MODIFIED TO EXPRESS CYTOSINE DEAMINASE

A human colorectal carcinoma cell line, WiDr, was genetically engineer ed to express the nonmammalian enzyme, cytosine deaminase (CD). Expres sion of CD in WiDr cells (WiDr/CD) did not alter the growth rate of th ese cells when grown in vitro or as solid tumor xenografts in nude mic e. However, expression of CD did increase the sensitivity of these cel ls to the nontoxic prodrug, 5-fluorocytosine (FCyt), decreasing the 50 % inhibitory concentration for FCyt from 26,000 muM in parental WiDr c ells to 27 muM in WiDr/CD cells. The increase in sensitivity to FCyt i n WiDr/CD cells was the result of the CD-mediated conversion of FCyt t o 5-fluorouracil (FUra) and subsequent FUra anabolites. The half-life of the prodrug, FCyt, was determined to be approximately 40 min in nud e mice. A single i.p. injection of 500 mg FCyt/kg body weight resulted in a transient FCyt plasma level of approximately 4000 muM while osmo tic minipumps or constant tail vein infusions of FCyt achieved continu al FCyt plasma levels of 5 muM and 50 muM, respectively, with no overt signs of toxicity. Significant antitumor effects were observed in nud e mice bearing tumors derived from WiDr/CD cells when these animals we re given 500 mg FCyt/kg i.p. for 10 consecutive days. These antitumor effects were demonstrated by decreases in tumor growth rate, tumor siz e, tumor weight, and thymidine incorporation into tumor DNA. This anti tumor effect was significant but less profound if FCyt was administere d by constant tail vein infusion. WiDr and WiDr/CD cells were very sen sitive to FUra in vitro (50% inhibitory concentration approximately 5 muM). However, no significant antitumor effects were observed in nude mice bearing tumors derived from either WiDr or WiDr/CD cells when the se animals were treated with various doses of FUra. Taken collectively , these data indicate that nontoxic plasma levels of FCyt can be attai ned which can produce profound antitumor effects on tumors engineered to express CD and that these antitumor effects are significantly bette r than those that can be achieved using FUra. These positive data supp ort the continued development of a gene therapy approach to colorectal carcinoma involving the selective expression of CD in colorectal tumo rs with subsequent administration of FCyt.