Enzyme pattern directed chemotheraphy: The effects of combinations of methotrexate, 5-fluorodeoxyuridine and thymidine on rat hepatoma cells in vitro

Abstract

The effects of methotrexate (MTX), 5-fluorodeoxyuridine (FUdR) and thymidine on the growth of four rat hepatoma lines were examined in vitro. In the rapidly growing 3924A and Novikoff lines, MTX exerted a marked anti-purine effect, as indicated by its continued toxicity in the presence of thymidine. Addition to the slower growing hepatoma lines 8999R and 8999S of 200 μM thymidine was able to sustain growth for 48 hr in the presence of MTX. However, at a lower thymidine concentration (20 μM) the MTX toxicity toward the 8999R and 8999S lines was not prevented, even in presence of hypoxanthine. Similarly, thymidine at 20 μM was able to protect the 3924A and Novikoff cell lines completely from the toxic effect of 10 μM FUdR, but this thymidine concentration did not protect the 8999R and 8999S lines. The failure of thymidine to protect the latter hepatoma lines was attributed to the rapid breakdown of thymidine in these cells. This interpretation was supported by experiments where addition of 5-diazouracil, an inhibitor of the rate-limiting enzyme of thymidine catabolism (dihydrothymine dehydrogenase), to the cultures resulted in an increased degree of rescue from FUdR by 20 μM thymidine. The combination of MTX and FUdR treatment in the slow growing hepatomas showed summation, but the two agents were less than additive in the rapidly growing lines. The results of these drug combination studies are interpreted in the context of the patterns of competing enzymes characteristic of the various tumor cell lines.