Proposed mechanism of therapeutic selectivity for 9-β-D-arabinofuranosyl-2-fluoroadenine against murine leukemia based upon lower capacities for transport and phosphorylation in proliferative intestinal epithelium compared to tumor cells

Studies have examined transport and phosphorylation of 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-Ara-A), a deaminase resistant adenosine analogue, as mechanisms that could mediate the observed therapeutic efficacy of this agent against murine tumor models. Earlier finds by Avramis and Plunkett (Cancer Res., 42: 2587-2591, 1982) showed markedly less accumulation in vivo of administered F-Ara-A as cytotoxic triphosphate in gastrointestinal mucosa and bone marrow compared to P388 cells. We have pursued the basis for this difference in vitro using L1210 ascites and proliferative epithelial cells (85-95% crypt cells) isolated from mouse small intestine as representative sample populations of drug-sensitive tumor and drug-limiting normal regenerative host tissue. Using a rapid sampling technique, linear initial rates of substrate uptake were established at 25 degrees C for radiolabeled F-Ara-A and adenosine at a concentration range of 1-1000 microM. The relationship between velocity of initial transport and substrate concentration is indicative of Michaelis-Menten saturation kinetics for both substrates. Competition studies between F-Ara-A and adenosine suggest a common route of entry for both substrates in crypt epithelial cells. Results from double-reciprocal analysis of the velocity versus concentration data are consistent with a simple carrier-mediated facilitated diffusion process with Km, V25max, and Ki values of 317 +/- 44 (SE) microM, 49 +/- 7 nmol/s/g dry weight, and 301 +/- 34 microM for F-Ara-A, and 264 +/- 14 microM, 44 +/- 5 nmol/s/g dry weight, and 225 +/- 44 microM for adenosine, respectively. The presence of a single low-affinity carrier in the proliferative epithelial cells contrasts sharply with the high affinity (Km, 68 +/- 14 microM; V25max, 48 +/- 4 nmol/s/g dry weight) and low-affinity (Km, 326 +/- 48 microM; V25max, 124 +/- 44 nmol/s/g dry weight) routes of entry documented for L1210 cells. This differential in transport kinetics conveys a 7- to 8-fold greater capacity to L1210 ascites compared with crypt epithelial cells for uptake of the antitumor agent F-Ara-A. At pharmacologically achievable concentrations of F-Ara-A and in view of this differential, influx of F-Ara-A would be more rate limiting to phosphorylation of F-Ara-A in epithelial cells than in L1210 cells. Metabolism studies with L1210 ascites and proliferative intestinal epithelial cells show that intracellular phosphorylation of F-Ara-A is also elevated in L1210 cells.