Carrier-mediated transport of folate compounds in L1210 cells. Initial rate kinetics and extent of duality of entry routes for folic acid and diastereomers of 5-methyltetrahydrohomofolate in the presence of physiological anions.

Comparison of the kinetic parameters for influx of highly purified [3H]folic acid versus [3H]methotrexate in L1210 cells under anionic buffer conditions showed a marked discordancy. In addition, the kinetics for influx of [3H]folic acid were unchanged in variant L1210 cells defective in [3H]methotrexate transport. In these variant cells, the Vmax for methotrexate was reduced 17-fold and the Km was increased 3-fold. The results show that [3H]folic acid influx is mediated by a system which has a low affinity, but a 20-fold higher capacity, for folate compounds than the classical high-affinity system mediating [3H]methotrexate influx. Since the latter system also exhibits very low affinity for [3H]folic acid, it would not be expected to contribute significantly to the total influx of [3H]folic acid. The high-capacity system for [3H]folic acid influx is different from that believed to mediate pterin influx in L1210 cells since it was not inhibited by adenine, a potent inhibitor of pterin influx. However, exposure of cells to [3H]folic acid in a nonanionic buffer resulted in marked stimulation of initial influx, and a fraction of influx under these conditions was inhibited by methotrexate. These results suggest that anions modulate the extent of multiplicity of [3H]folic acid influx by their known effects on the high-affinity, reduced folate/methotrexate system. The diastereomers, at carbon 6, of [14C]5-methyltetrahydrohomofolate shared both transport systems. The influx Km for the natural diastereomer was one-half that of the unnatural form for both transport systems. Both diastereomers showed a much greater differential in affinity between the two transport systems than did [3H]folic acid. Our results suggest that an analog which could be effectively transported by the low-affinity/high-capacity route may be useful in the treatment of tumors resistant to methotrexate due to a defective high-affinity/low capacity influx system. We also found that incubation of L1210 cells with [3H]folic acid or the natural diastereomer [14C]5-methyltetrahydrohomofolate for 10 min resulted in the formation of a nonexchangeable fraction of radioactivity amounting to 20-40% of the total accumulation. This non-exchangeable fraction may be explained by the accumulation of metabolites other than polyglutamates. Preloading of cells with methotrexate prior to incubation with [3H]folic acid prevented the accumulation of radioactivity as a nonexchangeable fraction.

Hydrolytic cleavage of methotrexate gamma-polyglutamates by folylpolyglutamyl hydrolase derived from various tumors and normal tissues of the mouse.

Folylpolyglutamate hydrolase (folyl hydrolase) activity derived from murine tumors and various normal tissues was measured by means of high performance liquid chromatography using methotrexate polyglutamates as substrate. Enzyme-mediated hydrolysis was considerably greater (10-20-fold) on a specific activity basis in extracts from all normal mouse tissues (kidney greater than bone marrow greater than small intestine approximately equal to liver) than from tumor cells (Sarcoma 180 greater than Ehrlich approximately equal to L1210 cells). Enzyme preparations from purified absorptive and crypt cell epithelium from mouse small intestine exhibited comparable levels of specific activity and were greater than that derived from the total organ. Folyl hydrolase from mouse kidney showed mixed endo- and exopeptidase activity while that derived from all other normal tissues and tumor cells was consistent with endopeptidase activity. Levels of cell-free folyl hydrolase activity derived from tumor cells varied substantially with the phase of growth in vivo. Also, levels were appreciably lower from the same cells grown in vitro. Hydrolysis by crude or partially purified enzyme preparations from mouse small intestine or tumor cells conformed to Michaelis-Menten kinetics (single saturable component). Rates of hydrolysis and Km values were proportional to gamma-glutamyl chain length in the case of L1210 cell-derived enzyme but not for enzyme derived from small intestine. Km values derived for 4-amino-10-methylpteroyldiglutamate were the same [Km = 80.4 +/- 9 (SE) microM] for small intestine and L1210 cells. However, with 4-amino-10-methylpteroyltetraglutamate Km values were 3-fold lower for tumor cell preparations and 8-fold lower for preparations derived from small intestine. Fourfold lower Km values for 4-amino-10-methylpteroyldiglutamate were obtained with enzyme derived from Sarcoma 180 cells as compared to the enzyme from L1210 or intestinal cells. Varying levels of folyl hydrolase activity for methotrexate polyglutamates in cell-free preparations from different tumor cells appeared to reflect differences in in situ hydrolytic activity shown for the same substrate when internalized. The relevance of these results to antifolate pharmacology and, specifically, to a role for polyglutamates of 4-aminofolate compounds in determining cytotoxicity and selective antitumor activity of these agents is discussed.

Extent of the requirement for folate transport by L1210 cells for growth and leukemogenesis in vivo.

In these studies the extent of the requirement for 5-methyltetrahydrofolate by L1210 cells for growth and leukemogenesis in vivo was addressed from the aspect of its cellular membrane transport. Growth characteristics and leukemogenesis in vivo were determined for parental and methotrexate-resistant L1210 cell variants with reduced capacity for folate coenzyme transport inward. These variants exhibited 6-, 16-, and 100-fold reductions compared to parental cells in influx Vmax for the high-affinity system transporting 5-substituted reduced folates and methotrexate. They also exhibited reduced saturability for methotrexate influx (3-fold higher Km), but not for influx of 5-formyltetrahydrofolate or 5-methyltetrahydrofolate. The reduced influx capacity in these variants correlated with their increased requirement for reduced folates during growth in vitro and with the ability of the variants to proliferate and develop leukemia in vivo. Lack of growth potential in vivo for one variant appears to reflect the inability for net intracellular accumulation of reduced folate per se, since growth of this variant could be restored by treatment of mice with folic acid, but not with 5-methyltetrahydrofolate or 5-formyltetrahydrofolate, and following reversion to a more transport-proficient phenotype.

New folate analogs of the 10-deaza-aminopterin series. Basis for structural design and biochemical and pharmacologic properties.

Structural modification of the N10 position of 4-amino folates affects mediated membrane transport in mammalian cells but has little or no effect on target enzyme (dihydrofolate reductase) inhibition. Some of these modifications have been associated with differential effects on transport in various cell types in a manner which favored greater accumulation and persistence of drug in responsive tumor cells than in normal proliferative tissue. With the aid of identifying new structures with greater potential for differential mediated accumulation, we have studied three new 10-alkyl analogs of 10-deaza-aminopterin. Two of these analogs showed therapeutic efficacy substantially greater than 10-deaza-aminopterin, an analog with antitumor properties superior to methotrexate. These analogs, the 10-methyl, 10-ethyl, and 10,10-dimethyl derivatives, were equivalent to the parent compound, 10-deaza-aminopterin, and aminopterin, and slightly more potent than methotrexate, as inhibitors of L1210 cell dihydrofolate reductase. The three new analogs, 10-deaza-aminopterin, and aminopterin exhibited similar transport properties in L1210, Ehrlich, and S180 cells. Efflux and influx Vmax were similar to those of methotrexate, but influx Km was 4- to 14-fold lower than for methotrexate. That is, substitution at N10, but not at C10, reduced influx potential in these tumor cells. These differences in transport properties among this group of analogs which determine net accumulation were reflected in the individual values for growth-inhibitory potency. In contrast to that seen in tumor cells, alkylation at both N10 and C10 reduced influx potential (increased Km) in isolated intestinal epithelial cells from mouse small intestine. Influx was in the order aminopterin greater than 10-deaza-aminopterin with further reduction in each series showing a magnitude in proportion to the size of the 10 substituent. Otherwise, influx Vmax and efflux were similar for the group. Accumulation of polyglutamates in small intestine was greater following aminopterin administration than following administration of other analogs (10-ethyl, 10-deaza-aminopterin less than methotrexate less than 10-deaza-aminopterin). Polyglutamate accumulation for all the analogs was greater in tumor cells, but accumulation of each varied between the two tumors (L1210 and S180) examined. Differences among the analogs were not as great in L1210 as in S180 cells, and their metabolism was not in the same relative order.(ABSTRACT TRUNCATED AT 400 WORDS)

New folate analogs of the 10-deaza-aminopterin series. Further evidence for markedly increased antitumor efficacy compared with methotrexate in ascitic and solid murine tumor models.

A group of folate analogs of the 10-deaza-aminopterin series, which were designed on the basis of the results of an intensive biochemical and pharmacokinetic program, have been examined in therapy experiments utilizing a group of murine tumor models. These new analogs were found to be markedly superior to methotrexate against four of five ascites tumors (L1210 leukemia, Sarcoma 180, Ehrlich carcinoma and Tapper carcinosarcoma) and against four of five solid tumors (S180, Tapper carcinosarcoma, E0771 mammary adenocarcinoma, Lewis lung carcinoma, and T241 sarcoma). Analogs alkylated (methyl or ethyl) at the 10 position of 10-deaza-aminopterin were found to be the most effective of the group. These analogs achieved log10 reduction in tumor burden several-fold greater in magnitude than methotrexate against L1210 and S180 ascites tumors and there were also long-term survivors. 10-Deaza-aminopterin itself gave a result intermediate between those obtained with the 10-alkyl derivatives and methotrexate. Against the solid forms of the Tapper tumor some partial regressions were obtained with methotrexate and 10-deaza-aminopterin, but a far greater number, extending over a longer period were obtained with the 10-ethyl derivative of 10-deaza-aminopterin. Against the E0771 tumor, 10-deaza-aminopterin was 2-fold and the ethyl derivative of 10-deaza-aminopterin was greater than 5-fold more effective than methotrexate in retarding tumor growth. Evidence for partial regressions and marked effects against metastatic disease were also obtained in the case of the 10-alkyl derivative. Similar results were also obtained with the T241 sarcoma. For Lewis lung carcinoma the relative potency was about the same but overall antitumor effects were more modest.

Biochemical correlates of responsiveness and collateral sensitivity of some methotrexate-resistant murine tumors to the lipophilic antifolate, metoprine.

The M5076 murine "ovarian" tumor which is naturally refractive to methotrexate was found to be highly responsive to the lipophilic antifolate, metoprine. M5076 cells were markedly deficient in mediated entry of methotrexate. This was in contrast to the L1210 leukemia, a tumor highly responsive to methotrexate but poorly responsive to metoprine. Two L1210 leukemia sublines, with acquired resistance to methotrexate by virtue of a deficiency in mediated entry of drug similar to that seen for M5076 cells, were found to be collaterally sensitive to metoprine. The insensitivity to methotrexate of the M5076 tumor and the two L1210 sublines is associated with low saturability (high Km) and reduced capacity (low Vmax) for mediated influx of drug. 5-Methyltetrahydrofolate, the major circulating folate in blood but not metoprine, shares this mediated route for entry. Therefore, a relatively low level of accumulation of this natural folate in these methotrexate-resistant tumors, in the face of a metoprine-induced blockade at the level of dihydrofolate reductase, probably accounts for the high sensitivity of these tumors to this lipophilic agent. Evidence for this notion was derived during transport and growth experiments in vitro using 5-formyltetrahydrofolate as a model folate coenzyme. The value for influx Vmax of this folate compound in a transport-deficient methotrexate-resistant subline compared to the parental L1210 was reduced to the same extent as that shown for methotrexate. Growth of this resistant L1210 subline showed a greater requirement for this model compound than did the parental line. Also, the concentration necessary for 50% inhibition by metoprine in the presence of this reduced folate was lower in the resistant subline. Inhibition of each cell line by metoprine, on the other hand, was the same when folic acid was used as the folate source. The implications of these findings for the use of lipophilic antifolates as alternative therapy for some methotrexate-resistant tumors are discussed.

Relative frequency and kinetic properties of transport-defective phenotypes among methotrexate-resistant L1210 clonal cell lines derived in vivo.

Information was sought on the relative extent to which transport-defective, methotrexate-resistant phenotypes emerge among the total subpopulation of resistant phenotypes during therapeutic challenge of leukemic cells in vivo. A number of monoclonal methotrexate-resistant sublines of the L1210 leukemia were derived during methotrexate therapy of leukemic mice and biochemically characterized. Of the total number of 14 sublines derived, five exhibited altered [3H]methotrexate transport alone, five exhibited increased dihydrofolate reductase content alone (2- to 18-fold), and four showed alterations in both of these properties. Methotrexate binding and substrate turnover rate for dihydrofolate reductase appeared to be unchanged in any of the resistant sublines. The relative resistance of each subline was accounted for by the biochemical alterations observed. Among the transport-defective sublines, one subcategory showed a 3- to 4-fold reduction in apparent influx Vmax for [3H]methotrexate, a second category showed both a 5-fold reduction in influx Vmax and a 3-fold increase in the apparent influx Km, and one subline showed only a 2-fold increase in Km. Otherwise, Michaelis-Menten saturation kinetics for influx was observed in each case and in the case of the parental line and the other resistant sublines. None of the resistant sublines exhibited altered efflux of [3H]methotrexate. Steady-state levels measured for intracellular exchangeable (osmotically active) fractions of drug accurately reflected the values for specific kinetic parameters determined for each sensitive and resistant cell line. These studies show that transport-defective phenotypes represent a major category of methotrexate-resistant cell types which emerge initially from leukemic cell populations under therapy in mice. Based on considerations discussed here, it is reasonable to assume that a similar relative occurrence of this phenotype would result during methotrexate therapy of leukemia patients.