Sirotnak F M, Moccio D M, Goutas L J, Kelleher L E, Montgomery J A
Cancer Res. 1982 Mar;42(3):924-8.
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.
对甲氨蝶呤天然耐药的M5076小鼠“卵巢”肿瘤被发现对亲脂性抗叶酸剂美托普林高度敏感。M5076细胞在介导的甲氨蝶呤摄取方面明显不足。这与L1210白血病形成对比，L1210白血病对甲氨蝶呤高度敏感，但对美托普林反应不佳。发现两个L1210白血病亚系，由于介导的药物摄取不足而获得了对甲氨蝶呤的耐药性，其情况与M5076细胞相似，它们对美托普林呈 collateral 敏感性。M5076肿瘤和两个L1210亚系对甲氨蝶呤的不敏感性与药物介导的流入的低饱和性（高Km）和降低的能力（低Vmax）有关。5-甲基四氢叶酸是血液中主要的循环叶酸，但美托普林不是，它共享这种介导的进入途径。因此，面对美托普林在二氢叶酸还原酶水平引起的阻断，这种天然叶酸在这些耐甲氨蝶呤肿瘤中的积累水平相对较低，这可能解释了这些肿瘤对这种亲脂性药物的高敏感性。这一观点的证据来自于使用5-甲酰四氢叶酸作为模型叶酸辅酶的体外转运和生长实验。与亲代L1210相比，这种叶酸化合物在转运缺陷的耐甲氨蝶呤亚系中的流入Vmax值降低的程度与甲氨蝶呤相同。这种耐药的L1210亚系的生长对这种模型化合物的需求比亲代细胞系更大。此外，在存在这种还原叶酸的情况下美托普林抑制50%所需的浓度在耐药亚系中更低。另一方面，当使用叶酸作为叶酸来源时，美托普林对每个细胞系的抑制作用是相同的。讨论了这些发现对于使用亲脂性抗叶酸剂作为某些耐甲氨蝶呤肿瘤替代疗法的意义。
