Goldman I D, Matherly L H
Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond 23298.
NCI Monogr. 1987(5):17-26.
Recent studies have clarified the critical role that polyglutamylation plays in methotrexate (MTX) action. Polyglutamate derivatives of MTX bind to dihydrofolate reductase (DHFR) with affinities comparable to the monoglutamate, but their retention in cells results in a sustained block in tetrahydrofolate (FH4) synthesis. One important element in the selectivity of MTX action is the preferential buildup and retention of these polyglutamyl forms in susceptible tumor cells as compared to host cells of the bone marrow or gastrointestinal mucosa. This selectivity in the accumulation of MTX polyglutamyl forms has now been further shown to play an important role in the selectivity of leucovorin rescue and may provide a unique new approach to nucleoside protection as well. This paper reviews the current understanding of the biochemical basis for leucovorin rescue and its selectivity. Important elements in leucovorin rescue are reactivation of DHFR with depression of cellular dihydrofolate (FH2) and provision of folate substrate to circumvent the block in FH4 synthesis. Selectivity of leucovorin rescue may be attributed to direct inhibition by MTX polyglutamyl forms, as well as FH2 polyglutamates that accumulate in their presence, at the levels of thymidylate synthase and transformylation during purine nucleotide biosynthesis. The presence of cellular MTX polyglutamates impairs reactivation of endogenous DHFR activity by leucovorin metabolites, and the resultant maintenance of high cellular levels of cellular FH2 and the polyglutamyl derivations of MTX impair the utilization of added FH4 in susceptible tumor cells. This paper also develops the concept of "early" nucleoside protection in antifolate therapy. In this approach, nucleosides are administered simultaneously with a pulse of MTX to provide early host protection from the cytotoxic effects of modest doses of MTX. Cessation of protection occurs at a time when extracellular and intracellular monoglutamate has fallen to low levels, and the polyglutamyl forms of the drug are present in susceptible tumors but not in host tissues of the gut and bone marrow. Data are presented to demonstrate that increased doses of MTX can be administered in normal and tumor-bearing animal systems as well as in humans by this technique.
近期研究已阐明聚谷氨酸化在甲氨蝶呤（MTX）作用中所起的关键作用。MTX的聚谷氨酸衍生物与二氢叶酸还原酶（DHFR）结合的亲和力与单谷氨酸相当，但其在细胞内的滞留导致四氢叶酸（FH4）合成持续受阻。MTX作用选择性的一个重要因素是，与骨髓或胃肠道黏膜的宿主细胞相比，这些聚谷氨酰形式在易感肿瘤细胞中优先积累和滞留。现在已进一步表明，MTX聚谷氨酰形式积累的这种选择性在亚叶酸救援的选择性中起重要作用，并且可能也为核苷保护提供一种独特的新方法。本文综述了目前对亚叶酸救援及其选择性生化基础的理解。亚叶酸救援的重要因素包括通过降低细胞二氢叶酸（FH2）来重新激活DHFR，以及提供叶酸底物以规避FH4合成的阻断。亚叶酸救援的选择性可能归因于MTX聚谷氨酰形式以及在其存在下积累的FH2聚谷氨酸在胸苷酸合成酶水平和嘌呤核苷酸生物合成过程中的转甲酰化作用的直接抑制。细胞内MTX聚谷氨酸的存在会损害亚叶酸代谢产物对内源性DHFR活性的重新激活，并且由此导致的细胞内FH2高水平的维持以及MTX的聚谷氨酰衍生物会损害易感肿瘤细胞中添加的FH4的利用。本文还提出了抗叶酸治疗中“早期”核苷保护的概念。在这种方法中，核苷与脉冲式MTX同时给药，以早期保护宿主免受中等剂量MTX的细胞毒性作用。当细胞外和细胞内单谷氨酸降至低水平且药物的聚谷氨酰形式存在于易感肿瘤中但不存在于肠道和骨髓的宿主组织中时，保护作用停止。提供的数据表明，通过该技术可以在正常和荷瘤动物系统以及人类中给予更高剂量的MTX。
