van der Wilt C L, Backus H H, Smid K, Comijn L, Veerman G, Wouters D, Voorn D A, Priest D G, Bunni M A, Mitchell F, Jackman A L, Jansen G, Peters G J
Department of Medical Oncology, University Hospital Vrije Universiteit, 1007 MB Amsterdam, the Netherlands.
Cancer Res. 2001 May 1;61(9):3675-81.
Plasma levels of folates and thymidine in mice are about 10-fold higher than in humans and may influence the therapeutic efficacy of thymidylate synthase (TS) inhibitors, such as 5-fluorouracil (5FU) and the antifolates pemetrexed (MTA) and raltitrexed (RTX). Therefore, we tested their therapeutic efficacy in various murine tumor models, grown in mice on a normal and a folate-depleted diet, with high and low thymidine kinase (TK) levels. MTA and RTX were inactive against Colon-26-10 [doubling times gained by treatment; growth delay factor (GDF), 0.5 and 0.3, respectively], whereas 5FU was very active (GDF, >10; complete cures). Colon-26-10/F, grown in mice on a folate-depleted diet, was more sensitive to RTX and MTA (GDF, 2.1 and 1.3, respectively) but not to 5FU (GDF, 1.2); however, leucovorin reversed the effect leading to cures. Folate depletion did not reverse resistance of Colon-26A and Colon-26G (low TK) to MTA and RTX, whereas leucovorin only enhanced the 5FU effect in Colon-26A and Colon-26A/F. Folic acid at 15 mg/kg did not improve the therapeutic efficacy of MTA in folate-deficient mice. The folate-depleted diet decreased the reduced folates in Colon-26A/F and Colon-26-G/F tumors less (4-5-fold; P < 0.01) than in Colon-26-10/F tumors (8-fold; P < 0.001). Folate depletion increased TS levels 2-3-fold in all of the models and TK levels 6-fold (P < 0.01) in Colon-26G/F, explaining the lack of activity of MTA and RTX in Colon-26G/F. In contrast, TK-deficient FM3A/TK tumors were much more sensitive to RTX, MTA, and 5FU than parent FM3A tumors, which have comparable TS levels. The rate of thymidine phosphorylysis varied considerably in all of the tumors without a clear relation to antitumor activity. In conclusion, tumor folates may potentiate (5FU) or protect (antifolates). Murine tumor models should combine low folates and low thymidine rescue to optimize preclinical testing of antifolates.
小鼠血浆中的叶酸和胸苷水平比人类高约10倍，这可能会影响胸苷酸合成酶（TS）抑制剂的治疗效果，如5-氟尿嘧啶（5FU）以及抗叶酸药物培美曲塞（MTA）和雷替曲塞（RTX）。因此，我们在各种小鼠肿瘤模型中测试了它们的治疗效果，这些肿瘤模型是在正常饮食和叶酸缺乏饮食的小鼠中生长的，胸苷激酶（TK）水平有高有低。MTA和RTX对Colon-26-10无效[治疗后获得的倍增时间；生长延迟因子（GDF）分别为0.5和0.3]，而5FU非常有效（GDF，>10；完全治愈）。在叶酸缺乏饮食的小鼠中生长的Colon-26-10/F对RTX和MTA更敏感（GDF分别为2.1和1.3），但对5FU不敏感（GDF为1.2）；然而，亚叶酸钙可逆转这种效应并导致治愈。叶酸缺乏并未逆转Colon-26A和Colon-26G（低TK）对MTA和RTX的耐药性，而亚叶酸钙仅增强了5FU在Colon-26A和Colon-26A/F中的作用。15mg/kg的叶酸并不能提高MTA在叶酸缺乏小鼠中的治疗效果。叶酸缺乏饮食使Colon-26A/F和Colon-２６-G/F肿瘤中还原型叶酸的减少程度（4-5倍；P<0.01）低于Colon-26-10/F肿瘤（8倍；P<0.001）。在所有模型中，叶酸缺乏使TS水平升高2-3倍，在Colon-26G/F中使TK水平升高6倍（P<0.01），这解释了MTA和RTX在Colon-26G/F中缺乏活性的原因。相比之下，TK缺陷的FM3A/TK肿瘤比具有相当TS水平的亲本FM3A肿瘤对RTX、MTA和5FU敏感得多。所有肿瘤中胸苷磷酸化的速率差异很大，与抗肿瘤活性没有明显关系。总之，肿瘤叶酸可能会增强（5FU）或保护（抗叶酸药物）。小鼠肿瘤模型应结合低叶酸和低胸苷挽救来优化抗叶酸药物的临床前测试。
