Romanini A, Li W W, Colofiore J R, Bertino J R
Department of Molecular Pharmacology, Memorial Sloan-Kettering Cancer Center, New York, NY 10021.
J Natl Cancer Inst. 1992 Jul 1;84(13):1033-8. doi: 10.1093/jnci/84.13.1033.
Increased response rates in studies of patients with colon cancer have indicated that the cytotoxic effects of fluorouracil (5-FU) are potentiated by leucovorin (LV) and by methotrexate (MTX). However, preliminary studies using a sequential combination of MTX, LV, and 5-FU showed no additional potentiation.
We hypothesized that the lack of additional cell kill with this combination could be due to competition of LV with MTX for cellular uptake and reduced folate polyglutamylation. We have tested this possibility by comparing the cytotoxicity of drug combinations containing MTX with that of drug combinations containing trimetrexate (TMTX), an antifolate that does not compete with LV for uptake or polyglutamylation.
Human lymphocytic leukemia CCRF-CEM cells were exposed to MTX or TMTX for 24 hours and to 5-FU during the last 4 hours of antifolate exposure. LV was administered 30 minutes before 5-FU.
After 20 hours of exposure to TMTX or MTX, intracellular levels of phosphoribosyl pyrophosphate were elevated to a similar degree, and these levels did not decrease after a 30-minute exposure to LV. No additional cell kill was observed when LV was added to the MTX/5-FU combination, but cytotoxicity was enhanced when LV was added to the TMTX/5-FU combination.
This study supports the hypothesis that the lack of additional cell kill when high-dose LV is added to the MTX/5-FU combination may be due to competition of MTX with LV for cellular uptake and/or competition of MTX or its polyglutamates with polyglutamylation of reduced folates. Inasmuch as TMTX does not compete with LV and reduced folates for uptake and polyglutamylation, the synergy obtained with the combination of TMTX plus 5-FU and high-dose LV further supports this hypothesis.
在结肠癌患者研究中反应率的提高表明，亚叶酸（LV）和甲氨蝶呤（MTX）可增强氟尿嘧啶（5-FU）的细胞毒性作用。然而，使用MTX、LV和5-FU序贯联合的初步研究未显示出额外的增效作用。
我们推测，这种联合方案缺乏额外的细胞杀伤作用可能是由于LV与MTX竞争细胞摄取以及叶酸多聚谷氨酸化减少所致。我们通过比较含MTX的药物组合与含三甲曲沙（TMTX，一种不与LV竞争摄取或多聚谷氨酸化的抗叶酸药物）的药物组合的细胞毒性来检验这种可能性。
将人淋巴细胞白血病CCRF-CEM细胞暴露于MTX或TMTX 24小时，并在抗叶酸药物暴露的最后4小时暴露于5-FU。在5-FU给药前30分钟给予LV。
暴露于TMTX或MTX 20小时后，磷酸核糖焦磷酸的细胞内水平升高至相似程度，并且在暴露于LV 30分钟后这些水平并未降低。当将LV添加到MTX/5-FU组合中时未观察到额外的细胞杀伤作用，但当将LV添加到TMTX/5-FU组合中时细胞毒性增强。
本研究支持以下假设，即当将高剂量LV添加到MTX/5-FU组合中时缺乏额外的细胞杀伤作用可能是由于MTX与LV竞争细胞摄取和/或MTX或其多聚谷氨酸与还原型叶酸的多聚谷氨酸化竞争所致。由于TMTX不与LV和还原型叶酸竞争摄取和多聚谷氨酸化，TMTX加5-FU与高剂量LV组合所获得的协同作用进一步支持了这一假设。
