Allegra C J, Kovacs J A, Drake J C, Swan J C, Chabner B A, Masur H
J Clin Invest. 1987 Feb;79(2):478-82. doi: 10.1172/JCI112837.
Trimetrexate, a highly lipid-soluble quinazoline antifolate now undergoing trials as an anticancer agent, was found to be a potent inhibitor of the dihydrofolate reductase (DHFR) isolated from Toxoplasma gondii. The concentration required for 50% inhibition of protozoal DHFR was 1.4 nM. As an inhibitor of this enzyme, trimetrexate was almost 600-fold (amount of antifolate required to inhibit catalytic reaction by 50%) and 750-fold (inhibition constant) more potent than pyrimethamine, the DHFR inhibitor currently used to treat toxoplasma infection. When the protozoan was incubated with 1 microM trimetrexate, the drug rapidly reached high intracellular concentrations. Since toxoplasma organisms lack a transmembrane transport system for physiologic folates, host toxicity can be prevented by co-administration of the reduced folate, leucovorin, without reversing the antiprotozoal effect. The effectiveness of trimetrexate against toxoplasma was demonstrated both in vitro and vivo. Proliferation of toxoplasma in murine macrophages in vitro was completely inhibited by exposure of these cells to 10(-7) M trimetrexate for 18 h. When used alone, trimetrexate was able to extend the survival of T. gondii-infected mice.
三甲曲沙是一种脂溶性很高的喹唑啉类抗叶酸剂，目前正作为抗癌药物进行试验，它被发现是从刚地弓形虫分离出的二氢叶酸还原酶（DHFR）的有效抑制剂。抑制原生动物DHFR活性50%所需的浓度为1.4 nM。作为这种酶的抑制剂，三甲曲沙的效力比目前用于治疗弓形虫感染的DHFR抑制剂乙胺嘧啶强近600倍（抑制催化反应50%所需的抗叶酸量）和750倍（抑制常数）。当原生动物与1 microM三甲曲沙一起孵育时，药物迅速达到较高的细胞内浓度。由于弓形虫生物体缺乏生理性叶酸的跨膜转运系统，通过联合给予还原型叶酸亚叶酸钙可以预防宿主毒性，而不会逆转抗寄生虫作用。三甲曲沙对弓形虫的有效性在体外和体内均得到了证实。将这些细胞暴露于10(-7) M三甲曲沙18小时，可完全抑制体外培养的小鼠巨噬细胞中弓形虫的增殖。单独使用时，三甲曲沙能够延长感染弓形虫的小鼠的存活时间。
