The protozoan parasite Trypanosoma brucei is the etiological agent of human African trypanosomiasis (HAT). HAT, together with other neglected tropical diseases, causes serious health and economic issues, especially in tropical and subtropical areas. The classical antifolates targeting dihydrofolate reductase (DHFR) are ineffective towards trypanosomatid parasites owing to a metabolic bypass by the expression of pteridine reductase 1 (PTR1). The combined inhibition of PTR1 and DHFR activities in Trypanosoma parasites represents a promising strategy for the development of new effective treatments for HAT. To date, only monocyclic and bicyclic aromatic systems have been proposed as inhibitors of T. brucei PTR1 (TbPTR1); nevertheless, the size of the catalytic cavity allows the accommodation of expanded molecular cores. Here, an innovative tricyclic‐based compound has been explored as a TbPTR1‐targeting molecule and its potential application for the development of a new class of PTR1 inhibitors has been evaluated. 2,4‐Diaminopyrimido[4,5‐b]indol‐6‐ol (1) was designed and synthesized, and was found to be effective in blocking TbPTR1 activity, with a K_i in the low‐micromolar range. The binding mode of 1 was clarified through the structural characterization of its ternary complex with TbPTR1 and the cofactor NADP(H), which was determined to 1.30 Å resolution. The compound adopts a substrate‐like orientation inside the cavity that maximizes the binding contributions of hydrophobic and hydrogen‐bond interactions. The binding mode of 1 was compared with those of previously reported bicyclic inhibitors, providing new insights for the design of innovative tricyclic‐based molecules targeting TbPTR1.

中文翻译：

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布鲁氏锥虫蝶呤还原酶1的高分辨率晶体结构与创新的基于三环的抑制剂复合。

原生动物寄生虫布鲁氏锥虫是人类非洲锥虫病（HAT）的病原体。HAT与其他被忽视的热带病一起，引起严重的健康和经济问题，尤其是在热带和亚热带地区。靶向二氢叶酸还原酶（DHFR）的经典抗叶酸药物由于通过蝶啶还原酶1（PTR1）的表达而绕过代谢，因此对锥虫的寄生虫无效。锥虫体内寄生虫对PTR1和DHFR活性的联合抑制代表了开发新的HAT有效治疗方法的有希望的策略。迄今为止，只有单环和双环芳族系统已被提议作为抑制剂锥虫PTR1（Tb的PTR1）；但是，催化腔的大小可以容纳膨胀的分子核。在此，已探索出一种创新的基于三环的化合物作为Tb PTR1靶向分子，并评估了其在开发新型PTR1抑制剂方面的潜在应用。设计并合成了2,4-二氨基嘧啶[4,5- b ]吲哚-6-醇（1），发现其在低微摩尔范围内的K _i可以有效地阻断Tb PTR1的活性。1与Tb的三元配合物的结构表征阐明了1的结合方式。PTR1和辅助因子NADP（H）的分辨率确定为1.30Å。该化合物在腔体内采用类似基质的取向，从而最大化了疏水和氢键相互作用的结合贡献。将1的结合模式与先前报道的双环抑制剂的结合模式进行了比较，为设计靶向Tb PTR1的基于三环的创新分子提供了新见识。