The neurotropic protozoan Toxoplasma gondii is the second leading cause of death due to foodborne illness in the US, and has been designated as one of five neglected parasitic infections by the Center for Disease Control and Prevention. Currently, no treatment options exist for the chronic dormant-phase Toxoplasma infection in the central nervous system (CNS). T. gondii cathepsin L (TgCPL) has recently been implicated as a novel viable target for the treatment of chronic toxoplasmosis. In this study, we report the first body of SAR work aimed at developing potent inhibitors of TgCPL with selectivity vs the human cathepsin L. Starting from a known inhibitor of human cathepsin L, and guided by structure-based design, we were able to modulate the selectivity for Toxoplasma vs human CPL by nearly 50-fold while modifying physiochemical properties to be more favorable for metabolic stability and CNS penetrance. The overall potency of our inhibitors towards TgCPL was improved from 2 μM to as low as 110 nM and we successfully demonstrated that an optimized analog 18b is capable of crossing the BBB (0.5 brain/plasma). This work is an important first step toward development of a CNS-penetrant probe to validate TgCPL as a feasible target for the treatment of chronic toxoplasmosis.

神经质原生动物弓形虫是美国食源性疾病导致的第二大死亡原因，并已被美国疾病控制与预防中心指定为五种被忽略的寄生虫感染之一。当前，对于中枢神经系统（CNS）中的慢性休眠期弓形虫感染尚无治疗选择。弓形虫组织蛋白酶L（TgCPL）最近被认为是治疗慢性弓形虫病的新型可行靶标。在这项研究中，我们报告了SAR研究的第一个目标，该研究旨在开发对人组织蛋白酶L具有选择性的强效TgCPL抑制剂。从已知的人组织蛋白酶L抑制剂开始，并在基于结构的设计指导下，我们能够进行调节弓形虫对人CPL的选择性提高了近50倍，同时修改了其理化特性，使其更有利于代谢稳定性和中枢神经系统渗透性。我们的抑制剂对TgCPL的总体效力从2μM降低至低至110 nM，我们成功地证明了优化的类似物18b能够穿越血脑屏障（0.5脑/血浆）。这项工作是开发CNS渗透探针以验证TgCPL作为治疗慢性弓形虫病可行目标的重要第一步。