Methionyl-tRNA Synthetase inhibitors can be developed as novel Giardiasis therapeutics

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Project Summary/Abstract Giardia lamblia is the causative agent of giardiasis, a gastrointestinal illness with symptoms including diarrhea and malabsorption. Chronic infections can lead to long term growth retardation or death in infants, with a recent estimate of global incidence of 280 million symptomatic cases per year. However, a substantial number of clinical infections are resistant to currently available treatments, especially metronidazole. We have shown that specific inhibitors of methionyl-tRNA synthetase (MetRS) representing 3 scaffolds in an available ~600 library prevent growth in wild-type and metronidazole resistant G. lamblia strains. The molecular mechanism of action seems to be the disruption of G. lamblia protein synthesis due to inhibition of GlMetRS enzyme activities. Proof of principle compound 1717 has decent oral bioavailability and is an effective treatment in a mouse model of giardiasis, showing complete clearance of G. lamblia after 3 days. This research proposal will capitalize on these encouraging preliminary data to develop compounds as novel anti-giardia drugs for alternative or complementary treatment of giardiasis. We have selected 18 compounds representing 3 distinct scaffolds based on chemical functional group diversity, GlMetRS IC50 ≤50nM, G. lamblia trophozoite EC50 ≤3000nM, and a selectivity index of ≥ 15 defined as CC50/EC50, for toxicity in HepG2 cell cultures. Preliminary data showed that the double-ring linker series tends to have better selectivity when compared to the other two series. We will therefore focus on the double-ring linker compounds. Since the pharmacokinetic correlations for effective anti-Giardia chemotherapy have not been well established, we will use these compounds to define the PK/PD properties necessary for optimum in vivo efficacy in Aim 1. Also, in Aim 1, we will determine structural activity relationships and select compounds with high potency against GlMetRS and multiple G. lamblia strains. In Aim 2, we will determine: static vs. cidal properties, rate of killing, propensity for acquired resistance and initial safety liabilities of the compounds. A combination of structure-based design, empirical SAR-driven approaches and automated quantitative tomography of G. lamblia will be used in Aim 3 to guide medicinal chemistry optimization of double- ring linker scaffold for improved efficacy and PK/ADMET properties, while addressing potential safety issues. We will determine potential toxicity and off-target effects of GlMetRS inhibitors in vitro and in rodent models. The compounds will be tested for hERG liabilities and CYP inhibition, as well as against the mutagenesis model and a safety panel of human receptors and ion channels. This will set the stage in Aim 4 for dose finding experiments in efficacy models, final toxicology studies, additional resistance studies and metronidazole combination studies. The proposed work will complete many of the steps necessary for selecting a preclinical candidate that will facilitate innovative, shorter course therapy for G. lamblia-associated chronic asymptomatic diseases, diarrhea, growth retardation, and poor cognitive function. The product will provide a great public health benefit.