Synthesis and In Vivo Profiling of Desymmetrized Antimalarial Trioxolanes with Diverse Carbamate Side Chains.

This article is licensed under CC-BY-NC-ND 4.0 pubs.acs.org/acsmedchemlett Letter Synthesis and In Vivo Profiling of Desymmetrized Antimalarial Trioxolanes with Diverse Carbamate Side Chains Matthew T. Klope, Juan A. Tapia Cardona, Jun Chen, Ryan L. Gonciarz, Ke Cheng, Priyadarshini Jaishankar, Julie Kim, Jenny Legac, Philip J. Rosenthal, and Adam R. Renslo* Cite This: ACS Med. Chem. Lett. 2024, 15, 1764−1770 ACCESS Metrics & More Read Online Article Recommendations sı Supporting Information * ABSTRACT: The recent withdrawal of artefenomel from clinical development leaves no endoperoxide-class agents in the antimalarial pipeline. Synthetic endoperoxides with a desymmetrized structure have demonstrated promising physiochemical and in vivo properties. Here we expand on our initial investigation of trans-3″ carbamate substitution with a diverse array of amine-, alcohol-, and sulfinyl-terminated analogues prepared in (S,S) and (R,R) configurations. In general, this chemotype combines low-nM antiplasmodial activity with excellent aqueous solubility but widely varying human liver microsome (HLM) stability. We evaluated 20 novel analogues in the P. berghei mouse malaria model, identifying new analogues such as RLA-4767 (9a) and RLA-5489 (9d), with HLM stability and pharmacokinetic profiles superior to analogues from our initial report (e.g., RLA-4776, 8a). These new leads approach or equal the efficacy of artefenomel after two daily oral doses of 10 mg/kg, thus revealing a promising chemotype with the potential to deliver development candidates. KEYWORDS: Antimalarials, endoperoxides, trioxolanes, lead optimization, stereoselective synthesis D espite notable progress in the preclinical arena, malaria remains a cause of significant mortality, particularly in sub-Saharan Africa.1 Artemisinin-based combination therapy, the standard treatment for uncomplicated malaria, and intravenous artesunate, the standard for severe malaria, are threatened by the increasing prevalence of artemisinin partial resistance (ART-R), which has recently emerged in eastern Africa.2,3 Among synthetic endoperoxide-class agents evaluated over the past two decades,4 the adamantyl-1,2,4-trioxolane pharmacophore identified by Vennerstrom and co-workers5,6 has produced the only clinical candidates: arterolane (OZ277)7 which is approved as combination therapy in certain regions, and artefenomel (OZ439),8 which exhibits a superior exposure profile and predicted efficacy in ART-R (Figure 1).9,10 Arterolane, used in combination with piperaquine in some countries, has theoretical efficacy concerns, and has had rather limited clinical impact.11 The long clinical development of artefenomel was recently discontinued, after failing to reach clinical pharmacokinetic and pharmacodynamic benchmarks developed around an admirable, if challenging,12 goal of achieving single-exposure efficacy. As well, food effects,13 and complex solution-phase behavior14 produced formulation challenges that contributed to a difficult clinical path for artefenomel. Accordingly, there are currently no endoperoxide agents in the clinical pipeline. Herein we report the further evaluation of desymmetrized trioxolane analogues related to arterolane but based on trans3″ substitution with heteroaliphatic carbamate side chains © 2024 The Authors. Published by American Chemical Society Figure 1. Structure of dihydroartemisinin, arterolane, and the closely related trans-3″ carbamate chemotype explored herein and in our preliminary report.15 Received: Revised: Accepted: Published: 1764 July 25, 2024 August 29, 2024 August 29, 2024 September 5, 2024 https://doi.org/10.1021/acsmedchemlett.4c00365 ACS Med. Chem. Lett. 2024, 15, 1764−1770 ACS Medicinal Chemistry Letters pubs.acs.org/acsmedchemlett (Figure 1). We hypothesized that 3″ substitution with trans stereochemistry should offer similarly stability of the endoperoxide bridge as with traditional cis-4″ substitution found in arterolane and artefenomel (Figure 2). Indeed, in our Letter the S configuration (Scheme 1). Oxidation of the C−B bond and protection of the resulting alcohol as a tert-butyldiphenylsilyl (TBDPS) ether afforded ketone 4. This material undergoes diastereocontrolled Griesbaum co-ozonolysis with adamantan-2-one O-methyloxime, to afford the desired trans intermediate 5 in a 12:1 diastereomeric ratio (dr) as determined by 1H NMR analysis. Subsequent deprotection of 5 and conversion to the p-nitrophenylcarbonate 7 allowed for late-stage diversification into the desired (S,S)-trans-3″carbamate analogues 9a-dd. An analogous approach, but using the antipode of the Taniaphos ligand, was used to prepare (R,R)-trans-3″-carbamates 8a-dd. In our preliminary report,15 carbamate analogues 8a, 8b, 8c, 8d, and 8i were all found to be more efficacious than the arterolane control when administered as a once daily 2 mg/kg oral dose for 4 days (Chart 1). With a single higher dose of 40 mg/kg, analogues such as 8c, 8j, 8k, and 8n cured 20−60% of animals at day 30, as compared to 100% cures for single-dose artefenomel at this dosage. Importantly, all of the carbamate analogues evaluated showed good aqueous solubility, and many (e.g., 8a/b, 9a/b, 9c) exhibited excellent stability in human liver microsome (HLM) preparations. From this auspicious starting point, we sought to explore a more diverse array of carbamate side chains and to evaluate all of the analogues in both enantiomeric forms (Chart 1). Since only three analogues with (S, S) stereochemistry had been prepared in our initial study, we began by synthesizing the (S, S) forms of previously reported (R, R) analogues and evaluating these compounds for their antiplasmodial effects against W2 strain P. falciparum (Chart 1, 9d−n). Similar to the (R, R) forms 8d−n, the (S, S) stereoisomers demonstrated potent antiplasmodial effect, with EC50 values of the enantiomer pairs generally within 2-fold of each other. Noting the potent antiparasitic effect and favorable solubility of analogues bearing terminal primary amines, we next explored additional (R, R) and (S, S) analogues bearing 2-substituted ethylenediamine substitutions (8o−8r, 9o−9r) and found that these analogues as well retained potent, low-nM antiparasitic activity. Replacing the terminal amino function with hydroxyl, as in the change from aminooxetanes 8r/9r to hydroxyoxetanes 8s and 9s resulted in a ∼ 10-fold decrease in potency, suggesting a role for basic amines in promoting cellular uptake or retention, although the potent morpholine analogues 8f and 9f proved an exception to the wider trend. Figure 2. Conformational dynamics of trioxolane antimalarials determine their antiplasmodial effects, with the minor, peroxideexposed conformer (top right) undergoing Fenton-like reactivity with ferrous iron sources in the parasite, leading to a pharmacodynamic effect. Shown at bottom are the 1,3-diaxial interactions that disfavor the iron-reactive conformer in artefenomel (left (...truncated)