Structural elucidation and antiviral activity of cathepsin L inhibitors with carbonyl and epoxide warheads (preprint)

Emerging RNA viruses including SARS-CoV-2 continue to be a major threat around the globe. The cell entry of SARS-CoV-2 particles via the endosomal pathway involves the cysteine protease cathepsin L (CatL) among other proteases. CatL is rendered as a promising drug target in the context of different viral and lysosome-related diseases. Hence, drug discovery and structure-based optimization of inhibitors is of high pharmaceutical interest. We herein verified and compared the anti-SARS-CoV-2 activity of a set of carbonyl and succinyl-epoxide-based inhibitors, which have previously been identified as cathepsin inhibitors. Calpain inhibitor XII (CI-XII), MG-101 and CatL inhibitor IV (CLI-IV) possess antiviral activity in the very low nanomolar IC50 range in Vero E6 cells. Experimental structural data on how these and related compounds bind to CatL are however notably lacking, despite their therapeutic potential. Consequently, we present and compare crystal structures of CatL in complex with 14 compounds, namely BOCA (N-BOC-2-aminoacetaldehyde), CLI-IV, CI-III, CI-VI, CI-XII, the main protease alpha-ketoamide inhibitor 13b, MG-101, MG-132 as well as E-64d (aloxistatin), E-64, CLIK148, CAA0225, TC-I (CID 16725315) and TPCK at resolutions better than 2 Angstrom. Overall, the presented data comprise a broad and solid basis for structure-guided understanding and optimization of CatL inhibitors towards protease drug development.

Calpeptin is a potent cathepsin inhibitor and drug candidate for SARS-CoV-2 infections (preprint)

Several drug screening campaigns identified Calpeptin as a drug candidate against SARS-CoV-2. Initially reported to target the viral main protease (Mpro), its moderate activity in Mpro inhibition assays hints at a second target. Indeed, we show that Calpeptin is an extremely potent cysteine cathepsin inhibitor, a finding additionally supported by X-ray crystallography. Cell infection assays proved Calpeptin’s efficacy against SARS-CoV-2. Treatment of SARS-CoV-2-infected Golden Syrian hamsters with sulfonated Calpeptin at a dose of 1 mg/kg body weight significantly reduces the viral load in the trachea. Our results show that the inhibition of cathepsins, a protein family of the host organism, is a promising approach for the treatment of SARS-CoV-2 and potentially other viral infections. An intrinsic advantage in targeting host proteins is their mutational stability in contrast to highly mutable viral targets.

Catalytic cleavage of HEAT and subsequent covalent binding of the tetralone moiety by the SARS-CoV-2 main protease (preprint)

Here we present the crystal structure of SARS-CoV-2 main protease (Mpro) covalently bound to 2-methyl-1-tetralone. This complex was obtained by co-crystallization of Mpro with HEAT (2-(((4-hydroxyphenethyl)amino)methyl)-3,4-dihydronaphthalen-1(2H)-one) in the framework of a large X-ray crystallographic screening project of Mpro against a drug repurposing library, consisting of 5632 approved drugs or compounds in clinical phase trials. Further investigations showed that HEAT is cleaved by Mpro in an E1cB-like reaction mechanism into 2-methylene-1-tetralone and tyramine. The catalytic Cys145 subsequently binds covalently in a Michael addition to the methylene carbon atom of 2-methylene-1-tetralone. According to this postulated model HEAT is acting in a pro-drug-like fashion. It is metabolized by Mpro, followed by covalent binding of one metabolite to the active site. The structure of the covalent adduct elucidated in this study opens up a new path for developing non-peptidic inhibitors.