Cyclophilins are peptidyl-prolyl cis/trans isomerases (PPIase) that catalyse the interconversion of the peptide bond at proline residues. Several cyclophilins play a pivotal role in the life cycle of a number of viruses. The existing cyclophilin inhibitors, all derived from cyclosporine A or sanglifehrin A, have disadvantages, including their size, potential for side effects unrelated to cyclophilin inhibition and drug–drug interactions, unclear antiviral spectrum and manufacturing issues. Here we use a fragment-based drug discovery approach using nucleic magnetic resonance, X-ray crystallography and structure-based compound optimization to generate a new family of non-peptidic, small-molecule cyclophilin inhibitors with potent in vitro PPIase inhibitory activity and antiviral activity against hepatitis C virus, human immunodeficiency virus and coronaviruses. This family of compounds has the potential for broad-spectrum, high-barrier-to-resistance treatment of viral infections.

Silibinin A and silibinin B, as well as Legalon SIL, inhibit HCV replicon and JFH1 replication in cell culture. This effect is at least partly explained by the ability of these compounds to inhibit HCV RNA-dependent RNA polymerase activity. Our results provide a basis for the optimization and subsequent development of members of the Flavonoid family as specific HCV antivirals.

24 25 Cyclophilins play a key role in the lifecycle of coronaviruses. Alisporivir (Debio 025) is 26 a non-immunosuppressive analogue of cyclosporin A with potent cyclophilin inhibition 27 properties. Alisporivir reduced SARS-CoV-2 RNA production in a dose-dependent manner in 28 VeroE6 cell line, with an EC 50 of 0.46±0.04 µM. Alisporivir inhibited a post-entry step of the 29 SARS-CoV-2 lifecycle. These results justify that a proof-of-concept Phase 2 trial be rapidly 30 conducted with alisporivir in patients with SARS-CoV-2 infection. 31 32 33 on June 9, 2020 by guest http://aac.asm.org/ Downloaded from 101 SARS-CoV-2 life cycle through mechanisms that remain to be unraveled. These results justify 102 that a proof-of-concept Phase 2 trial be rapidly conducted to assess the antiviral properties 103 and the effect of alisporivir on COVID-19 clinical outcomes in infected patients. 104 on June 9, 2020 by guest http://aac.asm.org/ Downloaded from 6Alisporivir has been shown to be well tolerated when administered as a 105 monotherapy 12 . Preclinical pharmacology data indicate that, after oral administration, 106 alisporivir is widely distributed in the whole body, including the lungs, and that its EC 90 107 against SARS-CoV-2 in VeroE6 cells is clinically achievable in patients. In addition, because 108 alisporivir inhibits all cellular cyclophilins, it also blocks mitochondrial cyclophilin-D, a key 109 regulator of mitochondrial permeability transition pore (mPTP) opening, a mechanism 110