Optimizing a small molecule inhibitor of SARS-CoV-2 replication and associated cytokine storm

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Our goal is to develop towards an IND a novel class of small molecule inhibitors of phosphoinositide (PI) 4 kinase IIIb (PI4KIIIb) with potent dual activity against both SARS-CoV-2 and the excess cytokine release associated with COVID-19 disease. Entry of SARS-CoV has been shown to depend on PI4KIIIb, and strong inhibition of entry was achieved following knockdown of PI4KIIIb via siRNA, and SARS-CoV-2 is believed to enter cells via a similar mechanism. This likely reflects a requirement for enrichment of phosphorylated isoforms of PI, such as PI-4, in the lipid organelle required for viral fusion upon entry. We have developed potent and specific small molecule inhibitors of PI4KIIIb, and optimized them for high oral bioavailability. Our lead inhibitor, STF-1019 has nanomolar efficacy against enteroviruses (EV) which are also dependent on PI4KIIIb, and is the only molecule to have demonstrated in vivo efficacy in the animal model of EV-71, and without toxicity. We have now shown that STF-1019’s EC50 against SARS-CoV-2 is 210 nM, with a CC50 of >100 microM, reflecting a therapeutic index (TI) of ~500. Finally, likely due to PI4KIIIb’s role in Golgi-mediated secretion, we have also recently shown that STF-1019 can potently inhibit the LPS-induced secretion of IL-6 from human PBMC. STF-1019’s metabolic stability, however, is suboptimal, requiring co-administration with an inhibitor (i.e. ritonavir) of its metabolism by CYP3A4 for optimal sustained tissue exposure. We hypothesize that: 1) STF-1019’s SAR and major metabolites indicates that our lead PI4KIIIb inhibitor can be further optimized to increase its activity and metabolic stability to achieve an optimal exposure profile; 2) modifications that further increase PI4KIIIb inhibition can provide a buffer for modifications that may increase metabolic stability at the expense of efficacy; 3) the optimized inhibitor will inhibit SARS-CoV-2 in vitro, and in vivo; 4) the optimized inhibitor will have a high barrier to the development of resistance; 5) because of its orthogonal mechanism of action, our PI4KIIIb inhibitor can be used in combination with other agents to maximize efficacy; 6) STF-1019’s inhibition of IL-6 reflects an ability to modulate the release of other cytokines, and this non- antiviral activity can be of great additional benefit in addressing the cytokine storm associated with severe COVID-19 infection; 7) determination of key pharmacokinetic, in vitro ADME-Tox parameters and initial preclinical in vivo toxicity assessment of our optimized lead can advance its translational development, and form the basis of a future IND package. We propose the test these hypotheses by: 1) Identifying the STF-1019 analog (and back-up compound) with greatest in vivo trough:EC90 ratios; 2) determining the in vivo activity of the optimized PI4KIIIb inhibitors against SARS-CoV-2 and their effect on cytokine production; 3) determining the relative barrier to resistance, and potential for synergy with other agents; and 4) nominating a PI4KIIIb inhibitor IND candidate by subjecting the optimized lead to initial in vitro ADME-tox and IND-enabling preclinical animal safety studies.