ABL and SRC inhibitors
ABL kinase inhibitors have been demonstrated to inhibit replication of several unrelated viruses at different stages of their life cycle, including the coxsackie virus and dengue, Ebola, and vaccinia, in in vitro cell-based studies (Table 1) (22) (23) (24) (25) (26). For the coxsackie virus, ABL is activated following attachment of the virus to the glycosylphosphatidylinositol (GPI)-anchored protein decay-accelerating factor (DAF) on the apical cell surface; the ABL activation in turn triggers Rac-dependent actin reassembly that allows delivery of the virus to the tight junction (22). FYN kinase is also activated in response to viral attachment to DAF, and this leads to phosphorylation of the plasma membrane protein, caveolin, and viral transport into the cell through caveolin-containing vesicles (22). Activation of ABL by the coxsackie virus and the role ABL plays in viral infection are independent of SRC kinases (22), whereas in contrast ABL kinases partner with SRC family kinases to stimulate the actin-based movement of vaccinia virus (23). In the case of Ebola virus, regulation of viral replication by ABL1 was demonstrated by ABL1-specific siRNA inhibition of the release of virus-like particles in a cell culture co-transfection system; nilotinib also showed antiviral activity in this assay, at μM concentrations that were not cytotoxic (24). In vivo antiviral efficacy of imatinib was shown in a model of vaccinia virus; testing of imatinib in this model was based on the demonstrated involvement of ABL in release of cell-associated enveloped virions from the host cell (25). In this study, a dose of 200 mg/kg/day of imatinib was able to reduce the number of viral genome copies by around 4 logs (25). Lack of efficacy of dasatinib in the same model was attributed to immunotoxicity due to Src inhibition, however it is believed that dasatinib could still be a candidate coronavirus treatment with a dosing regimen that effectively blocks viral dissemination while exhibiting minimal Src-related immunotoxicity (27).
The ABL inhibitors, imatinib and dasatinib, were identified in a screen as inhibitors of both SARS-CoV and MERS-CoV replication, and nilotinib was identified as an inhibitor of only SARS-CoV, in vitro (27). Investigation of the mechanism for imatinib against SARS-CoV and MERS-CoV revealed inhibition of the early stages of the virus life cycle, and inhibition of viral replication through blocking the fusion of the coronavirus virion with the endosomal membrane (28) (29). Importantly, authors show that targeted knockdown of ABL2, however not ABL1, significantly inhibited SARS-CoV and MERS-CoV replication/entry in vitro (29). The relatively high, albeit minimally toxic, μM range concentrations of imatinib and dasatinib required to inhibit SARS-CoV and MERS-CoV in the aforementioned cell-based studies may be attributable to experimental factors such as drug resistance of the cell lines used as tools for propagating the viruses (27) (29), and thus in vivo testing would be needed to determine optimal dosing. It is worth noting that in many cell-based assays measuring drug effects on virus titer, the antiviral activity is cell-type dependent, and there is also variability depending on which virus strain is used. Recent, unpublished results, reported as a preprint, suggest that imatinib inhibits SARS-CoV-2 in vitro, among 17 other FDA-approved drugs with IC50 values similar to those observed for SARS-CoV and MERS-CoV; concentrations showing antiviral activity were not cytotoxic (BioRxiv, 2020, 10.1101/2020.03.25.008482)
As discussed above, infection by SARS-CoV is the result of several steps, including receptor binding, S glycoprotein conformational alterations, and proteolysis within endosomes that is mediated by capthepsin L (30). SARS-CoV infection has been shown to be blocked by targeted inhibitors of cathepsin L (30). On a related note, it has been shown that complete inhibition of viral entry and replication can result from treatment of cells with a cathepsin inhibitor as well as treatment with the serine protease inhibitor, camostat, which blocks activity of the type II transmembrane serine protease (TTSP) TMPRSS2, a surface-expressed serine protease that cleaves the coronavirus S protein and is involved in viral entry into a host cell (31). It has been proposed that imatinib may inhibit the function, localization or activity of TMPRSS2 (29). This suggests that this may be a promising drug:target match that could be further explored as a potential treatment for SARS-CoV-2 infection, since SARS-CoV-2 uses the SARS-CoV receptor ACE2 and the protease TMPRSS2 to enter host cells. In addition, ABL and ARG kinases have been found, in cancer cells, to promote secretion of the endosomal protease cathepsin L (30) (32). Thus, the testing of the ability of ABL inhibitors to inhibit cathepsin L in the context of viral infection may be warranted. It may generally be worthwhile to evaluate each of these targets with respect to what is known about SARS-CoV-2 infection and conduct further studies to elucidate potential therapeutic approaches involving ABL inhibition.
Several of the SRC family kinases have been implicated in replication of viruses, including those related to SARS-CoV-2, as well as unrelated viruses. The ABL/SRC inhibitor, saracatinib, has been shown to inhibit MERS-CoV at early stages of the viral life cycle, at μM range concentrations (33). In this study, siRNA knockdown of SRC family proteins, LYN and FYN, the latter implicated in coxsackievirus entry through epithelial tight junctions (22), led to significant reductions in MERS-CoV titer, suggesting these proteins may be important for MERS-CoV replication (33). Saracatinib was also shown to synergize with gemcitabine, which also exhibits anti-MERS-CoV activity (33). SRC has been shown, through siRNA knockdown, to be important for replication of dengue virus; dasatinib inhibited dengue infection by preventing infectious virus particle formation within the virus replication complex (34) (35). Saracatinib and dasatinib were shown to exhibit activity against dengue virus in vitro, with FYN implicated as a target for RNA replication (36). YES was demonstrated, through genetic knockdown, to reduce West Nile virus titers through effects on the viral replication cycle and to attenuate viral assembly and egress (37). Finally, siRNA library screenings focused on identifying host factors required for replication of HCV and dengue revealed c-terminal SRC kinase (Csk) as being important (38) (35).