N‐Substituted Pyrrole-Based Heterocycles as Broad-Spectrum Filoviral Entry Inhibitors
J. Med. Chem.2024, 67, 13737-13764
Emerging pathogenic viruses of pandemic potential remain a global public health concern. In recent decades, outbreaks of 2 significant members of the Filovirus family (Filoviridae): Ebola virus (EBOV) and Marburg virus (MARV) have occurred, resulting in thousands of deaths in some cases. Host macrophages and dendritic cells are highly susceptible to these viruses, causing immune system dysregulation (MAPK activation), which can lead to serious complications, such as lethal multiorgan failure. Mortality rates for EBOV and MARV range between 40-90% of those infected, according to current estimates (See doi.org/10.1007/s001140050562 and www.cdc.gov/ebola/outbreaks/index.html).
With a substantial outbreak potential, the threat of emerging filoviruses warrants development of broad-spectrum antiviral drugs for prophylaxis, treatment and virus eradication. Vaccines and monoclonal antibody therapies (mAbs) for EBOV have been approved by the FDA, but these are not orally administered, and lack cross-protection against other filoviruses of concern, including MARV, Sudan virus (SUDV), Bundibugyo virus (BDBV) and Tai Forest virus (TFV). Small molecule therapeutics have the added advantage over vaccines and mAbs in that they can target numerous steps throughout the virus life cycle, rather than just the early stages of entry (assuming all compartments are accessible to the small molecule in question).
Due to the risks associated with handling infectious and potentially lethal pathogens, the authors opted for a pseudovirus system for initial screening, based on a HIV viral vector expressing the filovirus glycoprotein (GP), which controls virus entry and host tropism.
The start point for this study was a series of furopyrrole compounds (CBS1111, see figure 1), discovered to have anti-filoviral activity (pseudo EBOV (pEBOV) and infectious EBOV). This was developed by thorough SAR studies against mainly pEBOV and pMARV, but also later screened against pSUDV, pBDBV and pTFV. Final (lead) compounds were tested against infectious EBOV, SUDV and MARV showing reasonable potency and also lending the pseudovirus assay a degree of validation.
Figure 1: Hit to lead strategy for the development of antifiloviral compound CBS-1111(doi.org/10.1177/1087057113494405)
During development, it was noted that having a tethered basic group in region B (figure 1) was likely to help it to accumulate in the acidic endosome, which is the main site of filoviral accumulation and entry. The authors observed a loss of antiviral activity upon changing the amine linker to an aromatic ring (although this was partially confounded by cytotoxicity) where decreased basicity of the amine and thus less accumulation in the endosome were blamed. While plausible, there are other reasons why this structural change could alter the antiviral activity. Perhaps a fluoroalkyl-substituted amine or a morpholine could better probe the effects of amine basicity on antiviral activity / endosomal accumulation?
This of course represents an important fundamental point to consider in target-based drug discovery: Target inhibition and cellular activity are not guaranteed to agree with one another from the start (or at any point, for that matter). There are all sorts of nuanced reasons for this and accumulation in particular portions of the cell is one of them. Another source of complication, is drug induced phospholipidosis (PLD). This is where phospholipid homeostasis and metabolism are disrupted in the cell by the pharmaceutical agent. This can result in false positive results in-vitro that do not translate to in-vivo efficacy. As a result, there are now assays to screen for this and that’s exactly what the authors did with their antifiloviral compounds. The tested compounds were found to induce PLD, but only at concentrations that were 10x higher than their antiviral EC50s, so the compounds were progressed to further optimisation.