Covalent fragments vs chikungunya nsP2

Perhaps because it sounds like “chicken,” when I first heard of chikungunya I thought it was a joke. But there’s nothing funny about a disease whose name comes from a word meaning “to become contorted,” referring to contortion caused by pain, which can last for months. The mosquito-borne alphavirus was first identified in 1952 in West Africa, introduced to the Americas in 2013, and is now spreading rapidly worldwide. There is no specific treatment. In three recent papers, a large group of researchers mostly from the Structural Genomics Consortium take the first steps towards one.

 

Like many viruses, the chikungunya genome encodes polyproteins that are cleaved by viral proteases, in this case a domain of the nonstructural protein 2 (nsP2). This cysteine protease is essential for viral replication, and the three papers collectively describe finding and exploring selective probes against it.  

 

In Proc. Nat. Acad. Sci. USA, Kenneth Pearce (University of North Carolina at Chapel Hill) and collaborators describe a screen of 6120 covalent fragments from Enamine against this target. Compounds were preincubated in a FRET-based functional assay at 20 µM for 30 minutes, resulting in 153 hits that inhibited activity by at least 50%. 43 of these were repurchased for full-dose response curves, and 20 of these had IC₅₀ values < 20 µM. Of these, compound RA-0002034 was the most potent, with IC₅₀ = 180 nM.

The proper way to assess irreversible covalent inhibitors is not the time-dependent IC₅₀, but rather the (theoretically) invariant k_inact/K_I ratio. The researchers measured this for the best hits and found the value for RA-0002034 to be 6400 M^-1s^-1, which is not far below that for the approved covalent drug sotorasib for its target.

 

Mass spectrometry experiments after tryptic digestion revealed the compound binds to the catalytic cysteine of nsP2, as expected, and not to other cysteines. RA-0002034 contains a potentially reactive vinyl sulfone warhead, but the half-life against the biologically relevant nucleophile glutathione is a respectable 130 minutes. A screen against 13 other cysteine proteases was also quite clean, as was chemoproteomic profiling in human cells.

 

The compound was also tested in cellular viral replication assays and found to be remarkably potent, with a low nanomolar EC₅₀ value. Encouragingly, it was also potent against three other alphaviruses, Ross River virus, Venezuelan Equine Encephalitis virus, and Mayaro virus.

 

RA-0002034 appears to be an attractive chemical probe for exploring the biology of chikungunya. Best practices are to also have an inactive control molecule, and the researchers made a substitution off the central pyrazole ring to produce RA-0003161, which is 500-fold less active.

 

The paper includes some SAR-by-catalog, and the chemistry is more extensively explored in an open-access J. Med. Chem. paper by Timothy Willson (UNC Chapel Hill) and collaborators. Although no crystal structures of the compounds bound to nsP2 were available, the researchers used modeling to guide modification of all portions of the molecule. The most potent molecule was 8d, which is slightly more active than RA-0002034. Also, methyl substitution near the electrophilic center is tolerated, which could improve stability, as seen with the covalent WRN inhibitor from Vividion which we wrote about here.

 

One annoying feature of RA-0002034 is its tendency to cyclize to inactive compound 2, a process explored in an open-access Pharmaceuticals paper by Timothy Willson and collaborators. This occurs even at neutral pH. However, replacing the central pyrazole with an isoxazole (compound 10) fixes this problem.

 

Collectively these three publications provide new insights and tools for investigating chikungunya. RA-0002034 is a far more attractive starting point than a molecule Teddy described on Practical Fragments back in 2015. The pharmacokinetics of RA-0002034 need to be improved before in vivo experiments are warranted, but this seems achievable, and I look forward to watching this story develop.