6′-β-Fluoro-Homoaristeromycin and 6′-Fluoro-Homoneplanocin A Are Potent Inhibitors of Chikungunya Virus Replication through Their Direct Effect on Viral Nonstructural Protein 1

Alphaviruses are arthropod-borne, positive-stranded RNA viruses capable of causing severe disease with high morbidity. Chikungunya virus (CHIKV) is an alphavirus that causes a febrile illness which can progress into chronic arthralgia. The current lack of vaccines and specific treatment for CHIKV infection underscores the need to develop new therapeutic interventions. To discover new antiviral agents, we performed a compound screen in cell culture-based infection models and identified two carbocyclic adenosine analogues, 6′-β-fluoro-homoaristeromycin (FHA) and 6′-fluoro-homoneplanocin A (FHNA), that displayed potent activity against CHIKV and Semliki Forest virus (SFV) with 50% effective concentrations in the nanomolar range at nontoxic concentrations. The compounds, designed as inhibitors of the host enzyme S-adenosylhomocysteine (SAH) hydrolase, impeded postentry steps in CHIKV and SFV replication. Selection of FHNA-resistant mutants and reverse genetics studies demonstrated that the combination of mutations G230R and K299E in CHIKV nonstructural protein 1 (nsP1) conferred resistance to the compounds. Enzymatic assays with purified wild-type (wt) SFV nsP1 suggested that an oxidized (3′-keto) form, rather than FHNA itself, directly inhibited the MTase activity, while a mutant protein with the K231R and K299E substitutions was insensitive to the compound. Both wt nsP1 and the resistant mutant were equally sensitive to the inhibitory effect of SAH. Our combined data suggest that FHA and FHNA inhibit CHIKV and SFV replication by directly targeting the MTase activity of nsP1, rather than through an indirect effect on host SAH hydrolase. The high potency and selectivity of these novel alphavirus mRNA capping inhibitors warrant further preclinical investigation of these compounds.