Innovative Strategy toward Mutant CFTR Rescue in Cystic Fibrosis: Design and Synthesis of Thiadiazole Inhibitors of the E3 Ligase RNF5

In cystic fibrosis (CF), deletion of phenylalanine 508(F508del)in the CF transmembrane conductance regulator (CFTR) is associatedto misfolding and defective gating of the mutant channel. One of themost promising CF drug targets is the ubiquitin ligase RNF5, whichpromotes F508del-CFTR degradation. Recently, the first ever reportedinhibitor of RNF5 was discovered, i.e., the 1,2,4-thiadiazol-5-ylidene inh-2. Here, we designed and synthesized a series of new analoguesto explore the structure-activity relationships (SAR) of thisclass of compounds. SAR efforts ultimately led to compound 16, which showed a greater F508del-CFTR corrector activity than inh-2, good tolerability, and no toxic side effects. Analogue 16 increased the basal level of autophagy similar to whathas been described with RNF5 silencing. Furthermore, co-treatmentwith 16 significantly improved the F508del-CFTR rescueinduced by the triple combination elexacaftor/tezacaftor/ivacaftorin CFBE41o(-) cells. These findings validate the 1,2,4-thiadiazolylidenescaffold for the discovery of novel RNF5 inhibitors and provide evidenceto pursue this unprecedented strategy for the treatment of CF.