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 associated
to misfolding and defective gating of the mutant channel. One of the
most promising CF drug targets is the ubiquitin ligase RNF5, which
promotes F508del-CFTR degradation. Recently, the first ever reported
inhibitor of RNF5 was discovered, i.e., the 1,2,4-thiadiazol-5-ylidene inh-2. Here, we designed and synthesized a series of new analogues
to explore the structure–activity relationships (SAR) of this
class 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 what
has been described with RNF5 silencing. Furthermore, co-treatment
with 16 significantly improved the F508del-CFTR rescue
induced by the triple combination elexacaftor/tezacaftor/ivacaftor
in CFBE41o– cells. These findings validate the 1,2,4-thiadiazolylidene
scaffold for the discovery of novel RNF5 inhibitors and provide evidence
to pursue this unprecedented strategy for the treatment of CF.