Selection of GalNAc-conjugated siRNAs with limited off-target-driven rat hepatotoxicity

Small interfering RNAs (siRNAs) conjugated to a trivalent N -acetylgalactosamine (GalNAc) ligand are being evaluated in investigational clinical studies for a variety of indications. The typical development candidate selection process includes evaluation of the most active compounds for toxicity in rats at pharmacologically exaggerated doses. The subset of GalNAc-siRNAs that show rat hepatotoxicity is not advanced to clinical development. Potential mechanisms of hepatotoxicity can be associated with the intracellular accumulation of oligonucleotides and their metabolites, RNA interference (RNAi)-mediated hybridization-based off-target effects, and/or perturbation of endogenous RNAi pathways. Here we show that rodent hepatotoxicity observed at supratherapeutic exposures can be largely attributed to RNAi-mediated off-target effects, but not chemical modifications or the perturbation of RNAi pathways. Furthermore, these off-target effects can be mitigated by modulating seed-pairing using a thermally destabilizing chemical modification, which significantly improves the safety profile of a GalNAc-siRNA in rat and may minimize the occurrence of hepatotoxic siRNAs across species. A subset of chemically-modified siRNAs conjugated to trivalent GalNAc may fail during nonclinical development due to rat hepatotoxicity. Here, the authors show that hepatotoxicity may be accounted for by microRNA-like off-target effects of siRNA and can be mitigated by a thermally destabilizing modification in the siRNA seed region.