Humanized Na_V1.8 rats overcome cross-species potency shifts in developing novel Na_V1.8 inhibitors

Voltage-gated sodium channel isoform 1.8 (Na_V1.8) has emerged as a promising pharmaceutical target for the treatment of acute and chronic pain. However, highly selective and potent inhibitors for this channel have been difficult to develop and only recently have advanced to clinical testing. Our efforts to develop Na_V1.8 small molecule inhibitors yielded a series of molecules with favorable in vitro potency and selectivity against the human Na_V1.8 channel but exhibited dramatic rightward potency shifts against the rodent channel, severely limiting in vivo screening and candidate selection. In anticipation of supporting drug discovery efforts, a transgenic rat line expressing the human Na_V1.8 channel in lieu of the rodent channel was developed. Utilizing these humanized animals, the in vitro potency of our chemical matter in freshly isolated humanized rat DRG neurons was consistent with in vitro human potency values, enabling in vivo work to progress. We demonstrate capsaicin-induced nocifensive behaviors (CNB) as a moderate throughput in vivo screening assay, from which we demonstrate pharmacokinetic-pharmacodynamic (PK-PD) and in vitro-in vivo correlation (IVIVC) relationships. We identified MSD199 as a potent Na_V1.8 inhibitor with acute pain efficacy and assessed it in traditional inflammatory (Complete Freund's Adjuvant) and neuropathic (spinal nerve ligation) behavioral chronic pain assays where it was shown to significantly reduce pain-related behaviors. Overall, we demonstrate the utility of humanized transgenic animals when cross-species potency shifts are observed within an otherwise promising chemical series.