Repurposing Clinical Candidates for Nipah and Hendra Viruses.

Nipah virus (NiV) and Hendra virus (HeV) are bat-borne zoonotic paramyxoviruses that cause severe and often fatal respiratory and neurological disease in humans, for which no small-molecule therapies are currently approved. While host-protease-mediated viral entry has been implicated previously in henipavirus infection, the translational feasibility of targeting this pathway with clinically relevant compounds remains poorly defined. Here, we report a systematic evaluation of repurposed clinical-stage molecules using an integrated computational, in vitro, and in vivo framework to prioritize candidates with potential activity against NiV and HeV. Using machine-learning-guided prioritization and the targeted selection of known cathepsin L inhibitors, we evaluated multiple compounds for antiviral activity against NiV (Malaysia and Bangladesh strains) and HeV in cell-based assays. Potent and reproducible in vitro inhibition was observed for several cathepsin L inhibitors, with antiviral activity strongly correlating with cathepsin L potency and extending across henipaviruses. The clinical-stage cathepsin L inhibitor relacatib exhibited sub-micromolar to nanomolar inhibition of NiV and HeV and was substantially more potent than the nucleoside analogue control favipiravir in vitro. To assess translational feasibility, selected candidates were advanced to pharmacokinetic, tolerability, and efficacy studies in the Syrian hamster model of NiV infection. Despite favorable in vitro potency, neither relacatib nor pyronaridine conferred protection in vivo under the tested dosing regimens, a result in part attributable to limited unbound systemic exposure and rapid clearance rather than definitive biological inefficacy. Taken together, this study provides a clinically relevant assessment of host-directed entry inhibition for virulent henipaviruses, highlights key pharmacological barriers to in vivo translation, and offers empirical guidance for prioritizing future antiviral development efforts against NiV and HeV.