Targeting the RSV and hMPV L Protein: Cryo-EM and Structure-Based Approaches to Antiviral Drug Discovery

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV), members of the family Pneumoviridae, represent a foremost global cause of acute lower respiratory tract infection in infants, young children, the elderly, and immunocompromised individuals. Despite the recent approval of preventive vaccines and monoclonal antibody prophylactics targeting the viral fusion protein, no widely adopted, RSV-specific direct-acting antiviral is currently approved for routine post-infection treatment. The large (L) protein of the viral RNA polymerase complex, which catalyzes genome replication and mRNA transcription in concert with its obligate cofactor, the phosphoprotein (P), constitutes an ideal drug target owing to its essential and multifunctional enzymatic activities and its absence from host cells. Over the past decade, Cryo-electron microscopy (Cryo-EM) has yielded a series of landmark structures of Pneumoviridae L–P complexes, including apo forms of RSV (at 3.2–3.67 Å) and hMPV (at 3.7 Å) polymerases, among the first promoter-bound non-segmented negative-sense (nsNSV) RNA virus polymerase structures (at 3.40–3.41 Å), and inhibitor-bound complexes that illuminate the molecular basis of non-nucleoside inhibitor (NNI) action at sub-nanomolar potency. This review synthesizes the structural biology of Pneumoviridae RNA polymerases from a chronological and mechanistic perspective, compares RSV and hMPV L protein active sites at near-atomic resolution, and critically evaluates how structural insights are being translated into next-generation antiviral drug candidates, including nucleoside analog inhibitors, allosteric non-nucleoside inhibitors, and emerging candidates at various stages of preclinical and clinical investigation.