Evaluation of a monoclonal antibody against respiratory syncytial virus, clesrovimab, in infants and children: Comprehensive rationale and study design for the late-stage clinical trials

Background: Respiratory syncytial virus (RSV) is a leading cause of infant morbidity and mortality. Clesrovimab is a half-life-extended, RSV neutralizing monoclonal antibody for the prevention of RSV disease in infants. This article describes the methodology that enabled the acceleration of two pivotal late-stage clinical trials, CLEVER (MK-1654-004; NCT04767373) and SMART (MK-1654-007; NCT04938830), for the evaluation of clesrovimab. Methods: CLEVER is a placebo-controlled phase 2b/3 study in healthy preterm and full-term infants, evaluating the efficacy and safety of clesrovimab for the prevention of RSV-associated medically attended lower respiratory tract infection (RSV-MALRI) and RSV-associated hospitalization. SMART is a phase 3 palivizumab-controlled study evaluating the safety, tolerability, and efficacy of clesrovimab, compared with palivizumab, for the prevention of RSV-associated MALRI and RSV-associated hospitalization in infants and children at increased risk of severe RSV disease. Dose selection in these studies was informed using a model-based meta-analysis of phase 1 and 2 clesrovimab trial data. Program acceleration was enabled by designing CLEVER seamlessly, to rapidly progress from phase 2b to phase 3. Additionally, efficacy was extrapolated to the SMART population, based on pharmacokinetic bridging between CLEVER and SMART. Conclusion: The methodology of the accelerated late-stage development of clesrovimab, including the modelinformed dose selection approach, the seamless enrollment in the phase 3 portion of CLEVER, and the extrapolation of efficacy from the population in CLEVER to the population in SMART, may be used to inform future trial designs where acceleration is needed to address an unmet medical need.