Population Pharmacokinetic-Pharmacodynamic Model of Nirogacestat Effects on B-cell Maturation Antigen in Healthy Subjects.

8027 Background: B-cell maturation antigen (BCMA) is expressed on the membrane of normal plasma cells (PCs) and multiple myeloma (MM) cells and is the target of several investigational agents and approved products for MM treatment. Mechanistically, BCMA is cleaved from the cell surface by the enzyme gamma secretase, resulting in reduced levels of membrane-bound BCMA (mbBCMA) and release of soluble BCMA. Gamma secretase inhibitors (GSIs) have been shown to increase BCMA receptor density and potentiate the activity of several BCMA-targeted therapies in both clinical trials and in vitro studies. Although the effect of GSIs on BCMA receptor density has been well characterized in vitro, their effect on BCMA dynamics is incompletely understood in humans. This study sought to evaluate the pharmacodynamics (PD) of the GSI nirogacestat on mbBCMA in healthy subjects. Methods: A total of 23 healthy subjects were administered single doses of 50, 150, or 300 mg nirogacestat or multiple doses of 100 mg every 12 hours. Density of mbBCMA on PCs isolated from whole blood and bone marrow was measured by flow cytometry at baseline and 1, 2, 4, 8, 24, and 48 hours after single doses and 24 and 48 hours after the first dose in subjects receiving multiple doses. Nirogacestat serum concentrations were also measured at multiple time points to fully characterize the pharmacokinetics (PK). A population PK-PD model of the effect of nirogacestat on BCMA dynamics in humans was developed and used to simulate various administration dose levels and schedules to find an optimal nirogacestat regimen that continually increases mbBCMA over the dosing interval. Results: A 2-compartment model with linear absorption and elimination described serum nirogacestat in healthy subjects. A sequential PK-PD model that included an effect compartment and drug effect on mbBCMA cleavage rate described mbBCMA values. Simulations indicated that a 100-mg dose administered twice daily sufficiently maintained elevated and clinically relevant mbBCMA levels. Exposure and efficacy benchmarks from in vitro experiments and published clinical studies were used to contextualize simulations and hypothesize various dose and schedule scenarios to minimize dose of the BCMA-targeting therapy while maximizing BCMA potentiation. Conclusions: These results suggest that nirogacestat dose and schedule may have direct implications on the potentiation of BCMA-targeting therapies. Simulations suggest that the 100-mg, twice-daily nirogacestat dose results in elevated mbBCMA levels that could even allow for use of lower doses of BCMA-targeted agents while maintaining or improving efficacy. Study results along with this PK-PD model will be used to support nirogacestat dose and schedule in ongoing and planned studies in patients with MM.