(S-102) Mathematical Modeling to Optimize Receptor Occupancy Assay Development for Isatuximab

Min Hai – The Ohio State University; Justin Thomas – The Ohio State University; Nicole Abbott – The Ohio State University; Francesca Cottini – The Ohio State University; Douglas Sborov – Huntsman Cancer Institute; Karen Sweiss – University of Illinois Chicago; Mitch Phelps – The Ohio State University

Objectives: Multiple myeloma (MM) is a hematologic malignancy caused by abnormal plasma cells overproducing monoclonal immunoglobulins. Isatuximab, a chimeric immunoglobulin G1 monoclonal antibody, binds to human cluster of differentiation 38 (CD38) receptor and is approved in frontline and relapsed/refractory MM (RRMM). Receptor occupancy (RO, the proportion of total receptors occupied by a drug), is a key measure of target engagement and saturation. In a previous phase I clinical study evaluating isatuximab in patients with RRMM, RO was correlated with clinical response, but plateaued at only 80% [1]. The RO needed for clinical efficacy across other phases of MM treatment, such as the maintenance phase, remains unknown. In the context of a currently enrolling phase II study, “Post-Autologous Transplant Maintenance with Isatuximab and Lenalidomide in Minimal Residual Disease Positive Multiple Myeloma” (NCT05344833), our goal is to characterize isatuximab RO in the maintenance phase to inform its optimal use.

Methods: A series of anti-CD38 clones and labeled isatuximab were evaluated as spectral flow cytometry detection reagents to analyze free and total CD38 receptors, respectively. To support in vitro RO assay development and streamline conversion of flow cytometry data to functional binding estimates, we implemented a target-mediated drug disposition (TMDD) model using the rxode2 package in R to simulate isatuximab-CD38 binding dynamics, incorporating receptor turnover, internalization, and pharmacokinetics. Literature-informed parameters were used to explore RO with various combinations of CD38 expression levels and isatuximab concentrations. Simulations were performed to mimic the in vitro assay and gain insight into assay sensitivity and dynamic range.

Results: Our in-house labeled isatuximab, together with anti-human CD38 antibody, clone HB-7, adequately provided measurements of free and total CD38 receptors, respectively, as well as the ability to calculate RO for individual cells. Initial simulations showed a nonlinear, saturable relationship between isatuximab concentration and RO, consistent with expected target-mediated binding dynamics. Variations in kinetic parameters influenced assay sensitivity and the shape of the RO-concentration curve. Depending on the endpoint (median fluorescence intensity vs individual cell data), we observed significant discrepancies between theoretical and true RO for each simulated scenario. These in silico results are currently being used to guide experimental design, including selection of concentration ranges and detection thresholds.

Conclusions: Absolute quantification of CD38 receptors and RO determination in samples from patients enrolled in NCT05344833 are currently ongoing. To support assay development, we applied TMDD modeling to provide a mechanistic framework for simulating isatuximab-CD38 interactions. These simulations enable rational selection of experimental conditions and improve the interpretability of the RO assay results. Integration of modeling results with ongoing assay optimization will help improve the interpretation of RO measurements in patient samples and facilitate future quantitative PK/PD modeling.

Citations: [1] Martin, T.; Strickland, S.; Glenn, M.; Charpentier, E.; Guillemin, H.; Hsu, K.; Mikhael, J. Phase I Trial of Isatuximab Monotherapy in the Treatment of Refractory Multiple Myeloma. Blood Cancer J. 2019, 9 (4), 41. https://doi.org/10.1038/s41408-019-0198-4.

Keywords: target-mediated drug disposition, receptor occupancy, monoclonal antibody