Functional Imaging As a Key Diagnostic Tool in the Follow-up of RRMM Patients after CAR-T-Cell Therapy

Introduction:

Anti BCMA CAR T-cell therapy (CART) is establishing itself as a new standard of care for relapsed and refractory multiple myeloma (RRMM). However, the optimal follow-up strategy post CART has yet to be determined. Prior studies of our own and others have emphasized the role of functional imaging in monitoring RRMM patients post traditional chemotherapy, revealing residual disease in up to 50 % of otherwise minimal residual disease (MRD) negative patients (Rasche et al. Leukemia 2019). Whether a similar patchy disease pattern is relevant post CART remains unknown. To address this, we implemented a standardized follow-up program post CART, which includes 18F-FDG PET-CT, diffusion-weighted MRI, and bone marrow aspiration with MRD measurements at our center.

Methods:

We conducted monthly ambulatory visits and serological testing to monitor patients post CART infusion. Additionally, an extended follow-up visit at three months post CART infusion was performed, encompassing whole body PET-CT, whole body diffusion-weighted MRI, and bone marrow aspiration with MRD testing. Response assessments were based on IMWG criteria. MRI classified focal lesions as vital if they exhibited 1) significant diffusion-restriction, 2) a fat fraction less than 20 %, and 3) homogenous hypointensity on T2-weighted images. MRD measurements were performed with flow cytometry with a sensitivity of at least 10 ^-5 following the Euroflow recommendations.

Results:

From December 2021 to July 2023 we treated 37 heavily pretreated patients with idecabtagene vicleucel at our center. These patients had a median of 5 prior lines of therapy. High-risk cytogenetics, extra-medullary disease (EMD), and non-secretory disease were seen in 17, 14 and 6 patients, respectively. At data cut off, the ORR, VGPR and CR rates were 78 %, 27 % and 27 %, respectively. We observed primary refractory disease in 14 % of the patients. Among the 22 patients who underwent extended follow-up examinations at three months, the majority achieved MRD negativity, with only 7/22 (32 %) remaining MRD positive after CART. However, functional imaging revealed a higher number of patients with residual disease, with positive results on PET-CT and DW-MRI in 9/22 (41 %) and 10/22 (45 %) of patients, respectively. Notably, 6 of 22 (27 %) patients displayed active lesions on imaging despite MRD negativity at the iliac crest at this time point.

Next, we focused on patients who experienced relapse from CART and sought to determine the necessary technology to detect relapse in this setting. Thirteen patients experienced relapse during the study period, and in 8 (61%) of them, functional imaging was the sole method to detect progressive disease. Only 1 patient transitioned from MRD negative to positive during the follow-up period, while the remaining 4 patients showed a significant M protein increase as defined by the IMWG criteria.

Conclusion:

In our heavily pretreated RRMM population, functional imaging emerged as a crucial diagnostic tool, detecting relapse alone in more than half of the patients. Our data supports inclusion of cross-sectional imaging in a comprehensive follow-plan after CART.