P-128: 68Ga-Pentixafor PET/CT-based response assessment in multiple myeloma patients and its comparison with 18F-FDG PET/CT and IMWG based clinical response.

Introduction: ¹⁸F-FDG PET/CT is the currently recommended modality for response evaluation in patients with multiple myeloma (MM). However, it ails from low sensitivity and also shows early normalisation to therapy in patients with myeloma. ⁶⁸Ga-Pentixafor PET/CT; a CXCR4 targeted radiotracer has shown higher diagnostic accuracy compared to ¹⁸F-FDG PET/CT in MM, however, its utility in response assessment has not been evaluated. Our study evaluates the utility of ⁶⁸Ga-Pentixafor PET/CT for response evaluation and survival outcomes in patients with MM.

Methods: Forty (24M:16F; median age=56years; range=34-81years) treatment naive MM patients were recruited. ⁶⁸Ga-Pentixafor and ¹⁸F-FDG PET/CT imaging was performed at baseline and at median time of 7.2 months of induction therapy. The clinical response (IMWG criteria) was classified as complete response (CR) that included sCR and VGPR, partial response (PR), stable disease (SD) and progressive disease (PD) respectively. The PET/CT imaging data was reconstructed and semi-quantitative parameters (SUV_max) along with image based response (CR, PR, SD and PD) were evaluated. Progression-free survival (PFS) and overall survival (OS) was assessed and survival differences between groups were compared using Kaplan-Meier survival curves.

Results: Baseline ⁶⁸Ga-Pentixafor PET/CT was positive in a significantly higher number of patients (39/40; 97.5%) as compared to ¹⁸F-FDG PET/CT (31/40; 77.5%) (p=0.021). ⁶⁸Ga-Pentixafor PET/CT also showed significantly (p< 0.001) higher (by factor of 2.5) SUV_max values than ¹⁸F-FDG PET/CT (18.1±16.5 vs. 7.1±6.0). As per IMWG criteria, 34/40 (85.0%) patients showed CR, 5/40 (12.5%) PR and 1/40 (2.5%) showed PD. On ¹⁸F-FDG PET/CT based response evaluation, 21/31 (67.7%) patients showed CR, 7/31 (22.6%) PR and 3/31 (9.7%) showed PD. ⁶⁸Ga-Pentixafor PET/CT evaluation showed CR in 13/39 (33.3%), PR in 23/39 (60.0%) and PD in 3/39 (7.7%) patients. All three response assessment criteria demonstrated significant difference (log rank test p< 0.05) in PFS among their respective categories. ⁶⁸Ga-Pentixafor PET/CT identified 5/6 patients (83.3%) demonstrating poor response (PR, SD and PD group) whereas ¹⁸F-FDG PET/CT identified 4/6 (66.6%). ¹⁸F-FDG PET/CT and IMWG classified a higher proportion (67.7% & 85.0% respectively) of patients as CR, whereas ⁶⁸Ga-Pentixafor PET/CT demonstrated 60.0% patients as PR. On sub-analysis, ⁶⁸Ga-Pentixafor PET/CT further classified 34 patients with CR on IMWG into CR in 12/34, PR in 19/34 and PD in 2 patients (1-negative) which showed significant difference in PFS. Similarly, 21 patients with CR on ¹⁸F-FDG PET/CT were further classified into CR in 8 and PR in 12 patients (1-negative) which also showed significant difference in PFS.

Conclusions: ⁶⁸Ga-Pentixafor PET/CT showed excellent performance in PET-based response assessment and prediction of PFS in MM patients. Combining ⁶⁸Ga-Pentixafor with ¹⁸F-FDG PET/CT may provide a comprehensive response assessment and survival analysis in these patients.