Whole Body CD8+ T Cell PET Imaging in Patients with DLBCL before and during Anti-CD19 CAR T Cell Therapy

CAR T cell therapy (CART) has advanced the treatment of patients (pts) with refractory/relapsed diffuse large B-cell lymphoma (RR DLBCL), but long-term disease control eludes most recipients. Better clinical outcomes are associated with dense CD8⁺ T cell tumor immune contexture prior to treatment, high CD8⁺ fraction in the drug product, and high CD8⁺ CAR T cell expansion in blood (Scholler et al. 2022; Monfrini et al. 2022). Nevertheless, knowledge of CD8⁺ T cell dynamics and distribution in vivo before and during CART is lacking. Therefore, a single center clinical trial (NL9034) was conducted utilizing a ⁸⁹Zirconium-labeled one-armed anti-CD8 antibody (⁸⁹ZED88082A) imaged with whole-body PET/CT to assess the dynamics of T cell distribution in pts with RR DLBCL before and during treatment with axicabtagene ciloleucel.

Pts with RR DLBCL were included after at least 2 lines of therapy. Tracer injections (37 MBq, 10 mg, ⁸⁹ZED88082A) were administered 2 days prior to PET scan. The 1^st anti-CD8 PET scan (pre-CAR) was performed after apheresis and optional bridging therapy but before lymphodepleting chemotherapy. The 2^nd anti-CD8 PET scan (post-CAR) was initially performed 7 days after CAR T cell infusion (day +7; cohort 1). The interval between post-CAR scans was subsequently shortened to optimally study the (CAR) T cell influx in the tumors, as ultrasound-guided biopsies revealed rapid clinical response to CART. The PET scan in cohort 2 was performed on day +5 post-CAR. In cohort 3, CAR T cells and tracer were administered on day 0, followed by PET scans on post-CAR days +2, +4 & +7. When clinically feasible, tumor biopsies were performed on the same day as the PET scan.

Tumor lesions were segmented manually, normal tissues using TotalSegmentator (Wasserthal et al. 2023). Standardized Uptake Values (SUV) for tumor (SUVpeak) and normal tissue (SUVmean) regions were obtained. Linear mixed-effect models were utilized to obtain CD8 tracer uptake estimates in tumor and normal tissues and to study the relationship between tumor tracer uptake pre- and post-CAR, and uptake in irradiated and non-irradiated lesions. Lesion-wise correlation analysis (SUVpeak versus % CD8⁺ cells from immunohistochemistry) was performed.

Of the 27 pts, 23 underwent at least 2 tracer administrations with corresponding PET scans. Cohort 1 consisted of 7 pts, cohorts 2 and 3 were 8 pts each. 79% received bridging therapy; 12 (67%) got radiotherapy (8 – 20 Gy in 2 – 5 fractions). Twelve (52%) pts experienced CRS grade ≥ 2 and/or ICANS grade ≥ 2. The overall response rate was 92% (n = 21) and 16 (70%) pts obtained a CR.

CD8 tracer uptake was determined in normal tissues to study the distribution in non-tumoral regions. Uptake in spleen, bone marrow and tonsils did not change from pre-CAR to post-CAR (day +2, +5 & +7) (SUVmean range 19.6 - 26.2, 1.6 - 2.4, and 1.4 – 2.1, respectively; all p > 0.05). Although brain uptake is low, it increased from pre-CAR to post-CAR day +5 (SUVmean 0.2 to 0.4, p < 0.01), suggesting an increased CD8⁺ T cell infiltration post-CAR.

Tumor lesions in cohort 3 served to compare pre- and post-CAR SUVpeak (day +2), with 79 lesions in 8 pts. Heterogeneity in CD8 tracer uptake was observed between and within pts. Geometric mean SUVpeak pre-CAR was 2.4 [CI 1.9 – 3.0] and 1.6 [CI 1.3 – 2.1] post-CAR. Higher pre-CAR SUVpeak was associated with higher post-CAR tumor SUVpeak (β = 0.9 [CI 0.7 – 1.0]; p < 0.01). Much of the variability in post-CAR SUVpeak was explained by the pre-CAR SUVpeak (57%), implicating a potential predictive relationship between pre- and post-CAR tumor SUVpeak.

The 12 pts who received radiotherapy had 112 lesions, of which 64 were exposed to any radiotherapy dose. Pre-CAR these lesions had a 61% higher SUVpeak than non-irradiated lesions (p = 0.03), with a geometric mean SUVpeak for irradiated lesions of 3.8 [CI 2.3 – 6.2], versus 2.3 [CI 1.4 – 3.9] for non-irradiated lesions. Pre-CAR SUVpeak positively correlated with the % CD8⁺ cells in the tumor biopsies (n = 7; R = 0.82; p < 0.01).

This first anti-CD8 whole-body PET CAR T cell study shows that higher pre-CAR CD8⁺ T cell levels in tumor indicates enhanced post-CAR CD8⁺ T cell infiltration. Irradiated lesions show higher CD8 tracer uptake than non-irradiated lesions, suggesting priming by radiotherapy. Further analyses are ongoing, CD8⁺ T cell tumor distributions in the other 2 cohorts and associations with CAR T cell expansion, cytokine production, toxicities, and outcome will be presented.