Potential synergy between radiotherapy and CAR T-cells - a multicentric analysis of the role of radiotherapy in the combination of CAR T cell therapy

Background Chimeric antigen receptor (CAR) T-cell therapy has improved the limited overall survival (OS) of patients with intensively pretreated diffuse large B-cell lymphoma (DLBCL). However, the potentially life-threatening toxicities of CAR T-cells and early relapses remain a challenge. As suggested by smaller monocentric analyses, radiotherapy (RT) in combination with CAR T-cells may have an immunomodulatory effect. Method/ Results In this multicentric retrospective analysis, we investigated potentially synergistic effects of RT and CAR T-cells. Of 78 patients from four centers who received CAR T-cell therapy for DLBCL, 37 patients underwent bridging RT or received salvage RT. RTs (median 36 gray) were well tolerated. Therapy response and disease control of CAR T-cell therapy were comparable after bridging RT or bridging systemic therapy. High-grade neurotoxicity tended to occur less frequently after bridging RT. After further disease progression, patients with localized relapses showed better outcomes, compared to those in advanced stage. In the subgroup with localized relapse, patients receiving salvage RT had an increased OS, vs. those without salvage RT (1-year OS rate 89% vs. 38%, p = 0.03). Conclusion Our analysis demonstrated that RT in combination with CAR T-cells led neither to high-grade toxicities, nor to a decreased response rate. We observed better outcomes of salvage therapies in patients with localized relapses vs. those with advanced stage relapses. Especially the patients who received salvage RTs for localized relapses seem to benefit more. Further analyses are necessary to clarify whether specific synergistic effects exist, such as an enhanced anti-tumor effect of CAR T-cells from RT sensitizing.