Intratumoral cellular immunotherapy with autologous hyperactivated M1 SIRPα-low macrophages in non-Hodgkin lymphoma: Clinical results from a first-in-human Phase 1 study

Abstract Background. Many T cell Non-Hodgkin Lymphomas (NHL) lack an actionable target, and many B cell NHL show post-therapy antigen loss, limiting effective treatment options. RB-1355 (previously known as SIRPant-M) is a cellular product consisting of a homogenous population of autologous hyperactivated SIRPα-low M1 macrophages. Following a single apheresis, RB-1355 is generated ex-vivo using a proprietary macrophage-augmentation catalyst generally comprised of cytokines (“MACH-1™”) and is then cryopreserved. In pre-clinical models, administered by intratumoral injection, RB-1355 elicited a multi-pronged polyclonal response that transformed the tumor microenvironment (TME), catalyzing a robust neoantigen targeted cellular and humoral immune response against tumoral cells, without significant adverse effect on healthy tissue. Methods. Patients with relapsed or refractory B or T cell NHL, following at least two lines of systemic therapy (prior CAR T-cell therapy, bispecific antibodies and/or stem cell transplant were allowed), ECOG performance status ≤ 2, with injectable lesion(s) ≥ 1.5 and ≤ 5 cm, and adequate renal, hepatic, and hematological function, were eligible for this first-in-human phase 1 trial (NCT05967416). Four cohorts were investigated in a 3+3 design: 90×106 or 300×106 cell dose, administered by 3 intra-lesional injections (days 1, 3, 5), with or without single beam radiotherapy (SBRT) to the injected lesion in 3 X 2.5 Gy fractions. Patients with clinical benefit were allowed retreatment. The primary endpoint was dose limiting toxicity (DLT) over 30 days. Responses were evaluated by Lugano 2014 criteria for non-cutaneous NHL or modified severity-weighted assessment tool (mSWAT) for cutaneous NHL. Serial blood samples and tumor biopsies were collected for scRNA and TCR/BCR sequencing. Results. Thirteen patients were treated. Three patients had Diffuse Large B-cell Lymphoma (DLBCL),1 Peripheral T cell Lymphoma (PTCL), 1 double-hit Follicular Lymphoma, 1 skin-only Anaplastic Large-cell Lymphoma, 5 Mycosis Fungoides (MF), 1 large cell lymphoma transformed from Cutaneous T-cell Lymphoma, and 1 Lymphomatoid Papulosis. The median number of prior lines of systemic therapy was 3 (range 2-6). No DLT or Grade 3-4 treatment-related adverse events (AEs) were observed. AEs at least possibly related to RB-1355 included grade 2 neutropenia (N=2) and grade 1 chest wall pain, cytomegalovirus reactivation, and injection site reaction (N=1 each). Grade 1 cytokine release syndrome was reported in 1 DLBCL receiving 300×106 cells + SBRT who later developed complete response (CR). One patient with CAR-T refractory DLBCL in the 300×106 monotherapy cohort achieved CR. This patient recurred with a single lesion by day 84, then received his second cycle + SBRT resulting in a second CR. Another patient with CAR-T refractory DLBCL with CD19+/CD20+ antigen loss in the 300x106 + SBRT cohort also achieved CR. Both patients remain in CR at most recent follow-up. Partial responses (PRs) were seen in 1 PTCL and 1 MF, both treated at 90×106 cells + SBRT. Both PRs relapsed by day 84. The MF patient was retreated twice with resultant improvement in mSWAT. Two deaths have occurred to date, both related to disease progression. Paired scRNAseq and TCR/BCRseq performed on 8 patients showed that RB-1355: (i) drives in situ and de novo expansion of memory and effector T and B cell clones that egress from the treated tumor into circulation; (ii) increases intratumoral T cell clonal diversity; (iii) reduces malignant T and B cell clones; and (iv) confers a favorable CXCL9:SPP1 macrophage ratio in tumors that correlates with treatment response (r=0.96, p=0.0012). Conclusion. In this phase 1 first-in-human study RB-1355 has shown to be a safe and easily administered cellular therapy, not requiring lymphodepleting chemotherapy or specific antigen expression, with promising anti-tumoral activity and favorable immunological effects in patients with heavily pre-treated NHL. Higher doses and retreatment, with the goal of promoting secondary immune responses and further restructuring of the TME, may be needed to induce more frequent and durable responses. Forthcoming analysis of ctDNA, peripheral blood cytokines, and circulating T cells will further inform our understanding of RB-1355 mechanism of action and dose optimization.