Abstract CT187: Phase 1 Trial of RTX-240, allogeneic red blood cells engineered to express 4-1BBL and trans-presented IL-15, in patients (Pts) with advanced solid tumors
Authors: Hamid Omid , Johnson Melissa L. , Luke Jason , Maziarz Richard T. , Merchan Jamie , Lim Emerson E. , Patel Sandip P. , Kuesters Geoffrey M. , Sienczylo Iga , Gordon Gilad , Campbell Karen , Horton Kristin , Turka Laurence , Spira Alexander I. 
Published: June 2022

Background: RTX-240 is an allogeneic cellular therapy genetically engineered to express high copy numbers of trimeric 4-1BBL and IL-15/IL-15Rα fusion proteins on the RBC surface membrane. RTX-240 is designed to activate and expand CD8+ memory T cells and NK cells and is restricted to the normal biodistribution of red blood cells to mitigate toxicity. Safety, pharmacodynamic (PD) effects, pharmacokinetics (PK) and preliminary efficacy of RTX-240 were assessed in a Phase 1 study of pts with solid tumors.

Methods: Pts with relapsed/refractory solid tumors not eligible for standard therapy were treated with monotherapy RTX-240 in 10 cohorts utilizing 6 dose levels (1x108-5x1010 cells per dose) and 4 different schedules until disease progression or unacceptable toxicity. Peripheral blood and tumor tissue were collected at baseline and on-treatment.

Results: As of Dec 3, 2021, 30 pts with solid tumors (median age 58 yr, range 23-80, 17 males, 20 ECOG 1), were treated with single agent RTX-240. Pts had previously received a median of 3 therapies (range, 1-9); 21 pts had received prior PD-1/PD-L1 inhibitor (i) therapy. Common tumor types included NSCLC (n=7), melanoma (n=7), colorectal/lower GI (n=4) and RCC (n=3). Per RECIST v1.1, a confirmed partial response (PR) was observed in 1 pt with anal carcinoma and an unconfirmed PR in a uveal melanoma, both of whom had been previously treated with a PD-1. Disease control (PR or stable disease &gt; 12 weeks) occurred in 11/30 pts (37%), of which 7 had received a prior PD-1 for a median of 8 months. Peripheral blood PD studies demonstrate activation, expansion, and cytotoxic potential (GzB) of both NK and memory CD8+ T cells. Every pt with evaluable samples (n=29) had an increase in at least one of the activation or expansion parameters, and the majority had an increase in all. There was also a clear dose response effect in NK cell numbers and trends in other PD markers. Immune activation was observed by increases in plasma IFN-γ in a subset of patients. On-treatment biopsies in a subset (3/5) of evaluable pts show infiltration of NK and/or T cells into the tumor microenvironment (TME) after RTX-240 dosing. No DLTs or related grade (Gr)&gt;3 AEs were seen. The most common treatment-related AEs (Gr 1-2) were fatigue (n = 6); chills, nausea, decrease in appetite, arthralgia (n =3); and fever, myalgia, dysgeusia, and hyperhidrosis (n=2). Additional Gr 2 irAEs include pneumonitis, adrenal insufficiency, hypothyroidism, and worsening transaminitis.

Conclusions: RTX-240 is well tolerated at all tested doses, schedules and routes. The drug leads to activation, expansion and trafficking of two target cells (memory CD8+ T cells and NK cells) into the TME, exhibits a clear dose response effect with NK cells and demonstrates preliminary evidence of anti-tumor activity. Dose escalation continues and a PD-1 inhibitor combination arm is also enrolling.

Citation Format: Omid Hamid, Melissa L. Johnson, Jason Luke, Richard T. Maziarz, Jamie Merchan, Emerson E. Lim, Sandip P. Patel, Geoffrey M. Kuesters, Iga Sienczylo, Gilad Gordon, Karen Campbell, Kristin Horton, Laurence Turka, Alexander I. Spira. Phase 1 Trial of RTX-240, allogeneic red blood cells engineered to express 4-1BBL and trans-presented IL-15, in patients (Pts) with advanced solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT187.