Selective Allodepletion by TH9402-Mediated Photosensitization Results in Early Full Donor T Cell Reconstitution in the Absence of High-Grade, Acute GvHD and Is Associated with Favorable Outcome after HLA Matched Sibling SCT for Hematologic Malignancies

Selective allodepletion (SD) is a strategy to eliminate host-reactive donor T-cells from allografts to prevent graft versus host disease (GvHD) while conserving useful donor immunity. We developed a semi-closed, GMP-quality, clinical scale SD process where donor-derived lymphocytes are stimulated with patient-derived T-cell antigen presenting cells in an ex vivo mixed lymphocyte reaction (MLR). Alloactivated donor T cells preferentially retain the photosensitizer 4,5-dibromorhodamine 123 (TH9402), rendering them susceptible to elimination by exposure to visible light in a photodepletion device (Kiadis Pharma Inc, The Netherlands). After Food and Drug Administration and Institutional Review Board approval we initiated a clinical trial where HLA-identical sibling recipients with hematological (non T-cell) malignancies received a CD34-cell selected transplant (Miltenyi, Germany) containing less then 1 x 10⁴ T cells/kg together with 5 x 10⁶ /kg viable SD donor T cells on day 0, using an age-adapted, radiation-based preparative regimen (FluCyTBI). Low-dose cyclosporine was used as sole immunosuppression in the absence of GvHD. Eleven patients (median age 43 (28-68) years with ALL, MDS, CML, mantle cell lymphoma (MCL), or AML) were transplanted with a median follow-up of 240 (43-400) days. Nine patients were considered high risk. Patients received a stem cell product containing a median of 6.0 (3.9-9.5) x10⁶/kg CD34+ stem cells in addition to 5x10⁶/kg SD T cells. Absolute lymphocyte recovery was rapid (median 834 (384-2486) cells/µL day 30 post transplant) [Fig A]. Early T cell chimerism was donor-dominated (median 66% (6-95) on day 14, and 97% (82-100) on day 30, and 100% (92-100) on day 45 [Fig B]. One patient received an unmanipulated DLI to treat a delayed fall in T cell chimerism. Three patients developed steroid-sensitive grade II aGvHD of skin (N=2) and gut (N=1) but no grade III-IV aGvHD occurred after transfusion of the photodepleted lymphocytes [Fig C]. Two patients developed limited chronic GvHD. Only one patient, transplanted for refractory MCL, relapsed 340 days after transplant. One patient died of infectious complications and GvHD 330 days after transplant after receiving an unmanipulated DLI in her home country for suspected, but subsequently unconfirmed relapse. Eight patients reactivated CMV but were successfully treated. These results demonstrate for the first time clinical feasibility of photodepletion-based SD stem cell allotransplants in matched siblings. Robust lymphocyte recovery and early donor chimerism with a low relapse incidence in a high-risk population suggest functionality of SD T cells in the absence of severe GvHD, which should allow further reduction of immunosuppression to optimize disease control in future studies.