Hematopoietic stem cell transplantation using briquilimab (Anti-CD117 Antibody-Conditioning), immunosuppression and TCRαβ+ T-cell/CD19+ B-cell depleted haploidentical grafts in patients with fanconi anemia: An approach without irradiation, busulfan a

Abstract Introduction: Fanconi anemia (FA) is a genetic disorder that leads to DNA repair defect and is clinically characterized by progressive bone marrow failure (BMF) and a predisposition to malignancies. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is currently the only curative treatment for BMF. For patients with FA who do not have a matched sibling donor, most centers use unrelated donors and cord blood leaving a significant proportion of patients without a donor. Patients with FA have heightened sensitivity to traditional HSCT conditioning regimens such as total body irradiation (TBI) or busulfan with cyclophosphamide, fludarabine and anti-thymocyte globulin (ATG). This increased sensitivity enhances conditioning related toxicities and, graft vs host disease (GvHD) and heightens predisposition to malignancies. Prior attempts to remove TBI from FA conditioning regimens have been unsuccessful due to increased risk of graft rejection - and alternative approaches to replace TBI with busulfan remain genotoxic. This is the first HSCT regimen to use anti-CD117 mAb-based conditioning in immunocompetent patients without concurrent use of irradiation and busulfan in the setting of haploidentical donors. In addition, prophylaxis for GvHD commonly includes calcineurin inhibitors with associated toxicities. In our approach, TCRαβdepletion is used as effective GVHD prophylaxis without the toxicities of calcineurin inhibitors. FA patients have renal abnormalities and are prone to renal toxicities. Therefore, avoiding calcineurin inhibitors increases the renal safety during transplant. Objective: To reduce acute and long-term treatment-related toxicities, we have developed a first of its kind treatment intended to improve the safety of allo-HSCT using: 1) a TBI- and busulfan-free conditioning regimen consisting of briquilimab, rabbit ATG (rATG - Thymoglobulin), fludarabine, cyclophosphamide and rituximab - briquilimab is a monoclonal antibody (mAb) that targets human CD117 to clear host HSCs from their niches enabling blood and immune reconstitution with minimal toxicity with other agents being used for immune suppression to prevent immunologic rejection; 2) transplantation of TCRαβ+ T-cell/CD19+ B-cell hematopoietic grafts - a stem cell therapy that enhances donor hematopoietic and immune reconstitution while decreasing GvHD; and 3) pharmacokinetic analysis for briquilimab, rATG and fludarabine to understand niche clearance and level of immune suppression. Methods: This phase 1b/2a trial intended to treat 12 eligible patients with FA in BMF with allo-HSCT using TCRαβ+ T-cell/CD19+ B-cell depleted hematopoietic grafts from haploidentical family donors. In the current regimen, a fixed dose of briquilimab was administered at 0.6 mg/kg in combination with standard FA dosing of rATG, fludarabine, cyclophosphamide and rituximab as lymphodepletion. Prospective pharmacokinetic measurements of briquilimab, rATG and fludarabine were included in the protocol. Results: Treatment of all 12 patients on the phase 1b/2a study has been completed with each of the 12 patients showing promising safety and 11 of 12 patients showing efficacy. Importantly, briquilimab treatment was well tolerated without any complications with a steady and predictable antibody clearance of <1500 ng/ml prior to HSCT in all 12 patients. 11 of 12 patients achieved full blood count recovery by Day +15 post HSCT with haploidentical grafts that were within specifications of the protocol. Total, CD34+, and CD15+ donor chimerism of > 98% in the bone marrow has been observed in the 11 of 12 engrafted patients with > 91% chimerism in all immune subsets (CD3, CD19 and CD56) for the first 3 patients who have completed a 2-year follow-up period. Median follow-up for the phase 2a patients is 9 months post allo-HSCT. Incidence of acute and chronic GvHD requiring systemic treatment is <15%. Conclusions: This study shows the safety and efficacy of this briquilimab-containing HSCT approach in patients with FA in BMF. The regimen was well tolerated and 11/12 patients have shown prompt and durable donor engraftment. This data indicates that it is possible to remove irradiation or busulfan from the conditioning regimen in patients with FA thereby decreasing the cancer risk in patients with inherited DNA repair defects. Furthermore, TCRαβ depletion, in the haploidentical setting provides effective GVHD prophylaxis without the toxicities associated with calcineurin inhibitors.