Effect of rituximab on remissions without minimal residual disease and immunogenicity in patients with relapsed/refractory hairy cell leukemia receiving moxetumomab pasudotox.
Authors: Kreitman Robert J. , Yuan Constance , Wang Hao-Wei , Zhou Hong , Raffeld Mark , Xi Liqiang , Arons Evgeny , Feurtado Julie , James-Echenique Lacey , Steinberg Seth M. , Braylan Raul , Calvo Katherine R. , Maric Irina , Pastan Ira 
Published: June 2024

e19015 Background: Anti-CD22 recombinant immunotoxin moxetumomab pasudotox (Moxe) was FDA-approved for relapsed/refractory hairy cell leukemia (HCL) but is now unavailable due to vial expiration until a company resumes development. The phase 3 complete remission (CR) rate was 41%, higher in patients with lower anti-drug antibodies (ADA). Minimal residual disease (MRD) eradication led to longer CR duration. HCL and the poorer prognosis variant HCLv strongly express CD22 and CD20. A clinical trial was done to determine if Rituximab could decrease immunogenicity by killing normal B-cells in HCL patients receiving Moxe and hasten MRD-free CR by reducing HCL tumor burden. Methods: To permit Rituximab enough time to reduce ADA and tumor burden, it was administered 3 days before cycle 1 day 1 at 375 mg/m2, and Moxe was given by 30-minute infusion days 1, 3 and 5. On subsequent 28-day cycles, patients received Moxe days 1, 3 and 5, and Rituximab prior to Moxe on day 1. Patients received up to 4 cycles past MRD-free CR, or up to 8 total cycles if MRD-free CR was achieved after cycle 4. The ADA assay determined percent neutralization by serum of the cytotoxicity of Moxe on CD22+ Raji cells. Results: After 13 patients received Moxe-Rituximab (MoxeR) without dose-limiting toxicity (DLT), meeting the phase 1 endpoint, 5 additional patients received Moxe with the biosimilar Ruxience (MoxeR). The first 3 patients received Moxe at 30 mcg/Kg/dose and subsequent patients 40 mcg/Kg/dose. All 18 were evaluable for toxicity and response. Although no DLT, one patient had transient grade 3 hemolytic uremic syndrome during cycle 3 without significant symptoms or need for therapy. Of the 18 patients, 15 (83%) responded, 14 (78%) achieved CR and 13 (72%) MRD-free CR by blood and bone marrow (BM) aspirate flow cytometry and BM biopsy immunohistochemistry. MRD-free CRs included one with HCLv. Even though Moxe vial expiration prevented enrollment of the 26 planned patients needed for a 1-sided p-value <0.025 compared to Moxe alone where 30 (47%) of 64 phase 1-3 patients achieved MRD-free CR, the 53% relative improvement with MoxeR achieved a 1-sided p-value of 0.05. Compared to 29 (48%) of 61 evaluable phase 1-3 patients who received Moxe alone with high ADA levels, 4 (29%) of 18 patients had high ADA levels to MoxeR (p=0.048). At 14-51 (median 35.3) months of follow-up, all but 2 of 13 MRD-free CRs are continuing, with relapse-free survival 14-45 (median 34.5) months. Conclusions: Despite enrolling slightly fewer patients than planned, MoxeR was safe and more effective than Moxe alone at achieving MRD-free CR, probably due to lower immunogenicity and faster reduction of HCL/HCLv tumor burden. Since non-Hodgkin’s lymphoma (NHL) cells from patients are sensitive to Moxe like HCL, MoxeR could be tested after NHL treatment to convert MRD+ to MRD-free CRs. Clinical trial information: NCT03805932 .