Apamistamab (Iomab-B) conditioning treatment with targeted radioimmunotherapy to the bone marrow resulted in high rates of successful allogeneic hematopoietic stem cell transplants in patients with active, relapsed, or refractory acute myeloid leukemia (AML), according to interim results from the phase 3 SIERRA trial, which were presented virtually at the 2021 Transplant and Cellular Therapies Meetings.1

In these patients with relapsed or refractory AML, we observed high rates of allogeneic stem cell transplant with curative intent [in] 88% of patients on the Iomab-B arm, 18% of patients who were randomized to the conventional care arm achieved complete remission and received standard of care allo-transplant, and an overall rate of 79% of allo-transplant in all enrolled patients, Boglarka Gyurkocza, MD, said in a virtual presentation.

Investigators sought to prove with this study that targeted radiation to the marrow with apamistamab, a radioactive iodine (131I)labeled anti-CD45 antibody, could enable the successful engraftment of patients despite active disease in the marrow. Safety and robust efficacy had previously been demonstrated with the agent in 271 patients treated in 9 different phase 1 and 2 clinical trials.

The SIERRA trial is looking to enroll 150 patients, and the trial is already over 75% enrolled. Recently, an independent data monitoring committee recommended that the trial continue to the planned full enrollment based on a positive pre-planned ad-hoc analysis.2

In the study, patients with active, relapsed, refractory AML are randomized 1:1 to receive either apamistamab conditioning therapy and allogeneic HCT or conventional care. In the control arm, patients who do not achieve a complete remission (CR) by day 42 are allowed to cross over to receive Iomab-B, and those who do have a CR undergo HCT or receive standard-of-care therapy of the physicians choice.

Durable CR (dCR) rate is the primary end point of the study, characterized as complete response at 6 months after initial CR, and the secondary end point is overall survival (OS) rate at 1 year.

Patients are eligible for enrollment if they have marrow blast count 5% or the presence of peripheral blasts, age 55 years, a Karnofsky score 70, and related/unrelated donor matching at human leukocyte antigen (HLA)-A, HLA-B, HLA-C, and DRB-1. Active, relapsed, or refractory AML was defined for the sake of the trial as primary induction failure after 2 cycles of therapy including chemotherapy or 2 cycles of venetoclax (Venclexta) with a hypomethylating agent or low-dose cytarabine, first early relapse after first CR of less than 6 months, relapse refractory to salvage chemotherapy regimen, or second or subsequent relapse. Secondary or treatment-related AML was also allowed.

In the SIERRA trial, patient-specific dosimetry was used to generate an individualized therapeutic dose to target marrow and spare non-hematopoietic organs. Patients in the investigational arm received a dosimetric dose of apamistamab ( 20 mCi) approximately 19 days prior to HCT followed by a therapeutic dose of apamistamab, which is individually calculated for each patient based on an upper limit of 24 Gy to the liver. After, patients remain on radiation isolation for several days before receiving fludarabine conditioning therapy (30 mg/m2/day for 3 days) and finally low-dose total body irradiation (200 cGy) prior to HCT.

Among the first 75% of enrolled patients (n = 113), patients in the apamistamab arm (n = 56) had a median age of 63 years (range, 55-77), 35% had intermediate risk and 61% had adverse risk, the median

percent of marrow blasts at baseline was 29% (range, 4%-95%), and had received a median of 3 prior treatment regimens (range, 1-7). At randomization, 56% were in primary induction failure, 16% were in first early relapse, 15% had relapsed or refractory disease, and 13% were in their second or later relapse.

In the conventional care arm, the median age was 65 years (range, 55-77), 32% had intermediate risk and 63% had adverse risk, median marrow blasts was 20% (range, 5%-97%), and had received a median of 3 prior regimens (range, 1-6). At randomization, 49% were in primary induction failure, 21% were in first early relapse, 21% had relapsed or refractory disease, and 8.8% were in their second or later relapse. Patients who crossed over to receive apamistamab (n = 30) had similar baseline characteristics.

Forty-nine patients in the apamistamab-randomized arm were able to go on and undergo allogeneic HCT compared with 10 patients in the conventional care arm. In the investigational arm, a median of 646 mCi (range, 3541027) of apamistamab was infused at a dose of 14.7 Gy (range, 4.6-32) to the marrow. The median infused CD34-positive cell count was 5.6 x 106/Kg (range, 1.8-208). Forty-five patients received peripheral blood stem cells (PBSCs), 3 received marrow grafts, 17 had related donors, and 31 had unrelated.

Individualized therapy of Iomab-B provided myeloablative doses of radiation to the marrow, Gyurkocza, a medical oncologist at Memorial Sloan Kettering Cancer Center, commented.

These patients had a median of 30 days (range, 23-60) to HCT after randomization and 14 days (range, 9-22) to neutrophil engraftment, with no graft failure reported. Patients also had 18 days (range, 4-39) until platelet engraftment.

We also observed 100% neutrophil and platelet engraftment in patients who received Iomab-B conditioning, despite a heavy leukemia burden, Gyurkocza said.

In patients in the conventional arm who went on to HCT, conditioning regimens for HCT consisted of fludarabine/melphalan in 2, fludarabine/melphalan/total body irradiation in 1, busulfan/fludarabine in 1, cyclophosphamide/fludarabine/total body irradiation in 2, and 4 had no data on conditioning regimens available. Eight of these patients had PBSCs, 2 had marrow, 3 had related donors, 6 had unrelated, and 1 was unreported.

Median days to HCT was 67 (range, 52-104) with 17 days (range, 13-83) to neutrophil engraftment and 22 days (range, 8-35) to platelet engraftment. There was 1 graft failure.

Among the patients who crossed over to receive apamistamab before HCT, the median infused dose was 592 mCi (range, 313-1013) with 15.5 Gy (range, 6.3-42) to the marrow. The median infused CD34-positive cell count was 5.1 x 106/Kg (range, 1.8-16.1). Twenty-eight patients had PBSCs, 2 had marrow, 10 had related donors, and 20 had unrelated.

Patients had a median of 62 days (range, 36-100) to HCT, 14 days (range, 10-37) to neutrophil engraftment, and 19 days (range, 1-38) to platelet engraftment. No graft failure was reported in this group.

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Apamistamab Conditioning Treatment Induces High Rates of HCT Success in AML - OncLive