Iomab-B

Iomab-B (apamistamab-I131) is Actinium Pharmaceuticals' anti-CD45 radioimmunoconjugate designed to replace toxic myeloablative chemotherapy conditioning before allogeneic stem cell transplant in older patients with active, relapsed or refractory acute myeloid leukemia. The Phase 3 SIERRA trial (NCT02665065, n=153) met its primary endpoint of durable complete remission, with 22% (n=13) of Iomab-B patients achieving dCR at 180+ days versus 0% in the conventional-care arm (p<0.0001) [1][2]. Despite that, in August 2024 the FDA informed Actinium that SIERRA is not sufficient to support a BLA filing and that an additional randomized head-to-head trial powered for overall survival is required - a meaningful capital and timeline ask for a microcap [3][8]. Actinium has since announced it is seeking a U.S. strategic partner for Iomab-B and is reallocating internal development resources to Actimab-A and earlier programs; EMEA rights are already licensed to Immedica [9][10]. The asset is also being studied as a CAR-T conditioning agent at Memorial Sloan Kettering and UT Southwestern, which could expand reach beyond AML if Phase 1 data hold up [4][5]. The clinical story is settled. The U.S. commercial story now depends on a partner and a second trial, not a BLA.

SIERRA Phase 3 is completed - full enrollment of 153 patients, topline reported at the 2022 Tandem Meetings (TCT/ASTCT) and updated at 2023 Tandem, peer-reviewed publication in Journal of Clinical Oncology in September 2024 [1][2]. Iomab-B is a first-in-class anti-CD45 radioimmunotherapy with FDA Orphan Drug and Fast Track designations for AML conditioning [6]. As of mid-2026, no BLA has been submitted in the U.S. In an August 5, 2024 announcement, Actinium disclosed that following Type C and Type B meetings the FDA concluded SIERRA alone does not support a BLA, and that an additional randomized trial of Iomab-B + reduced-intensity Flu/TBI versus Cy + Flu/TBI powered for overall survival would be required [3][8]. Actinium responded by initiating a process to identify a U.S. strategic partner for Iomab-B and announced it would focus internal spend on Actimab-A, Iomab-ACT, and preclinical programs [8][9]. EMEA commercialization is licensed to Immedica under an agreement signed in 2023, and Actinium retains royalties and milestones for that geography [10]. Two combination programs continue independently: Iomab-ACT at MSK (NCT04512716, n=12, B-ALL and DLBCL) and IOMAB-CAR-T at UT Southwestern (NCT06768905, n=30, R/R DLBCL) [4][5]. The gating clinical event in the U.S. is no longer BLA timing - it is whether a partner agrees to fund the FDA-requested confirmatory trial.

CD45 is a protein tyrosine phosphatase that sits on the surface of essentially every nucleated hematopoietic cell, including 85-90% of AML blasts - a near-universal flag on blood-forming and immune cells [7]. Iomab-B exploits that flag by chemically conjugating radioactive iodine-131 to an antibody (BC8/apamistamab) that binds CD45. When infused, the antibody finds CD45-bearing cells throughout the body and delivers a lethal dose of beta radiation specifically to them. The result: leukemic cells die AND the patient's normal bone marrow gets ablated, clearing the runway for donor stem cells to engraft. The mechanistic case is straightforward and well-validated. Targeted radiation to blood-forming tissue has decades of academic precedent - the BC8 antibody was originally developed at Fred Hutchinson, and academic groups have run I-131-BC8 studies for years [6]. What Iomab-B does is industrialize that approach into a commercial product with standardized dosing and centralized supply. The rationale matters most for older patients: standard myeloablative conditioning with high-dose busulfan or total body irradiation kills too many of them through organ toxicity. A targeted radiation delivery vehicle that hits blood cells while sparing gut, lung, and liver opens transplant eligibility to a population currently excluded from curative therapy [6].

SIERRA (NCT02665065) randomized 153 patients aged 55 and older with active relapsed or refractory AML to either Iomab-B plus reduced-intensity conditioning followed by allogeneic HCT, or to conventional care (investigator's choice salvage chemotherapy) [1]. Primary endpoint was durable complete remission lasting at least 180 days. The trial allowed control-arm patients who failed salvage to cross over to Iomab-B, which is the right ethical call but complicates interpretation of survival endpoints. Active-disease AML was the key inclusion criterion - these are exactly the patients other transplant trials exclude because their disease burden makes standard conditioning futile. Primary endpoint result: 22% of Iomab-B patients (n=13) achieved dCR at 180 days versus 0% of conventional-care patients, p<0.0001 [2]. Secondary endpoints including event-free survival and HCT rates also favored Iomab-B. Overall survival was not formally powered, and the crossover design substantially attenuates any OS comparison. The FDA's August 2024 feedback made the structural concerns about the trial explicit: small n for a Phase 3 oncology trial (153), a single primary endpoint, no OS-powered analysis, a heterogeneous control arm of "investigator's-choice" chemotherapy, and the surrogate-like nature of dCR for FDA's recent OS-focused preferences in AML [3][8]. The agency concluded these limitations preclude using SIERRA as the basis for approval.

The model gives this drug a 20% chance of eventually being approved. Drugs at this stage in this area succeed about 57% of the time historically, and the model adjusts that starting point using ten facts about the trial and its sponsor. The trial's open-label design works in its favor, while the sponsor's thin approval record, weak earlier-phase results, and randomized design pull the estimate down. The remaining factors are close to average for this stage, so they leave the number roughly where the historical rate set it.

Regulatory risk is no longer hypothetical: as of August 2024 the FDA has explicitly stated SIERRA does not support a BLA in its current form, and a new randomized OS-powered trial is required [3][8]. Without a partner, that trial does not get funded by Actinium alone. Efficacy risk in the conditioning indication is mostly resolved given SIERRA's primary endpoint hit (22% dCR vs. 0%) [2]; durability of any future approval depends on whether the confirmatory trial replicates that effect with OS support. Safety risk is the standard transplant stack - graft-versus-host disease, infection from prolonged neutropenia, transplant-related mortality - plus a radiation-specific layer for long-term effects in transplant survivors. Execution risk is concentrated in the sponsor and the partner search. Actinium's Q1 2026 10-Q reported approximately $49.4M in short-term assets against modest short-term liabilities, with internal spend now redirected to Actimab-A, so cash itself is not the immediate killer - but funding a second Phase 3 SIERRA-style trial without a partner is not a realistic ask of that balance sheet [11]. Commercial risk has direct historical precedent that an informed investor will raise: Bexxar (tositumomab-I131) and Zevalin (ibritumomab tiuxetan), the two approved radiolabeled antibodies in hematologic malignancies, both failed commercially despite regulatory approval. Bexxar was withdrawn by GSK in 2013/2014 after sales collapsed (only 75 patients dosed in 2012); Zevalin's availability has been minimal for years [12]. The failure modes were operational: complex radiopharmaceutical logistics, multi-party reimbursement, inpatient I-131 requirements at many sites, and competition from newer cleaner modalities. Iomab-B's use case differs structurally - it is a one-time conditioning agent administered at a transplant center already set up for radiation handling, not a repeat-dose outpatient lymphoma treatment in community oncology - which removes some of the failure modes that killed Bexxar but not all of them. Convincing transplant centers to adopt a new conditioning modality requires both data and operational simplicity; Iomab-B will be challenged on the second.

Watch the partner search, not the BLA. As of mid-2026, no BLA has been filed and there is no FDA pathway for Iomab-B in the U.S. without a new randomized OS-powered trial that Actinium is not funding on its own [3][8]. The signal that matters is a U.S. licensing or acquisition announcement to a transplant-focused or radiopharm-focused buyer - a category that has gotten meaningfully more crowded since Novartis built Pluvicto into a billion-dollar franchise. EMEA is already licensed to Immedica, so the European route is independent of the U.S. partner search [10]. The combination programs at MSK and UT Southwestern matter for the longer story: if Iomab-ACT shows that targeted CD45 radiation can substitute for high-dose chemo before CAR-T, the addressable population expands well beyond AML conditioning [4][5]. Addressable market context for an investor: SIERRA-eligible patients (age 55+, active R/R AML, fit enough for transplant) are a small fraction of the roughly 20,000 new U.S. AML cases per year; reasonable estimates put the addressable population in the low thousands annually, concentrated at large academic transplant centers. That ceiling matters when modeling any future deal economics. Specific datapoints to track: any 8-K disclosing U.S. partnership terms for Iomab-B, design of any post-FDA-feedback confirmatory trial, Iomab-ACT and IOMAB-CAR-T readouts, and Actinium quarterly cash burn against the Actimab-A reprioritization.