The Alpha Emitter Astatine-211 Targeted to CD38 Can Eradicate Multiple Myeloma in Minimal Residual Disease Models

Introduction

Multiple myeloma (MM) is considered incurable but patients achieving minimal-residual disease (MRD) negative status following treatment have significantly better overall and progression-free survival. MM is highly heterogeneous both between and within patients, limiting the curative potential of novel agents targeting specific pathways. However all MM is highly sensitive to radiation. The α-emitter astatine-211 (²¹¹At) deposits a very large amount of energy (~100 keV/μm) within a few cell diameters (50-90 μm) resulting in irreparable double strand DNA breaks, making ²¹¹At, targeted to MM cells, particularly suited to eliminating MRD. CD38 is expressed on malignant plasma cells regardless of mutational status, and CD38 monoclonal antibodies (mAbs) constitute a proven targeted therapy for MM but do not alone eradicate disease. We proposed that ²¹¹At conjugated to an anti-CD38 mAb could effectively eliminate MM MRD, and tested this hypothesis in cellular and murine models.

Methods

We conjugated the anti-CD38 mAb OKT10 and an isotype matched control mAb, BHV1, to the amine-reactive labeling agent B10-NCS and labeled the final constructs with ²¹¹At. To assess in vitro cell binding we incubated each labeled construct with CD38+ cell lines, washed, and then measured cell pellet radioactivity in a gamma counter. To assess cytotoxicity we incubated CD38+ and CD38- cell lines with unlabeled or ²¹¹At-labeled OKT10-B10 for 60 hrs, then assayed viability. NOD.Cg-Rag1^tm1MomIl2rg^tm1Wjl/SzJ (NRG) mice bearing H929^luc or OPM-2^luc MM xenografts were generated by subcutaneous (SQ) flank injection of 10⁷ cells 7 days prior to treatment. MRD was modeled by intravenous (IV) injection of 2.5 - 5 x 10⁵ cells 5 days prior to treatment. Radioimmunotherapy (RIT) was administered by IV injection of 7.5 - 45 µCi of ²¹¹At-OKT10-B10 or ²¹¹At-BHV1-B10. For biodistribution studies (n = 5/group) mouse tissues were harvested 24 hrs post RIT and measured in a gamma counter. For therapy studies (n = 8-10/group), all mice received syngeneic bone marrow transplant 3 days post RIT. Disease progression was assessed by tumor dimensions, luminescence imaging and survival.

Results

²¹¹At-CD38 mAb selectively bound and killed CD38+ but not CD38- MM cells in vitro. In vivo, biodistribution experiments demonstrated that ²¹¹At-CD38 RIT delivered 2.4 times more radiation to MM xenografts than did control ²¹¹At-BHV1 RIT (p = 0.007), and delivered significantly higher dose to tumor than to healthy tissues including lung (p = 0.04) and kidney (p = 0.015). In murine therapy studies, ²¹¹At-CD38 RIT at 15 - 45 µCi at least doubled median survival relative to untreated controls in each of two MM SQ xenograft models (p < 0.003). However, no mice in these models achieved complete remission and all eventually died of disease. In contrast, therapy studies using MRD models showed that 30 µCi ²¹¹At-CD38 RIT eliminated detectable disease in 80% of mice at day 21, compared to 20% of mice receiving nontargeted ²¹¹At RIT and 0% of untreated mice. At day 130, 50% of the 30 µCi ²¹¹At-CD38-RIT mice remained alive and disease-free, while all nontargeted and untreated mice died before day 85 (Fig. 1, survival of 30 µCi > 15 µCi ²¹¹At-CD38-RIT [p = 0.016] and all other groups [p < 0.0007]). The impact of therapy on body weight as well as hematopoietic, liver and kidney function was mild and returned to normal within 32 days of treatment.

Conclusions

The efficacy of CD38 targeted ²¹¹At appears to be a function of disease distribution and malignant plasma cell access, as compellingly demonstrated by our models. Bulky tumor geometry reduces mAb penetration. In contrast, the isolated cells and small tumor clusters that define MRD are readily accessible to mAbs, creating optimal conditions for α-emitter cell kill. In an era of highly potent MM therapy, preventing relapse remains frustratingly rare. Our approach is both agnostic to high-risk cytogenetic features and offers the potential to eliminate all residual MM cell clones. These encouraging findings will be explored in a clinical trial of ²¹¹At-CD38 RIT.

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