Acute Myeloid Leukemia (AML) is the most common, highly aggressive, and genetically heterogeneous hematologic malignancy, characterized by the uncontrolled proliferation of immature myeloid cells in the bone marrow and peripheral blood. With an estimated five-year overall survival (OS) rate of only 32%, AML is the most lethal adult leukemia. While intensive chemotherapy and stem cell transplantation have improved outcomes for a subset of younger, fit patients, the majority—particularly older adults—face poor long-term survival and limited therapeutic options. [1][2][3]
Although the disease can occur in children, it is uncommon in people under 45 years of age. The average age at first diagnosis of the disease, which is slightly more common in men than women, is about 69 years. The American Cancer Society’s estimates for acute myeloid leukemia (AML) in the United States for 2026 suggest that about 22,720 people will be diagnosed with AML, while 11,500 will die from the disease.[4][5][6]
Recent advancements, including targeted therapies against FLT3, IDH1/2, and KMT2A mutations, have provided new hope but are applicable only to patients with specific molecular profiles. Many patients lack these mutations or relapse despite therapy, underscoring an urgent need for novel, broadly effective treatments.
Myelodysplastic syndromes (MDS) are a group of bone marrow disorders often considered precursors to AML. High-risk MDS is similarly challenging to treat, with limited available therapies and a substantial risk of progression to AML. Both AML and MDS present a critical unmet medical need, driving the search for new therapeutic strategies.[7][8]
One promising avenue is the targeting of CD37, a tetraspanin family member broadly expressed on malignant myeloid blasts but with limited expression on normal hematopoietic stem cells. Debio 1562M,** a second-generation novel antibody-drug conjugate (ADC) developed by Debiopharm, is designed to target CD37 and deliver a potent cytotoxic payload directly to tumor cells. Here, we review the preclinical development, mechanism of action, and therapeutic potential of Debio 1562M in AML and MDS.[9][10][11]
Why Targeting CD37 in AML and MDS
CD37 (TSPAN26) is a cell-surface glycoprotein primarily expressed on mature B cells, with lower levels on T cells, monocytes, and granulocytes. It plays a role in immune cell signaling, survival, and adhesion, influencing both B-cell and T-cell biology. Notably, CD37 is highly expressed in B-cell neoplasms such as chronic lymphocytic leukemia (CLL) and diffuse large B-cell lymphoma (DLBCL), where it has been associated with poor prognosis.[12][13][14]
Recent studies have revealed that CD37 is also expressed on AML and MDS blasts, including leukemic stem cells (LSCs), and its expression correlates with adverse outcomes in AML. Importantly, CD37 is largely absent from normal hematopoietic stem cells, making it an attractive and potentially safe target for therapeutic intervention in myeloid malignancies.[12][15]
Precision Delivery of Cytotoxic Agents
ADCs are a class of targeted cancer therapies that deliver cytotoxic agents directly to malignant cells via antibodies directed against specific cell surface antigens. The efficacy of ADCs depends on the selective expression of the target, efficient internalization, and the ability to release the cytotoxic payload within the tumor cell. [16]
To date, gemtuzumab ozogamicin (Mylotarg®; Pfizer/Wyeth), an ADC targeting CD33, is the only ADC approved for AML, but its use is limited by myelosuppressive toxicity and variable efficacy. [17] Previous attempts to target CD37 with ADCs, such as AGS67E*, showed promise in preclinical models for the treatment of hematological malignancies, including non-Hodgkin lymphoma (NHL), chronic lymphocytic leukemia (CLL), and acute myeloid leukemia (AML), but failed to demonstrate sufficient clinical benefit. Debio 1562M was developed to overcome these challenges through improved design and engineering, including enhanced target binding, payload delivery, and linker stability.[18]
Debio 1562M: Design and Mechanism of Action
Molecular Construction
Debio 1562M** consists of naratuximab, a humanized IgG1 monoclonal antibody directed against CD37, conjugated to a maytansinoid cytotoxic payload (DM1) via Debiopharm’s proprietary Multilink™, a modular, peptide-based, cathepsin B-cleavable linker platform technology. The technology improves manufacturing to achieve a homogeneous high drug-antibody ratio (DAR), with each antibody carrying eight DM1 molecules. The Multilink™ linker ensures rapid and specific intracellular payload release while maintaining high solubility and stability in systemic circulation, and favorable biologic characteristics, with preserved internalization kinetics on target cells.
Cytotoxic Payload
DM1 (Mertansine) is a potent, thiol-containing maytansinoid microtubule polymerization inhibitor, originally developed to overcome systemic toxicity associated with maytansine and to enhance tumor-specific delivery. Upon internalization and cleavage, DM1 disrupts microtubule dynamics, causing G2/M cell cycle arrest, mitotic catastrophe, and cell death. Unlike payloads with bystander effects, DM1 is membrane-impermeable, minimizing off-target toxicity.
Preclinical Features
- Efficient Targeting and Internalization
- Debio 1562M binds specifically and robustly to CD37 on AML and MDS blasts and is efficiently internalized into malignant cells.
- Superior Cytotoxicity
- In vitro studies show that Debio 1562M exhibits up to a 100-fold greater cytotoxicity against CD37-positive leukemia cells than first-generation ADCs.
- Broad Activity
- Debio 1562M demonstrates potent anti-leukemic effects across all AML and MDS subtypes, regardless of genotype, disease stage, or prior therapy.
- Favorable Pharmacokinetics and Stability
- The Multilink™ technology confers low aggregation, high plasma stability, and prolonged circulation time, leading to sustained tumor exposure and activity.
- Safety Profile
- Preclinical toxicology studies reveal good tolerability, minimal impact on hematopoietic stem cells, and low off-target toxicity, particularly compared to earlier-generation ADCs and standard therapies.
Preclinical Efficacy in AML and MDS
In Vitro Activity
Debio 1562M binds and is internalized by AML and MDS blasts, with a median IC50 in the sub-nanomolar range. Importantly, Debio 1562M is effective even in primary blasts from patients with diverse genetic backgrounds, including those with TP53, FLT3, IDH1/2, and KMT2A mutations, and those who are refractory to standard therapies.
Furthermore, Debio 1562M effectively targets leukemic stem cells, demonstrated by a significant reduction in colony-forming potential in primary AML samples.
In Vivo Efficacy
- Cell Line-Derived Xenograft (CDX) Models
In murine models engrafted with luciferase-expressing AML cells (e.g., MOLM-13, OCI-AML-3), Debio 1562M induces dose-dependent tumor regression in both peripheral blood and bone marrow, with survival significantly prolonged compared to standard-of-care (SOC), including Venetoclax (Venclexta®; AbbVie and Genentech) + Azacitidine (Vidaza®; Bristol-Myers Squibb/BMS)*** and targeted therapies (Gilteritinib (Xospata®; Astellas Pharma), Revumenib (Revuforj®; Syndax)). The ADC outperforms first-generation Debio 1562 (αCD37-DM1; naratuximab emtansine, formerly IMGN529) at DAR-equivalent doses. - Patient-Derived Xenograft (PDX) Models
In multiple PDX models representing both de novo and relapsed/refractory AML, Debio 1562M markedly reduces tumor burden in blood and bone marrow, translating into substantial survival benefit. Notably, single or multiple weekly doses of Debio 1562M resulted in extended survival in aggressive AML subtypes, including those refractory to venetoclax-azacitidine and post-transplant relapse.
Activity in MDS
Primary MDS blasts also express CD37 and exhibit internalization and sensitivity to Debio 1562M treatment similar to those of AML cells. All tested MDS samples showed dose-dependent cell death upon ADC exposure, with minimal impact on normal lymphocytes.[19]
Pharmacokinetics and Safety
- Pharmacokinetics
Debio 1562M exhibits a favorable biphasic pharmacokinetic profile in mice, with an initial distribution phase followed by first-order elimination, with a half-life of approximately 120 hours. Plasma concentrations remain higher and clearance lower than those of first-generation Debio 1562, supporting sustained drug exposure.[19] - Safety in Preclinical Models
Toxicology studies in mice demonstrate that Debio 1562M is well tolerated at high doses, with no significant loss of body weight, minimal hematologic toxicity, and acceptable clinical chemistry parameters. Platelet counts and liver enzyme levels are less affected than with Debio 1562, and organ pathology reveals a reduced incidence and severity of findings.[19]
In ex vivo studies using human peripheral blood mononuclear cells, Debio 1562M selectively depletes B cells (the normal cell type with the highest CD37 expression), with minimal impact on T cells or other lineages at concentrations exceeding those likely to be achieved in humans.
Comparative Efficacy: Debio 1562M vs. Standard and Targeted Therapies
In direct comparisons using multiple AML models, Debio 1562M consistently demonstrates superior anti-leukemic activity and survival benefit over:
- Venetoclax plus Azacitidine: The current standard of care for older adults and those unfit for intensive chemotherapy. Debio 1562M achieves more profound tumor regression and longer survival with better tolerability.
- Gilteritinib, an oral, targeted FLT3 inhibitor: Debio 1562M outperforms gilteritinib in FLT3-mutated AML models, with greater tumor reduction and survival extension.
- Revumenib, a menin inhibitor used to treat relapsed or refractory AML with a susceptible NPM1 mutation or KMT2A translocation in patients 1 year and older: In models with KMT2A rearrangements, Debio 1562M induces deeper responses and improved outcomes.
Importantly, Debio 1562M maintains activity regardless of mutation status, offering a potential option for patients ineligible for mutation-specific therapies.
Mechanistic Insights and Biomarker Considerations
While CD37 expression levels are lower in AML and MDS than in B-cell malignancies, the optimized design of Debio 1562M—including a high, homogeneous DAR and efficient internalization—ensures effective delivery of DM1 even at low antigen densities. Internalization studies reveal that AML and MDS blasts internalize Debio 1562M more efficiently than healthy B cells, possibly due to differences in CD20 expression and Fcγ receptor-mediated endocytosis. [19][13]
Interestingly, there is no clear correlation between CD37 expression level or internalization and sensitivity to Debio 1562M, suggesting that downstream factors, such as payload sensitivity, also influence the response. These findings support the broad applicability of Debio 1562M across genetically and phenotypically diverse patient populations.[18
Clinical Development: The Path Forward
Based on compelling preclinical data, Debio 1562M has entered a phase 1/2 multicenter, open-label, single-arm clinical study (NCT06969430) to evaluate its safety, tolerability, pharmacokinetics, and preliminary efficacy in patients with relapsed/refractory AML and high-risk MDS. The study aims to generate early clinical insights and inform future development, with the potential to establish Debio 1562M as a first-in-class therapy for these challenging diseases.[19]
Study Limitations and Future Directions
While the preclinical data for Debio 1562M are robust, several limitations must be acknowledged:
- Limited Safety Data in Non-Human Primates
- Due to species specificity, toxicology studies relied on murine models and in vitro human cell assays. A comprehensive safety assessment will be addressed in ongoing clinical trials.
- Lack of In Vivo MDS Models
- While MDS blasts are sensitive to Debio 1562M in vitro, in vivo efficacy could not be directly assessed due to a lack of suitable animal models.
- Translation of Dose and Exposure
- The impact of variable tumor burden on drug disposition (TMDD) in AML could complicate dose translation from preclinical to clinical settings.
- Need for Biomarkers
- Further research is needed to identify reliable predictors of response and to optimize patient selection.
Debio 1562M represents a promising advancement in the treatment of AML and MDS, addressing a critical unmet need for patients with limited options and poor prognosis. Its next-generation ADC design enables efficient targeting and killing of malignant blasts and LSCs across all subtypes, with favorable pharmacokinetics and a well-tolerated safety profile in preclinical studies. As Debio 1562M advances through clinical evaluation, it has the potential to become a first-in-class, broadly applicable therapy, offering new hope for patients facing these life-threatening hematologic malignancies.[19]
__
Note: * AGS67E, an anti-CD37-MMAE ADC, is a fully human IgG2 antibody conjugated to monomethyl auristatin E (MMAE) via a protease-cleavable linker. The investigational agent was developed by Agensys, a Santa Monica-based biotechnology company acquired by Astellas Pharma in 2007. Agensys operations were shuttered in 2018 as Astellas Pharma shifted priorities.
** Debio 1562M is a second-generation, optimized antibody-drug conjugate (ADC) designed to treat AML/MDS by targeting CD37. Compared to the first-generation Debio 1562, Debio 1562M is an optimized version that incorporates Debiopharm’s proprietary Multilink™ linker platform technology to achieve higher stability, better tolerability, and improved antitumor efficacy in preclinical models. In vitro studies have shown that Debio 1562M exhibits improved cytotoxicity compared to the previous generation, indicating greater efficacy in killing target cells (IC50 < 1nM in AML models).
*** The combination of azacitidine and venetoclax is a US Food and Drug Administration (FDA-) approved combination therapy for newly diagnosed acute myeloid leukemia (AML) in adults 75 years and older or those ineligible for intensive chemotherapy. It combines a hypomethylating agent (azacitidine) with a BCL-2 inhibitor (venetoclax) to induce remission. [DiNardo CD, Jonas BA, Pullarkat V, Thirman MJ, Garcia JS, Wei AH, Konopleva M, Döhner H, Letai A, Fenaux P, Koller E, Havelange V, Leber B, Esteve J, Wang J, Pejsa V, Hájek R, Porkka K, Illés Á, Lavie D, Lemoli RM, Yamamoto K, Yoon SS, Jang JH, Yeh SP, Turgut M, Hong WJ, Zhou Y, Potluri J, Pratz KW. Azacitidine and Venetoclax in Previously Untreated Acute Myeloid Leukemia. N Engl J Med. 2020 Aug 13;383(7):617-629. doi: 10.1056/NEJMoa2012971. PMID: 32786187.]
Clinical trials
A Study to Assess the Safety, Tolerability, and Antileukemic Activity of Debio 1562M in Participants With Acute Myeloid Leukemia (AML) – ClinicalTrials.gov ID NCT06969430
Highlights of prescribing information
Gemtuzumab ozogamicin (Mylotarg®; Pfizer/Wyeth)[Prescribing information]
Venetoclax (Venclexta®; AbbVie and Genentech)[Prescribing Information]
Azacitidine (Vidaza®; Bristol-Myers Squibb/BMS)[Prescribing Information]
Gilteritinib (Xospata®; Astellas Pharma)[Prescribing Information]
Revumenib (Revuforj®; Syndax)[Prescribing Information]
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DOI 10.14229/jadc.2026.21.04.001
