What CD33 modulators are in clinical trials currently?

Overview of CD33 and Its Role

CD33 Structure and Function
CD33, also known as Siglec-3, is a transmembrane receptor characterized by an amino-terminal V-set immunoglobulin domain that mediates sialic acid binding, followed by a C2-set domain, a single transmembrane segment, and a cytoplasmic tail with immunoreceptor tyrosine-based inhibitory motifs (ITIMs). These ITIMs recruit phosphatases such as SHP-1 and SHP-2 upon phosphorylation, which play a role in inhibitory signaling. CD33’s structure is particularly important not only because it defines its function but also because it impacts the internalization process—a key mechanism exploited by several CD33 modulators.

Importance in Disease Context, Especially in Hematologic Malignancies
CD33 is prominently expressed on the majority of malignant blasts in acute myeloid leukemia (AML), as well as on leukemic progenitor cells and certain myeloid-derived suppressor cells (MDSCs). Its expression on these cells, contrasted with more restricted expression on normal hematopoietic stem and progenitor cells (HSPCs), renders it an attractive target for immunotherapy. The rationale originated from both diagnostic and therapeutic perspectives—CD33 has been used as a marker to help distinguish malignant from normal myeloid cells and, importantly, as a therapeutic target for antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T cells, and bispecific T cell engagers. In AML, targeting CD33 has already translated into clinically approved treatments such as gemtuzumab ozogamicin (GO) that have demonstrated improvements in event-free survival when combined with standard chemotherapy. Recent insights into CD33 biology also suggest a potential role in other hematologic malignancies, reinforcing its importance in the treatment landscape.

Current CD33 Modulators in Clinical Trials

Types of CD33 Modulators
A variety of therapeutic modalities are currently under clinical investigation to modulate CD33 activity. These can be broadly categorized as follows:

• Antibody–Drug Conjugates (ADCs)
 – Gemtuzumab Ozogamicin (GO): Although initially withdrawn from the market due to safety concerns, GO was re-approved in 2017 and is subject to ongoing clinical investigations both as a single agent and in combination with chemotherapy for various treatment settings in AML.
 – SGN-CD33A: An ADC that was evaluated for potency in targeting CD33 but encountered challenges related to toxicity, leading to its clinical development being halted after phase III trials.
 – IMGN779: A novel CD33-targeted ADC that utilizes a maytansinoid derivative (DM4) conjugated to an anti-CD33 antibody. Early-phase trials have been conducted to optimize dosing schedules and assess safety and efficacy, with promising early reductions of bone marrow blast counts being reported in relapsed/refractory AML patients.

• Radioimmunoconjugates
 – 225Ac-lintuzumab: This construct is designed wherein a radioactive payload is attached to an anti-CD33 antibody (lintuzumab). Although clinical data is limited, preclinical reports have suggested moderate single-agent activity. Current investigations are evaluating its integration into combination regimens.

• CAR-modified Immune Effector Cells
 – Anti-CD33 CAR T Cells: Several generations of CAR T cell therapies targeting CD33 have been developed. Notably, third-generation CAR T cells (3G.CAR33-T) have shown increased viability, proliferation, and cytotoxic activity when compared with second-generation products, and are currently under investigation in early-phase clinical trials.
 – Strategies to mitigate on-target off-leukemia toxicity (e.g., combining CAR T therapy with CD33 knockout HSPCs) are also under clinical exploration, aiming to allow more aggressive targeting of AML blasts while preserving normal hematopoiesis.

• Bispecific T Cell Engagers (BiTEs) and Related Constructs
 – AMG330: A bispecific antibody construct that engages both CD33 on AML blasts and CD3 on T cells, facilitating T cell-mediated cytotoxicity. Early clinical data have demonstrated tolerable safety profiles and evidence of antileukemic activity.
 – AMG673 and AMV564: Other bispecific constructs similar in design to AMG330 are under investigation and have demonstrated promising early-phase in vitro and in vivo cytotoxicity, offering a potential treatment modality for patients with relapsed/refractory AML.

• Antibody-Drug Conjugate-like Radioisotope Conjugates
 – Actimab-A: Developed by Actinium Pharmaceuticals, this medicinal product comprises the anti-CD33 monoclonal antibody lintuzumab linked to the alpha-emitting isotope Actinium-225. The program is exploring its utility as a single agent at high doses for targeted conditioning prior to bone marrow transplant or at lower doses for therapeutic combinations in AML and related hematologic malignancies.

• Tri-specific Engagers
 – GTB-3550: This is a Tri-specific Killer Engager (TriKE) that incorporates anti-CD16 and anti-CD33 scFv domains linked with a modified form of IL-15, thus concurrently targeting AML cells and promoting natural killer (NK) cell activation. It is currently being evaluated in phase I/II trials for CD33-positive leukemias and MDS.

• Unconjugated, Fc-engineered antibodies
 – BI 836858: This type of antibody is engineered to enhance effector functions via Fc modifications. While it may not be directly cytotoxic, its increased ability to engage immune effector cells is being investigated in early clinical studies and may pave the way for combination approaches.

Clinical Trial Phases and Status
The clinical programs for CD33-based modulators span early-phase dose-escalation studies through to phase III randomized studies, although many remain in the early clinical phase due to challenges in striking the optimal balance between efficacy and safety. For example:

• Gemtuzumab Ozogamicin (GO) is further evaluated in both adult and pediatric AML clinical settings. In pediatric patients with newly diagnosed AML, GO is being used in combination with standard chemotherapy regimens, and the outcomes have shown improved relapse-free survival and acceptable toxicity profiles.
• Phase I/II studies evaluating CAR T cell therapies targeted against CD33 have demonstrated rapid clearance of blasts in some cases, but longer-term efficacy and toxicity profiles remain under careful scrutiny.
• AMG330 and other BiTE constructs have been tested in early-phase trials with encouraging data on cytokine release syndrome (CRS) management and overall tolerability at escalated doses. The safety profiles are acceptable, though the efficacy endpoints in terms of overall response rates are still being optimized.
• IMGN779 has been evaluated in phase I trials with schedules ranging from weekly to every two weeks. The results indicate reductions in bone marrow blast percentages in certain patients, and dose-escalation efforts are still ongoing to optimize clinical benefits and minimize hematologic toxicities.
• Actimab-A and other novel agents such as GTB-3550 are in the early phases of clinical testing. Their trial designs focus on determining maximally tolerated doses, response rates in combination therapies, and the feasibility of integrating them into existing treatment protocols for AML and MDS.
• Unconjugated antibodies and Fc-engineered constructs, while promising in preclinical models, are being advanced in early human studies to determine their ability to mediate antibody-dependent cellular cytotoxicity (ADCC) specifically in the context of CD33-positive malignancies.

Mechanisms of Action

How CD33 Modulators Work
The therapeutic modalities targeting CD33 utilize several mechanisms to neutralize or eradicate malignant cells:

• ADCs such as GO and IMGN779 deliver cytotoxic payloads selectively into CD33-positive cells. Upon binding to CD33, the complex is internalized, leading to the intracellular release of the toxic drug (typically calicheamicin for GO or DM4 for IMGN779) that induces double-strand DNA breaks and cell death.
• Radioimmunoconjugates like 225Ac-lintuzumab similarly depend on CD33-mediated internalization, but instead deliver a radioactive isotope to induce lethal DNA damage by alpha particle emission.
• CAR T cell therapies exploit genetically engineered T cells that directly recognize CD33 on leukemia cells. These modified T cells are designed to activate upon antigen engagement, leading to targeted cell lysis. The third-generation CAR designs incorporate multiple costimulatory domains to achieve increased proliferation, persistence, and cytotoxicity.
• Bispecific T cell engagers (BiTEs) such as AMG330 operate by simultaneously binding CD33 on the target cell and CD3 on T cells, creating a synapse that facilitates T cell activation and the subsequent release of cytolytic granules, leading to apoptosis of the malignant cell.
• TriKEs like GTB-3550 additionally include an IL-15 moiety that enhances NK cell activation and proliferation, complementing the direct targeting of CD33 with an immune-stimulating component that may overcome resistance mechanisms.

Comparison of Different Modulators
Each CD33-targeted modality offers distinct advantages and challenges:

• ADCs have the clear advantage of delivering potent cytotoxins directly to malignant cells, but their efficacy heavily depends on the rate of CD33 internalization and the release kinetics of the toxic payload. GO has shown clinically meaningful improvements in survival in certain AML subtypes, but the toxic effects (e.g., hepatotoxicity) require careful management.
• Radioimmunoconjugates are promising for their ability to provide localized radiation; however, they require precise dosing to avoid off-target effects.
• CAR T cell therapies offer the prospect of durable responses through self-propagating cellular therapies, yet the risks of prolonged cytopenias and the potential for severe cytokine release syndrome remain concerns. Strategies such as transient CAR expression have been explored to mitigate these risks.
• Bispecific antibodies have the benefit of off-the-shelf availability and a generally more manageable safety profile. They can be titrated according to dose and are reversible in many cases compared to cellular therapies. The immune synapse formation they induce is highly efficient even at low antigen densities, although overcoming mechanisms such as PD-L1 up-regulation on target cells may be necessary to enhance efficacy.
• Tri-specific engagers add an extra layer by engaging NK cells and providing cytokine support, which can be especially useful in patients where T cell responses are diminished. Early data from these trials suggest that they could achieve a broader immune activation while maintaining specificity for CD33-positive cells.
• Unconjugated, Fc-engineered antibodies are designed to modulate CD33 function or enhance immune cell recruitment without necessarily delivering a cytotoxic payload. Their role is being explored as part of combination regimens where synergy with standard chemotherapy or other immunotherapeutic modalities may maximize clinical benefit.

Clinical Trial Outcomes and Implications

Efficacy and Safety Data
The clinical trials conducted so far with CD33 modulators have provided important insights into both their efficacy and safety profiles:

• Gemtuzumab Ozogamicin (GO) has shown a significant improvement in event-free survival when used in combination with standard induction chemotherapy for newly diagnosed AML, particularly in patients with favorable or intermediate-risk cytogenetics. Pediatric studies have also reported better relapse-free survival with acceptable toxicity. Nonetheless, adverse effects such as hepatotoxicity and cytopenias remain clinically significant and necessitate meticulous supportive care.
• Early-phase studies with CAR T cell therapies against CD33 have demonstrated rapid and potent antileukemic activity; however, single case reports have also noted eventual disease relapse, and the persistence of CAR T cells in the presence of low levels of CD33 on residual normal progenitors poses risks of myeloid toxicity. This has spurred the development of strategies combining CD33 knockout HSPCs with CAR T therapy to mitigate prolonged myelosuppression.
• Bispecific engagers such as AMG330 have produced encouraging results with regards to in vitro cytotoxicity and early clinical responses in relapsed/refractory AML patients. The incidence of cytokine release syndrome (CRS) is noted, but it is mostly manageable at therapeutic doses. Notably, these constructs have also demonstrated an ability to stimulate T cell activation and proliferation, a feature that is especially critical in heavily pre-treated patients.
• Preliminary outcomes from IMGN779 trials indicate that reductions in bone marrow blasts are achievable at tolerable dose ranges. The overall response rates suggest that optimizing dosing schedules—whether weekly or biweekly—plays a crucial role in balancing efficacy with hematologic toxicities.
• The Actimab-A program, exploring the use of Ac-225 conjugates, and the TriKE approaches are still in early-phase clinical trials. Although comprehensive efficacy data are not yet mature, early indications of response rates and manageable adverse event profiles are promising. These modalities are particularly intriguing for their potential dual effects: direct cytotoxicity and immune cell engagement.

Potential Impact on Treatment Protocols
The ongoing clinical investigations suggest that CD33 modulators could further refine treatment protocols for AML and related hematologic malignancies in several ways:

• They offer a targeted approach that may allow for intensification of therapy in patients with high CD33 expression while sparing HSPCs, thereby reducing long-term myelosuppression when combined with strategies such as CD33 knockout.
• The success of bispecific antibodies and tri-specific engagers could lead to the integration of immune modulatory approaches into frontline therapies, especially for relapsed/refractory patient populations who have limited treatment options.
• Combination strategies involving CD33 modulators with traditional chemotherapeutic regimens, hypomethylating agents, or checkpoint inhibitors may yield synergistic effects that enhance overall survival and reduce relapse rates.
• The versatility in the types of CD33 modulators (ADC, CAR T, BiTE, radioimmunoconjugate) allows for customization of therapy based on patient-specific factors such as antigen density, disease stage, and prior treatment history, thereby potentially improving personalized medicine approaches.

Future Directions and Challenges

Ongoing Research and Development
The clinical landscape continues to evolve with several promising avenues:

• Multiple strategies are under investigation to optimize CD33-targeted therapy. The development of next-generation ADCs with more stable linkers and less off-target toxicity remains a priority.
• There is active research on refining CAR T cell engineering techniques. Approaches such as transient CAR expression and the integration of safety switches (suicide genes) are under exploration to enhance the safety and control of CD33-directed cellular therapy.
• Bispecific and tri-specific engagers are being further optimized to not only enhance efficacy but also to mitigate immunosuppressive factors in the tumor microenvironment (for example, by combining with PD-1/PD-L1 blockade).
• New radioisotope conjugates are designed to leverage advances in radionuclide chemistry that may lead to more effective delivery of cytotoxic radiation while minimizing harm to healthy tissues.
• The Actimab-A program and similar efforts are exploring the role of alpha-emitting isotopes in conditioning regimens, which could revolutionize pre-transplant protocols for high-risk AML patients.

Challenges in CD33 Targeting and Modulation
Despite the significant progress, several challenges remain:

• Safety Concerns: One of the primary issues with CD33 targeting is the non-exclusivity of CD33 expression. Although malignant blasts express CD33 at high levels, normal myeloid progenitors and mature cells also display it, increasing the risk for hematologic toxicity. Optimization of dosing schedules and incorporation of safety switches are important measures to address these concerns.
• Heterogeneity of Antigen Expression: In AML, CD33 expression can vary widely among patients and even within different cell populations in the same patient, potentially affecting the efficacy of CD33-targeted therapies. Future clinical trials will need to include robust diagnostic assessments (e.g., quantitative CD33 expression analyses) to better select patients who are most likely to benefit.
• Resistance Mechanisms: Tumor cells may develop resistance mechanisms such as antigen modulation, efflux of cytotoxic payloads (in the case of ADCs), or upregulation of inhibitory ligands. Addressing these resistance pathways, perhaps by combining CD33 modulators with other therapeutic agents, is a critical area of ongoing research.
• Complex Clinical Trial Design: As multiple modalities progress concurrently, designing head-to-head and combination clinical trials will be increasingly complex. Particular attention will need to be given to defining clinical endpoints, managing combination toxicities, and balancing patient selection criteria to ensure that positive effects in one subgroup are not diluted by heterogeneity in the study population.
• Manufacturing and Scalability: Cellular therapies such as CAR T cells require customized manufacturing processes that can be resource-intensive and may lead to variability in clinical outcomes. Standardization of production protocols and scaling manufacturing processes under GMP conditions are therefore essential to broaden their clinical adoption.

Conclusion

In summary, current clinical trials for CD33 modulators are exploring an impressive array of therapeutic modalities, each with unique mechanisms designed to target CD33-positive hematologic malignancies, primarily AML. The modalities include:

• Antibody–Drug Conjugates (GO, IMGN779, SGN-CD33A) that offer direct cytotoxicity through the intracellular delivery of potent drugs. GO remains in clinical use with multiple combination studies in both adult and pediatric populations despite past challenges, whereas newer ADCs like IMGN779 are in early-phase testing with promising results.
• Radioimmunoconjugates such as 225Ac-lintuzumab that leverage localized radiation therapy to drive DNA damage once internalized have been explored preclinically, with ongoing clinical interests.
• CAR T cells targeting CD33, including third-generation constructs showing enhanced proliferation and cytotoxicity along with innovative strategies combining CAR T therapy with genetically modified HSPCs to avoid myelosuppression, are in early-phase trials, offering hope for durable responses.
• Bispecific T cell engagers (AMG330, AMG673, AMV564) and tri-specific NK cell engagers (GTB-3550) are under clinical evaluation, presenting off-the-shelf solutions that facilitate efficient immune synapse formation and consequent tumor cell lysis, with safety profiles that are manageable and promising early response rates.
• Programs like Actimab-A, which incorporates alpha-emitting Actinium-225 linked to an anti-CD33 antibody, are actively being pursued to enhance both the cytotoxic efficacy and versatility of CD33-targeted therapies.

The overall therapeutic potential of these modulators lies in their ability to selectively eradicate malignant cells while preserving normal hematopoiesis—a balance that remains challenging due to the shared antigenic profile of normal and malignant myeloid cells. Current clinical trials continue to define the optimal dosing, combination regimens, and patient selection strategies to maximize the clinical benefit of CD33 modulation.

From a general perspective, the scientific rationale behind targeting CD33 is underpinned by extensive preclinical evidence and early-phase clinical data, which support the notion that CD33 modulation can achieve meaningful antileukemic effects. On a specific level, each therapeutic approach is being refined to address inherent challenges such as hematologic toxicity, antigen heterogeneity, and resistance mechanisms. Finally, in a general future outlook, these ongoing advancements are expected to pave the way for more personalized and effective treatment protocols in AML and other CD33-positive hematologic malignancies.

In conclusion, the field of CD33 modulation is moving rapidly, with multiple promising agents in clinical trials employing advanced engineering designs and novel therapeutic concepts. While challenges remain—particularly regarding safety and patient variability—the robust clinical programs and innovative approaches currently under investigation promise to significantly impact future treatment paradigms. Their success will likely lead to improved outcomes for patients with challenging hematologic malignancies, making CD33 a central focus of ongoing research and clinical development.