What HER3 modulators are in clinical trials currently?

Introduction to HER3 and Its Role in Cancer
HER3 (ErbB3) is a unique member of the human epidermal growth factor receptor (HER/ErbB) family. It has a non‐functional intracellular kinase domain that requires partnering with other receptor tyrosine kinases (RTKs) such as HER2 or EGFR to initiate signal transduction. This atypical feature makes HER3 an important coreceptor and “signaling hub” that modulates downstream phosphoinositide 3-kinase (PI3K)/AKT, MEK/MAPK, and other pathways critical for cell survival, proliferation, and differentiation.

HER3 Structure and Function
Structurally, HER3 is composed of an extracellular domain (ECD) that binds ligands like neuregulins (NRG), a single-span transmembrane region, and an impaired intracellular kinase domain. Due to mutations in conserved kinase catalytic residues, HER3 possesses minimal intrinsic enzymatic activity, which necessitates heterodimerization with catalytically active partners (e.g., HER2) to promote phosphorylation events. The receptor’s extracellular domain is segmented into four subdomains (I–IV) that primarily mediate ligand binding and receptor–receptor interactions. This configuration enables HER3 to contribute decisively to the activation of PI3K/AKT signaling via the recruitment of the p85 regulatory subunit, even though it cannot autophosphorylate effectively on its own.

HER3 in Cancer Pathophysiology
HER3 is overexpressed or aberrantly activated in several cancers, including breast, colorectal, lung, and gastric cancers. Overexpression is often observed during tumor progression, metastasis, and acquired resistance to treatments targeting EGFR and HER2. This overexpression can facilitate compensatory activation of key proliferative and anti-apoptotic signaling pathways. Furthermore, mutations in HER3 – although less frequent than those in its family members – can drive oncogenesis and therapy resistance, making it an attractive target for novel therapeutic approaches. The pivotal role of HER3 in circumventing blockade strategies aimed at its dimerization partners has been well documented, which underscores both its importance as a biological marker for aggressive disease and as a promising focal point for emerging treatments.

HER3 Modulators Overview
The development of HER3 modulators is an area of intense research, especially given the receptor’s role in mediating drug resistance and sustaining oncogenic signaling in various cancers. These modulators are designed to inhibit HER3 function either by blocking ligand binding, preventing dimerization, or delivering cytotoxic payloads selectively to HER3-expressing cells.

Types of HER3 Modulators
The modulators currently under investigation can largely be grouped by their modality and mechanism:

• Monoclonal Antibodies (mAbs):
Multiple anti-HER3 antibodies have been developed. These target the extracellular domain of HER3 to block ligand binding or interfere with receptor dimerization. For example, antibodies such as patritumab and MM-121 have been widely studied. Patritumab deruxtecan (also known as HER3-DXd) is not only an antibody but is also conjugated to a cytotoxic payload, forming an antibody–drug conjugate (ADC) that selectively eliminates HER3-expressing tumor cells.

• Antibody–Drug Conjugates (ADCs):
ADCs are an advanced modality where an anti‐HER3 antibody is chemically linked to a potent cytotoxic agent. By using HER3 as a selector, these conjugates allow targeted delivery of the drug while sparing normal tissues. Patritumab deruxtecan is one such example, which is undergoing clinical evaluation in HER3-positive lung and colorectal cancers. Other ADC candidates include DB-1310 from Duality Bio, YL202 from MediLink Therapeutics/BioNTech, and BL-B01D1 from SystImmune/Bristol Myers Squibb.

• Bispecific Antibodies:
Another approach involves bispecific antibodies that target HER3 and another co-expressed antigen, such as HER2 or CD3 on T-cells. These agents facilitate cancer cell killing either by simultaneous inhibition of multiple signaling pathways or by recruiting immune cells to target the tumor cells more effectively. Some studies have also explored bispecific formats that may enhance binding avidity or improve selectivity.

• Radiolabeled Agents:
Although less common in treatment modalities compared to ADCs and mAbs, radiolabeled HER3-targeting molecules (for imaging) have been investigated for positron emission tomography (PET) imaging. These agents help in non-invasive assessment of HER3 status, which indirectly helps in patient stratification and therapy monitoring.

• Small Molecule Ligands:
Despite HER3 being historically perceived as a “pseudokinase,” efforts have been made to develop small molecules that bind to its ATP pocket or disrupt its heterodimerization interface. However, due to the low intrinsic kinase activity of HER3, these small molecule approaches face challenges and are less advanced relative to antibody-based therapeutics.

Mechanisms of Action
The available HER3 modulators act through multiple mechanistic routes:

• Ligand Blockade and Dimerization Inhibition:
By binding to the extracellular domain, monoclonal antibodies can sterically hinder the binding of ligands such as neuregulins or disrupt HER3’s capacity to form heterodimers with HER2, leading to diminished downstream PI3K/AKT signaling.

• Receptor Internalization and Degradation:
Certain HER3 antibodies induce rapid internalization of the receptor leading to its degradation. This not only reduces surface HER3 levels but also prevents downstream signaling cascades from being activated. For example, U3-1287 has been reported to trigger receptor internalization effectively.

• Targeted Cytotoxic Delivery:
ADCs leverage the binding specificity of anti-HER3 antibodies to deliver potent cytotoxic agents directly to cancer cells that overexpress HER3. Once the ADC is internalized, the linker is cleaved releasing the cytotoxic drug intracellularly, leading to cell death.

• Engagement of Immune Effector Functions:
Some HER3-targeting antibodies are engineered with Fc modifications to enhance antibody-dependent cellular cytotoxicity (ADCC) or other immune effector functions, thereby recruiting natural killer (NK) cells and other immune components to eliminate tumor cells.

Current Clinical Trials Involving HER3 Modulators
The clinical trial landscape for HER3 modulators has evolved significantly over the past several years. Both early phase trials (Phase I) and later stage evaluations (Phase II/III) are ongoing across multiple tumor types. The selection of patients is often driven by HER3 overexpression, the presence of HER3 mutations, or an established role for HER3 in driving tumor resistance, particularly in the context of EGFR/HER2 targeting.

Phase I Trials
In the early phases of clinical evaluation, the focus is primarily on safety, pharmacokinetics (PK), pharmacodynamics (PD), and establishing the maximum tolerated dose (MTD) of HER3 modulators. Several studies from synapse-sourced literature have provided valuable insights into phase I studies:

• Patritumab Deruxtecan/HER3-DXd:
One of the most promising HER3 modulators in clinical development is patritumab deruxtecan. Early clinical studies have focused on assessing its safety, tolerability, and preliminary efficacy in patients with advanced cancers that express HER3. These phase I trials are crucial for establishing the recommended dose for subsequent trials and understanding the biodistribution profile and internalization dynamics of the ADC. The design of these trials often incorporates measurements such as HER3 expression via immunohistochemistry (IHC), circulating biomarker profiling, and evaluation of dose-limiting toxicities (DLTs). The radiotracer studies also attached to HER3-targeting mAbs provide an example of using advanced imaging techniques to monitor tumor uptake and receptor occupancy in vivo.

• Other ADC Candidates:
Alongside patritumab deruxtecan, other HER3-targeting ADCs such as DB-1310 (from Duality Bio) and YL202 (from MediLink Therapeutics/BioNTech) have entered early-phase clinical trials. These agents are designed using cleavable linker-payload systems that allow targeted delivery of cytotoxic drugs to HER3-expressing tumors. Phase I trials for these agents include dose-escalation studies in solid tumors such as non–small cell lung cancer (NSCLC), colorectal cancer, and possibly breast and gastric cancers. The early trials are designed to document pharmacokinetics, safety parameters, and early signals of antitumor activity, while also incorporating modern biomarker analyses to correlate HER3 expression with response.

• Bispecific Antibody Approaches:
Some phase I studies also involve bispecific antibody constructs that target HER3 in conjunction with another antigen (or with CD3 on T-cells for direct immune engagement). These trials are exploring whether bivalent binding to HER3 can improve specificity, enhance receptor internalization, and overcome resistance mechanisms in combination with standard-of-care treatments. Preclinical data supporting effective HER3 blockade by these bispecific constructs have led to the initiation of early phase evaluations where safety and immunogenicity are key endpoints.

Phase II and III Trials
Advanced clinical trials (Phase II/III) focus on demonstrating efficacy and further evaluating safety in well-defined patient populations. Recent and ongoing studies are aiming to assess the clinical benefits of HER3 modulators in various cancer settings:

• Patritumab Deruxtecan in Phase II/III Settings:
Patritumab deruxtecan has progressed into later-stage clinical trials, particularly in patients with cancers known for HER3 overexpression, such as NSCLC and colorectal cancer. In these studies, the ADC is being evaluated both as a monotherapy and in combination with other therapies (e.g., HER2-targeted agents or conventional chemotherapy) to overcome acquired resistance. The patient populations are carefully stratified using biomarkers that quantify HER3 expression on tumor tissues, as upregulation of HER3 is often implicated in resistance to other targeted agents. The trial endpoints typically include progression-free survival (PFS), overall response rate (ORR), and overall survival (OS), and are designed in a randomized, controlled manner.

• Other HER3-Targeting ADCs:
In addition to patritumab deruxtecan, clinical trials are ongoing with ADC candidates like DB-1310 and YL202. These molecules are being tested in phase II studies to determine their antitumor efficacy in HER3-positive tumors and to identify potential biomarkers predictive of response. Early signals from these studies have indicated that proper patient selection based on HER3 expression, as well as the use of companion diagnostic assays, is critical for optimizing therapeutic outcomes. These phase II trials also contribute to dosage refinement and the evaluation of treatment regimens in the context of combination therapies.

• Combinational Strategies:
Several phase II/III trials investigate HER3 modulators in combination with other targeted agents. Because HER3 signaling is often implicated in resistance to EGFR and HER2 inhibitors, combining HER3 modulators with these agents is a strategic endeavor meant to synergize the blockade of compensatory pathways. For example, trials combining patritumab deruxtecan with EGFR/HER2 inhibitors are designed to mitigate resistance mechanisms while providing an improved safety profile and enhanced tumor response. Such studies incorporate complex dosing schedules and require careful patient monitoring to assess for overlapping toxicities and to optimize the therapeutic window.

• Biomarker-Driven Trials:
Phase III trials are increasingly using robust biomarker assessments to determine patient eligibility and to monitor treatment response comprehensively. HER3 expression levels, assessed by IHC or fluorescence in situ hybridization (FISH), and assays for phosphorylated HER3 or downstream effectors (e.g., p-AKT) are being incorporated into trial designs. These biomarker-driven designs are intended to enrich the study population for those most likely to benefit from HER3-targeted therapy, thereby improving the risk–benefit ratio and overall trial success.

Challenges and Future Directions
While the development and clinical testing of HER3 modulators are promising, several challenges remain that affect clinical development and future research directions.

Clinical Development Challenges
• Biomarker Identification and Patient Stratification:
One of the most significant challenges in evaluating HER3 modulators is the identification and validation of predictive biomarkers. Unlike HER2, where amplification can be measured with high specificity, HER3 expression is more variable and may be influenced by dynamic factors such as prior therapies or tumor microenvironment interactions. The lack of standardized, robust assays for HER3 expression and activation complicates patient selection and may result in heterogeneous trial populations.

• Resistance and Redundant Signaling Pathways:
HER3’s role as a central signaling hub means that it participates in multiple redundant pathways. Tumors can develop compensatory mechanisms that bypass HER3 inhibition, such as upregulation of parallel receptor tyrosine kinases (e.g., EGFR, HER2, c-Met) or by secreting alternative ligands. This interplay of multiple signaling networks presents a challenge to achieving durable responses with HER3 modulators as monotherapies. As a result, combinational treatment strategies are needed, yet they further complicate trial design, safety assessments, and dosage optimization.

• Safety Concerns and Off-Target Effects:
The balance between therapeutic efficacy and safety is also critical for ADCs and bispecific antibodies. Although ADCs like patritumab deruxtecan are designed to minimize toxicity by exploiting the selective expression of HER3, dose-limiting toxicities and off-target cytotoxicity need to be closely monitored, particularly in combination therapy settings. Furthermore, the immune-mediated effects of bispecific antibodies demand rigorous immunogenicity assessments in early phase trials.

• Regulatory Hurdles and Trial Design Complexity:
Advanced-phase clinical trials of HER3 modulators often adopt adaptive and biomarker-enriched designs. While this can increase the efficiency of the study, it also complicates statistical analyses and regulatory approval processes. The current clinical trials must balance traditional endpoints such as ORR and PFS with newer surrogate markers of efficacy that reflect target engagement and signaling inhibition.

Future Research and Development Trends
• Integration of Precision Medicine Approaches:
Looking ahead, the trending direction in HER3 modulator development is the integration of precision medicine approaches. This involves using next-generation sequencing, advanced imaging, and multiplexed biomarker assays to precisely identify which patients are most likely to benefit from HER3-targeted therapy. Future trials will likely incorporate serial biopsies and liquid biopsies to track changes in HER3 expression and receptor activation over time.

• Development of Novel ADCs and Bispecific Formats:
The evolution of ADC technology, with more stable linkers and novel payloads, is expected to improve the therapeutic index of HER3 modulators. In parallel, the prolonged half-life and enhanced immune recruitment by next-generation bispecific antibodies are being optimized to overcome issues of resistance and limited efficacy. Research is also focused on designing ADCs that can be effectively combined with checkpoint inhibitors, thereby harnessing synergistic anticancer immune responses that may further extend the duration of response.

• Combination Therapies to Overcome Resistance:
Due to the multifaceted role of HER3 in resistance mechanisms, future clinical strategies will likely involve combination regimens. By pairing HER3 modulators with inhibitors of EGFR, HER2, or even downstream effectors such as PI3K/AKT/mTOR inhibitors, these studies aim to forestall the emergence of resistance pathways and achieve more robust antitumor effects. Ongoing and planned trials are investigating such combinational approaches in both first- and later-line therapy settings in various tumor types.

• Advancements in Imaging as a Companion Diagnostic:
The progress in developing radiolabeled HER3-targeting compounds for imaging purposes suggests that future clinical trials may incorporate PET imaging as a companion diagnostic tool. This will allow non-invasive assessments of HER3 expression, quantify receptor occupancy, and provide real-time feedback on treatment efficacy – a trend anticipated to improve patient outcomes in HER3 modulator trials.

• Addressing Safety and Toxicity in Novel Formats:
Engineering antibodies with optimized Fc regions and designing ADCs that cleave their payload only within the tumor microenvironment are likely to reduce the incidence of off-target toxicity. Future directions also include exploring alternative routes of administration and dosing regimens that maintain efficacy while minimizing adverse effects. Regulatory agencies are expected to work closely with developers on adaptive trial designs that can flexibly incorporate emerging safety data.

Conclusion
In summary, several HER3 modulators are currently in clinical trials spanning from Phase I to Phase III. These include monoclonal antibodies and advanced antibody–drug conjugates such as patritumab deruxtecan (HER3-DXd), as well as other emerging candidates like DB-1310, YL202, and BL-B01D1 from companies such as Duality Bio, MediLink Therapeutics/BioNTech, and SystImmune/Bristol Myers Squibb respectively. These modulators operate by blocking ligand binding and heterodimerization, inducing VEG signalling inhibition, promoting receptor internalization and degradation, or delivering cytotoxic payloads selectively to tumors. The Phase I trials are focused on safety, PK/PD profiling, and dose escalation, while later-phase trials seek to confirm efficacy through rigorous biomarker-driven designs and combination therapy strategies.

Challenges remain regarding robust patient stratification, overcoming compensatory resistance mechanisms, and managing safety profiles in complex combinational settings. Future trends include precision diagnostics, next-generation ADCs, bispecific antibody formats, and more adaptive clinical trial designs – all aimed at exploiting HER3’s critical role in cancer pathophysiology to improve patient outcomes. Overall, the evolving landscape of HER3 modulators represents a promising frontier in oncology, with the potential to address unmet needs in patients whose tumors are driven by HER3-mediated pathways.