What TriTAC are being developed?

Introduction to TriTAC Technology

Definition and Mechanism of Action
TriTACs (Tri-specific T cell Activating Constructs) represent an innovative class of engineered proteins that couple the functionality of T cell engagers with three binding domains. In essence, a TriTAC simultaneously binds to a target tumor antigen, engages the CD3 receptor on T cells, and incorporates a half-life–extending or modulatory component (frequently achieved through albumin-binding or other strategies) to improve in vivo stability and therapeutic window. This triple engagement facilitates the formation of a productive ternary complex that redirects a patient’s own T cells to recognize and kill tumor cells. The mechanism of action relies on the recruitment and activation of T cells in the tumor microenvironment, leading to direct cytotoxicity against cells expressing the target antigen. Moreover, the TriTAC platform has been adapted in certain cases into a prodrug format, known as ProTriTAC, wherein the construct remains inactive until it encounters tumor-specific proteases, thus limiting off-tumor toxicities.

Historical Development and Key Milestones
The development of TriTAC technology has evolved over recent years as the need for more selective and potent immunotherapies has grown. Early T cell engagers paved the way for TriTAC constructs by demonstrating that engagement of CD3 on T cells can be harnessed for direct anti-tumor activity. Building on these successes, companies such as Harpoon Therapeutics advanced their TriTAC platform into preclinical and early clinical development in a series of well-documented milestones. Notably, preclinical work on FLT3 TriTACs established the feasibility of using nanobody-based constructs for targeting hematologic malignancies. Subsequent advancements include the clinical evaluation of HPN217, a BCMA-targeting TriTAC now in Phase 1/2 trials for multiple myeloma, and HPN328, a DLL3-targeting TriTAC for small cell lung cancer (SCLC). These developments mark significant progress in translating the TriTAC concept from bench to bedside over the past few years.

Current TriTAC Therapeutics

Overview of TriTACs in Development
Today, several TriTAC-based therapeutics are being developed for a broad range of indications, especially in the realms of solid tumors and hematologic malignancies. The portfolio includes:
- FLT3 TriTACs: Designed to target FLT3 in certain neoplasms and hematologic diseases, this candidate employs a nanobody format that couples FLT3 modulation with CD3 and albumin binding to enhance in vivo stability.
- HPN217 (BCMA TriTAC): Focusing on multiple myeloma, HPN217 targets the B cell maturation antigen (BCMA). Early clinical data from Phase 1/2 studies have shown promising anti-cancer activity with manageable adverse event profiles.
- HPN328 (DLL3 TriTAC): Under development for small cell lung cancer (SCLC) and other DLL3-expressing tumors, HPN328 is an open-label candidate advancing in Phase 1/2 dose escalation studies. Preliminary observations include partial responses and signs of anti-tumor activity in DLL3-positive tumors.
- HPN424 (PSMA TriTAC): A candidate targeting the prostate-specific membrane antigen (PSMA) for metastatic castration-resistant prostate cancer, HPN424 is being evaluated in early-phase clinical trials. It has demonstrated promising efficacy and safety signals in dose-escalation cohorts.
- HPN536 (Mesothelin TriTAC): With an aim to treat mesothelin-expressing solid tumors, HPN536 has progressed from preclinical characterization into Phase 1 testing. Its design leverages the TriTAC structure to improve T cell engagement and tumor specificity.
- TROP2 ProTriTAC™: An innovative adaptation of the TriTAC platform, the TROP2 ProTriTAC is a protease-activated T cell engager prodrug candidate that remains masked until encountering tumor-specific proteases, allowing for improved safety by reducing off-target effects.
- HPN601 (EpCAM-targeting ProTriTAC): Emerging as part of the ProTriTAC strategy, HPN601 targets EpCAM expressed on a variety of solid tumors, combining the advantages of conditional activation with efficient T cell recruitment.

Leading Companies and Research Institutions
The development of TriTAC technology is primarily led by Harpoon Therapeutics, Inc., a clinical-stage immunotherapy company that has become synonymous with the advancement of novel Tri-specific T cell engagers. In addition, significant collaborations and partnerships have bolstered the platform’s progress. For instance, strategic alliances with Merck Sharp & Dohme LLC and previously with AbbVie have supported the broad development and clinical evaluation of TriTAC candidates such as HPN217 and HPN328. Research institutions and industry partners further contribute through collaborative preclinical studies that validate novel targets, enhance the design of TriTAC constructs (such as the ProTriTAC and extended release TriTAC-XR strategies), and optimize their clinical profiles.

Developmental Stages and Clinical Trials
The TriTAC candidates are progressing through various stages of development, with several advancing into early clinical evaluation:
- Preclinical Stage: FLT3 TriTACs are currently in the preclinical phase, where detailed mechanistic studies and in vivo efficacy work are being conducted to establish proof-of-concept.
- Phase 1/2 Trials:
- HPN217 (BCMA TriTAC): Underwent dose escalation and is now providing early signs of clinical activity in multiple myeloma patients, with favorable response rates and pharmacokinetic profiles being reported at major scientific meetings such as ASH.
- HPN328 (DLL3 TriTAC): Being tested in an open-label, dose-escalation setting, HPN328 has shown early partial responses in SCLC cohorts, and combination strategies are being explored in subsequent trial phases.
- HPN424 (PSMA TriTAC) and HPN536 (Mesothelin TriTAC): These candidates are also in Phase 1/2 studies, with data being continuously updated at scientific conferences (e.g., ASCO and AACR), reflecting dose escalation outcomes, tolerability, and preliminary efficacy signals.
- Innovative Platforms: The ProTriTAC and extended release TriTAC-XR platforms represent novel strategies aiming to further optimize the therapeutic index of TriTACs by controlling activation and reducing cytokine release syndrome (CRS). Such strategies are integrated into candidates like the TROP2 ProTriTAC and HPN601, which are in the early exploratory stages of development.

Efficacy and Safety of TriTACs

Preclinical and Clinical Trial Results
Preclinical studies of TriTACs have demonstrated the feasibility of simultaneous multi-target engagement. For example, the FLT3 TriTAC has shown promising nanomolar potency in vitro by effectively directing T cells to target FLT3-positive cells while leveraging albumin-binding to extend serum half-life.
Clinical trial data, particularly from Phase 1/2 studies, have highlighted encouraging efficacy:
- HPN217: Clinical data have revealed an objective response rate (ORR) of up to 77% in the highest dose cohorts, with patients exhibiting durable responses and a manageable safety profile.
- HPN328: In the context of SCLC and DLL3 expression, early reports indicate that dose escalation cohorts have achieved partial responses, and combinational studies (such as with checkpoint inhibitors like atezolizumab) are being planned to further enhance clinical benefit.
- HPN424 and HPN536: Data emerging from these trials underscore a balanced safety profile with indications of T cell activation and cytokine modulation. Dose-limiting toxicities have been observed at higher doses; however, modifications like step dosing and incorporation of half-life extension maneuvers have contributed to safety improvements.
The ProTriTAC approach, as seen with the TROP2 ProTriTAC, has shown improved binding kinetics in vitro and potent T cell–mediated cytotoxicity in preclinical models, setting the stage for future clinical evaluations.

Comparative Analysis with Other Therapies
In comparison to traditional bispecific T cell engagers (BiTEs) and other T cell–engaging formats, TriTACs offer several potential advantages:
- Enhanced Stability and Pharmacokinetics: The inclusion of albumin-binding domains or engineered half-life extension peptides typically lengthens the serum half-life of TriTACs (with reported half-lives ranging from several days to even weeks in non-human primates), which can translate into less frequent dosing and steadier exposure compared to BiTE molecules that often require continuous infusion.
- Improved Selectivity and Activity: TriTACs can achieve higher selectivity through simultaneous binding of three distinct epitopes, reducing off-tumor activity, and potentially mitigating the risk of adverse effects such as cytokine release syndrome. The conditional activation seen in ProTriTACs further enhances safety over conventional T cell engagers.
- Modular Design for Target Versatility: By simply exchanging the tumor-targeting domain, the TriTAC platform can be adapted to a wide range of indications—from hematologic malignancies (e.g., FLT3 and BCMA targeting) to solid tumors (e.g., DLL3, PSMA, mesothelin, TROP2, and EpCAM targets).
- Comparative Efficacy: Early clinical response rates and pharmacodynamic markers (such as T cell activation and cytokine profiles) suggest that TriTACs are at least comparable to, and in some cases superior to, other T cell engager platforms in preclinical and early clinical settings. Their design addresses some of the limitations observed with other immunotherapies, such as rapid clearance, limited tumor penetration, or off-target toxicities, thereby offering a promising alternative approach.

Challenges and Future Directions

Current Challenges in TriTAC Development
Despite the promising attributes, several challenges remain in the development of TriTAC therapeutics:
- Manufacturing and Stability: The multivalent and modular nature of TriTACs requires sophisticated protein engineering and manufacturing processes. Maintaining a consistent structure with the correct folding, stability, and bioactivity throughout the production and formulation phases is complex.
- Safety and Cytokine Release: Even though TriTACs are designed to minimize off-tumor toxicities, high potency T cell activation raises concerns regarding cytokine release syndrome (CRS). The development of extended release (TriTAC-XR) and prodrug (ProTriTAC) strategies aims to mitigate these adverse events, but fine-tuning the balance between efficacy and toxicity remains a key hurdle.
- Tumor Microenvironment Resistance: The immunosuppressive environment within tumors poses an inherent challenge. Resistance mechanisms, including antigen loss or heterogeneity, may limit the efficacy of TriTACs. Combination strategies with immune checkpoint inhibitors or other immunomodulatory agents are under investigation to overcome this barrier.
- Clinical Endpoint Establishment: As TriTACs progress through clinical trials, establishing robust biomarkers and endpoints to evaluate efficacy across different tumor types is essential. Variability in patient responses adds another layer of complexity that necessitates personalized approaches and adaptive trial designs.
- Regulatory and Strategic Commercialization: With the novelty of the TriTAC approach, regulatory pathways and commercial strategies need further consolidation. Engaging regulatory authorities in understanding these novel modalities, as well as integrating feedback from early clinical trials, is vital to streamline the approval process.

Future Prospects and Innovations
Looking forward, the TriTAC platform is poised to evolve with several promising innovations on the horizon:
- Next-Generation Constructs: Advances in protein engineering could lead to the design of even more sophisticated TriTAC and ProTriTAC molecules that incorporate additional safety switches, improved tumor penetration, and sharper specificity. Ongoing research is exploring strategies to integrate small-molecule modulators or to engineer constructs that can be selectively activated in the tumor microenvironment.
- Combination Therapies: Future clinical investigations are expected to incorporate TriTACs into combination regimens with checkpoint inhibitors, cytokine modulators, or chemotherapeutic agents. Such approaches aim to overcome resistance mechanisms and enhance overall anti-tumor efficacy. For instance, combining HPN328 with atezolizumab is an area of active interest.
- Broader Therapeutic Targets: The modular nature of the TriTAC platform allows rapid adaptation to new targets. Researchers are evaluating TriTACs against emerging oncologic targets (such as TROP2, EpCAM, and others) across a spectrum of solid tumors and hematologic malignancies. This broad target adaptability is likely to drive expansion into therapeutic areas beyond cancer, potentially addressing autoimmune and inflammatory diseases as well.
- Improved Patient-Centric Formats: Innovations in the administration route—such as subcutaneous formulations or less frequent dosing schedules enabled by extended half-life modifications—could improve patient adherence and quality of life. Continuous refinement of pharmacokinetic and pharmacodynamic parameters will be crucial in this regard.
- Regulatory Harmonization and Data Integration: As more clinical data become available, efforts to standardize efficacy endpoints and harmonize regulatory requirements are expected to accelerate. This will also help in the early identification of responsive patient subsets and in the refinement of dosing regimens.
- Digital and AI-Driven Optimization: The integration of digital tools and artificial intelligence in protein engineering could facilitate rapid optimization of TriTAC designs and streamline clinical trial development, ultimately reducing costs and improving efficiency in drug development pipelines.

Conclusion
In summary, the current landscape of TriTAC therapeutics is marked by a diverse portfolio of innovative candidates being developed primarily by Harpoon Therapeutics and its collaborators. The TriTAC platform leverages a unique triple-binding mechanism that enhances T cell activation against tumors while incorporating strategies to extend serum half-life and improve safety. Key candidates include FLT3 TriTACs in preclinical stages, HPN217 for multiple myeloma, HPN328 for small cell lung cancer, HPN424 for prostate cancer, HPN536 for mesothelin-expressing tumors, as well as innovative prodrug formats such as the TROP2 ProTriTAC and HPN601 targeting EpCAM.
From preclinical promise to early clinical success—evidenced by encouraging response rates, durable anti-tumor activity, and manageable adverse events—the TriTAC platform is rapidly evolving. However, challenges remain in optimizing manufacturing, mitigating cytokine-related toxicities, and overcoming the suppressive tumor microenvironment. Future directions point towards next-generation constructs with enhanced safety features, more personalized combination therapies, and broader therapeutic applications possibly extending to non-oncology indications. With continuing strategic collaborations, regulatory clarity, and technological innovations, TriTACs are poised to become a significant modality in cancer immunotherapy and beyond.

The evolution of TriTAC development exemplifies a general-specific-general progression: starting from an innovative theoretical framework (general) through rigorous preclinical and clinical validation (specific), and moving toward broad therapeutic application with continual improvements informed by real-world data (general). This comprehensive overview underlines the promising landscape as well as the hurdles that remain, while highlighting the dynamic interplay between scientific innovation, clinical translation, and future directions in T cell engager therapeutics.