What are the preclinical assets being developed for PSMA?

Introduction to PSMA

Definition and Biological Role
Prostate‐specific membrane antigen (PSMA) is a type II transmembrane glycoprotein characterized by a large extracellular domain endowed with enzymatic functions such as folate hydrolase and glutamate carboxypeptidase activity. PSMA is highly expressed not only on the surface of malignant prostate epithelial cells but also on the neovasculature of a number of solid tumors. Its complex biochemical functionality involves binding and hydrolyzing folates and other substrates, which underlies its potential role in modulating nutrient uptake and cellular signaling. PSMA’s ability to internalize rapidly upon ligand engagement makes it an excellent conduit for targeted delivery approaches. In addition, the receptor’s overexpression in cancer cells – particularly in cells from advanced, metastatic, and castration‐resistant prostate cancers – emphasizes its utility as a molecular target for both diagnostic and therapeutic purposes.

Importance in Prostate Cancer
The integral role of PSMA in prostate cancer is supported by its marked upregulation in malignant tissues relative to normal prostatic cells. Studies have demonstrated that PSMA expression is up to 100–1000 times higher in prostate cancer cells compared with benign epithelial cells, an observation that has been exploited in imaging and therapeutic strategies. Because the expression of PSMA correlates with tumor aggressiveness, androgen independence, and metastatic potential, it serves not only as a diagnostic marker but also as a critical target for therapeutic intervention. This selective overexpression enables a focused delivery of cytotoxic agents and radionuclides, thereby minimizing collateral damage to healthy tissues. PSMA’s endocytic properties ensure that once a ligand (be it a radioligand, antibody, or small molecule drug) binds to the target, the entire complex is internalized, triggering intracellular release of a therapeutic payload.

Preclinical Development Landscape

Current Preclinical Assets
Over the past decade, a wide array of preclinical assets targeting PSMA has been developed using different strategic modalities. These modalities span small molecule inhibitors, antibody–drug conjugates (ADCs), radioimmunoconjugates, bispecific T-cell engagers (BiTEs), and targeted alpha therapies. In preclinical settings, each asset is optimized for better binding affinity, improved pharmacokinetics, and enhanced therapeutic index. Among the most notable preclinical assets are:

• PSMA-targeting bispecific T-cell engagers (BiTEs): One of the promising preclinical assets is PSMA XPAT, a bispecific T-cell engager designed to simultaneously bind to PSMA on tumor cells and to CD3 on T cells. This strategy recruits the body's immune effector cells to selectively kill PSMA-expressing prostate cancer cells. Preclinical data for PSMA XPAT suggest that it can redirect T-cell cytotoxicity specifically against tumor cells, paving the way for a potentially effective immunotherapy that could complement or even bypass conventional treatment modalities.

• PSMA-targeted alpha conjugates: Another major asset in the development pipeline is a PSMA-targeted thorium-227 conjugate (PSMA-TTC), which is built upon an antibody that specifically recognizes PSMA and is coupled to an alpha-emitting radionuclide (thorium-227). The rationale behind such an approach is that alpha particles deliver high linear energy transfer (LET) over a very short range, resulting in potent cytotoxicity within tumor cells while sparing surrounding normal tissues. Preclinical investigations have demonstrated promising antitumor efficacy with rapid tumor growth inhibition in xenograft models, indicating its viability as a therapeutic asset in radioimmunotherapy.

• Novel small molecules and radioligands: Beyond the assets designed for therapeutic purposes, preclinical development has also produced a spectrum of high-affinity small molecules and radioligands for diagnostic imaging. Although many such agents (e.g., 68Ga-PSMA-11, 18F-DCFPyL) have progressed to clinical phases, ongoing preclinical research into next-generation compounds aims to reduce off-target uptake (such as in the salivary glands and kidneys), improve tumor-to-background ratios, and optimize clearance profiles. These compounds are often evaluated for their binding kinetics, stability, and potential for theranostic applications, where the same molecule can be labeled with either imaging or therapeutic isotopes.

• Antibody–drug conjugates (ADCs): Preclinical assets also include ADCs in which antibodies directed against PSMA are conjugated to cytotoxic drugs. The design of these ADCs is focused on achieving selectivity by ensuring that the antibody has high affinity for PSMA-expressing cells. Once bound, the ADC is internalized and the toxic payload is released intracellularly, directly triggering apoptosis or other forms of cell death. Optimization efforts in preclinical studies involve fine-tuning the linker stability and ensuring that the conjugate maintains its activity in the harsh biological environment.

• Emerging cellular therapeutics: Although still in conceptual and early preclinical evaluation, there are approaches leveraging genetically engineered cells – such as CAR-T cells – that target PSMA. Early evidence suggests that modifying T cells to express receptors which recognize PSMA might enhance the immune-mediated elimination of prostate cancer cells. However, while several studies have shown promising cytotoxic effects in vitro, these strategies are at a relatively nascent stage and require further validation regarding their in vivo safety and efficacy.

Each of these assets reflects a different therapeutic modality, yet all share the common goal of exploiting the high expression and internalization properties of PSMA to deliver an effective therapeutic payload directly to prostate cancer cells.

Key Players in Development
The preclinical development of PSMA-targeted assets is a collaborative effort facilitated by both academic institutions and biopharmaceutical companies. Key industry players include:

• Amunix, Inc.: With products such as PSMA XPAT, Amunix is at the forefront of developing bispecific T-cell engagers that bind both PSMA and CD3. The preclinical data from these agents reveal promising immune-mediated cytotoxicity against PSMA-positive tumor cells, demonstrating the potential to harness the immune system in a highly targeted manner.

• Organizations focusing on targeted alpha therapies: Several entities are involved in the development of antibody-based PSMA targeting conjugates – such as the PSMA-TTC asset. The preclinical investigations performed on these thorium-227 conjugates are representative of a broader industry strategy to refine radiotherapeutic paradigms that deliver lethal doses of radiation directly into the tumor microenvironment.

• Pharmaceutical companies with a strong focus on radioligand therapy: While some radiolabeled compounds have made it to clinical testing, ongoing preclinical research is being managed by companies and research organizations aiming to improve these diagnostic and therapeutic agents. Advanced chelation chemistry is being employed to improve the stability and biodistribution of PSMA-targeted radioligands, with several groups contributing to this area of research.

• Emerging start-ups and biotechnology companies: In addition to established players, there are numerous smaller biotechnology companies focusing on next-generation immunotherapy platforms and ADC development targeting PSMA. These companies are leveraging novel linker technologies, improved conjugation chemistries, and advanced molecular design strategies to produce assets with superior efficacy and safety profiles. The diverse portfolio emerging from these preclinical efforts underpins a vibrant innovation ecosystem fueled by both academic research and industrial investment.

Mechanisms of Action

Targeting PSMA in Cancer Therapy
The success of any PSMA-targeted therapy hinges on its ability to exploit the receptor’s differential expression on cancer cells and its rapid internalization upon ligand binding. The mechanisms of action observed in preclinical assets include:

• Receptor-mediated internalization: Once a PSMA-targeting agent (whether a small molecule, antibody, or BiTE) engages the antigen, the receptor-ligand complex undergoes internalization. This process facilitates the intracellular release of a therapeutic payload, which can be a cytotoxic drug, radionuclide, or immune-activating molecule. For instance, in PSMA-TTC, the internalized complex delivers high-energy alpha particles directly into the cell nucleus, leading to double-stranded DNA breaks and subsequent cell death.

• Immune cell recruitment and activation: In the case of bispecific T-cell engagers like PSMA XPAT, the agent serves as a molecular bridge between PSMA-positive tumor cells and T cells. By binding to CD3 on T cells, these bispecific molecules enable the formation of an immunological synapse that results in T-cell activation and the selective release of cytotoxic molecules. This mechanism not only promotes the targeted destruction of tumor cells but also potentially enhances systemic antitumor immunity.

• Localized radiotoxicity: Radioligand therapies developed for PSMA typically involve either beta-emitting or alpha-emitting isotopes linked to a targeting moiety. While beta emitters (like lutetium-177) have been clinically approved, the development of PSMA-TTC utilizes alpha emitters that have a shorter path length but deliver a more potent, localized radiotoxic effect. This focused irradiation maximizes tumor cell kill while sparing adjacent normal tissues, capitalizing on the localized overexpression of PSMA.

• Antibody–drug conjugate design: ADCs developed in preclinical settings harness the specificity of antibodies for PSMA to deliver highly potent cytotoxic compounds directly into cancer cells. Once the antibody binds to PSMA, internalization occurs and the cytotoxic payload is released intracellularly after cleavage of a chemically optimized linker. This strategy significantly amplifies the therapeutic index by concentrating the drug’s activity on malignant cells and reducing systemic exposure.

Innovative Approaches
Innovation in preclinical PSMA targeting is not limited to conventional targeting strategies but also embraces novel mechanisms and combination approaches:

• Hybrid theranostic platforms: A strong focus lies in integrating diagnostic imaging with targeted therapy. This theranostic approach involves the use of agents that can be alternately labeled with isotopes for imaging or therapy without altering their molecular structure. New chemical modifications and chelator optimizations are being investigated to maximize these dual functions while maintaining high affinity and specificity for PSMA.

• Multimodal immunotherapy: The development of bispecific T-cell engagers represents a cutting-edge immunotherapeutic approach. Innovations in the design of these molecules, such as optimizing the spacing between binding domains and fine-tuning the affinity for both PSMA and CD3, aim to achieve maximum T-cell engagement while limiting cytokine release syndrome and off-target toxicity. These agents are being designed to work synergistically with other immunomodulatory strategies, such as checkpoint inhibitors, which could further enhance their clinical efficacy.

• Next-generation radioconjugates: In addition to thorium-227-based targeted alpha therapies, novel radioconjugates are undergoing preclinical testing to address issues related to pharmacokinetics and off-target toxicity. Research is focused on modular linker designs, improved chelators, and conjugation techniques that yield enhanced tumor-to-background ratios and more predictable biodistribution profiles. Such refinements are critical for minimizing radiation exposure to normal tissues, particularly in organs that physiologically express low levels of PSMA.

• Advanced ADCs and nanoparticle formulations: Nanotechnology is being leveraged to develop ADCs and nanoparticle-based delivery systems that can encapsulate cytotoxic agents and release them in a controlled manner inside PSMA-positive cells. These platforms are designed to overcome obstacles associated with drug resistance and to target cancer stem cell populations that are often implicated in tumor relapse. Preclinical studies employing such formulations have shown promising results in terms of both efficacy and safety, as they provide a means to deliver high-dose chemotherapy directly to the tumor microenvironment.

Challenges and Opportunities

Preclinical Development Challenges
Despite impressive progress in preclinical asset development for PSMA, significant challenges persist. Some of the key challenges include:

• Off-target toxicity and non-specific uptake: One of the foremost challenges is the potential for off-target uptake of PSMA-targeted agents in tissues that express low levels of PSMA, such as salivary glands, kidneys, and the small intestine. This unwanted accumulation can lead to dose-limiting toxicities, especially in radioligand therapies and ADCs. Optimizing ligand specificity, modulating linker chemistry, and improving clearance kinetics remain active areas of research aimed at reducing these risks.

• Heterogeneity of PSMA expression: Tumor heterogeneity poses a significant challenge in preclinical studies. Variability in PSMA expression among different tumor foci and within the same tumor may lead to suboptimal binding and therapeutic efficacy. This heterogeneity necessitates the development of combination and multi-targeted strategies, as well as robust patient stratification methods during future clinical trials.

• Stability and pharmacokinetic optimization: For radioconjugates and ADCs, stability in the bloodstream and predictable pharmacokinetics are paramount. The preclinical optimization of these assets involves enhancing the stability of the conjugated complexes (e.g., through improved chelator chemistry for radioligands) and ensuring rapid clearance from non-target tissues. Inadequate stability may result in premature release of the payload, reducing efficacy and increasing systemic toxicity.

• Manufacturing and regulatory challenges: The production of complex bioconjugates, such as bispecific T-cell engagers and targeted alpha therapies, often requires state-of-the-art manufacturing processes that guarantee uniformity, high purity, and batch-to-batch reproducibility. Scaling up such technologies from preclinical studies to clinical-grade production is not trivial and is accompanied by strict regulatory scrutiny. These challenges must be overcome to achieve successful translation from bench to bedside.

• Immunogenicity concerns: For protein-based therapies, including bispecific antibodies and ADCs, immunogenicity is a major concern. Although techniques such as humanization of antibodies have mitigated these risks, unexpected immune reactions can still occur in preclinical models and ultimately in clinical trials. Sustained research into the immunological profile of these agents is essential to minimize adverse outcomes.

Future Opportunities and Trends
The current challenges are accompanied by multiple opportunities that could redefine the future of PSMA-targeted therapies:

• Combination therapy strategies: There is an increasing trend toward the combination of PSMA-targeted therapies with other treatment modalities. Preclinical studies are investigating the synergistic potential of combining targeted alpha therapies with immunomodulatory agents such as checkpoint inhibitors to overcome resistance mechanisms and boost overall efficacy. Likewise, the pairing of ADCs or radioconjugates with conventional cytotoxic chemotherapy or androgen deprivation therapy could enhance tumor eradication while reducing the likelihood of treatment resistance.

• Precision patient stratification: Advances in genomics and molecular imaging are opening avenues for more precise patient stratification. In the future, the effective application of PSMA-targeted assets could be significantly improved by identifying patients whose tumors exhibit high, homogeneous PSMA expression. Translational and preclinical models that mimic patient heterogeneity will provide valuable insights into which therapeutic combinations are most likely to succeed in distinct patient subsets.

• Nanomedicine and innovative delivery systems: The integration of nanotechnology into drug development presents a promising opportunity. Nanoparticle formulations and other advanced drug delivery systems are under investigation to improve the targeted delivery of cytotoxic agents to PSMA-positive cancer cells. Such systems have the potential to mitigate systemic side effects and improve the therapeutic index by releasing the payload in a controlled manner once the target site is reached.

• Next-generation chelator and linker technologies: In the context of radioligand therapy, technological advancements in chelation chemistry are crucial for the stability and specificity of novel agents. Future developments in chelator design could provide radioconjugates with improved in vivo stability, higher tumor accumulation, and faster clearance from non-target tissues. Similarly, innovations in linker technology for ADCs promise to minimize premature drug release and ensure that the cytotoxic agent is released only upon internalization into the tumor cell.

• Expanding the asset portfolio: Preclinical research is not limited to the assets discussed so far. There is an ongoing exploration into novel modalities such as CAR-T cell therapies targeting PSMA, dual-targeting constructs that address multiple tumor antigens simultaneously, and new classes of antibody fragments and minibodies offering improved tissue penetration and reduced immunogenicity. These expanding avenues illustrate the dynamic and multifaceted approach to PSMA targeting that is emerging from both academic and industrial research communities.

• Enhanced preclinical models: Finally, one of the most significant opportunities for the advancement of PSMA-targeted therapies lies in the development of more sophisticated preclinical models. Patient-derived xenografts, three-dimensional organotypic cultures, and genetically engineered mouse models that accurately mimic the human tumor microenvironment will enable more reliable prediction of clinical success. Innovations in imaging modalities and pharmacodynamic markers in these models will enhance our ability to assess therapeutic efficacy and optimize dose regimens well before human trials commence.

Conclusion
In summary, the spectrum of preclinical assets being developed for PSMA is both broad and innovative, reflecting the intense focus on exploiting this marker’s unique biological properties for therapeutic gain. PSMA’s inherent enzymatic activity, rapid internalization, and overexpression in aggressive prostate cancer underpin the rationale for its use as a target in a variety of modalities—from bispecific T-cell engagers such as PSMA XPAT, which engage the immune system to directly kill tumor cells, to targeted alpha conjugates like PSMA-TTC, which deliver potent cytotoxic radiation in a highly localized manner. Alongside these, there is a robust pipeline of novel small molecules, ADCs, and radioligands currently undergoing optimization for improved stability, specificity, and reduced off-target toxicity.

Key players in the field include established biopharmaceutical companies and emerging start-ups that are leveraging advancements in molecular biology, immunotherapy, and nanotechnology. Their collaborative preclinical research efforts are enhancing our understanding of the complex pharmacokinetics and biodistribution profiles required for successful clinical translation. The ability to fine-tune these assets through innovations in chelator and linker chemistry, as well as the use of sophisticated preclinical models, points to a future in which PSMA-targeted therapies could be tailored to individual patients’ tumor characteristics through precision medicine.

Despite significant progress, challenges such as managing off-target toxicities, overcoming tumor heterogeneity, ensuring product stability, and addressing immunogenicity concerns remain. However, these challenges are being met with innovative solutions, including combination therapy approaches, advanced drug delivery systems, and modular platforms that promise to improve the therapeutic index of these agents. The ongoing refinement in preclinical asset development – as demonstrated by robust data in both in vitro and in vivo models – is likely to translate into more effective clinical interventions for patients with advanced prostate cancer.

In conclusion, the preclinical landscape for PSMA-targeted therapies is highly dynamic and promising. With the development of advanced bispecific T-cell engagers, innovative targeted alpha therapies, next-generation ADCs, and novel small molecule radioligands, researchers are paving the way for transformative treatments that offer improved efficacy and reduced toxicity. These preclinical assets, supported by rigorous academic and industrial research, represent the next wave of targeted cancer therapeutics that could fundamentally alter the treatment paradigm for prostate cancer. The future opportunities presented by precision patient stratification, enhanced delivery systems, and combination strategies further underscore the potential impact of these innovations. Overcoming the challenges inherent in translating these assets from preclinical evaluation to clinical application will require continued multidisciplinary collaboration, but the progress to date is a solid foundation for a new era of personalized, effective prostate cancer therapy.