AM109: A PSMA‑Targeted CD137 Bispecific Antibody for Metastatic Castration‑Resistant Prostate Cancer (mCRPC / mAPMR)

AM109 is designed as a bispecific agent that links a PSMA‑targeting humanized antibody to a CD137 (4‑1BB)‑targeting affibody, effectively creating a molecular bridge between prostate cancer cells and T cells in the tumor microenvironment. The rationale is straightforward: by restricting CD137 activation to sites where PSMA is expressed, the therapy aims to drive potent anti‑tumor immunity locally while avoiding widespread systemic immune activation that has historically limited the utility of CD137 agonists. Preclinical experiments show that AM109 induces robust CD8⁺ T‑cell activation and cytokine secretion, including IFN‑γ, IL‑2, and Granzyme B, when cultured with PSMA‑positive LNCaP prostate cancer cells, whereas PSMA‑negative MKN45 gastric cancer cells show no such response, confirming its target‑dependent mode of action. Cytotoxicity assays further demonstrate dose‑dependent tumor cell killing that correlates with PSMA expression levels, reinforcing the idea that the compound’s activity is tightly coupled to the presence of its antigenic target.

In vivo, the potency of AM109 becomes even more striking. In human CD137 transgenic mice bearing hPSMA/MC38 tumors, treatment with AM109 leads to complete tumor regression at doses as low as 0.1–0.3 mg/kg. This level of activity significantly outpaces that of a reference CD137 agonist used as a comparator, highlighting the advantage of tumor‑restricted costimulation over untargeted CD137 engagement. The tumor regression observed is associated with evidence of CD8⁺ T‑cell–mediated tumor rejection, with minimal signs of off‑tumor immune activation in these models, suggesting a more favorable therapeutic window for this bispecific approach.

Beyond efficacy, the biophysical and stability characteristics of AM109 appear favorable for clinical translation. Structural and functional analyses indicate that the molecule maintains its conformation and activity for at least 12 weeks when stored at temperatures between 4 and 40 °C, a range compatible with standard biologic storage and distribution.

Within the broader context of mCRPC therapy, AM109 fits into a growing toolkit of PSMA‑directed interventions that now includes radioligand therapies, antibody–drug conjugates, and various bispecific T‑cell engagers. What distinguishes this agent is its focus on costimulation rather than direct cytotoxicity or T‑cell recruitment via CD3. By engaging CD137 selectively at PSMA‑expressing tumor sites, AM109 aims to convert the immunosuppressive tumor microenvironment of mCRPC into a more “hot” one, enhancing endogenous T‑cell function, promoting tumor‑specific memory, and potentially improving the durability of responses.

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