Phase 1 Trial: Boosting Sipuleucel‑T with N‑803

Fifteen years after sipuleucel‑T became the first cellular immunotherapy to show an overall‑survival benefit in metastatic castration‑resistant prostate cancer (mCRPC), a new phase Ib trial at Washington University is testing whether adding the IL‑15 superagonist N‑803 (Anktiva) can turn that modest, durable survival signal into a stronger, more consistent immune attack on metastatic disease. The logic is straightforward: sipuleucel‑T “teaches” the immune system to recognize prostate cancer by priming antigen‑presenting cells against prostatic acid phosphatase (PAP), while N‑803 expands and activates the effector cells (CD8⁺ T cells and NK cells) needed to carry out that instruction.

Sipuleucel‑T is an autologous dendritic‑cell–based vaccine in which a patient’s peripheral blood mononuclear cells are exposed ex vivo to PA2024 and reinfused to drive PAP‑specific T‑cell responses and antigen spread. In the IMPACT trial it reduced the risk of death by ~22% and extended median overall survival by 4.1 months, with real‑world data suggesting even larger gains in some cohorts, despite low rates of PSA decline or radiographic response. Immune‑correlative analyses show that stronger antigen‑presenting‑cell activation and PAP‑specific T‑cell responses track with longer survival, consistent with an immunity‑driven mechanism rather than rapid tumor shrinkage.

N‑803 was engineered to overcome the short half‑life and complex receptor biology of native IL‑15, a cytokine that sustains CD8⁺ T‑cell and NK‑cell fitness. As an IL‑15 receptor superagonist, it preferentially expands cytotoxic lymphocytes without substantially stimulating regulatory T cells, and is already FDA‑approved with BCG for BCG‑unresponsive non‑muscle‑invasive bladder cancer. Preclinically, IL‑15–based agonists synergize with tumor‑targeted vaccines by increasing the number, persistence, and function of CD8⁺ T cells and NK cells and by making “cold” tumors more inflamed and T‑cell–rich.

The combination rationale in metastatic androgen‑pathway–modulation–resistant prostate cancer (mAPMR) is complementary: sipuleucel‑T provides antigen specificity and a primed T‑cell repertoire, while N‑803 supplies the cytokine “fuel” to expand and sustain those clones and recruit NK cells that can kill via innate and antibody‑dependent mechanisms. In prostate cancer models, IL‑15 co‑administration amplifies antigen‑specific CD8⁺ responses and improves tumor control when paired with immune activators, and N‑803 has shown enhanced activity with antibodies and checkpoint blockade across tumor types. Layering N‑803 onto sipuleucel‑T aims to deepen and prolong immune pressure, potentially converting sipuleucel‑T’s survival benefit into higher response rates and longer progression control.

Clinical trial.