Some men with advanced prostate cancer hoped a one-two punch of vaccines might improve their odds. A new trial tested that idea. It found that adding a second DNA vaccine did not extend time without progression compared with a single vaccine. However, both approaches showed signs of anti-tumor activity, including drops in PSA and some tumor shrinkage.
Prostate cancer that has spread and no longer responds to standard hormone therapy is called metastatic castration-resistant prostate cancer, or mCRPC. It affects tens of thousands of men each year in the United States. Treatments can help, but the disease often becomes resistant. Patients and doctors are eager for therapies that harness the immune system to keep cancer in check longer, with fewer side effects.
The study tested a combination approach: a DNA vaccine plus pembrolizumab, an immune checkpoint inhibitor that helps T cells attack tumors. The idea is to train the immune system to recognize prostate cancer–related proteins while removing the brakes that cancer exploits. In a prior trial, outcomes were linked to T-cell activation, suggesting the vaccine’s immune response matters.
But here’s the twist: would broadening the immune response with a second vaccine do better? This trial compared one vaccine to two. The goal was to see if targeting more cancer-associated antigens would lead to longer control of the disease.
Think of the immune system as a security team patrolling the body. A vaccine hands the guards a “wanted poster” of a cancer protein, like prostatic acid phosphatase (PAP). Pembrolizumab then takes the handcuffs off the guards, letting them act. In this study, researchers tried handing out two wanted posters—one for PAP and one for the androgen receptor (AR)—to see if that made the team more effective.
The trial enrolled 60 patients with mCRPC. All received pembrolizumab on day 1 of each 3-week cycle. Everyone also got the PAP vaccine (pTVG-HP) on days 1 and 8 of each cycle. Half of the patients (Arm A) stayed on that single vaccine. The other half (Arm B) alternated between the PAP vaccine and an AR vaccine (pTVG-AR) every two cycles. After eight cycles (about six months), patients continued pembrolizumab alone and could receive booster shots later if their PSA rose again. Researchers measured progression-free survival at six months, along with PSA responses, tumor shrinkage, overall survival, and immune activity.
The headline result was clear: the two-vaccine approach did not improve time to progression. Median time to progression was 24 weeks in both arms. At six months, 51% of patients on the single vaccine were progression-free, compared with 45% on the two-vaccine strategy. These results were not statistically different.
Overall survival also favored the single vaccine numerically, though the difference was not significant. Median overall survival was 2.5 years in Arm A versus 3.7 years in Arm B (p=0.16). About 20% of patients in each arm saw their PSA drop by more than half. Among the 20 patients with measurable disease, 6 (30%) had a partial response, meaning their tumors shrank. Eight patients received booster shots after six months, and three of them later saw their PSA decline.
There’s a catch. Arm B, which used the alternating AR vaccine, had side effects not seen in Arm A. Three patients in Arm B had creatine kinase elevations, sometimes with increased liver enzymes (transaminases). Creatine kinase is a muscle enzyme; when it rises, it can signal muscle inflammation. This suggests the AR vaccine may have targeted tissues that also express the androgen receptor, not just the cancer.
Immune testing showed that Arm B did generate T cell responses to both antigens. So the second vaccine did activate the intended immune response. Yet that extra immune activity did not translate into better clinical outcomes and came with added toxicity.
Experts note that this study reinforces a key lesson: more immune activation is not always better. The goal is to direct the immune response precisely against cancer while sparing healthy tissue. In this case, the added immune response to AR may have engaged normal tissues that also carry that receptor.
What does this mean for patients and families? The single-vaccine combination showed meaningful activity, with some patients seeing tumor shrinkage and PSA declines. But the two-vaccine strategy did not offer a clear advantage and raised safety concerns. Patients considering experimental vaccines should discuss the potential benefits and risks with their oncologist, including what monitoring is needed for muscle or liver side effects.
It’s important to set expectations. This is a mid-size trial with a specific patient population. The results suggest the PAP vaccine plus pembrolizumab has activity, but they do not prove it extends life or is ready for widespread use. The alternating AR approach, at this dose and schedule, did not help and may add risk.
Limitations are part of the picture. The study was relatively small, and the two arms were not perfectly balanced for all factors. The primary endpoint was progression-free survival at six months, which is an early measure. Overall survival results were not statistically significant. The findings need confirmation in larger, randomized trials.
What happens next? Researchers will likely refine vaccine strategies to focus on targets with cleaner safety profiles or adjust dosing to avoid off-tissue effects. Larger trials will be needed to confirm whether the PAP vaccine plus pembrolizumab can improve outcomes, and to identify which patients benefit most. For now, this study adds a careful step forward: it shows that DNA vaccines can work with checkpoint inhibitors, but adding more antigens is not automatically better—and can introduce new risks.
