Can grape seed extract slow the growth of prostate cancer after treatment?

University of Colorado Cancer Center urologic oncology surgeon Dr. Paul Maroni studied the extract’s benefits if prostate cancer progresses after local therapy

Grape seed extract – a dietary supplement made from the seeds of wine grapes – has been investigated to see if it can help reduce blood cholesterol levels and high blood pressure. But can it also help slow down the growth of prostate cancer in men who have already undergone treatment?

That’s the question that University of Colorado Cancer Center member Dr. Paul Maroni, and his colleagues have set out to address in a study of the extract’s impact on certain cases of prostate cancer.

Maroni presented the results of the phase II trial at the annual meeting of the American Association for Cancer Research, in Chicago in April 2025 – a manuscript on the study is being prepared for publication.

Maroni is an associate professor in the CU Department of Surgery’s Division of Urology, where he is clinical director of urologic oncology. A specialist in robotic surgery for prostate and kidney cancer, Maroni’s research includes studying quality-of-life outcomes for patients with prostate and testicular cancer.

Prostate cancer occurs in the prostate, a walnut-sized gland just below the bladder that produces seminal fluid. It is the second most common type of cancer among men both in the United States and worldwide after lung cancer. Indeed almost 50 per cent of men over 65 years of age are found to have some evidence of a prostate cancer at death. But prostate cancer is highly treatable – especially when detected early – often through a blood test for prostate specific antigen (PSA).

Initial treatments vary for prostate cancer, Maroni says. When patients have early-stage, low-grade cancer that poses a low risk for many years, the disease might be monitored through active surveillance instead of direct treatment. Otherwise, depending on the cancer’s severity, treatment might include surgery to remove the prostate – a so-called ‘radical prostatectomy’ which is often performed robotically – or radiological treatment.

If prostate cancer comes back after local treatment, doctors often turn next to androgen deprivation therapy (ADT), a hormone therapy that seeks to reduce levels of male hormones, called androgens, to slow the cancer’s growth. But ADT can have quality-of-life impacts through reduction of testosterone possibly leading to hot flashes, fatigue, weight gain, weakened bones, increased risk for metabolic problems and heart disease.

So, when the recurrent cancer is progressing slowly and has not spread to other parts of the body – known as metastasis – doctors may wait and monitor patients before starting ADT.

“In this situation, we usually just watch those guys, which gives us an opportunity to try out some lower impact medications or supplements to see if they might further slow the progression of their prostate cancer, because many of these men will live for a very long time anyway, even though they may have recurrent cancer,” Maroni said.

“If we can delay the need to start hormonal therapy, which can be costly and be accompanied by a lot of side effects, that was an interesting question to ask,” he added.

Maroni and his collaborators wanted to see if grape seed extract could help slow down prostate cancer growth during this waiting period. Prior research had been conducted by CU Cancer Center colleague, Dr. Rajesh Agarwal, into the extract’s potential anti-cancer properties because of constituent antioxidant compounds called proanthocyanidins.

Maroni and Agarwal collaborated on a phase II trial involving 20 patients, which showed promising results. The updated trial focused on 41 men whose prostate cancer had returned after treatment. The subjects had no signs of metastasis and their PSA levels were rising slowly enough that it was not expected they would develop metastatic cancer within the following year.

Patients were given 150 mg of grape seed extract twice daily for up to 12 months, in a formulation intended to maximize absorption. Researchers measured how quickly their PSA levels were rising, expressed as PSA doubling time which was the number of months it took for PSA levels to increase twofold. That data was compared to their PSA doubling time before taking the extract.

The study found that PSA doubling time for the men taking the grape seed extract increased on average from 5.71 months to 6.86 months. About 78% of the patients saw their PSA doubling time increase, indicating slower cancer growth, of which 37% had significant improvement – at least 30 per cent – of their PSA doubling time. Patients with higher starting PSA levels and higher testosterone levels tended to respond better. The grape seed extract treatment was also found to be well tolerated by the patients with minimal side effects.

As a result, Maroni reported that grape seed extract showed promise as a way to help slow prostate cancer progression in some patients, especially those with higher baseline PSA levels.

But he cautioned that further, larger studies are needed to confirm these results.

“This is not ready for prime time yet. People should not just start taking grape seed extract on the assumption that it’s going to work for them.”

To read the team’s AACR abstract please visit here