Metformin Linked to Prostate Cancer Survival

August 6, 2013

News Review From Harvard Medical School -- Metformin Linked to Prostate Cancer Survival

Men with prostate cancer and diabetes may survive longer if they take metformin instead of other diabetes drugs, a study finds. The study included more than 3,800 men ages 67 or older who had diabetes. They later developed prostate cancer, at an average age of 75. About one-third were taking metformin when the study started. Others used different diabetes drugs. Researchers kept track of the men for more than 4 years after their cancer diagnosis. Men who took metformin were less likely to die of prostate cancer or any cause than men who took other diabetes drugs. The death risk was 24% lower for each 6 months they took metformin after their cancer diagnosis. Metformin helps the body to use insulin better to turn glucose (sugar) into energy. This means the pancreas needs to make less insulin. Research suggests that insulin might help cancer cells to grow. However, the new study does not prove that metformin causes longer survival with prostate cancer. It was not designed to show cause and effect. The Journal of Clinical Oncology published the study online. HealthDay News wrote about it August 5.

By Howard LeWine, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

Metformin is the best medicine to use first for people with type 2 diabetes. It also may help improve survival in people with some types of cancer, especially prostate cancer.

That's the finding of a study of 3,837 diabetic men who were later diagnosed with prostate cancer. The men were at least 66 years old at the time of cancer diagnosis. Their average age was 75.

The researchers compared men taking metformin to those taking other drugs for diabetes. The men taking metformin had better overall survival and less risk of prostate cancer death than  men who took any other diabetes medicine. And the longer they took metformin after prostate cancer was discovered, the better their outcome.

This is an observational study. This type of study looks at what happens when people choose different behaviors or treatments. It cannot prove a cause and effect.

In this case, there are some biologically sound reasons why metformin could slow down the growth and spread of prostate cancer. Metformin makes cells more responsive to insulin. This means the pancreas needs to make and release less insulin to keep blood sugar levels closer to normal. And insulin has been shown to stimulate the growth of some types of cancer cells.

But there might be something else that's different between the men who took metformin and those who took other diabetes drugs. So it's too soon to conclude that metformin is the direct reason for the improved survival of diabetic men with prostate cancer.

The researchers noticed something else interesting. Men who took statins also had longer survival after they were diagnosed with prostate cancer. Statins are the most effective drugs that lower cholesterol. Since this was not part of the original study design, the researchers did not provide details about this link. Nor did they speculate about the possible reasons.

What Changes Can I Make Now?

Here's my approach for all people, men and women, with newly diagnosed type 2 diabetes. I discuss two treatment options:

• For a few months, try diet and exercise, without any diabetes medicine.
• Start metformin right away, combined with diet and exercise.

When making the choice, be realistic. Will you really stick with a strict diet and exercise program over the long haul?

If you doubt it, then start the metformin now.

If you think you can do it, there's nothing wrong -- and a lot right -- with first trying a weight-loss program and exercise. You can start metformin later if needed.

However, if you are a man with prostate cancer and develop type 2 diabetes, you probably should start metformin right away. If you have both type 2 diabetes and prostate cancer now and are not taking metformin, talk with your doctor.  

Not everyone can take metformin. This includes people with:

• Impaired kidney function
• Alcohol abuse
• Severe liver disease
• Advanced heart failure

Metformin probably helps prevent the growth and spread of some prostate cancers once they have started. But studies show that the drug does not prevent prostate cancer.

What Can I Expect Looking to the Future?

It's likely that new studies of metformin in men with prostate cancer are already in progress. Besides studying metformin in men with prostate cancer and diabetes, I suspect that studies also will include men with pre-diabetes and men without any obvious risk for diabetes.