The value of metformin in management of prostate cancer: the debate continues

Available data offer both positive and negative evidence from observational studies regarding the value of metformin in the management of men newly diagnosed with prostate cancer.

Because of these conflicting data, a group of Canadian researchers (Bensimon et al.) used information from four linked databases in the United Kingdom to try and work out whether use of metformin after a diagnosis of prostate cancer is associated with a decreased risk of cancer-specific and all-cause mortality.

Specifically, Bensimon et al. studied data from a cohort of 935 men, all newly diagnosed with non-metastatic prostate cancer between April 1, 1998 and December 31, 2009, and all of whom had a prior history of treatment for Type II diabetes. The patients were followed for at least 33 months (through October 1, 2012).

Here are the key study findings:

  • The average (mean) follow-up was 3.7 years.
  • 258/935 patients (27.6 percent) died during the follow-up period.
  • 112/935 patients (12.0 percent) died of prostate cancer during the follow-up period.
  • There was no evidence that use of metformin after diagnosis was associated with
    • A decrease in risk for prostate cancer-specific mortality (relative risk [RR] = 1.09)
    • Any significant change in risk for all-cause mortality (RR = 0.79)
  • A cumulative duration of treatment (before and after diagnosis) with metformin of ≥ 938 days was actually associated with an increased risk for prostate cancer-specific mortality (RR = 3.20).

Bensimon et al. conclude, based on these data, that

The use of metformin after a prostate cancer diagnosis was not associated with an overall decreased risk of cancer-specific and all-cause mortality.

We do not expect this paper to end the debate on the potential value of metformin in the prevention or the management of prostate cancer. That is going to take clinical trials, and some of these trials are already ongoing. However, in the interim, this new study does add evidence to the argument that the potential benefits of metformin may be limited to a small subset of patients, and it also suggests that men with diabetes who have been managed for long periods on metformin prior to their diagnosis with prostate cancer may actually at be increased risk of shorter life expectancy after diagnosis with prostate cancer.

2 Responses

  1. Sitemaster:

    One result of this study seems quite remarkable … that 12% of the sample, 112/935 men, died of prostate cancer. This seems abnormally high, compared to a random sample of 935 newly diagnosed men with non-metastatic prostate cancer.

    Surely the norm would be way less than 3% if the sample was restricted to non-metastatic disease.

    Now granted, this group all had diabetes; so, as you point out, isn’t this telling us more about the lethal aspect of diabetes and prostate cancer. Is all cancer more lethal to diabetics?

  2. Dear Rick:

    I don’t think it is possible, from the available information, to know quite what these data are telling us. Another way to look at these data would be to say that the cause of death listed for 112/935 men in this cohort was prostate cancer, but we don’t have a clue how many men in the cohort died with a cause listed as, for example, “diabetes and related complications” or “complications of diabetes and prostate cancer”.

    My point is, we don’t know how many of these men really did have metastatic, castration-resistant prostate cancer at the time of their deaths; we just know what was listed as the cause of their death. Causes of death are not always particularly accurate.

    This study, from 2004, appears to show no increase in risk for prostate cancer-specific mortality among men with a prior diagnosis of diabetes.

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