The relationship between the exposure time of insulin glargine and risk of breast and prostate cancer: An observational study of the time-dependent effects of antidiabetic treatments in patients with diabetes

Aims: To elucidate methodological questions in assessing the relationship between insulin treatment and cancer, since the risk of tumour growth generally increases with longer exposure time and higher dose of a growth promoting substance. Methods: Continuous hazard functions for risk of breast and prostate cancer were estimated in relation to exposure of insulin glargine among diabetic patients included in the record system, Diab-Base, as well as in the general population in Sweden. Results: In 7942 female diabetic patients, mean follow-up 7.0 years, 2014 patients initiated insulin glargine with a mean follow-up of 3.5 years. Among 11,613 men, mean follow-up 6.9 years, 2760 had a mean follow-up with glargine of 3.4 years. Risk of prostate cancer decreased significantly with longer exposure to insulin glargine (p = 0.032), although average risk versus non-glargine was non-significantly higher (HR 1.37, 95% CI 0.78-2.39). The breast cancer risk did not change with longer exposure to insulin glargine (p = 0.35) and the mean risk was similar for glargine and non-glargine (p = 0.12). With higher dose of insulin glargine, there was an increase in risk of prostate (p = 0.037) and breast cancer (p = 0.019). In diabetics, the mean risk of prostate cancer was decreased (HR 0.68, 95% Cl 0.59-0.79) but similar for breast cancer (HR 0.95, 95% Cl 0.78-1.14) compared to the general population and did not change with longer diabetes duration (p = 0.68 and p = 0.53 respectively). Conclusions: Analysing continuous hazard functions for cancer risk in relation to exposure time to an antidiabetic agent is an important complementary tool in diabetes and cancer research. (C) 2011 Primary Care Diabetes Europe. Published by Elsevier Ltd. All rights reserved.