According to a study published in British Medical Journal (BMJ), a drug for treating type 2 diabetes called Pioglitazone is linked to a higher risk of bladder cancer and taking the drug continuously for longer than two years doubles the risk. The researchers stress, however, that the risk in absolute terms is relatively low with up to 137 extra cases per 100,000 person years.

Rosiglitazone, which is a similar drug, showed no increased risk. Both pioglitazone and rosiglitazone are thiazolidinediones, which help control blood sugar levels in patients with type 2 diabetes and are known to increase the risk of heart failure, yet following a safety review, the European Medicines Agency (EMA) decided to keep pioglitazone on the market.

A Canadian team of researchers decided to investigate if pioglitazone was linked to a higher risk of bladder cancer in people with type 2 diabetes and after obtaining data from the General Practice Research Database (GPRD), they evaluated 115,727 patients who initiated diabetes therapy from 1988 to 2009. The GPRD contains anonymous patient records from over 600 UK GPs. The team identified cases of bladder cancer and matched them to up to 20 healthy control patients.

The results revealed that from 376 cases that were matched against 6,699 controls, 470 patients were diagnosed with bladder cancer during the average 4.6 years of follow-up, which translates to a rate of 89 per 100,000 person years, whilst the rate of bladder cancer in the general UK population aged 65 years and above is 73 per 100,000 person years.

Patients who had taken pioglitazone at any time were found to have a 83% higher risk of bladder cancer, which translates into 74 per 100,000 person years. The figures increased to 88 per 100,000 person years for those who had taken the drug for two years or longer and increased even further, to 137 per 100,000 person years those who had taken 28,000mg or more.

The researchers cross-checked the results in several further analyses, but the results remained consistent, and “provide evidence that pioglitazone is associated with an increased risk of bladder cancer, whereas no increased risk was observed with the thiazolidinedione rosiglitazone.”

They believe that these links may have been underestimated during earlier observational studies and comment that doctors, patients and regulatory agencies “should be aware of this association when assessing the overall risks and benefits of this therapy.”

Dominique Hillaire-Buys and Jean-Luc Faillie from the Department of Medical Pharmacology and Toxicology in Montpellier, France comment in a linked editorial: “It can confidently be assumed that pioglitazone increases the risk of bladder cancer. It also seems that this association could have been predicted earlier.”

They continue saying that in view of pioglitazone’s benefits of reducing cardiovascular events being questionable, they remark that, “prescribers who are ultimately responsible for therapeutic choices can legitimately question whether the benefit-risk ratio of pioglitazone is still acceptable for their patients with diabetes.”

Written By Petra Rattue