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The British Medical Journal recently published results from a study providing evidence that Actos (pioglitazone), an oral antidiabetic agent in the thiazolidinedione class, is associated with an increased risk of bladder cancer. The cohort included 115,727 diabetics newly treated with oral hypoglycemic agents between 1988 and 2009. The study criteria excluded patients who were below the age of 40 at the time of receiving their first oral antidiabetic prescription in addition to those previously diagnosed with bladder cancer at any time before the cohort entry. The mean duration of using Actos was 2.2 years (range 28 days to 6.7 years) and was comparable to rosiglitazone, with a mean duration of 2.3 years (range 28 days to 7.6 years). This study, unlike previous studies, did not combine new and prevalent users of antidiabetic drugs. Therefore, previous studies examining the association between Actos and bladder cancer may have underestimated the association between the two.

The results showed that 470 patients were diagnosed with bladder cancer during 526,559 person years of follow-up (rate 89.4 per 100,000 person years). The 376 patients diagnosed with bladder cancer beyond one-year of follow-up were matched to 6699 controls. Exclusive ever use of Actos resulted in an 83% increased rate of bladder cancer which corresponded to an absolute adjusted rate difference of 74 per 100,000 person years. This association was not seen for exclusive ever use of rosiglitazone, the other thiazolidinedione available in the UK during the timeframe of the study.

In terms of cumulative duration of use, the rate of bladder cancer increased with duration of use, with the highest rate observed in patients exposed for more than 24 months and in patients with a cumulative dosage of 28,000 mg or greater. This corresponded to an absolute adjusted rate difference of 88 per 100,000 person years. No evidence of a dose-response relation with rosiglitazone for both cumulative duration of use and cumulative dosage was seen.

Thiazolidinediones are indicated for those intolerant of metformin or sulfonylureas, and for those with poorly controlled diabetes already taking other oral hypoglycemic agents. Therefore, patients receiving thiazolidinediones generally are at a more advanced stage of the disease than those receiving other oral agents, which perhaps by itself could lead to an increase risk of bladder cancer. However, because of the lack of association between bladder cancer and rosiglitazone, the evidence strongly suggests that the severity of the disease does not distort the evidence showing the association between Actos and bladder cancer.

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