A Swedish study has rekindled concerns about a possible link between Sanofi's top-selling insulin drug Lantus and cancer, but the French drugmaker said on Wednesday the observational research was seriously flawed.

It said separate data would be presented on December 8 showing no increased risk of cancer among patients taking its $4.7 billion-a-year product.

The Swedish research, which looked at the medical records of 23,000 people in the southwest of the country in an attempt to find common links to cancer, found an apparent doubling of cancer risk in those taking Sanofi's long-acting insulin.

The results, which also showed, more broadly, that having diabetes and being overweight increased the chance of cancer, were presented by Hakan Olsson of Lund University at the San Antonio Breast Cancer Symposium.

Sanofi said the nature of the Swedish study meant the analysis was limited and inconclusive. It looked at 2,724 women who developed cancer and 20,542 who did not.

Riccardo Perfetti, head of medical affairs at Sanofi's diabetes division, said the study had "major flaws". In particular, the sample size was inadequate, since it included a very wide range of women and not just those taking Lantus.

"This study was too small to say anything," he said in a telephone interview. "The study also seems to be poor from the methodological point of view, not only from the sample size."

Olsson acknowledged more research was needed to clarify specific cancers at increased risk with Lantus use, adding that the number of patients in his study who developed breast cancer was too small to make any link to a specific breast cancer risk.


Lantus has been linked to cancer before. In 2009, shares in Sanofi plunged when another study highlighted the possible risk posed by the long-acting insulin, which competes with Novo Nordisk's Levemir.

U.S. and European regulators, however, both went on to back the Sanofi product, and prescriptions for Lantus held up despite the controversy. Lantus sales reached 3.51 billion euros ($4.70 billion) last year and are expected to peak at $6.5 billion in 2014, according to consensus forecasts from Thomson Reuters Pharma.

Mark Clark, an industry analyst at Deutsche Bank, said the concerns raised by the Swedish research were likely to prove unfounded, since such retrospective studies were often unreliable.

In order to address the concerns raised in 2009, Sanofi is sponsoring a large research programme, with final results of the first study based on Nordic databases, representing 1 million patient years of Lantus treatment, due to be presented to international regulators by the end of this month.

In the meantime, Sanofi said updated data from a separate pooled analysis of individual studies with Lantus showing no increased risk of cancer would be presented at the World Diabetes Congress in Dubai on December 8 by Peter Boyle, president of the Lyon-based International Prevention Research Institute.