Sanofi drug may increase cancer risk, studies find

* European studies see possible cancer link

* More research needed as findings not conclusive

* Patients told to stay on treatment, consider options * Sanofi stands behind safety profile of Lantus

By Ben Hirschler

LONDON, June 27 (Reuters) - Sanofi-Aventis's (SASY.PA) diabetes drug Lantus may increase the risk of cancer, according to European studies involving some 300,000 insulin-treated patients, prompting a call from experts for more research.

The European Association for the Study of Diabetes (EASD), which released details online of four studies from its journal Diabetologia, said they were "far from conclusive but they do indicate the need for further investigation of this issue".

The new research was released after mounting speculation that damaging data was about to be published over a cancer link with Sanofi's modern long-acting insulin analogue, sinking the the French drugmaker's share price by 12.3 percent in two days.

Lantus, which sold 2.45 billion euros ($3.41 billion) in 2008, is a key driver for Sanofi as top drugs like Plavix and Lovenox face the threat of generic competition. Analysts have been expecting sales to grow strongly for the next five years.

A German study of 127,031 diabetics on insulin found malignancies were more common in patients treated with Lantus, also known as glargine, than in those prescribed a comparable dose of old-style human insulin.

"Our analysis does not provide absolute proof that glargine promotes cancer," said Peter Sawicki, director of Germany's Institute for Quality and Efficiency in Health Care and co-author of the study.

"Our study does, however, arouse an urgent suspicion which should have consequences for the treatment of patients."

Following that original German study -- which was presented to the European diabetes journal last year -- further research was carried out using three other large patient databases, or registries, in Sweden, Scotland and the UK.

In the Swedish study, involving 114,841 insulin-treated patients, those on Lantus alone were almost twice as likely to be diagnosed with breast cancer.

The 49,197-patient Scottish study also found patients on Lantus were more likely to have cancer, including breast cancer, although the difference did not reach statistical significance.

The UK study -- the smallest of the four, involving 10,067 patients -- found no link.