LONDON - Sanofi-Aventis's 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.

TREATMENT OPTIONS

The EASD said patients should not stop taking treatment but the organisation added they could consider using a long-acting human insulin or a mixture of long- and short-acting human insulin twice a day instead of once-daily Lantus.

The EASD said it had already communicated the latest study results to the European Medicines Agency and had started discussions with Sanofi as to how further studies might be conducted to get to the bottom of the issue.

A large combined analysis of the best available databases worldwide is the best way forward, EASD president Ulf Smith said in a statement.

He added there was no evidence that Lantus actually causes cancer, but it might possibly cause any existing cancer cells to grow and divide more rapidly.

Sanofi said late on Friday it had just been made aware of the latest data and it stood by the safety profile of Lantus, given the extensive evidence from past clinical trials and post-marketing surveillance.

Tim Anderson, an analyst with Sanford Bernstein, said the new research was not alarmist, but it would still likely have a negative impact on Lantus demand.

As the core driver of Sanofi-Aventis' earnings going forward, this will probably continue to weigh on the stock, he said.

Denmark's Novo Nordisk also has a long-acting insulin analogue called Levemir, which is modified in a different way from Lantus. Levemir was launched more recently, however, so was not assessed in the European registry studies.

For an earlier analysis on Lantus, click on (Editing by Sue Thomas)

United kingdom Medicine Cancer