Failing of the cancer drug worth $800 million triggered the fall of shares of Sanofi-Aventis, the French Drug company which is eyeing to buy Genzyme, the U. S. biotech.

Sanofi developed the Iniparib, or BSI-201 for treating advanced breast cancer, failed to control the progress of the cancer.

Earlier this month, Multaq, the recent medicine developed by the company for heart problems caused liver failure in the two patients.

Deutsche Bank analyst Mark Clark said, "We cannot view this as anything other than a major setback as we had expected BSI-201 to lead a revival in Sanofi-Aventis' formerly No. 2-ranked cancer franchise."

Tim Anderson, Bernstein Research analyst said, "Sanofi-Aventis is a large company, with a total revenue base of around 32 billion euros ($43.9 billion), so a setback with a single pipeline drug like iniparib is not likely to change the intermediate-term financials much."

On Friday the shares of the company were at 49.26 euros by 0829 GMT after slipping by 3.7 percent, thus making the shares as the largest decliners on CAC 40 index.

Bernstein analysts had claimed that the sales of the Iniparib will be around 644 million euros by 2015 year.

Genzyme has already rejected the Sanofi's offer of $18.5 billion, worth $69 a share.

BSI-201 is the main product of BiPar Sciences, which has been taken over by Sanofi in 2009.

Sanofi was planning to submit its treatment for the U. S. regulatory approval during the first quarter of the current year and during the second quarter it was expecting to submit the same for European approval.