FDA Extends Review of Multiple Sclerosis Pill

The Food and Drug Administration has extended Merck's priority review of the drug cladribine, a sphingosine 1-phosphate receptor modulator, by three months to examine additional information on the product.  The FDA's decision puts Merck further behind in the race to provide MS patients in the United States with a first-line oral treatment for relapsing forms of multiple sclerosis.

In MS, the immune system damages the covering that protects nerve fibers in the central nervous system (CNS), which includes the brain and spinal cord. Sphingosine 1-phosphate receptor (S1PR) modulators  reduce the immune system's attack on the CNS by retaining certain white blood cells (lymphocytes) in the lymph nodes. This prevents the white blood cells from reaching the CNS, where they could potentially attack the protective covering around the nerve fibers, resulting in less inflammatory damage to the nerve cells. The white blood cell retention is reversible if treatment is stopped.


European regulators rejected cladribine in September, saying the drug's benefits didn't outweigh the risks.  U.S. regulators didn't request more clinical trials, and  Merck representatives declined to elaborate on what additional information the FDA is reviewing.   The FDA had originally granted cladribine a priority review in July, shortening the standard 10 month review period to six months.  Based on this request for additional information, the FDA review is expected to now end on February 28, 2011.