Amiselimod (former MT-1303) is an investigational sphingosine 1-phosphate (S1P) receptor modulator that was in development by Biogen for people with relapsing-remitting muscular sclerosis RRMS, inflammatory bowel disease, and other autoimmune conditions.

S1P receptor modulators interfere with S1P signaling. They block lymphocytes (immune cells) from receiving exit signals inside the lymph nodes, which keeps the immune cells trapped inside the node. The result is a reduction of circulating T- and B-cells leading to anti-inflammatory activity because it stops the migration of certain immune cells to the actual sites of inflammation.

A Phase 2 study (NCT01742052) of amiselimod in patients with RRMS was completed in September with successful and promising results. Amiselimod (0.2 mg and 0.4 mg) significantly reduced the total number of brain lesions and produced a safety and effectiveness profile that suggested it should be furthered studied as  a new potential treatment for people with RRMS.

Although the trial results showed positive results, Biogen has discontinued development of amiselimod without explanation, according to a press release.

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