The Evidence: Rigel Pharmaceuticals’ (RIGL) Fight for Fostamatinib’s FDA Approval

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Fostamatinib disodium (known as R935788 and R788 during development) is the generic name for Tavalisse, an oral medication sponsored by Rigel Pharmaceuticals, Inc. (Ticker: RIGL) with an expected PDUFA action date of 4/17/2018.

Fostamatinib is a prodrug. That is to say, it only becomes an active compound once it’s processed by intestinal enzymes into the active form of the drug, known as R406.

This medication is indicated for the treatment of primary persistent/chronic immune thrombocytopenia, sometimes called immune thrombocytopenic purpura and idiopathic thrombocytopenic purpura, (ITP).

In other words, fostamatinib is used to treat a condition that results in the long-term (> 3 month) depletion of platelets, sometimes despite numerous treatments.

Moreover, while the initial cause of primary ITP is unknown, we do know that the immune system ends up depleting the platelets in this condition anyways.


A macrophage deforming itself in order to capture and destroy a platelet.

A macrophage deforming itself in order to capture and destroy a platelet.

And so, fostamatinib works by counteracting an aberrant immune response by inhibiting a biological catalyst called spleen tyrosine kinase (Syk).

Syk is necessary for white blood cells, called macrophages, to properly function. When triggered, Syk signals these cells to re-arrange their shape in order to capture and destroy anything the immune system has marked for elimination.

In ITP, the immune system erroneously marks platelets for eradication. So if fostamatinib stops Syk from working properly, we can minimize the destruction of platelets as a result.


In ITP, a person loses platelets through a combination of possible mechanisms, including:

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Published: 3/14/2018

Last Updated: 3/14/2018

No one associated with this article has any financial stake in, or ties to, RIGL.