**5.2 Targeting S1P receptors**

S1P and its receptors are powerful regulators of various critical functions of RA synoviocytes. Targeting these cells with specific S1P receptor agonists or antagonists can modulate specific inflammatory responses locally and systemically by altering cell migration, cytokine and chemokine secretion, and proliferation/survival. RP-001, a potent S1P1 receptor agonist with properties similar to those reported for FTY-720 on S1P1 receptor degradation and lymphocyte sequestration from the blood, was recently characterized (Cahalan et al., 2011). The short half-life of RP-001 *in vivo*, however, disqualifies this compound for clinical use.
