Targeting the S1P receptor signaling pathways as a promising approach for treatment of autoimmune and inflammatory diseases

The past two decades of intense research have revealed a key role of the sphingolipid molecule sphingosine 1-phosphate (S1P) in regulating multiple physiological and pathophysiological processes including cell proliferation and survival, cell migration, inflammatory mediator synthesis and tissue remodeling. S1P mainly acts through five high-affinity G protein-coupled S1P receptors, which are ubiquitously expressed and mediate a complex network of signaling in a cell type dependent manner. S1P receptors have become an attractive pharmacological target to interfere with S1P-mediated cellular responses, which contribute to various autoimmune and inflammatory diseases. Pioneering in this field was the synthesis of FTY720 (fingolimod, Gilenya (R)) from myriocin, one of the metabolites of the fungus Isaria sinclairii known from traditional Chinese medicine for its antibacterial and energy boosting effect. Fingolimod turned out as a very potent immunomodulatory agent that subsequently passed all clinical trials successfully and is now approved for the treatment of relapsing-remitting multiple sclerosis. Pharmacologically, fingolimod was characterized as a non-selective agonist of all of the SIP receptors (S1PR), with the exception of S1P(2), and in addition, as a selective S1P(1) functional antagonist by induction of irreversible S1P(1) internalization and degradation. Since proper lymphocyte trafficking depends on the expression of S1P(1) on lymphocytes, the degradation of S1P(1) leads to trapping and accumulation of lymphocytes in secondary lymphoid tissue, and consequently to a depletion of lymphocytes from the blood. Novel S1PR modulators are now being developed with a more selective receptor activation profile and improved pharmacokinetic characteristics. In this review, we will summarize the state-of-the-art approaches that target directly or indirectly SIP signaling and may be useful as novel strategies to treat autoimmune and inflammatory diseases.