Full Pharmacological Efficacy of a Novel S1P1 Agonist That Does Not Require S1P-Like Headgroup Interactions

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Abstract
Strong evidence exists for interactions of zwitterionic phosphate and amine groups in sphingosine-1 phosphate (S1P) to conserved Arg and Glu residues present at the extracellular face of the third transmembrane domain of S1P receptors. The contribution of Arg120 and Glu121 for high-affinity ligand-receptor interactions is essential, because single-point R120A or E121A S1P1 mutants neither bind S1P nor transduce S1P function. Because S1P receptors are therapeutically interesting, identifying potent selective agonists with different binding modes and in vivo efficacy is of pharmacological importance. Here we describe a modestly water-soluble highly selective S1P1 agonist [2-(4-(5-(3,4-diethoxyphenyl)-1,2,4-oxadiazol-3-yl)-2,3-dihydro-1H-inden-1-yl amino) ethanol (CYM-5442)] that does not require Arg120 or Glu121 residues for activating S1P1-dependent p42/p44 mitogen-activated protein kinase phosphorylation, which defines a new hydrophobic pocket in S1P1. CYM-5442 is a full agonist in vitro for S1P1 internalization, phosphorylation, and ubiquitination. It is noteworthy that CYM-5442 was a full agonist for induction and maintenance of S1P1-dependent blood lymphopenia, decreasing B lymphocytes by 65% and T lymphocytes by 85% of vehicle. Induction of CYM-5442 lymphopenia was dose- and time-dependent, requiring serum concentrations in the 50 nM range. In vitro measures of S1P1 activation by CYM-5442 were noncompetitively inhibited by a specific S1P1 antagonist [(R)-3-amino-(3-hexylphenylamino)-4-oxobutylphosphonic acid (W146)], competitive for S1P, 2-amino-2-(4-octylphenethyl)propane-1,3-diol (FTY720-P), and 5-[4-phenyl-5-(trifluoromethyl)-2-thienyl]-3-[3-(trifluoromethyl)phenyl]-1,2, 4-oxadiazole (SEW2871). In addition, lymphopenia induced by CYM-5442 was reversed by W146 administration or upon pharmacokinetic agonist clearance. Pharmacokinetics in mice also indicated that CYM-5442 partitions significantly in central nervous tissue. These data show that CYM-5442 activates S1P1-dependent pathways in vitro and to levels of full efficacy in vivo through a hydrophobic pocket separate from the orthosteric site of S1P binding that is headgroup-dependent.

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