Sphingosine 1-phosphate
(S1P) is a pleiotropic signaling molecule
that interacts with five native G-protein coupled receptors (S1P1–5)
to regulate cell growth, survival, and proliferation. S1P has been
implicated in a variety of pathologies including cancer, kidney fibrosis,
and multiple sclerosis. As key mediators in the synthesis of S1P,
sphingosine kinase (SphK) isoforms 1 and 2 have attracted attention
as viable targets for pharmacologic intervention. In this report,
we describe the design, synthesis, and biological evaluation of sphingosine
kinase 2 (SphK2) inhibitors with a focus on systematically introducing
rigid structures in the aliphatic lipid tail present in existing SphK2
inhibitors. Experimental as well as molecular modeling studies suggest
that conformationally restricted “lipophilic tail” analogues
bearing a bulky terminal moiety or an internal phenyl ring are useful
to complement the “J”-shaped sphingosine binding pocket
of SphK2. We identified 14c (SLP9101555) as a potent
SphK2 inhibitor (K
i = 90 nM) with 200-fold
selectivity over SphK1. Molecular docking studies indicated key interactions:
the cyclohexyl ring binding in the cleft deep in the pocket, a trifluoromethyl
group fitting in a small side cavity, and a hydrogen bond between
the guanidino group and Asp308 (amino acid numbering refers to human
SphK2 (isoform c) orthologue). In vitro studies using
U937 human histiocytic lymphoma cells showed marked decreases in extracellular
S1P levels in response to our SphK2 inhibitors. Administration of 14c (dose: 5 mg/kg) to mice resulted in a sustained increase
of circulating S1P levels, suggesting target engagement.