Structure-activity relationship studies and biological evaluation of selective sphingosine kinase inhibitors
Sphingosine 1-phosphate (S1P) has become a prevalent drug discovery target due to studies implicating it to several disease pathologies such as fibrosis, sickle cell disease, inflammation, diabetes, and cancer. S1P functions to induce cell proliferation and migration. S1P signaling occurs through intracellular targets or transport outside of the cell via ABC transporters, where it acts as a ligand to G-protein coupled receptors (S1P1-5). Sphingosine kinase (SphK) 1 and 2 phosphorylate sphingosine to S1P; these are the only enzymes known to mediate the phosphoryl transfer. Inhibiting either or both SphKs helps to modulate S1P, which may be useful as a therapeutic avenue for disease states where S1P signaling has gone awry.
Herein, we document our efforts in profiling the structure-activity relationships (SAR) of SphK2 through an iterative process of synthesis and biological testing. First, an SAR structured around the head and linker region of our lead molecule, SLR080811, was performed. SLR080811 has a Ki of 1.3 µM and is 5-fold selective for SphK2. The modifications performed on SLR080811 yielded two promising inhibitors: SLP120701 (SphK2 selective with a Ki of 1.2 µM) and SLP7111228 (>200 fold selective for SphK1 with a Ki of 48 nM). In vitro studies in U937 cells yielded a decrease in S1P levels with the introduction of inhibitors. Mouse studies provided insight into the pharmacokinetic effect of our SphK2-selective inhibitors, revealing an increase in S1P levels in the blood. When in vivo studies were performed with the SphK1 selective inhibitor, S1P levels in blood decreased. These molecules provide the chemical biology tools to determine the effect of modulating S1P levels in vivo.
We also focused our investigation on the tail region of the pharmacophore. From this study, SLM6031434 and SLM6041418 were discovered and both proved to be more potent and selective SphK2 inhibitors than SLR080811. SLM6031434 has a Ki of 370 nM and is 23-fold selective for SphK2. SLM6041418 has a Ki of 430 nM and is 24-fold selective for SphK2. Consistent with our previous observations, in vitro studies showed a decrease in S1P levels when inhibitor was introduced. Similarly, in vivo studies resulted in an increase of S1P levels in the blood. These compounds are positioned towards animal models of disease.