| Abstract: | 2-Acetyl-4(5)-tetrahydroxybutyl imidazole (THI) has been shown to reduce rodent peripheral blood lymphocytes through increasing lymphoid sphingosine 1-phosphate (S1P) by inhibiting S1P lyase. The objective of this study was to characterize the relationship between systemic THI exposure, splenic S1P concentrations, and lymphopenia in rats. Following the oral administration of 10 and 100?mg kg?1 THI to male rats, THI was rapidly absorbed and reached a plasma peak level at 1?h post-dosing. Splenic S1P increased and reached the peak level at 24?h. Blood lymphocyte count decreased as the splenic S1P level increased. THI plasma concentration was linked to splenic S1P concentration using an indirect model incorporated with a four-step signal transduction model. In turn, the S1P level was directly coupled with blood lymphocyte number. The integrated model simultaneously captured the splenic S1P and blood lymphocyte responses. This pharmacokinetic–biomarker–pharmacodynamic model resolved the remarkable discrepancy between plasma THI concentration and the pharmacological response and quantitatively described the relationship of THI exposure, S1P, and lymphopenic response.
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