Guanfacine hydrochloride (Intuniv) is a centrally acting antihypertensive agent with specificity towards ADRENERGIC ALPHA-2 RECEPTORS.

Cells (1 mL) are seeded into 24-well plates at a density of 1 × 105 cells/mL. Culture medium is replaced every two days until cells reached confluence. To investigate the potential influence of P-gp on guanfacine transport, intracellular accumulation of guanfacine is measured in LLC-PK1/MDR1 cells and P-gp-negative LLC-PK1 cells. Rhodamine6 g, a well-known P-gp substrate, is included as a positive control. After cells reach confluence, the culture medium is replaced by transport buffer for a 30-minute preincubation at 37°C. Transport buffer is then removed and the test compounds (5 μM rhodamine6 g, 5 μM guanfacine, or 50 μM guanfacine) are added, and an additional 60-minute incubation period commenced. Next, the solutions are discarded, and the cells are washed three times with ice-cold Dulbecco's phosphate-buffered saline and solubilized with 1% Triton X-100. Intracellular accumulation of rhodamine6 g is measured using a fluorescent microplate reader with a 530 nm excitation and a 580 nm emission wavelength, which is a well established method. Cellular retention of guanfacine is analyzed by the high-performance liquid chromatographic assay described below. The concentrations of rhodamine6 g and guanfacine are calculated from their respective standard curves, and standardized with the cellular protein content determined by a Pierce bicinchoninic acid protein kit.(Only for Reference)