An in vitro evaluation of guanfacine as a substrate for P-glycoprotein

Nancy K Gillis¹, Hao-Jie Zhu^1,2, John S Markowitz^1,2
1Department of Pharmacotherapy and Translational Research, ²Center for Pharmacogenomics, University of Florida, Gainesville, FL, USA

Background: With a US Food and Drug Administration-labeled indication to treat attention-deficit/hyperactivity disorder (ADHD), the nonstimulant guanfacine has become the preferred α₂-agonist for ADHD treatment. However, significant interindividual variability has been observed in response to guanfacine. Consequently, hypotheses of a contributing interaction with the ubiquitously expressed drug transporter, P-glycoprotein (P-gp), have arisen. We performed an in vitro study to determine if guanfacine is indeed a substrate of P-gp.
Methods: Intracellular accumulation of guanfacine was compared between P-gp expressing LLC-PK1/MDR1 cells and P-gp-negative LLC-PK1 cells to evaluate the potential interaction between P-gp and guanfacine. Cellular retention of guanfacine was analyzed using a high-performance liquid chromatographic-ultraviolet method. Rhodamine6G, a known P-gp substrate, was included in the study as a positive control.
Results: At guanfacine concentrations of 50 µM and 5 µM, intracellular accumulation of guanfacine in LLC-PK1/MDR1 cells was, 35.9% ± 4.8% and 49.0% ± 28.3% respectively, of that in LLC-PK1 cells. In comparison, the concentration of rhodamine6G, the positive P-gp substrate, in LLC-PK1/MDR1 cells was only 5% of that in LLC-PK1 cells.
Conclusion: The results of the intracellular accumulation study suggest that guanfacine is, at best, a weak P-gp substrate. Therefore, it is unlikely that P-gp, or any genetic variants thereof, are a determining factor in the interindividual variability of response observed with guanfacine therapy.

Keywords: guanfacine, P-glycoprotein, substrate, intracellular uptake