The ATP-dependent drug efflux transporter P-glycoprotein (Pgp) plays a significant role in the absorption and disposition of many compounds. The purpose of this study was to investigate
the possible interaction of P-gp with each of four major marijuana constituents: 9-tetrahydrocannabinol (THC), 11-nor-9-tetrahydrocannabinol-carboxylic acid (THC-COOH), cannabinol (CBN), and cannabidiol (CBD). The results of a P-gp ATPase
activity screen showed that THC-COOH, CBN, THC, and CBD
all stimulated P-gp ATPase activity with a Michaelis-Menten
parameter (Vmax/Km) value of 1.3, 0.7, 0.1, and 0.05, respectively. Furthermore, CBD showed a concentration-dependent
inhibitory effect on verapamil-stimulated ATPase activity with
an IC50 value of 39.6 M, whereas all other tested cannabinoids
did not display appreciable inhibitory effects. Thus, the inhibitory effects of CBD on P-gp transport were further studied. At concentrations ranging from 5 to 100 M, CBD robustly enhanced the intracellular accumulation of known P-gp substrates rhodamine 123 and doxorubicin in a concentrationdependent manner in Caco-2 and LLC-PK1/MDR1 cells. An IC50 value of 8.44 M was obtained for inhibition of P-gp
function in LLC-PK1/MDR1 cells as determined by flow cytometry using rhodamine 123 as a fluorescence probe. Following exposure to 30 M CBD, the apparent permeability coefficient
of rhodamine 123 across Caco-2 and rat brain microvessel
endothelial cell monolayers was increased to 2.2- and 2.6-fold
in the apical-to-basolateral direction but decreased to 0.69-
and 0.47-fold in the basolateral-to-apical direction, respectively. These findings indicate that CBD significantly inhibits
P-gp-mediated drug transport, suggesting CBD could potentially influence the absorption and disposition of other coadministered compounds that are P-gp substrates.
