Cannabis and cannabis-derived products are increasingly used for recreational and medical purposes including anxiety and epilepsy and are often concurrently used with other types of conventional medications. The concomitant use of cannabis with other therapeutic drugs has been shown to increase the likelihood of deleterious drug-drug interactions. Oxazepam is a well-known benzodiazepine used in clinical practice as an anxiolytic, sedative, and anticonvulsant agent. Oxazepam is a racemic mixture of the S-and R-enantiomers and is primarily metabolized via glucuronidation in an enantiomeric-specific manner, where S-oxazepam is mainly glucuronidated by UDP-glucuronosyltransferase (UGT) 2B15 and R-oxazepam is glucuronidated primarily by UGT1A9 and UGT2B7. Recent comprehensive in vitrostudies using probe substrates have shown that the major cannabinoids Δ9 - tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN) can inhibit several of the primary hepatic and renal UGT enzymes. The objective of the present study was to evaluate the potential inhibitory effect of cannabinoids and their major circulating metabolites, 11-hydroxy-Δ9 -tetrahydrocannabinol (11-OH-THC), 11- nor-9-carboxy-Δ9 -tetrahydrocannabinol (11-COOH-THC), and 11-nor-Δ9 -tetrahydrocannabinol-carboxylic acid glucuronide (THC-COOH-Gluc) on the UGT enzymes involved in R,S-oxazepam metabolism. We hypothesize that major cannabinoids and their metabolites will inhibit R,S-oxazepam metabolism, leading to decreased clearance. To investigate potential drug-drug interactions, cannabinoids and their major plasma metabolites were screened as inhibitors of R- and S-oxazepam glucuronidation in human liver microsomes (HLM). In vitro inhibition screening assays with 10 μM or 100 μM of CBD and 11-OH-THC inhibited both R- and S-oxazepam glucuronidation in HLM by up to 90%. The IC50 values 11-OH-THC and CBD inhibition of R-oxazepam glucuronidation were 10.0 ± 7.8 μM and 7.0 ± 4.2 μM, respectively, in HLM; the respective IC50 values for the inhibition of S-oxazepam glucuronidation by 11-OH-THC and CBD were 15.0 ± 6.4 μM and 52.2 ± 22.6 μM in HLM. These data indicate that co-administration of oxazepam with cannabis may result in inhibition of oxazepam clearance, suggesting a strong potential for drug-drug interactions in vivo.

The UDP-glucuronosyltransferase (UGT) family of enzymes play a central role in the metabolism and detoxification of a wide range of endogenous and exogenous compounds. UGTs exhibit a high degree of structural similarity and display overlapping substrate specificity, often making estimations of potential drug-drug interactions difficult to fully elucidate. One such interaction yet to be examined may be occurring between UGTs and cannabinoids, as the legalization of recreational and medicinal cannabis and subsequent co-usage of cannabis and therapeutic drugs increases in the U.S. and internationally. In the present study, the inhibition potential of the major cannabinoids Δ9-tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN), as well as their major metabolites, was determined in microsomes isolated from HEK293 cells over-expressing individual recombinant UGTs and in microsomes from human liver and kidney specimens. The highest inhibition was seen by CBD against the glucuronidation activity of UGTs 1A9, 2B4,1A6 and 2B7, with binding- corrected IC50,u values of 0.12 {plus minus} 0.020 µM, 0.22 {plus minus} 0.045 µM, 0.40 {plus minus} 0.10 µM and 0.82 {plus minus} 0.15 µM, respectively. Strong inhibition of UGT1A9 was also demonstrated by THC and CBN, with IC50,u values of 0.45 {plus minus} 0.12 µM and 0.51 {plus minus} 0.063 µM, respectively. Strong inhibition of UGT2B7 was also observed for THC and CBN; no or weak inhibition was observed with cannabinoid metabolites. This inhibition of UGT activity suggests that in addition to playing an important role in drug-drug interactions, cannabinoid exposure may have important implications in patients with impaired hepatic or kidney function.