Received on December 5, 2008
Accepted on September 22, 2009

Drug Monitoring and Toxicology

⁹-Tetrahydrocannabinol (THC), 11-Hydroxy-THC, and 11-Nor-9-carboxy-THC Plasma Pharmacokinetics During and After Continuous High-Dose Oral THC

¹ Chemistry and Drug Metabolism, Intramural Research Program, National Institute on Drug Abuse, NIH, Biomedical Research Center, 251 Bayview Blvd., Suite 200, Baltimore, MD
² Maryland Psychiatric Research Center, University of Maryland School of Medicine, Baltimore, MD
³ Office of the Scientific Director, Intramural Research Program, National Institute on Drug Abuse, NIH, Biomedical Research Center, 251 Bayview Blvd., Suite 200, Baltimore, MD

^* To whom correspondence should be addressed. E-mail: mhuestis{at}intra.nida.nih.gov.

BACKGROUND: ⁹-Tetrahydrocannabinol (THC) is the primary psychoactive constituent of cannabis and an active cannabinoid pharmacotherapy component. No plasma pharmacokinetic data after repeated oral THC administration are available.

METHODS: Six adult male daily cannabis smokers resided on a closed clinical research unit. Oral THC capsules (20 mg) were administered every 4–8 h in escalating total daily doses (40–120 mg) for 7 days. Free and glucuronidated plasma THC, 11-hydroxy-THC (11-OH-THC), and 11-nor-9-carboxy-THC (THCCOOH) were quantified by 2-dimensional GC-MS during and after dosing.

RESULTS: Free plasma THC, 11-OH-THC, and THCCOOH concentrations 19.5 h after admission (before controlled oral THC dosing) were mean 4.3 (SE 1.1), 1.3 (0.5), and 34.0 (8.4) μg/L, respectively. During oral dosing, free 11-OH-THC and THCCOOH increased steadily, whereas THC did not. Mean peak plasma free THC, 11-OH-THC, and THCCOOH concentrations were 3.8 (0.5), 3.0 (0.7), and 196.9 (39.9) μg/L, respectively, 22.5 h after the last dose. Escherichia coli -glucuronidase hydrolysis of 264 cannabinoid specimens yielded statistically significant increases in THC, 11-OH-THC, and THCCOOH concentrations (P < 0.001), but conjugated concentrations were underestimated owing to incomplete enzymatic hydrolysis.

CONCLUSIONS: Plasma THC concentrations remained >1 μg/L for at least 1 day after daily cannabis smoking and also after cessation of multiple oral THC doses. We report for the first time free plasma THC concentrations after multiple high-dose oral THC throughout the day and night, and after Escherichia coli -glucuronidase hydrolysis. These data will aid in the interpretation of plasma THC concentrations after multiple oral doses.