|SOFT - TIAFT 1998||Scientific Session 2||Wednesday October 7, 1998|
|LACK OF CORRELATION BETWEEN PERFORMANCE IMPAIRMENT AND PHARMACOKINETIC MEASURES DURING THE DISTRIBUTION PHASE FOLLOWING MARIJUANA SMOKING|
|Stephen J. Heishman, Marilyn A. Huestis, and Edward J. Cone, Dennis J. Crouch*|
Addiction Research Center, IRP, NIDA, Baltimore, MD, USA
The relationship between pharmacokinetic measures and performance impairment is not well understood for smoked marijuana. This is especially true of the first 30 minutes after smoking when impairment is most reliably observed and D9-tetrahydrocannabinol (THC) is being rapidly distributed from the plasma throughout the body. We have conducted two clinical studies to explore these relationships.
In the first study, 6 healthy research volunteers smoked on separate sessions one marijuana cigarette containing either 0, 1.75, or 3.55% THC. Plasma THC and 11-nor-9-carboxy-D9-tetrahydrocannabinol (THCCOOH) were measured by GC-MS, and a battery of performance measures was assessed before and after marijuana smoking. At 20 min postsmoking, mean plasma THC levels were 27.5 and 48.8 ng/ml for 1.75 and 3.55% THC, respectively, and at 30 min postsmoking, mean THC levels were 17.3 and 29.7 for 1.75 and 3.55% THC, respectively. At these same times, accuracy on a test of grammatical reasoning was significantly impaired.
In the second study, 16 healthy research volunteers smoked on separate sessions two marijuana cigarettes that contained either 0, 1.75, or 3.55% THC. Plasma THC and THCCOOH were measured by GC-MS, and subjects performed four field sobriety tests (FST) 20 min after smoking ended. Mean plasma THC levels obtained immediately before FST testing were 15.4 and 28.3 ng/ml for 1.75 and 3.55% THC, respectively. Marijuana produced dose-related impairment on two of the FST, One Leg Stand and Finger to Nose.
In both studies, although significant marijuana-induced performance impairment was documented, there was no consistent correlation between any of the performance measures and plasma THC, THCCOOH, or the ratio of THC/THCCOOH across both marijuana doses. Thus, during the early distribution phase after marijuana smoking, there is no correlation between pharmacokinetic measures and performance impairment. Additionally, large inter-individual variation precluded identifying a threshold plasma THC level for impairment.