The Primate THC Study Did Not Directly Measure Drug Metabolism as a Tolerance Mechanism
A behavioral pattern consistent with receptor adaptation does not independently rule out pharmacokinetic contributions. A primate study explicitly states that drug metabolism and disposition were not assessed.
The behavioral comparison supported a receptor hypothesis
McMahon found different degrees of tolerance and cross-tolerance among THC and synthetic cannabinoid agonists. The pattern was interpreted as consistent with differences in CB1 agonist efficacy and receptor function.
The study acknowledged what it did not measure
The discussion notes that metabolism and disposition were not examined to determine whether chronic treatment selectively increased THC metabolism. Prior studies in other species were discussed, but they were not concentration or clearance measurements from these monkeys.
A plausible mechanism is not an exclusive mechanism
Evidence favoring a pharmacodynamic explanation can be valuable without proving that exposure was identical before and during treatment. Direct pharmacokinetic measurements would be needed to test that part of the explanation in the same setting. Conversely, the absence of those measurements is not evidence that metabolism caused the tolerance. The defensible conclusion preserves the supported behavioral pattern and the unmeasured alternative, rather than assigning all adaptation to either receptors or clearance.
Related: Interpreting cannabinoid tolerance evidence.
Research cited
Lance R. McMahon. Chronic Δ9-tetrahydrocannabinol treatment in rhesus monkeys: differential tolerance and cross-tolerance among cannabinoids (2011).
