Absorption
Dronabinol (delta-9-THC) is almost completely absorbed (90 to 95%) after single oral doses. Due to the combined effects of first pass hepatic metabolism and high lipid solubility, only 10 to 20% of the administered dose reaches the systemic circulation. Concentrations of both parent drug and its major active metabolite (11-hydroxy-delta-9-THC) peak at approximately 0.5 to 4 hours after oral dosing and decline over several days.
The pharmacokinetics of dronabinol after single doses (2.5, 5, and 10 mg) and multiple doses (2.5, 5, and 10 mg given twice a day) have been studied in healthy subjects.
Summary of Multiple-Dose Pharmacokinetic Parameters of Dronabinol in Healthy
Subjects (n=34; 20-45 years) under Fasted Conditions
Mean (SD) PK Parameter Values
|
Twice Daily Dose
| Cmax ng/mL
| Median Tmax (range), hr
| AUC(0-12) ng•hr/mL
|
2.5 mg
| 1.32 (0.62)
| 1.00 (0.50-4.00)
| 2.88 (1.57)
|
5 mg
| 2.96 (1.81)
| 2.50 (0.50-4.00)
| 6.16 (1.85)
|
10 mg
| 7.88 (4.54)
| 1.50 (0.50-3.50)
| 15.2 (5.52)
|
Cmax: maximum observed plasma concentration; Tmax: time to maximum observed plasma concentration; AUC(0-12): area under the plasma concentration-time curve from 0 to 12 hours.
|
A slight increase in dose proportionality on mean Cmax and AUC(0-12) of dronabinol was observed with increasing dose over the dose range studied.
Effect of Food: In a published study, the effect of food on the pharmacokinetics of dronabinol was studied by concomitant dosing of dronabinol capsules with a high-fat (59 grams of fat, approximately 50% of total caloric content of the meal), high calorie meal (approximately 950 calories). An appreciable food effect was observed, resulting in a 4-hour delay in mean Tmax and 2.9-fold increase in total exposure (AUCinf), but Cmax was not significantly changed [see Dosage and Administration (2.2)].
Distribution
Dronabinol has an apparent volume of distribution of approximately 10 L/kg, because of its lipid solubility. The plasma protein binding of dronabinol and its metabolites is approximately 97% [see Drug Interactions (7.4)].
Elimination
The pharmacokinetics of dronabinol can be described using a two compartment model with an initial (alpha) half-life of about 4 hours and a terminal (beta) half-life of 25 to 36 hours. Values for clearance average about 0.2 L/kg-hr, but are highly variable due to the complexity of cannabinoid distribution.
Metabolism
Dronabinol undergoes extensive first-pass hepatic metabolism, primarily by hydroxylation, yielding both active and inactive metabolites. Dronabinol and its principal active metabolite, 11-hydroxy-delta-9-THC, are present in approximately equal concentrations in plasma. Published in vitro data indicates that CYP2C9 and CYP3A4 are the primary enzymes in the metabolism of dronabinol. CYP2C9 appears to be the enzyme responsible for the formation of the primary active metabolite [see Clinical Pharmacology (12.5)].
Excretion
Dronabinol and its biotransformation products are excreted in both feces and urine. Biliary excretion is the major route of elimination with about half of a radio-labeled oral dose being recovered from the feces within 72 hours as contrasted with 10 to 15% recovered from urine. Less than 5% of an oral dose is recovered unchanged in the feces.
Due to its re-distribution, dronabinol and its metabolites may be excreted at low levels for prolonged periods of time. Following single dose administration, low levels of dronabinol metabolites have been detected for more than 5 weeks in the urine and feces.
In a study of dronabinol capsules involving AIDS patients, urinary cannabinoid/creatinine concentration ratios were studied bi-weekly over a six week period. The urinary cannabinoid/creatinine ratio was closely correlated with dose. No increase in the cannabinoid/creatinine ratio was observed after the first two weeks of treatment, indicating that steady-state cannabinoid levels had been reached. This conclusion is consistent with predictions based on the observed terminal half-life of dronabinol.
Drug Interaction Studies
Formal drug-drug interaction studies have not been conducted with dronabinol.
The enzyme inhibition and induction potential of dronabinol and its active metabolite are not completely understood.
Published data showed an increase in the elimination half-life of pentobarbital by 4 hours when concomitantly dosed with dronabinol [see Warnings and Precautions (5.1)].