An understanding of the PK of CBD and its major metabolites is crucial in assessing the therapeutic effects and potential side effects of CBD administration in clinical settings. An appreciation of these parameters is also potentially important relative to dosing considerations and differential response. Although the pharmacology of CBD and its derivatives has been well studied, it remains incompletely understood. In the CNS, CBD does not bind appreciably to CB1 and CB2 receptors, unlike THC, which is an agonist of these receptors [18]. Pharmacological studies suggest that CBD acts as a negative allosteric modulator of CB1 receptors [6]. This indicates a longer action and structural modification by CBD in the CNS. The overall biological function of CB receptors in CNS is not well understood. Animal studies suggest that activation of CB receptors may alter dopamine release in the reward system [19], which is related to biological functions such as food intake, learning, sexual behaviors, and multiple neurological disorders, including Parkinson’s disease and depression [20]. The biological consequences of CB receptor activation and regulation still require further study. The major 7-OH-CBD metabolite has similar biological effects to the parent drug in both the CNS and periphery [21]. At present, 7-COOH-CBD is considered a pharmacologically inactive metabolite due to its lower lipid solubility and uncertainties regarding its transport across the blood-brain barrier [22]. However, in vitro research suggests that 7-COOH-CBD has a higher binding affinity for peripheral CB1 receptors compared to CBD [21], supporting further investigations into the biological activities of 7-COOH-CBD in the periphery.

Although CBD appears to be a relatively safe drug for clinical use, its associated side effects and potentially those of its metabolites must be considered. In human studies for epilepsy and psychiatric disorders, CBD has been variously reported to cause hepatic abnormalities, diarrhea, fatigue, vomiting, and somnolence in some subjects [23]. Additionally, in vitro studies show an inhibition of CBD and its major metabolites on selected CYP enzymes [24] and CES1 [25], which may pose some potential liability for drug-drug interactions.

Consistent with other clinical studies of CBD and its major metabolites [11, 16, 26, 27], results of this study showed that 7-COOH-CBD attains the highest plasma concentration, followed by the parent drug CBD, with 7-OH-CBD having the lower concentration. Similar to the report by Schultz et al. [12], the significant differences in AUC, C_max for CBD with respect to race, sex, age, or BMI were unable to be identified. Additionally, no significant differences were identified in C_max and AUC of CBD and 7-OH-CBD when analyzing age, sex, and BMI. There was no significant difference in C_max and AUC for 7-COOH-CBD concerning age and BMI. Published clinical studies have reported that the absorption and elimination of CBD vary depending on the formulation, dose, and fasting conditions [11]. However, unlike the parent drug, the formation and metabolism of secondary and tertiary metabolites are largely dependent on the metabolic activity of individual subjects, so their effects are likely to be more pronounced than those of the parent drug. For 7-COOH-CBD, a 2.25-fold higher C_max and 1.97-fold higher AUC for female subjects after normalizing body weight. These results are consistent with recently published clinical research by MacNair et al. [28] on the administration of cannabis oil. The reason for the sex difference in C_max and AUC is unclear, but it could be due to sex differences in UGT activity. As the terminal metabolite of the phase I reaction, 7-COOH-CBD is further metabolized through glucuronidation and eliminated by the renal system [22]. Previous research has reported sex differences in UGT activity, such as slower UGT2B15 metabolism in females [29], which may contribute to the observed PK differences between male and female subjects for 7-COOH-CBD.

In the treatment of seizure disorders, the dose of CBD in clinical practice is based on the patients’ body weight. However, in the context of this study, a fixed CBD dose (i.e., 750 mg twice daily) was administered to all participants, irrespective of their body weight. Interestingly, the results revealed that there was no noteworthy linear relationship observed between the body weight and the C_max and AUC of CBD. Significant linear correlations between 7-OH-CBD AUC and body weight were observed among females, whereas such correlations were not found to be significant in males. This trend is also mirrored in the C_max results for females, although statistical significance was not reached. There were no substantial linear associations observed in CBD and 7-COOH-CBD C_max and AUC with body weight. The underlying reason for this finding remains uncertain, although it might be attributed to the difference in fat tissue distribution between females and males. Females tend to have a greater proportion of fat tissue, leading to an increased volume of distribution for lipid-soluble drugs [30]. Consequently, this higher volume of distribution potentially contributes to lower AUC and C_max values for 7-OH-CBD.

There are several limitations of the study. First, as previously noted in the background, the investigation into CBD and metabolite PK utilized samples from a prior CBD-MPH drug interaction assessment. Following the administration of CBD at 750 mg twice daily for 3 days, subjects were given a single 10 mg dose of immediate-release MPH on the 4th day concurrently with a final 750 mg dose of CBD. Although MPH is not recognized as a significant metabolic inhibitor of any drug-metabolizing enzyme and has a relatively short half-life (2–3 h), there nonetheless exists the possibility of some minor metabolic effect that all subjects might have experienced. Secondly, the half-life of orally administered CBD has been reported to range from 14.43 to 60.54 h at varying dosages [13]. This variability makes it challenging to ensure that all subjects had attained a steady state by the 4th clinical day. A pharmacokinetic study of a more extended duration may be warranted to validate the results from the current study. Third, the sample size in the current study (6 women, 6 men) is insufficient to draw statistically reliable conclusions to represent a large population. It was noted that the normalized C_max for 7-COOH-CBD displayed two distinct clusters in women, with no significant differences observed by race, BMI, or body weight. The reason for these separated clusters is unknown, and it may be attributed to the variability and limited sample size. Likewise, conclusive statements about different race and ethnicities are not drawn due to the small sample size. A larger population PK study is necessary to validate the results of the current study.