This is the first pharmacokinetic serum assessment of cannabinoids and major metabolites using a whole hemp plant derived extract in dogs, most notably for the major cannabinoid acid forms, CBDA and THCA, which are not routinely assessed. The increase in oral use of whole hemp extract in veterinary and human medicine warrants further investigation into the native acid derivatives of CBD and THC, since existing products that do not undergo heat extraction will contain CBDA and THCA. In much of the prior research on inhaled forms of cannabis, these derivatives were not assessed since the heat-induced decarboxylation of CBDA to CBD and THCA to THC (5, 20). In the human literature, there is very little investigation into CBDA, and it was previously thought that when providing CBDA it either improved the absorption of CBD or rapidly became CBD due to potential for gastric or hepatic conversion (6, 7). Our data shows that the absorption of CBDA does occur in dogs and is absorbed at least twice as well as CBD in 24-h kinetic examination. One limitation of our study design is that we did not do more frequent sampling at 15 and 30 min post-dosing, as it is entirely possible that we missed the Cmax and Tmax for the cannabinioids, particularly since CDBA Cmax is at our first sampling time point. It is unlikely that we missed the Cmax or Tmax for CBD and THC since recent work in the fed canine model shows similar results to ours with Tmax in the 1–2 h range (21); however, the only other pharmacokinetics examining CBDA in humans with more frequent sampling in a fasted model suggesting 0.8–1 h to be the Tmax (22). Therefore, future experiments for CBDA should incorporate these earlier time points in fed or fasted models.

The pharmacokinetics of CBDA over a 2-week period shows nearly equal serum concentrations to CBD, suggesting absorption and retention of oral dosing. More interestingly, is that Form 2, which contains a 25% sunflower lecithin base, showed slightly higher AUC₀₋₂₄ and mean retention time in 24-h pharmacokinetics, which may have translated into the significantly higher weeks 1 and 2 serum concentrations when compared to Form 1 (25% medium chain triglyceride base) or Form 3. The implications of this are unknown from a clinical perspective; however, CBDA may have some unique properties as an anti-inflammatory as well the ability to mitigate nausea through 5HT 1a receptor activation (23–27). CBDA pharma-biological understanding is in its infancy, but it may have equal, if not better absorption when compared to CBD which is worth further investigation.

This investigation is the first of its kind to assess both THC and THCA as minor cannabinoids that are found in most CBD-rich hemp products at lower than 0.3% combined. The quantity of THC and THCA delivered to these dogs was <0.1 mg/kg body weight, which is far lower than prior long-term dosing (25 mg/kg) that resulted in minimal clinical side effects (28). In the prior 52-week study, dogs became habituated to these larger doses and exhibited THC serum concentrations in the tens of ng/mL while our THC concentrations were within 2–5 ng/mL regardless of the form used (28). In addition, no side effects such as ataxia or somnolence was observed in the beagles used in our study at any point during treatment. Interestingly, the THCA concentrations in the dogs were noticeably higher in the 10–25 ng/mL range at weeks 1 and 2 with slightly better retention of THCA in Form 2 and Form 3 treated groups. THCA appearance in the bloodstream at these concentrations may be of therapeutic value since THCA has been suggested to be a non-intoxicating and a neuroprotective cannabinoid, but research into THCA is relatively sparse (15, 29, 30). It must also be recognized that these exploratory investigations into canine serum cannabinoids are not validated procedures (1, 4, 17, 21). Although R² values of ≥0.98 were achieved for all cannabinoids when examining goodness of fit calculations, and the accuracy of 75% of calibration standards was within ±15% of the nominal concentrations in our study, there may be higher inconsistencies at data points particularly when many of THC cannabinoids measured were near the lower limit of quantitation (due to very low THC concentrations in the initial formulation products), except for THCA.

Not surprisingly, the 11-OH-THC metabolite, which has been associated with intoxicating effects, showed variable measurable quantities in the 24-h pharmacokinetic profile and could not be universally detected in all dogs at multiple time points, which did not allow for 24-h pharmacokinetic analysis. However, weeks 1 and 2 concentrations were above the lower limit of quantitation (2.5 ng/mL) in a few dogs on Form 2 (n = 3) and Form 3 (n = 1) with a range of 2.6–6.7 ng/mL. The only other long-term canine assessment of 11-OH-THC suggests that dogs can have concentrations between 6 and 46 ng/mL in their serum and not display outward clinical side effects (26). Humans with 11-OH-THC concentrations in the 2–20 ng/mL concentrations will begin to exhibit cognitive and recall issues when tested without any physiological changes (31, 32). These disparities suggest the possibility of species-dependent sensitivities and metabolism, particularly in light of the evidence that dogs, rats, and humans metabolize CBD and THC differently (33, 34), and due to the insufficient sensitivity of our assay in detecting 11-OH-THC, other methods are necessary to truly evaluate this metabolite (and/or dosing with higher THC levels may be used). The THCA serum concentrations were higher than expected for all forms of treatment when compared to the THC concentrations, yet the mean retention time for THCA was ~70–100% longer than other cannabinoids, which might allow for slightly higher bioaccumulation. Furthermore, we cannot rule out whether THCA is metabolized to THC or eventually to 11-OH-THC, which may be the reasons the occasionally higher 11-OH-THC than THC that was encountered sporadically, though unlikely.

From a metabolic perspective, the metabolism of CBD was also interrogated by assessing the major metabolites identified in humans (7-OH-CBD and 7-COOH-CBD), which are not prevalent in dogs. The levels of 7-OH-CBD could not be measured in our dogs, while 7-COOH-CBD was in the 20–30 ng/mL range as a mean concentration after 1 and 2 weeks of use. These results are dramatically higher than what was recently observed in a study where dogs received 62 mg/kg in a short-term study, suggesting that there may be some adaptation to CBD metabolism or differences in absorption and retention in this study (17). Comparatively, the 7-COOH-CBD is about 1–2% of what has been observed in humans when giving similar doses of CBD (35). This lack of 7-hydroxylation or carboxylation has been observed previously in comparative literature, highlighting that the metabolites are likely to be different and may result in different pharmacological effects across species, which has not been investigated (28). In-vitro liver homogenate experiments and metabolite generation suggest hydroxylation and/or glucuronidation at the 4 or 6 position rather than the 7 position (33, 34). If and when standards for such metabolites are developed further, serum analysis may reveal the primary metabolites in dogs, which are currently unknown.

It must also be noted that our experimental design utilized feeding at the time of administration to promote cannabinoid absorption. These are not new findings since older literature suggests that THC uptake is 3- to 4-fold greater in primates when provided with a mixed meal in the form of a cookie (9). More recent data in rats shows that a lipid-based excipient will increase absorption 4-fold in rats (13). Prior work by our group, when using an oil base in fasted dogs at 2 mg/kg body weight as an equal mix of CBD and CBDA, showed a CBD Cmax of ~100 ng/mL, while other groups have published that 10 mg/kg body weight of CBD resulted in a serum Cmax of ~500–600 ng/mL (1, 21). More recent canine data regarding provision of 2 mg/kg as an equal mix of CBD and CBDA in a soft chew showed serum Cmax of ~300 ng/mL of CBD, further suggesting promotion of absorption with a mixed-meal soft chew (4). The use of the soft chew matrix may have produced the higher maximal concentrations observed; however, these dogs were fasted before administration of the soft chew with no follow-up feeding, which may have led to the quickened half-life observed (4). The current study utilized oils (Forms 1 and 2) or soft chews (Form 3) with a small mixed meal (110 g of canned wet dog food), which showed similar Cmax, Tmax, T½ life, and AUC for each form of treatment used; with trends toward Form 3 showings slightly better absorption of CBD in the pharmacokinetic models based on increased Cmax, while Form 2 has the highest weeks 1 and 2 mean CBD and CBDA concentrations.

The use of cannabinoid-rich hemp-based oils for health and disease is in its infancy in human and veterinary clinical medicine. CBD use is proving efficacious in seizure disorders, multiple-sclerosis-based spasm and pain, oncologic side effects of chemotherapy, with a more recent indication in anxiety and schizophrenic disorders using single or twice-a-day dosing between 5 and 20 mg/kg per day in humans (15, 36–38). In veterinary medicine, three clinical studies have suggested potential use of CBD-rich hemp oils in seizure and osteoarthritis disorders in canines (1–3). The only side effects noted in these studies were rises in the alkaline phosphatase enzyme presumably due to hepatic cytochrome p450 enzyme upregulation. These hepatic effects are observed rather universally at doses of 10 mg/kg or higher in dogs (19). The use of 2 mg/kg of a CBD/CBDA-rich cannabinoid oil did show some modest rises in ALP in a geriatric population (1). However, the current study, and a prior study in healthy young beagles, have not shown any rises in any enzymes outside of normal reference ranges associated with the hepatobiliary system, signifying that the CBD-rich hemp product used in this study at this concentration is likely to be safe (4), particularly in light of the recent long-term GW pharmaceutical study in dogs using 10–100 mg/kg of CBD daily for 39 weeks proving safe with a no observed adverse effects limits being set at 100 mg/kg in that toxicity trial (39).

Overall, this study is the first to show that CBDA and THCA are readily absorbed and retained in dogs with some differences observed in CBDA absorption and/or retention depending on the medium used to deliver the oral treatment. The finding of mid dosing concentrations of 75 ng/mL of CBD and CBDA or greater suggests potential for therapeutic use when delivered at 1 mg/kg body weight for each of these cannabinoids with food. A more interesting finding is the retention of THCA in the serum of between 10 and 25 ng/mL. The exact functions of CBDA and THCA physiologically suggest similar therapeutic benefits to CBD that may have the potential to work synergistically with CBD. These synergistic properties known as the “entourage effect” are currently thought to be the primary reason that lower CBD whole hemp extract dosing can be therapeutic when compared to purified CBD (40, 41). Although the hemp extract product used in this study appears to be generally safe, the results of this study cannot be translated to other products due to differences in variable absorption dependent on carrier oils, and cannabinoid and terpene profiles. This fact makes recommendations of CBD-rich hemp products globally tenuous, and veterinarians should become versed on products available before using them clinically.