Three studies have attempted to answer this question by testing in vitro the affinity of THCA-A toward CB1 and CB2, yielding contradictory results. In 2006, Verhoeckx et al. reported THCA-A to be a weak agonist of CB1 and CB2 receptors compared with THC (Ki_CB1=630 vs. 3.5 nM; Ki_CB2=890 vs. 3.2 nM).¹¹ The authors mentioned that affinity values were determined by binding assays using membranes isolated from Sf9 cells stably expressing human CB1 receptors, but neither the actual data nor the experimental procedure was included in the article. In 2008, Ahmed et al. isolated 11 novel cannabinoid esters from high-potency Cannabis sativa, 8 of which were derivatives of THCA-A. The authors clearly stated: “Both the esters and the parent acids were not active, as indicated by a CB1 receptor binding assay.”³² Again, neither the experimental conditions of the assay nor the affinity values for any compound were reported in the text or referenced. More recently in 2014, Rosenthaler et al. tested seven different phytocannabinoids, including THCA-A, for their ability to bind CB1 and CB2 receptors in vitro. The binding experiment measured the competitive displacement of the radioligand [3H] CP-55,940 (1 nM) by several concentrations of the phytocannabinoids in membranes prepared from Sf9 cells expressing either CB1 or CB2. The study concluded that THCA-A effectively binds to both cannabinoid receptors, displaying a higher affinity for CB1, with Ki values of 23.51±3.5 nM and 56.13±8.2 nM, respectively. In fact, THCA-A (logIC₅₀=1.793±0.00) and THC (logIC₅₀=1.941±0.01) displaced CP-55,940 from CB1 in a similar range of concentrations.³³

This finding is in agreement with the results obtained in the single in vivo preclinical study performed with THCA-A to date.¹⁵ In 2013, Rock et al. found that THCA-A attenuated nausea-induced gaping in rats and vomiting in shrews through a mechanism that required CB1 activation. The effect could be reverted by the coadministration of SR141716, a selective CB1 antagonist. The authors acknowledged in their discussion the discrepancy between their observation and the in vitro data available at the time, since the study by Rosenthaler et al. was not yet published. They even ruled out the possibility of THCA-A converting to THC and activating CB1, a transformation previously suggested not to occur in vivo,⁸ by confirming (i) the absence of THC in blood and (ii) the lack of cannabimimetic responses, like hypothermia or reduced locomotion, after THCA-A injection in rats.¹⁵

Based on the results suggesting that THCA-A binds to CB1 receptors in vitro with an affinity similar to that of THC, a plausible explanation to these contradictory observations could be that THCA-A is a CB1 agonist with restricted access to the central nervous system (CNS). If this were true, THCA-A would still be able to activate peripheral, but not central, CB1 receptors. Three different lines of evidence support this hypothesis: (i) peripheral mechanisms were shown to contribute to CB1-mediated antiemetic actions^34–37; (ii) THC causes reductions in body temperature and motor activity by engaging CB1 receptors in central brain areas, such as the hypothalamus and basal ganglia^38,39; and (iii) brain disposition of THC is augmented in knockout mice lacking P-glycoprotein (P-gp/abcb1) and/or breast cancer resistance protein (Bcrp/abcg2).⁴⁰ Both abcb1 and abcg2 belong to the ATP-binding cassette (ABC) family of efflux transporters and are critical to blood–brain barrier (BBB) function, where they impede the passage of their substrates to the brain.⁴¹ Addition of polar residues, such as a carboxylic group, to the scaffold of abcb1/abcg2 substrates markedly decreases CNS penetration,⁴² which could be the case for THCA-A. Interestingly, brain distribution has been reported for several phytocannabinoids but not for THCA-A.^43,44 Therefore, further studies are required to confirm the ability of THCA-A to activate peripheral CB1 receptors in vivo and to characterize its central distribution.