Affinity and Efficacy Studies of Tetrahydrocannabinolic Acid A at Cannabinoid Receptor Types One and Two

Introduction: Cannabis biosynthesizes Δ(9)-tetrahydrocannabinolic acid (THCA-A), which decarboxylates into Δ(9)-tetrahydrocannabinol (THC). There is growing interest in the therapeutic use of THCA-A, but its clinical application may be hampered by instability. THCA-A lacks cannabimimetic effects; we hypothesize that it has little binding affinity at cannabinoid receptor 1 (CB(1)). Materials and Methods: Purity of certified reference standards were tested with high performance liquid chromatography (HPLC). Binding affinity of THCA-A and THC at human (h) CB(1) and hCB(2) was measured in competition binding assays, using transfected HEK cells and [(3)H]CP55,940. Efficacy at hCB(1) and hCB(2) was measured in a cyclic adenosine monophosphase (cAMP) assay, using a Bioluminescence Resonance Energy Transfer (BRET) biosensor. Results: The THCA-A reagent contained 2% THC. THCA-A displayed small but measurable binding at both hCB(1) and hCB(2), equating to approximate K(i) values of 3.1μM and 12.5μM, respectively. THC showed 62-fold greater affinity at hCB(1) and 125-fold greater affinity at hCB(2). In efficacy tests, THCA-A (10μM) slightly inhibited forskolin-stimulated cAMP at hCB(1), suggestive of weak agonist activity, and no measurable efficacy at hCB(2). Discussion: The presence of THC in our THCA-A certified standard agrees with decarboxylation kinetics (literature reviewed herein), which indicate contamination with THC is nearly unavoidable. THCA-A binding at 10μM approximated THC binding at 200nM. We therefore suspect some of our THCA-A binding curve was artifact—from its inevitable decarboxylation into THC—and the binding affinity of THCA-A is even weaker than our estimated values. We conclude that THCA-A has little affinity or efficacy at CB(1) or CB(2).