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

Summary & key facts

Researchers tested whether THCA-A — the natural, acidic form of THC made by the cannabis plant — sticks to and activates the two main human cannabinoid receptors called CB1 and CB2. They used lab cell tests that measure how well a compound binds to a receptor and whether it turns the receptor on. They found THCA-A barely sticks to either receptor and only weakly activates CB1 at a high dose. The sample also had about 2% THC from THCA-A turning into THC, so some of the tiny activity seen might actually be from that contamination. Overall, the study suggests THCA-A itself has very little ability to act at CB1 or CB2, which are the receptors behind most classic cannabis effects.

Key facts:

• THCA-A is the natural acidic form of THC made by the cannabis plant and can turn into THC over time or when heated.
• The THCA-A sample used in the study contained about 2% THC, which the authors say is hard to avoid because THCA-A tends to convert to THC.
• Researchers measured how well THCA-A binds to the human CB1 and CB2 receptors and how well it activates them using lab tests on human receptors in cells.
• THCA-A showed very weak binding: roughly 3 micromolar at CB1 and roughly 13 micromolar at CB2. Lower numbers would mean stronger sticking.
• THC stuck much better to those receptors than THCA-A did — about 60 times stronger at CB1 and about 125 times stronger at CB2.
• In tests of activation, a high concentration of THCA-A slightly reduced a signaling molecule linked to CB1, suggesting only weak activation, and it showed no measurable activation at CB2.
• Because THCA-A easily converts to THC, the authors warn that some of the small binding or activity they measured for THCA-A might actually come from the tiny amount of THC present.
• The study’s main conclusion is that THCA-A by itself has little ability to bind to or activate the main cannabinoid receptors that produce the typical effects of cannabis.

Abstract

Introduction:Cannabis biosynthesizes Δ9-tetrahydrocannabinolic acid (THCA-A), which decarboxylates into Δ9-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 (CB1). 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) CB1 and hCB2 was measured in competition binding assays, using transfected HEK cells and [3H]CP55,940. Efficacy at hCB1 and hCB2 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 hCB1 and hCB2, equating to approximate Ki values of 3.1μM and 12.5μM, respectively. THC showed 62-fold greater affinity at hCB1 and 125-fold greater affinity at hCB2. In efficacy tests, THCA-A (10μM) slightly inhibited forskolin-stimulated cAMP at hCB1, suggestive of weak agonist activity, and no measurable efficacy at hCB2. 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 CB1 or CB2.

Topics

Cannabis and Cannabinoid Research GABA and Rice Research Neurotransmitter Receptor Influence on Behavior

Categories

Health Sciences Medicine Pharmacology

Tags

Agonist Binding site Biochemistry Cannabinoid Cannabinoid receptor Chemistry Ligand binding assay Radioligand Assay Receptor Stereochemistry

Substances

Cannabis