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INTRODUCTION

Currently available cannabinoid drugs consist of one major subgroup that act at the CB1 receptors. However, there are rapidly developing alternative groups of drugs that target newly discovered receptors as well as new drugs that are designed to enrich endogenous cannabinoids.

Delta-9-tetrahydrocannabinol (THC) may be considered the prototype that acts directly at both CB1 and CB2 receptors. However, THC has made little progress in clinical use due to the psychoactive component, which is thought to be due mainly to action at the CB1 receptor. Novel cannabinoids are being discovered that lack this unwanted (in clinical usage) effect.

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DIRECT-ACTING CANNABINOID RECEPTOR AGONISTS

A. Mechanism of Action

Studies have identified two unique G-protein-coupled receptors termed CB1 and CB2, that when activated demonstrate Gi/o coupling and a corresponding decrease in cAMP levels followed by decreased protein kinase A (PKA) activity, as well as an indirect decrease in voltage-gated calcium channel activity and an increase in potassium channels on neurons. In spite of similar coupling, the receptors have very different expression patterns. CB1 demonstrates widespread expression in the majority of the organs of the body as well as high expression on the neurons of the central and peripheral nervous system. CB2 receptor expression is far less extensive, and demonstrates activity mainly on immune cells (ie, macrophages, monocytes, organ-specific resident immune cells), spleen, lymph nodes, microglia of the CNS, and specialized immune cells such as osteoclasts.

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High-Yield Terms to Learn
Cannabis Cannabis sativa, a tall plant of the family Cannabaceae, used for its tough fiber (hemp) and a psychoactive (mind-altering) drug, delta-9-THC
Cannabinoid Any of a group of closely related chemical compounds which include the active constituents of cannabis including delta-9-THC, cannabinol, and cannabidiol
Endogenous cannabinoids Endogenous lipids that engage cannabinoid receptors, affecting behavior in a fashion that at least partially recapitulates the effects produced by cannabinoids
Delta-9-THC (−)-trans-Δ9 -tetrahydrocannabinol is identified in the cannabis plant, engages the cannabinoid receptors, and is the principal psychoactive constituent of cannabis

SKILL KEEPER: DRUG METABOLISM (SEE CHAPTER 3)

Cannabinoids are very lipophilic compounds that can be detected in the blood stream for many hours and even days after a single administration. Based on this information what type of volume of distribution and half-life might cannabinoids have? The Skill Keeper Answer appears at the end of the chapter.

B. Classification

Three cannabinoids are currently approved by the FDA for medicinal use in the USA. The first two, nabilone and dronabinol, are schedule II medications. Dronabinol is the synthetic form of delta-9-tetrahydrocannabinol (THC), whereas nabilone is a synthetic substance with a chemical structure similar to dronabinol. The primary target for both dronabinol and nabilone is the CB1 receptor. The third ...

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