NESS-0327 is a drug used in scientific research which acts as an extremely potent and selective antagonist of the cannabinoid receptor

Cannabinoid receptor

The cannabinoid receptors are a class of cell membrane receptors under the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid receptors contain seven transmembrane spanning domains...

 CB₁. It is much more potent an antagonist, and more selective for the CB₁ receptor over CB₂, than the more commonly used ligand rimonabant

Rimonabant

Rimonabant is an anorectic antiobesity drug that has been withdrawn from the market. It is an inverse agonist for the cannabinoid receptor CB1...

, with a K_i

Dissociation constant

In chemistry, biochemistry, and pharmacology, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into...

 at CB₁ of 350fM (i.e. 0.00035nM) and a selectivity of over 60,000x for CB₁ over CB₂..
Independently, two other groups have described only modest nanomolar CB₁ affinity for this compound (125nM and
18.4nM).
Also unlike rimonabant, NESS-0327 does not appear to act as an inverse agonist

Inverse agonist

In the field of pharmacology, an inverse agonist is an agent that binds to the same receptor as an agonist but induces a pharmacological response opposite to that agonist....

 at higher doses, instead being a purely neutral antagonist which blocks the CB₁ receptor but does not produce any physiological effect of its own.