Naltrindole is a highly potent, highly selective delta opioid receptor

Delta Opioid receptor

The δ-opioid receptors, also known as delta opioid receptor or simply delta receptor, abbreviated DOR, is an opioid receptor that has enkephalins as its endogenous ligands.-Function:...

 antagonist

Competitive antagonist

A competitive antagonist is a receptor antagonist that binds to a receptor but does not activate the receptor. The antagonist will compete with available agonist for receptor binding sites on the same receptor...

 used in biomedical research.

Drug design

Since peptide

Peptide

Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

 compounds are unable to cross the blood brain barrier, researchers developed naltrindole to be a non-peptide antagonist analog of the delta

Delta Opioid receptor

The δ-opioid receptors, also known as delta opioid receptor or simply delta receptor, abbreviated DOR, is an opioid receptor that has enkephalins as its endogenous ligands.-Function:...

-preferring endogenous opiate enkephalin

Enkephalin

An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephalins are termed endogenous ligands, or specifically endorphins, as they are internally derived and bind to the body's opioid receptors. Discovered in 1975, two forms of enkephalin were revealed, one...

. Enkephalin contains an aromatic phenyl group on its Phe

Phenylalanine

Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

⁴ residue, which was hypothesized to be the "address" sequence responsible for the opiate's delta opioid receptor affinity. Thus, attachment of a phenyl-containing indole

Indole

Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...

 molecule to the C-ring of naltrexone

Naltrexone

Naltrexone is an opioid receptor antagonist used primarily in the management of alcohol dependence and opioid dependence. It is marketed in generic form as its hydrochloride salt, naltrexone hydrochloride, and marketed under the trade names Revia and Depade...

's morphinan

Morphinan

Morphinan is the base chemical structure of a large chemical class of psychoactive drugs, consisting of opioid analgesics, cough suppressants, and dissociative hallucinogens, among others.- Chemical Derivatives :Immediate derivatives of morphinan include:...

base successfully produced a drug with the high receptor affinity of naltrexone, but which binds almost exclusively to the delta opioid receptor.