Quinpirole (LY 171,555) is a psychoactive drug and research chemical which acts as a selective D2 and D3 receptor agonist. Quinpirole is the most widely used D2 agonist in in vivo and in vitro studies. Specific quinpirole binding in rat brain was saturable, and dependent on temperature, membrane concentration, sodium concentration and guanine nucleotides. Saturation analysis revealed high affinity binding characteristics (KD = 2.3 nM) which were confirmed by association-dissociation kinetics. The regional distribution of [3H]quinpirole binding sites roughly paralleled the distribution of [3H]spiperone binding sites, with greatest densities present in the striatum, nucleus accumbens and olfactory tubercles. A variety of drugs, most notably monoamine oxidase inhibitors (MAOls), inhibit the binding of [3H]quinpirole, but not [3H]spiperone or [3H](-)N-n-Propylnorapomorphine, in rat striatal membranes by a mechanism that does not appear to involve the enzymatic activity of MAO. Clinically antidepressant MAOIs exhibited selectivity between sites labeled by [3H]quinpirole and [3H]spiperone as did a number of structurally related propargylamines and N-acylethylenediamine derivatives and other drugs such as debrisoquin and phenylbiguanide. Quinpirole has been shown to increase locomotion and sniffing behavior in mice and induces compulsive behavior symptomatic of obsessive compulsive disorder in rats.

Ocfentanil, a compound structurally similar to the opioid analgesic fentanyl, was developed in the early 1990’s with the hope that it would provide a better clinical safety profile than fentanyl. The receptor pharmacology of ocfentanil appears to share pharmacodynamic effects with fentanyl and other μ opioid agonists, including analgesia, sedation, and respiratory depression. In rodents, ocfentanil was approximately 2.5 times more potent as an analgesic than fentanyl and had a shorter duration of action. Because the preclinical research suggested that ocfentanil had a better safety profile than fentanyl, it was selected for clinical evaluation. Like other μ opioid agonists, ocfentanil has been reported to produce itching, nausea, sedation, and severe respiratory depression. Chest pain, psychosis, and agitation have also been reported. In humans, however, ocfentanil had a similar potency (3 ug/kg ocfentanil produced effects that were comparable to 5 ug/kg fentanyl) and side-effects profile as fentanyl so further clinical development was discontinued. Ocfentanil is not approved in any country for medical useand is under national control in Canada, the United Kingdom, and China.

HU-308 ([(1R,2R,5R)-2-[2,6-dimethoxy-4-(2-methyloctan-2-yl)phenyl]-7,7-dimethyl-4-bicyclo[3.1.1]hept-3-enyl]methanol) is a potent, selective agonist for the CB2 receptor. The synthesis and characterization took place in the laboratory of Prof. Mechoulam at the Hebrew University of Jerusalem in the late 1990s. It has analgesic effects, promotes proliferation of neural stem cells,[3] and protects both liver and blood vessel tissues against oxidative stress via inhibition of TNF-α. In vivo, HU-308 has hypotensive, analgesic, and anti-inflammatory activities in mice that can be reversed by the CB2 receptor antagonist SR 144528 but not the CB1 receptor antagonist rimonabant. Pretreatment of mice with HU-308 decreases the I/R-induced tissue damage, inflammatory cell infiltration, tissue, and serum TNF-α, MIP-1, and MIP-2 levels, tissue lipid peroxidation, and apoptosis. HU-308 increases proliferation of HT29 colon cancer cells and growth of tumors in an HT29 mouse xenograft model. The physiological and toxicological properties of this compound have not been evaluated in humans.

Bifeprunox, code name DU-127,090 is an atypical antipsychotic agent, which combines minimal D2 receptor agonism with 5-HT receptor agonism. Bifeprunox was in phase III of clinical trials for the treatment of schizophrenia, Bipolar Depression and in phase I for Parkinson's disease, but these studies were discontinued because efficacy data did not support pursuing the existing development strategy of stabilization of non-acute patients with schizophrenia.

Pridopidine is an experimental drug candidate belonging to a class of agents known as dopidines, which act as dopaminergic stabilizers in the central nervous system. As a dopamine stabilizer, pridopidine is thought to reduce the effects of dopamine when there’s too much and increase its effects when there’s too little. Pridopidine, therefore, plays two opposing roles in the brain, which stabilize dopamine levels. In this way, pridopidine is thought to help the brain reestablish a normal balance of neurotransmitters, and thus regain control over motion. Pridopidine intended to treat Huntington’s disease movement symptoms. Pridopidine was well tolerated and had an adverse event profile similar to a placebo.

Etaqualone (Aolan, Athinazone) is an analog of the hypnotic methaqualone, a non-barbiturate sedative it was developed and marked in France. Etaqualone possesses sedative and hypnotic properties and was used to treat insomnia resulting from its agonist activity at the β subtype of the GABAa receptor.

Loreclezole was found to be active in all animal models of seizures, whether a genetic model of reflex epilepsy was used or models where seizures were elicited by chemical or electrical stimulation. In addition, loreclezole was active against both TON and CLON seizures and increased the threshold for behavioral as well as EEG seizures. Loreclezole has a very rapid onset of action and a duration of the activity, which in certain tests last for more than 24 hr. Chronic administration for 5-7 days did not lead to tolerance to the anticonvulsant action of loreclezole. Loreclezole potentiates gamma-aminobutyric acid (GABA) type A receptor function, by interacting with a specific allosteric modulatory site on receptor beta-subunits. It also acts as a negative modulator at a novel regulatory site, enhancing GABAA receptor sensitization. Inhibits homomeric ρ1 GABAC receptors.

Dimenoxadol is an opioid analgesic which produces typical opioid effects such as analgesia and sedation. It is structurally similar to methadone and is a benzilic acid derivative. In the United States it is classified as a Schedule I controlled drug.

UK-432097 is a selective adenosine A2a agonist which was in development with Pfizer as an inhaled treatment for asthma and chronic obstructive pulmonary disease. UK-432097 had been in phase II clinical trials by Pfizer for the treatment of chronic obstructive pulmonary disease (COPD). However, this study was terminated prematurely due to futility based on results of interim analysis.

Dexfosfoserine (Phosphoserine, L-Serine-O-Phosphate, O-Phosphoserine), the most abundant phosphoamino acid in the eukaryotic phosphoproteome, is not encoded in the genetic code but synthesized posttranslationally. Dexfosfoserine is an agonist of the group III metabotropic glutamate receptors. This endogenous compound inhibits neural stem cells proliferation and promotes survival of nascent neurons thus it has potential therapeutic value in addition to its basic utility as a probe for dissecting molecular mechanisms underlying neurogenesis.