Belaperidone (LU111995) is a recently identified antipsychotic agent with high 5-hydroxytryptamine2 and dopamine D4 receptor affinities as well as D4 versus D2 receptor selectivity. The drug did not produce catalepsy. LU111995 prolongs the Q-T interval to a limited degree and is not arrhythmogenic over the physiological range of cycle lengths. Belaperidone had been in phase II clinical trials for the treatment of schizophrenia. However, the study about this drug candidate was discontinued.

Setoperone is and antagonist of the brain serotonin 5-HT2 receptor and particular the 5-HT2A isoform. Setoperone is radiolabeled with the radioisotope fluorine-18 and is used in positron emission tomography (PET) in neuroimaging for the study neuropsychiatric disorders, such as schizophrenia and depression.

Mesulergine, an antagonist of 5-HT2C, and dopamine receptors was studied in clinical trials for the treatment of Parkinson's disease. However, further, development was discontinued due to toxicological observations in animal experiments.

Aplindore (DAB-452) is a small molecule that displays potent dopamine D2 receptor partial agonist activity in in vitro and in vivo assays and is predicted to have antipsychotic efficacy without motor side effects. Aplindore had been in phase II clinical trials for the treatment of Parkinson's disease and restless legs syndrome. Aplindore was generally well tolerated and there were no withdrawals due to adverse events and no serious adverse events.

Fananserin is a potential antipsychotic compound with a high affinity for both D4 and 5-HT2A receptors, and negligible affinity for D2 receptors. Fananserin has been researched for the treatment of schizophrenia. Fananserin was the first selective D4/5-HT2A antagonist to undergo clinical trials for schizophrenia. It has a high affinity for D4 (Ki 2.9 nM) and 5-HT2A (Ki 0.37 nM) receptors and is over 100-fold selective versus H1, a1 adrenergic, 5-HT1A and D2 dopamine receptors. Development of this compound was halted following phase II clinical trials due to lack of efficacy.

Eplivanserin (SR 46349) is an antagonist of serotonin 2A receptor. Eplivanserin was previously in development by Sanofi-aventis in an effort to educate the public regarding this new mechanism of action for sleep aids. Eplivanserin was reviewed by the FDA as a potential treatment for patients with chronic insomnia, but the FDA requested additional information regarding benefit-risk and development of the drug has been discontinued.

Quizapine is a piperazine-based nonselective serotonin (5-HT) receptor agonist with antidepressant and oxytocic activities. Quipazine targets and binds to serotonin receptors, particularly to the 5HT2A and 5HT3 receptors. Quipazine is a potential anti-parkinsonian agent. Serotonin receptor activation by quipazine may lead to smooth muscle contraction and antidepressant effects. quipazine, a central serotonergic agent, counteracted some of the drug-induced symptoms of pseudoparkinsonism in laboratory animals. Cholinergic, dopaminergic and histaminergic receptors play an important role in the manifestations of these symptoms.

Urocanic acid is a breakdown (deamination) product of histidine. In the liver, urocanic acid is an intermediate in the conversion of histidine to glutamic acid, whereas in the epidermis, it accumulates and may be both a UV protectant and an immunoregulator. Urocanic acid (UA) exists as a trans isomer (t-UA, approximately 30 mg/cm2) in the uppermost layer of the skin (stratum corneum). t-UA is formed as the cells of the second layer of skin become metabolically inactive. During this process, proteins and membranes degrade, histidine is released, and histidase (histidine ammonia lyase) catalyzes the deamination of histidine to form t-UA. t-UA accumulates in the epidermis until removal by either the monthly skin renewal cycle or sweat. Upon absorption of UV light, the naturally occurring t-UA isomerizes to its cis form, c-UA. Because DNA lesions (e. g. , pyrimidine dimers) in the lower epidermis can result from UV-B absorption, initial research proposed that t-UA acted as a natural sunscreen absorbing UV-B in the stratum corneum before the damaging rays could penetrate into lower epidermal zones. c-UA also suppresses contact hypersensitivity and delayed hypersensitivity, reduces the Langerhans cell count in the epidermis, prolongs skin-graft survival time, and affects natural killer cell activity. It has also been proposed that c-UA may mediate the transient alteration in immune surveillance resulting in immunosuppression induced after UV-irradiation, by interacting with immune cells locally and/or systemically to generate T cells with suppressor function.

ORG 37684 is a selective full agonist of 5-hydroxytryptamine receptor 5-HT2 family with a high selectivity for 5-HT2C in comparison with 5-HT2B and 5-HT2A receptors. It was studied for treatment of depression, but that investigations were discontinued. On animal studies ORG 37684 possessed hypophagic effect, which was exclusively mediated by activation of 5-HT2C receptors. Thus, this compound can be further investigated in using for weight loss.