Rislenemdaz, also known as MK-0657 or CERC‐30, is an orally active, selective NMDA receptor subunit 2B (NR2B) antagonist which was a study for the treatment of Parkinson's disease and major depressive disorder (MDD). The data from the phase I clinical trials have shown that drug was not effective in improving motor symptoms in patients with Parkinson disease. In case of using this drug to treat MDD, in spite of the Missing Primary Endpoint in phase II clinical trials, it was shown, that MK-0657 had possessed a potential clinical meaningfulness, that is why it was suggested to continue studying it for MDD patients.

MK-2048, diketo acid derivative, is a second-generation integrase strand transfer inhibitor (INSTI) developed to retain activity against HIV containing mutations associated with resistance to first-generation INSTIs, raltegravir (RAL) and elvitegravir (EVG). MK-2048 implements its inhibitory mechanism by modifying viral integrase-DNA interactions, the important step of the linear HIV-1 cDNA integration into the host genome. It binds to and inactivates the synaptic complex, an intermediate in the concerted integration pathway in vitro thereby preventing target DNA binding and concerted integration. MK-2048 is active against viruses resistant to RAL and EVG. MK-2048 is equally potent against wild-type IN and raltegravir-resistant IN mutant N155H with a low IC50 value of 42 nM for inhibiting concerted integration. It inhibits R263K mutants slightly more effectively than G118R mutants. MK-2048 exposure leads to the selection of G118R as a possible novel resistance mutation.

MK-212 is a 5HT2C-receptor agonist. It displays selectivity for 5-HT2C over 5-HT2A (IC50 values are 0.028 and 0.42 uM for human 5-HT2C and 5-HT2A receptors expressed in HEK293 cells respectively). A dose-dependent the effect of 5HT2C-receptor agonist MK-212 on mouse behavior was demonstrated. Intraperitoneal injection of MK-212 in high doses (0.5 and 1.0 mg/kg) increased blood level of corticosterone in mice and reduced their motor activity. In low doses of 0.1 and 0.2 mg/kg, the agonist reduced anxiety, but had no effect on motor activity. It is hypothesized that low doses of MK-212 exhibited anxiolytic activity in mice.

Naxagolide (MK-458; L 647,339; (+)-PHNO) is a dopamine D2/D3-receptor agonist, which was studied for the treatment of patients with Parkinson's disease, but further study was discontinued. In addition, was discovered, that Naxagolide C-11 ([(11)C]-(+)-PHNO) was a potential radiotracer for imaging the high-affinity state of dopamine D2 receptors with positron emission tomography (PET) in human subjects. This radiotracer is a suitable for imaging the agonist binding sites (denoted D(2HIGH) and D(3)) of these receptors. PET studies in nonhuman primates documented that, in vivo, [(11)C]-(+)-PHNO displays a relative selectivity for D(3) compared with D(2HIGH) receptor sites and that the [(11)C]-(+)-PHNO signal is enriched in D(3) contribution compared with conventional ligands such as [(11)C] raclopride. Recently was published article reflects the relationship between social attachment and dopamine D2/3 receptor availability in the brains of healthy humans using [11C]-(+)-PHNO.

MK-3207 represents the third CGRP receptor antagonist to display clinical efficacy in migraine trials. It is a potent CGRP receptor antagonist with IC50 and Ki of 0.12 nM and 0.022 nM, highly selective versus human AM1, AM2, CTR, and AMY3. MK-3207 had been in phase II clinical trials by Merck Sharp & Dohme for the treatment of migraine. But the company had discontinued the research due to asymptomatic liver test abnormalities in 2010.

Meribendan is a phosphodiesterase-III inhibitor. It exerts chrono-inodilatory response.

Ibutamoren (L-163,191 MK-0677) is a spiropiperidine agonist of the ghrelin receptor and a growth hormone secretagogue. Ibutamoren mimics the actions of growth hormone releasing peptide-6 to increase serum levels of serum insulin-like growth factor-I (IGF-I). Orally active Ibutamoren was being developed by Merck & Co. for a variety of indications, including fibromyalgia, muscle wasting/weakness in patients with chronic kidney disease, Alzheimer's disease and fractures.However, there has been no recent development reported or development has been discontinued for these indications.

MK-0773 is an orally active selective androgen receptor modulator. The safety and efficacy of MK-0773 was evaluated in sarcopenic elderly women. The MK-0773- induced improvements in lean body mass were not accompanied by statistically significant improvements in physical function. Higher dose of MK-0773 or longer duration of therapy might have resulted in improvements in physical function, but liver transaminase elevations likely preclude further development of MK-0773. Drug-candidate had been in phase I clinical trials for the treatment of osteoporosis.

Telcagepant (MK-0974) is a calcitonin gene-related peptide receptor antagonist. Merck & Co was developing telcagepant for the treatment of pain. Telcagepant is an extremely potent CGRP antagonist with a Ki = 0.77 (0.07 nM). Telcagepant showed efficacy against acute migraines; however, different patient populations may show more beneficial effects with telcagepant versus triptans. In the acute treatment of migraine, Telcagepant was found to have equal potency to rizatriptan and zolmitriptan in two Phase III clinical trials. Merck & Co has now terminated development of the drug.

Pruvanserin (EMD 281014, LY-2422347) is a selective serotonin 5-HT2A receptor antagonist. Pruvanserin was originated by Merck KGaA. Eli Lilly had been developing pruvanserin, under a global licence from Merck KGaA, for the treatment of primary insomnia and major depressive disorder. Phase II trials were completed in the US, Hungary and Spain. However, development appears to have been discontinued.