Carfentanil is a synthetic fentanyl analog. It is a mu-opioid receptor agonist with an estimated analgesic potency approximately 10,000 times that of morphine and 20-30 times that of fentanyl, based on animal studies. Receptor binding studies have shown that carfentanil binds selectively and competitively to the μ subtype of opioid receptors relative to δ and κ opioid receptors. Preclinical studies have demonstrated that the pharmacodynamic effects, such as analgesia and constipation, produced by carfentanil are similar to other μ opioid agonists. Its extreme potency and propensity to produce rapid and profound respiratory depression has prompted recommendations that an opioid antagonist, such as naloxone or naltrexone, be available whenever carfentanil is used or suspected to be present. Carfentanil (Wildnil) has been used in veterinary as a prescription-only general anesthetic for intramuscular injection in large animals. Carfentanil is no longer FDA-approved for use in animals after Wildlife Laboratories withdrew the application for Wildnil. Carfentanyl is increasingly involved in opioid overdose deaths among illicit opioid users.

Tazomeline (also known as LY 287041), a neuropsychiatric agent, is a muscarinic M1 acetylcholine receptor agonist that was studied in patients with cognitive dysfunction. Tazomeline participated in clinical trials for the treatment of Alzheimer’s disease and dementia. However, all these studied were discontinued for unknown reasons.

TAS-108 (also known as SR-16234) is a selective estrogen receptor modulator (SERM) and has been reported to have estrogen receptor (ER) α antagonistic activity and a strong affinity with a weak partial agonistic activity to ERβ receptor. It is known that ERs play a central role in the diverse actions of estrogen. TAS-108 was studied in phase II clinical trials to treat recurrent or recurrent inoperable breast cancer. In addition, TAS-108 participated in Japan in a phase II clinical trial in Endometriosis patients. The phase III studies are being planned with the drug.

Biricodar (also known as VX-710) was developed by Vertex as a chemosensitizing agent designed to restore the effectiveness of chemotherapeutic agents in tumor multidrug resistance. The phase II trials had commenced for biricodar, in combination with chemotherapy, for five common cancer indications: breast, ovarian, soft-tissue sarcomas, small cell lung cancer, and prostate cancer. In spite of completed trials, development of biricodar was discontinued because of the adverse effects.

Cizolirtine is a potent analgesic in mice and rats, with an efficacy superior to that of aspirin and other nonsteroid anti-inflammatory drugs. Recent studies have shown that the analgesic effect of cizolirtine could be related, at least partially, to an inhibition of spinal substance P and calcitonin gene-related peptide release. Cizolirtine has been in clinical trials for the treatment of pain and overactive bladder. Reported adverse events are: dry mouth, dizziness, nausea, vomiting, blurred vision.

Elzasonan (CP 448187) is a serotonin 1B/1D receptor antagonist. Elzasonan was primarily metabolized via oxidative N‐demethylation, N‐oxidation, and aryl hydroxylation. Pfizer was developing elzasonan for the treatment of anxiety and affective disorders however development has been discontinued.

Enclomiphene (Androxal), in development by Repros Therapeutics Inc, is a non-steroidal estrogen receptor antagonist that promotes gonadotropin-dependent testosterone secretion by the testes. Enclomiphene constitutes the trans-stereoisomer of clomiphene citrate, a drug that has been widely prescribed for several decades for the treatment of female ovulatory dysfunction. Because of the antagonistic effects of enclomiphene, the drug has the potential to increase serum testosterone levels in men with secondary hypogonadism by restoring physiological endogenous testosterone secretion while maintaining testicular volume and, potentially, spermatogenesis. In clinical trials conducted to date, enclomiphene demonstrated significant efficacy in the physiological restoration of testosterone levels in males with secondary hypogonadism. The compound also exhibited an unanticipated favorable effect on fasting plasma glucose; this result has been accompanied by rapidly accumulating evidence from other researchers for a bidirectional relationship between low serum testosterone and obesity/metabolic syndrome (syndrome X) in men.

Droloxifene, a derivative of the triphenylethylene drug tamoxifen, is a novel selective estrogen receptor modulator (SERM). Droloxifene also exhibits more rapid pharmacokinetics, reaching peak concentrations and being eliminated much more rapidly than tamoxifen. Its higher affinity to the estrogen receptor, higher anti-estrogenic to estrogenic ratio, more effective inhibition of cell growth and division in estrogen receptor-positive cell lines, and lower toxicity give it theoretical advantages over tamoxifen in the treatment of human breast cancer. Short-term toxicity was generally mild, and similar to that seen with other antiestrogens. Droloxifene appears active and tolerable. It may have a particular role in situations in which rapid pharmacokinetics, or an increased antiestrogenic to estrogenic ratio, are required. Droloxifene may also be a potentially useful agent for the treatment of postmenopausal osteoporosis because it can prevent estrogen deficiency-induced bone loss without causing uterine hypertrophy. Droloxifene may have an effect on bone and breast tissue because it induces apoptosis. Droloxifene has an anti-implantation effect in rats, and the effect appears to be not completely due to its anti-estrogenic activity.

Nitromiphene (NIT; CI 628) is a triarylethylene antiestrogen shown to be effective in treatment of experimental breast cancer. Nitromiphene is one of the earliest nonsteroidal selective estrogen receptor modulators (SERMs). It is an anti-estrogen capable to translocate the estrogen receptor to the nucleus and to induce the replenishment of the cytosol receptor. Nitromiphene inhibited the uptake of [3H]-estradiol in rat whole homogenates and isolated cell nuclei tissues and the pituitary, and inhibited estradiol-induced female sexual behavior. Nitromiphene has thus been shown to suppress the growth of chemically induced and ransplantedmammary tumors in rodents. Also, Nitromiphene was shown to have potent, prolonged antiuterotropic effects in immature rats. Nitromiphene has been shown to undergo conversion to demethyl Nitromiphene (CI628M), a phenolic metabolite which had greater affinity for estrogen receptors and greater biological potency in vitro than did Nitromiphene. However, the in vivo antiestrogenic effects of Nitromiphene and demethyl Nitromiphene were similar, possibly due to facile O-demethylation of the former compound after administration.