OAT: organic anion transporter.

Obligate nose breathers: animals such as rats and mice which have to breathe through their noses rather than through their mouths.

Occamer: any compound which is less complex and/or less expensive than a therapeutic lead but which has the same pharmacological profile.

Occupancy theory: a theory proposed by Clark in 1926 which states that a drug-receptor response is a function of the number of receptors occupied by a ligand and that the maximum achievable effect of a drug is attained when it occupies all receptors. The addition of more drug will have no effect. According to occupancy theory, the formation of the ligand-receptor complex is reversible, all receptors of a given class are identical and they can bind ligand independently of each other. The formation of the ligand-receptor complex does not affect the free concentration of the ligand or the affinity of the receptor for the ligand. This theory, however, does not explain several important features of drug action such as why agonists produce an effect and antagonists do not, and why some drugs produce maximal effect while some receptors are not occupied (spare receptors).
Clark AJ. The Mode of Action of Drugs on Cells. Arnold, London (1933)

Ochratoxins: a group of mycotoxins obtained from Aspergillus ochraceus which cause food spoilage. They are hepatotoxic in mice and rats.

17-Octadecynoic acid: also known as 17-ODYA and 17-ODA, a fatty acid analogue and a suicide substrate inhibitor which is used experimentally as a non-selective cytochrome P450 monooxygenase (CYP) inhibitor.

Octreotide: a cyclic cationic octapeptide and a long-acting somatostatin (a hypothalamic release-inhibiting hormone) analogue. It is used clinically to treat symptoms related to carcinoid tumours and acromegaly.

ODQ: 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a potent and selective inhibitor of nitric oxide-sensitive guanylyl cyclase.

Oestradiol (estradiol):  an endogenous oestrogen and the principal oestrogen secreted by the ovaries. It is used clinically (with or without progestogen) in hormone replacement therapy via nasal spray and can be given topically into the vagina in the form of a cream to treat menopausal atrophic vaginitis.

Oestrogens (estrogens): a group of sex hormones which are synthesized from cholesterol in the ovary and placenta and which comprise oestradiol, oestrone and oestriol. They induce the synthesis of progesterone receptors in the uterus, vagina, anterior pituitary and hypothalamus. They are used clinically in hormone replacement therapy, contraception and prostrate and breast cancers. In veterinary pharmacology, oestrogens are used in mismating therapy in dogs (where the animal is bred against the wishes of its owner), to treat anoestrus in cows, for the treatment of persistent corpus luteum in cows, for the treatment of pyometra (a disease of the uterus showing accumulation of pus in the lumen) and mummified foetus, treatment of disorders related to spaying such as urinary incontinence and vaginitis), for the treatment of prostate hyperplasia and tumours and in the promotion of growth (mainly of ruminants and include the use of estradiol and zeranol in calves, heifers and steers).

Oestrogen analogues: compounds which are structurally similar to oestrogens and which mimic their effect. Examples include mestranol (used an an oral contraceptive) and stilbestrol (diethylstilbestrol, used rarely to treat prostate cancer. In veterinary pharmacology, oestrogen analogues such are used to treat postpartum metritis in cattle and, if given within 72 hours of mating, to prevent embryo implantation in dogs and cats.

Oestrogen receptors: a family of two nuclear receptors know as ERα and ERβ which mediate the effects of oestrogens.
Warner M, Nilsson S, Gustafsson JA. The estrogen receptor family. Curr Opin Obstet Gynecol (1999) 11(3); 249-54

Okadaic acid: a polyether fatty acid, an ionophore and tumour promoter. It potently inhibits serine threonine specific protein phosphatases 1 and 2A.
Cohen P, Holmes CF, Tsukitani Y. Okadaic acid: a new probe for the study of cellular regulation. Trends Biochem Sci (1990) 15(3); 98-102

Olanzapine: an atypical antipsychotic compound used clinically to treat schizophrenia, in monotherapy for mania and in the control of agitation and disturbed behaviour.

Oleamide: also known as oleic acid amide and cis-9,10-octadecenoamide, an endogenous sleep-inducing brain lipid ((Z)-9-octadecenamide) which allosterically modulates GABA_A receptors and potentiates serotonin receptor responses.
Lees G et al. Modulation of GABA_A receptors and inhibitory synaptic currents by the endogenous CNS sleep regulator cis-9,10-octadecenoamide (cOA). Br J Pharmacol (1988) 124; 873-882

Olfactory bulbectomy: surgical removal of the olfactory bulb. This is associated with variety of behavioural abnormalities such as hyperactivity in an open field test. This hyperactivity is decreases by chronic administration of antidepressants.
Kelly JP, Wrynn AS, Leonard BE. The olfactory bulbectomized rat as a model of depression: an update. Pharmacol Ther (1997) 74(3):299-316

Oligosyndactyly mouse: a mouse model showing a 50% reduction in nephron number, glomerular hypertrophy and severe glomerulosclerosis which is used as a model for kidney disease.

Olvanil: also known as NE 19550, a local anaesthetic and a capsaicin analogue (N-vanillyloleoylamide, N-(4-hydroxy-3-methoxyphenyl) methyl-9Z-octadecenamide) with approximately the same in vivo antinociceptive activity as capsaicin. Olvanil stimulates the efferent function of cutaneous sensory nerves, causing the release of calcitonin gene-related peptide which causes vasodilation. Olvanil is as potent as resiniferatoxin and about 10 times more potent than capsaicin. It only weakly activates nociceptors. It is a potent vanilloid receptor agonist and inhibitor of anandamide transport.
Hughes SR, Buckley TL, Brain SD. Olvanil: more potent than capsaicin at stimulating the efferent function of sensory nerves. Eur J Pharmacol (1992) 219(3); 481-4

Omeprazole: a substituted benzimidazole and a proton pump inhibitor used clinically to treat benign gastric and duodenal ulcers, acid-related dyspepsia, NSAID-related gastric erosion and gastrooesophageal reflux disease (GERD). It acts by irreversibly inhibiting H⁺/K⁺-ATPase in the acid secretory pathway in the stomach wall thereby inhibiting the release of hydrogen ions into the fluid of the stomach. The benzimidazole moiety is shown in blue.



Oncolytic: relating to the destruction of tumour cells. For example, oncolytic viruses can be engineered to replicate only in tumour cells leading to tumour cell death and lysis. See en.wikipedia.org/wiki/Oncolytic_virus

Oncoprotein: a protein which induces the formation of cancers. Examples of oncoproteins include p21 and p53.

Oncoprotein inhibitors: substances which inhibit the cancer-promoting effects of oncoproteins and so substances with anticancer effects.

Oncostatin M (OSM): a member of the interleukin-6 (IL-6) cytokine subfamily which was identified as a polypeptide cytokine wit inhibitory effects on the in vitro growth of melanomas and other solid tumors.
Loy JK et al. Oncostatin M: development of a pleiotropic cytokine. Toxicol Pathol (1999) 27(2);151-155

Ondansetron:  a 5-HT₃ receptor antagonist used clinically to treat nausea and vomiting particularly those associated with chemotherapy and radiotherapy of cancer and in the prevention of postoperative nausea. Other 5-HT₃ antagonists with similar actions include dolaserton, granisetron, tropisetron and palonosetron. These compounds block 5-HT₃ receptor in the gastrointestinal tract and CNS.

One-compartment model: a pharmacokinetic term used to describe a model of drug distribution in which a drug is assumed to be distributed in one compartment, ie it assumes that the human or animal body is a simple box and is not divided up into different areas where the drug may accumulate or may not permeate.

Op/op mouse: an osteoporotic mouse and a model of this bone disorder. This mouse shows a deficiency in macrophages due to the genetic absence of colony-stimulating factor (CSF-1) and this leads to a decrease in the number of osteoclasts and osteopetrosis (also known as marble bone disease and Albers-Schonberg disease, a bone disorder where the bones harden).

Open field test: a test using animal such as mice or rats which investigates behavioural responses such as locomotor activity, hyperactivity and exploratory behaviour and which is also a measure of anxiety. Rats and mice prefer to avoid brightly illuminated open spaces so open spaces acts as an anxiogenic stimulus. Measurement of their movements in and out of the open spaces is a measure of anxiety-induced locomotor activity and exploratory behaviour. The open field test is used to investigate the effects of anxiolytics.

Opiates: a class of narcotic alkaloids obtained from opium, the resinous latex obtained from poppy pods, which includes morphine (the most abundant), heroin, codeine, thebaine, papaverine, noscapine and narceine.

Opioid analgesics: a class of analgesic (painkiller) used to relieve moderate to severe pain particularly visceral pain often associated with cancer. Examples include morphine, buprenorphine, codeine, diamorphine (heroin), alfenatil and methadone. They are derived from opium and several are synthetic analogues or morphine. They act as agonists (eg, morphine, codeine and methadone), partial/mixed agonists (nalbuphine and buprenorphine) or antagonists (naloxone and naltrexone) at opioid receptor sites in the brain and periphery.



Opioid peptides: a group of peptides present in the brain, hypophysis, adrenal medulla, intestines, urine and blood which act as opiates and which bind to opioid receptors often having analgesic actions (eg, the endogenous pentapeptide enkephalins).

Opioid receptors: a family of G protein-coupled receptors present in the brain and periphery which mediate the effects of opioids (ie, analgesia, respiratory depression, suppression of the cough reflex, pupil constriction, a decrease in gut motility, nausea and vomiting, dysphoria, sedation, euphoria and physical dependence). They are divided into three main subtypes; μ, σ and κ which mediate different effects of opioids although these is some overlap in receptor action. For example, euphoria and supraspinal analgesia are mediated by μ receptors but sedation and peripheral analgesia are mediated by μ and κ receptors.

Opioidergic: relating to the synthesis, storage and effects of opioids.

Opium: the name given to a narcotic drug obtained from the poppy plant (Papaver somniferum) and used in ethnopharmacology. It is, in fact, a mixture of opioid present in the resinous latex liquid in immature poppy pods and contains about 15% morphine amongst other narcotic alkaloids. See en.wikipedia.org/wiki/Opium.

Opium alkaloid: also known as Papaveraceae alkaloids present in the opium poppy and which include morphine.

Oral contraceptives: a form of chemical contraceptive commonly known as 'the pill' and a type of ovulation inhibitor.

Oral hypoglycaemics: drugs which lower blood sugar levels and which can be given enterally unlike insulin. They are divided into two main groups, ie biguanides and sulphonylureas but also include glitazones. Biguanides include metformin which increases glucose uptake and utilization in skeletal muscle and also reduces hepatic glucose production. Sulphonylureas include tolbutamide and chlorpropamide (first generation sulphonylureas) and glibenclamide and glipazide (second generation). Sulphonylureas stimulate B cells in the islets of Langerhans to secrete insulin via a sulphonylurea receptor. Other drugs used to treat hyperglycaemia include repaglinide and nateglinide which act like sulphonylureas. The glitazones are chemically thiazolidinediones and include ciglitazone, troglitazone, rosglitazone and pioglitazone. The glitazones bind to the peroxisome proliferator-activated receptor-γ (PPARγ) which is a nuclear receptor which controls lipid and glucose metabolism.

Orexigenic: relating to the effects of orexins, ie substances which increase appetite and food intake. The opposite is anorexigenic.

Orexins: excitatory neuropeptides which stimulate food intake. Two are known, orexin A (also known as hypocretin-1) and orexin B (hypocretin-2). They are active in the orexin/hypocretin system which controls food intake, are important in the sleep disorder narcolepsy and play important roles in circadian rhythms. Orexin antagonists such as ACT-078573 (also known as almorexant), SB649868 and SB334867 (a selective non-peptide orexin OX₁ receptor antagonist) are being developed to treat sleep disorders such as primary insomnia.

Orexin receptors: endogenous receptors for orexin of which OX₁ and OX₂ are known.

Orexin receptor agonists: compounds which stimulate orexin receptors.

Orexin receptor antagonists: compounds which block the effects of orexin ligands at orexin receptors. They have potential used in the treatment of sleep, insomnia and neurological disorders.
Roecker AJ, Coleman PJ. Orexin receptor antagonists: medicinal chemistry and therapeutic potential. Curr Top Med Chem. 2008; 8(11):977-87

Organic nitrates: a group of organic compounds which all contain an NO₂ group. They are used to treat angina as they causes smooth muscle relaxation and increase cGMP formation due to the release of nitric oxide. Organic nitrates include amyl nitrate, glyceryl trinitrate and isosorbide mononitrate.



Organoleptic: relating to substances which can be detected by the senses such as a flavour or smell. Organoleptic compounds are neither micronutrients or macronutrients but include food flavours.

Organometallic drugs: drugs which comprise organometallic compounds as the active component. A good example is cisplatin which has been used for a long time in the therapy of tumours. A wide variety of organometallic drugs are known and contain metals such as platinum, tin, ruthenium, gadolinium.
Desoize B. Metals and metal compounds in cancer treatment. Anticancer Res (2004) 24(3a);1529-44

Organotypic: resembling an organ in vivo in shape and/or function.

Orotic acid (OA): a toxic compound which can be used in rat model of microvesicular steatosis produced by the intake of diets containing 1% orotic acid when administered for 5, 10, or 21 days.

Orvinols: a class of potent opioids derived from thebaine and include buprenorphine, etorphine, dihydroetorphine and diprenorphine.

Osmotic diuretics: a type of diuretic.

OTC: over the counter. Drugs which are obtainable without a prescription. At one time only aspirin and relatively harmless drugs were available over the counter but now more and more drugs are coming off prescription (often because their patent protection is due to expire) and become available for self-medication. Examples include Zantac for the treatment of ulcers.

Ototoxicity: toxic to the ear and hearing mechanism.

Oubain:  also known as g-stropanthin, a stropanthus glycoside obtained from the seeds of Stropanthus gratus and Acokanthera ouabaio and a potent cardiac glycoside which has been used to treat heart failure and as a poison on arrows. It acts by inhibiting the transport of sodium and potassium ions across membranes by blocking Na⁺/K⁺-ATPase. It has also been used as a pharmacological tool to study membrane sodium pumps in myocardial cells.

Overton-Meyer hypothesis (correlation): a hypothesis which states that the potency of a general anaesthetic is directly related to its lipid solubility. This hypothesis was made independently by Overton and Meyer but is usually referred to as a joint effort.

Ovicide: a compound which kills the eggs of pests or parasites and which are used primarily in veterinary pharmacology where they are used, for instance, in flea control in cats and dogs. Examples include acetamipid, thiodicarb and lambda-cyhalothrin.

Ovine: relating to sheep.

OVLT: organum vasculosum laminae terminalis, an area of the brain outside the blood-brain barrier which mediates central hyperosmolality-induced increases in sympathetic nerve activity and arterial blood pressure.

Ovotoxic: showing deleterious actions on the ova at any stage prior to fertilization.

Ovulation inhibitors: a group of steroids which inhibit ovulation by a feedback mechanism on the synthesis of leutenizing hormone and follicle stimulating hormone. The are used as chemical contraceptives and are popularly known as 'the pill'. Most commonly they comprise a combination of a progestin and an oestrogen. They are usually taken daily but can be used as intramuscular depot preparations.

Owl monkey:an animal used as a model of cholesterol cholelithiasis. The owl monkey (Aotus nancymae) has a bile acid and biliary lipid profile resembling that of humans and, like humans, spontaneously develops cholesterol gallstones.

Oxacephems: a class of cephem antibiotics. Examples include latamoxef.

Oxazepam: a benzodiazepine anxiolytic used clinically in the short-term treatment of anxiety.

Oxazolidinones: a class of antibiotics. Examples include linezolid.

Oxazolone-induced colitis: a model of colitis used to investigate the effects of drugs on ulcerative colitis.

Oxicams: a class of 4-hydroxy-1,2-benzothiazines with analgesic and anti-inflammatory actions. Examples include piroxicam.

2,3 Oxidosqualene lanosterol cyclase inhibitor: compounds which inhibit the actions of oxidosqualene lanosteol cyclase, an important enzyme in cholesterol synthesis. These inhibitors are being developed to treat hypercholesterolaemia.

Oxonate potassium: a uricase inhibitor used to induce hyperuricaemia in experimental models of gout.

Oxoeicosanoids: a family of biologically active arachidonic acid derivatives that which are involved in cellular migration. They are potent chemotaxins, elicit oxygen radical production and induce secretory events in different cells. The most potent native ligand reported is 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE).
Brink C et al. International Union of Pharmacology XLIV. Nomenclature for the oxoeicosanoid receptor. Pharmacol Rev. 2004. 56;149-157

Oxotremorine: a compound which is a selective agonist at muscarinic receptors relative to its effect at nicotinic receptors.

Oxymetazoline: a selective antagonist of α_1A-adrenoceptors and a selective but partial agonist of α_2A-adrenoceptors.

Oxytocic: a drug with an oxytocin-like effect.

Oxytocin: a neurohypophyseal octapeptide hormone (mwt 1007) which causes contraction of mammary and uterine smooth muscle.

Oxytocin receptors: receptors which mediate the actions of oxytocin.
The oxytocin receptor system: structure, function, and regulation. Gerald Gimpl and Falk Fahrenholz.
http://physrev.physiology.org/cgi/content/full/81/2/629

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