Benzodiazepines, sometimes called "benzos", are a class of psychoactive drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. The first such drug, was discovered accidentally by Leo Sternbach in 1955, made available in 1960 by Hoffmann–La Roche, since 1963, has marketed the benzodiazepine diazepam. In 1977 benzodiazepines were globally the most prescribed medications, they are in the family of drugs known as minor tranquilizers. Benzodiazepines enhance the effect of the neurotransmitter gamma-aminobutyric acid at the GABAA receptor, resulting in sedative, anxiolytic and muscle relaxant properties. High doses of many shorter-acting benzodiazepines may cause anterograde amnesia and dissociation; these properties make benzodiazepines useful in treating anxiety, agitation, muscle spasms, alcohol withdrawal and as a premedication for medical or dental procedures. Benzodiazepines are categorized as either intermediary, or long-acting. Short- and intermediate-acting benzodiazepines are preferred for the treatment of insomnia.
Benzodiazepines are viewed as safe and effective for short-term use, although cognitive impairment and paradoxical effects such as aggression or behavioral disinhibition occur. A minority of people can have paradoxical reactions such as worsened panic. Benzodiazepines are associated with increased risk of suicide. Long-term use is controversial because of concerns about decreasing effectiveness, physical dependence, an increased risk of dementia. Stopping benzodiazepines leads to improved physical and mental health; the elderly are at an increased risk of both short- and long-term adverse effects, as a result, all benzodiazepines are listed in the Beers List of inappropriate medications for older adults. There is controversy concerning the safety of benzodiazepines in pregnancy. While they are not major teratogens, uncertainty remains as to whether they cause cleft palate in a small number of babies and whether neurobehavioural effects occur as a result of prenatal exposure. Benzodiazepines can cause dangerous deep unconsciousness.
However, they are less toxic than their predecessors, the barbiturates, death results when a benzodiazepine is the only drug taken. When combined with other central nervous system depressants such as alcoholic drinks and opioids, the potential for toxicity and fatal overdose increases. Benzodiazepines are misused and taken in combination with other drugs of abuse. Benzodiazepines possess psycholeptic, hypnotic, anticonvulsant, muscle relaxant, amnesic actions, which are useful in a variety of indications such as alcohol dependence, anxiety disorders, panic and insomnia. Most are administered orally. In general, benzodiazepines are well-tolerated and are safe and effective drugs in the short term for a wide range of conditions. Tolerance can develop to their effects and there is a risk of dependence, upon discontinuation a withdrawal syndrome may occur; these factors, combined with other possible secondary effects after prolonged use such as psychomotor, cognitive, or memory impairments, limit their long-term applicability.
The effects of long-term use or misuse include the tendency to cause or worsen cognitive deficits and anxiety. The College of Physicians and Surgeons of British Columbia recommends discontinuing the usage of benzodiazepines in those on opioids and those who have used them long term. Benzodiazepines can have serious adverse health outcomes, these findings support clinical and regulatory efforts to reduce usage in combination with non-benzodiazepine receptor agonists; because of their effectiveness and rapid onset of anxiolytic action, benzodiazepines are used for the treatment of anxiety associated with panic disorder. However, there is disagreement among expert bodies regarding the long-term use of benzodiazepines for panic disorder; the views range from those that hold that benzodiazepines are not effective long-term and that they should be reserved for treatment-resistant cases to those that hold that they are as effective in the long term as selective serotonin reuptake inhibitors. The American Psychiatric Association guidelines note that, in general, benzodiazepines are well tolerated, their use for the initial treatment for panic disorder is supported by numerous controlled trials.
APA states that there is insufficient evidence to recommend any of the established panic disorder treatments over another. The choice of treatment between benzodiazepines, SSRIs, serotonin–norepinephrine reuptake inhibitors, tricyclic antidepressants, psychotherapy should be based on the patient's history and other individual characteristics. Selective serotonin reuptake inhibitors are to be the best choice of pharmacotherapy for many patients with panic disorder, but benzodiazepines are often used, some studies suggest that these medications are still used with greater frequency than the SSRIs. One advantage of benzodiazepines is that they alleviate the anxiety symptoms much faster than antidepressants, therefore may be preferred in patients for whom rapid symptom control is critical. However, this advantage is offset by the possibility of developing benzodiazepine dependence. APA does not recommend benzodiazepines for persons with depressive
Pharmacokinetics, sometimes abbreviated as PK, is a branch of pharmacology dedicated to determine the fate of substances administered to a living organism. The substances of interest include any chemical xenobiotic such as: pharmaceutical drugs, food additives, etc, it attempts to analyze chemical metabolism and to discover the fate of a chemical from the moment that it is administered up to the point at which it is eliminated from the body. Pharmacokinetics is the study of how an organism affects a drug, whereas pharmacodynamics is the study of how the drug affects the organism. Both together influence dosing and adverse effects, as seen in PK/PD models. Pharmacokinetics describes how the body affects a specific xenobiotic/chemical after administration through the mechanisms of absorption and distribution, as well as the metabolic changes of the substance in the body, the effects and routes of excretion of the metabolites of the drug. Pharmacokinetic properties of chemicals are affected by the route of administration and the dose of administered drug.
These may affect the absorption rate. Models have been developed to simplify conceptualization of the many processes that take place in the interaction between an organism and a chemical substance. One of these, the multi-compartmental model, is the most used approximations to reality; the various compartments that the model is divided into are referred to as the ADME scheme: Liberation – the process of release of a drug from the pharmaceutical formulation. See IVIVC. Absorption – the process of a substance entering the blood circulation. Distribution – the dispersion or dissemination of substances throughout the fluids and tissues of the body. Metabolism – the recognition by the organism that a foreign substance is present and the irreversible transformation of parent compounds into daughter metabolites. Excretion – the removal of the substances from the body. In rare cases, some drugs irreversibly accumulate in body tissue; the two phases of metabolism and excretion can be grouped together under the title elimination.
The study of these distinct phases involves the use and manipulation of basic concepts in order to understand the process dynamics. For this reason in order to comprehend the kinetics of a drug it is necessary to have detailed knowledge of a number of factors such as: the properties of the substances that act as excipients, the characteristics of the appropriate biological membranes and the way that substances can cross them, or the characteristics of the enzyme reactions that inactivate the drug. All these concepts can be represented through mathematical formulas that have a corresponding graphical representation; the use of these models allows an understanding of the characteristics of a molecule, as well as how a particular drug will behave given information regarding some of its basic characteristics such as its acid dissociation constant and solubility, absorption capacity and distribution in the organism. The model outputs for a drug can be used in industry or in the clinical application of pharmacokinetic concepts.
Clinical pharmacokinetics provides many performance guidelines for effective and efficient use of drugs for human-health professionals and in veterinary medicine. The following are the most measured pharmacokinetic metrics: In pharmacokinetics, steady state refers to the situation where the overall intake of a drug is in dynamic equilibrium with its elimination. In practice, it is considered that steady state is reached when a time of 4 to 5 times the half-life for a drug after regular dosing is started; the following graph depicts a typical time course of drug plasma concentration and illustrates main pharmacokinetic metrics: Pharmacokinetic modelling is performed by noncompartmental or compartmental methods. Noncompartmental methods estimate the exposure to a drug by estimating the area under the curve of a concentration-time graph. Compartmental methods estimate the concentration-time graph using kinetic models. Noncompartmental methods are more versatile in that they do not assume any specific compartmental model and produce accurate results acceptable for bioequivalence studies.
The final outcome of the transformations that a drug undergoes in an organism and the rules that determine this fate depend on a number of interrelated factors. A number of functional models have been developed in order to simplify the study of pharmacokinetics; these models are based on a consideration of an organism as a number of related compartments. The simplest idea is to think of an organism as only one homogenous compartment; this monocompartmental model presupposes that blood plasma concentrations of the drug are a true reflection of the drug's concentration in other fluids or tissues and that the elimination of the drug is directly proportional to the drug's concentration in the organism. However, these models do not always reflect the real situation within an organism. For example, not all body tissues have the same blood supply, so the distribution of the drug will be slower in these tissues than in others with a better blood supply. In addition, there are some tissues (s
Controlled Substances Act
The Controlled Substances Act is the statute establishing federal U. S. drug policy under which the manufacture, possession and distribution of certain substances is regulated. It was passed by the 91st United States Congress as Title II of the Comprehensive Drug Abuse Prevention and Control Act of 1970 and signed into law by President Richard Nixon; the Act served as the national implementing legislation for the Single Convention on Narcotic Drugs. The legislation created five schedules, with varying qualifications for a substance to be included in each. Two federal agencies, the Drug Enforcement Administration and the Food and Drug Administration, determine which substances are added to or removed from the various schedules, although the statute passed by Congress created the initial listing. Congress has sometimes scheduled other substances through legislation such as the Hillory J. Farias and Samantha Reid Date-Rape Prevention Act of 2000, which placed gamma hydroxybutyrate in Schedule I and sodium oxybate in Schedule III.
Classification decisions are required to be made on criteria including potential for abuse accepted medical use in treatment in the United States, international treaties. The nation first outlawed addictive drugs in the early 1900s and the International Opium Convention helped lead international agreements regulating trade; the Food and Drugs Act of 1906 was the beginning of over 200 laws concerning public health and consumer protections. Others were the Federal Food and Cosmetic Act, the Kefauver Harris Amendment of 1962. In 1969, President Richard Nixon announced that the Attorney General, John N. Mitchell, was preparing a comprehensive new measure to more meet the narcotic and dangerous drug problems at the federal level by combining all existing federal laws into a single new statute. With the help of White House Counsel head, John Dean; the CSA not only combined existing federal drug laws and expanded their scope, but it changed the nature of federal drug law policies and expanded Federal law enforcement pertaining to controlled substances.
Title II, Part F of the Comprehensive Drug Abuse Prevention and Control Act of 1970 established the National Commission on Marijuana and Drug Abuse—known as the Shafer Commission after its chairman, Raymond P. Shafer—to study cannabis abuse in the United States. During his presentation of the commission's First Report to Congress and Shafer recommended the decriminalization of marijuana in small amounts, with Shafer stating, he criminal law is too harsh a tool to apply to personal possession in the effort to discourage use, it implies. The actual and potential harm of use of the drug is not great enough to justify intrusion by the criminal law into private behavior, a step which our society takes only with the greatest reluctance. Rufus King notes that this stratagem was similar to that used by Harry Anslinger when he consolidated the previous anti-drug treaties into the Single Convention and took the opportunity to add new provisions that otherwise might have been unpalatable to the international community.
According to David T. Courtwright, "the Act was part of an omnibus reform package designed to rationalize, in some respects to liberalize, American drug policy." It provided support for drug treatment and research. King notes that the rehabilitation clauses were added as a compromise to Senator Jim Hughes, who favored a moderate approach; the bill, as introduced by Senator Everett Dirksen, ran to 91 pages. While it was being drafted, the Uniform Controlled Substances Act, to be passed by state legislatures, was being drafted by the Department of Justice. Since its enactment in 1970, the Act has been amended numerous times: The 1976 Medical Device Regulation Act; the Psychotropic Substances Act of 1978 added provisions implementing the Convention on Psychotropic Substances. The Controlled Substances Penalties Amendments Act of 1984; the 1986 Federal Analog Act for chemicals "substantially similar" in Schedule I and II to be listed The 1988 Chemical Diversion and Trafficking Act added provisions implementing the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances that went into force on November 11, 1990.
1990 The Anabolic Steroids Act, passed as part of the Crime Control Act of 1990, which placed anabolic steroids into Schedule III The 1993 Domestic Chemical Diversion and Control Act in response to methamphetamine trafficking. The 2008 Ryan Haight Online Pharmacy Consumer Protection Act The 2010 Electronic Prescriptions for Controlled Substances; the 2010 Secure and Responsible Drug Disposal Act, to allow pharmacies to operate take-back programs for controlled subtance medications in response to the US opioid epidemic. The Controlled Substances Act consists of 2 subchapters. Subchapter I defines Schedules I-V, lists chemicals used in the manufacture of controlled substances, differentiates lawful and unlawful manufacturing and possession of controlled substances, including possession of Schedule I drugs for personal use.
Route of administration
A route of administration in pharmacology and toxicology is the path by which a drug, poison, or other substance is taken into the body. Routes of administration are classified by the location at which the substance is applied. Common examples include intravenous administration. Routes can be classified based on where the target of action is. Action may be enteral, or parenteral. Route of administration and dosage form are aspects of drug delivery. Routes of administration are classified by application location; the route or course the active substance takes from application location to the location where it has its target effect is rather a matter of pharmacokinetics. Exceptions include the transdermal or transmucosal routes, which are still referred to as routes of administration; the location of the target effect of active substances are rather a matter of pharmacodynamics. An exception is topical administration, which means that both the application location and the effect thereof is local. Topical administration is sometimes defined as both a local application location and local pharmacodynamic effect, sometimes as a local application location regardless of location of the effects.
Administration through the gastrointestinal tract is sometimes termed enteral or enteric administration. Enteral/enteric administration includes oral and rectal administration, in the sense that these are taken up by the intestines. However, uptake of drugs administered orally may occur in the stomach, as such gastrointestinal may be a more fitting term for this route of administration. Furthermore, some application locations classified as enteral, such as sublingual and sublabial or buccal, are taken up in the proximal part of the gastrointestinal tract without reaching the intestines. Enteral administration can be used for systemic administration, as well as local, such as in a contrast enema, whereby contrast media is infused into the intestines for imaging. However, for the purposes of classification based on location of effects, the term enteral is reserved for substances with systemic effects. Many drugs as tablets, capsules, or drops are taken orally. Administration methods directly into the stomach include those by gastric feeding tube or gastrostomy.
Substances may be placed into the small intestines, as with a duodenal feeding tube and enteral nutrition. Enteric coated tablets are designed to dissolve in the intestine, not the stomach, because the drug present in the tablet causes irritation in the stomach; the rectal route is an effective route of administration for many medications those used at the end of life. The walls of the rectum absorb many medications and effectively. Medications delivered to the distal one-third of the rectum at least avoid the "first pass effect" through the liver, which allows for greater bio-availability of many medications than that of the oral route. Rectal mucosa is vascularized tissue that allows for rapid and effective absorption of medications. A suppository is a solid dosage form. In hospice care, a specialized rectal catheter, designed to provide comfortable and discreet administration of ongoing medications provides a practical way to deliver and retain liquid formulations in the distal rectum, giving health practitioners a way to leverage the established benefits of rectal administration.
The parenteral route is any route, not enteral. Parenteral administration can be performed by injection, that is, using a needle and a syringe, or by the insertion of an indwelling catheter. Locations of application of parenteral administration include: central nervous systemepidural, e.g. epidural anesthesia intracerebral direct injection into the brain. Used in experimental research of chemicals and as a treatment for malignancies of the brain; the intracerebral route can interrupt the blood brain barrier from holding up against subsequent routes. Intracerebroventricular administration into the ventricular system of the brain. One use is as a last line of opioid treatment for terminal cancer patients with intractable cancer pain. Epicutaneous, it can be used both for local effect as in allergy testing and typical local anesthesia, as well as systemic effects when the active substance diffuses through skin in a transdermal route. Sublingual and buccal medication administration is a way of giving someone medicine orally.
Sublingual administration is. The word "sublingual" means "under the tongue." Buccal administration involves placement of the drug between the cheek. These medications can come in the form of films, or sprays. Many drugs are designed for sublingual administration, including cardiovascular drugs, barbiturates, opioid analgesics with poor gastrointestinal bioavailability and vitamins and minerals. Extra-amniotic administration, between the endometrium and fetal membranes nasal administration (th
Excretion is a process by which metabolic waste is eliminated from an organism. In vertebrates this is carried out by the lungs and skin; this is in contrast with secretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life. For example, in mammals urine is expelled through the urethra, part of the excretory system. In unicellular organisms, waste products are discharged directly through the surface of the cell. During life activities such as cellular respiration, several chemical reactions take place in the body; these are known as metabolism. These chemical reactions produce waste products such as carbon dioxide, salts and uric acid. Accumulation of these wastes beyond a level inside the body is harmful to the body; the excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion. Green plants produce carbon water as respiratory products. In green plants, the carbon dioxide released during respiration gets utilized during photosynthesis.
Oxygen is a by product generated during photosynthesis, exits through stomata, root cell walls, other routes. Plants can get rid of excess water by guttation, it has been shown that the leaf acts as an'excretophore' and, in addition to being a primary organ of photosynthesis, is used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants — resin, latex, etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high. Plants excrete some waste substances into the soil around them. In animals, the main excretory products are carbon dioxide, urea, uric acid and creatine; the liver and kidneys clear many substances from the blood, the cleared substances are excreted from the body in the urine and feces. Aquatic animals excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution.
In terrestrial animals ammonia-like compounds are converted into other nitrogenous materials as there is less water in the environment and ammonia itself is toxic. Birds excrete their nitrogenous wastes as uric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficient water retention and it can be stored more in the egg. Many avian species seabirds, can excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak. In insects, a system involving Malpighian tubules is utilized to excrete metabolic waste. Metabolic waste diffuses or is transported into the tubule, which transports the wastes to the intestines; the metabolic waste is released from the body along with fecal matter. The excreted material may be called ejecta. In pathology the word ejecta is more used. UAlberta.ca, Animation of excretion Brian J Ford on leaf fall in Nature
Drug metabolism is the metabolic breakdown of drugs by living organisms through specialized enzymatic systems. More xenobiotic metabolism is the set of metabolic pathways that modify the chemical structure of xenobiotics, which are compounds foreign to an organism's normal biochemistry, such as any drug or poison; these pathways are a form of biotransformation present in all major groups of organisms, are considered to be of ancient origin. These reactions act to detoxify poisonous compounds; the study of drug metabolism is called pharmacokinetics. The metabolism of pharmaceutical drugs is an important aspect of medicine. For example, the rate of metabolism determines the duration and intensity of a drug's pharmacologic action. Drug metabolism affects multidrug resistance in infectious diseases and in chemotherapy for cancer, the actions of some drugs as substrates or inhibitors of enzymes involved in xenobiotic metabolism are a common reason for hazardous drug interactions; these pathways are important in environmental science, with the xenobiotic metabolism of microorganisms determining whether a pollutant will be broken down during bioremediation, or persist in the environment.
The enzymes of xenobiotic metabolism the glutathione S-transferases are important in agriculture, since they may produce resistance to pesticides and herbicides. Drug metabolism is divided into three phases. In phase I, enzymes such as cytochrome P450 oxidases introduce reactive or polar groups into xenobiotics; these modified compounds are conjugated to polar compounds in phase II reactions. These reactions are catalysed by transferase enzymes such as glutathione S-transferases. In phase III, the conjugated xenobiotics may be further processed, before being recognised by efflux transporters and pumped out of cells. Drug metabolism converts lipophilic compounds into hydrophilic products that are more excreted; the exact compounds an organism is exposed to will be unpredictable, may differ over time. The major challenge faced by xenobiotic detoxification systems is that they must be able to remove the almost-limitless number of xenobiotic compounds from the complex mixture of chemicals involved in normal metabolism.
The solution that has evolved to address this problem is an elegant combination of physical barriers and low-specificity enzymatic systems. All organisms use cell membranes as hydrophobic permeability barriers to control access to their internal environment. Polar compounds cannot diffuse across these cell membranes, the uptake of useful molecules is mediated through transport proteins that select substrates from the extracellular mixture; this selective uptake means that most hydrophilic molecules cannot enter cells, since they are not recognised by any specific transporters. In contrast, the diffusion of hydrophobic compounds across these barriers cannot be controlled, organisms, cannot exclude lipid-soluble xenobiotics using membrane barriers. However, the existence of a permeability barrier means that organisms were able to evolve detoxification systems that exploit the hydrophobicity common to membrane-permeable xenobiotics; these systems therefore solve the specificity problem by possessing such broad substrate specificities that they metabolise any non-polar compound.
Useful metabolites are excluded since they are polar, in general contain one or more charged groups. The detoxification of the reactive by-products of normal metabolism cannot be achieved by the systems outlined above, because these species are derived from normal cellular constituents and share their polar characteristics. However, since these compounds are few in number, specific enzymes can remove them. Examples of these specific detoxification systems are the glyoxalase system, which removes the reactive aldehyde methylglyoxal, the various antioxidant systems that eliminate reactive oxygen species; the metabolism of xenobiotics is divided into three phases:- modification and excretion. These reactions act in concert to remove them from cells. In phase I, a variety of enzymes act to introduce polar groups into their substrates. One of the most common modifications is hydroxylation catalysed by the cytochrome P-450-dependent mixed-function oxidase system; these enzyme complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups or N-, O- and S-dealkylation of substrates.
The reaction mechanism of the P-450 oxidases proceeds through the reduction of cytochrome-bound oxygen and the generation of a highly-reactive oxyferryl species, according to the following scheme: O2 + NADPH + H+ + RH → NADP+ + H2O + ROHPhase I reactions may occur by oxidation, hydrolysis, cyclization and addition of oxygen or removal of hydrogen, carried out by mixed function oxidases in the liver. These oxidative reactions involve a cytochrome P450 monooxygenase, NADPH and oxygen; the classes of pharmaceutical drugs that utilize this method for their metabolism include phenothiazines and steroids. If the metabolites of phase I reactions are sufficiently polar, they may be excreted at this point. However, many phase I products are not eliminated and undergo a subsequent reaction in which an endogenous substrate combines with the newly incorporated functional group to
Simplified molecular-input line-entry system
The simplified molecular-input line-entry system is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules; the original SMILES specification was initiated in the 1980s. It has since been extended. In 2007, an open standard called. Other linear notations include the Wiswesser line notation, ROSDAL, SYBYL Line Notation; the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. Acknowledged for their parts in the early development were "Gilman Veith and Rose Russo and Albert Leo and Corwin Hansch for supporting the work, Arthur Weininger and Jeremy Scofield for assistance in programming the system." The Environmental Protection Agency funded the initial project to develop SMILES. It has since been modified and extended by others, most notably by Daylight Chemical Information Systems.
In 2007, an open standard called "OpenSMILES" was developed by the Blue Obelisk open-source chemistry community. Other'linear' notations include the Wiswesser Line Notation, ROSDAL and SLN. In July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is considered to have the advantage of being more human-readable than InChI; the term SMILES refers to a line notation for encoding molecular structures and specific instances should be called SMILES strings. However, the term SMILES is commonly used to refer to both a single SMILES string and a number of SMILES strings; the terms "canonical" and "isomeric" can lead to some confusion when applied to SMILES. The terms are not mutually exclusive. A number of valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol. Algorithms have been developed to generate the same SMILES string for a given molecule; this SMILES is unique for each structure, although dependent on the canonicalization algorithm used to generate it, is termed the canonical SMILES.
These algorithms first convert the SMILES to an internal representation of the molecular structure. Various algorithms for generating canonical SMILES have been developed and include those by Daylight Chemical Information Systems, OpenEye Scientific Software, MEDIT, Chemical Computing Group, MolSoft LLC, the Chemistry Development Kit. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database; the original paper that described the CANGEN algorithm claimed to generate unique SMILES strings for graphs representing molecules, but the algorithm fails for a number of simple cases and cannot be considered a correct method for representing a graph canonically. There is no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, double bond geometry; these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES.
A notable feature of these rules is. The term isomeric SMILES is applied to SMILES in which isotopes are specified. In terms of a graph-based computational procedure, SMILES is a string obtained by printing the symbol nodes encountered in a depth-first tree traversal of a chemical graph; the chemical graph is first trimmed to remove hydrogen atoms and cycles are broken to turn it into a spanning tree. Where cycles have been broken, numeric suffix labels are included to indicate the connected nodes. Parentheses are used to indicate points of branching on the tree; the resultant SMILES form depends on the choices: of the bonds chosen to break cycles, of the starting atom used for the depth-first traversal, of the order in which branches are listed when encountered. Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, have no formal charge, have the number of hydrogens attached implied by the SMILES valence model, are the normal isotopes, are not chiral centers.
All other elements must be enclosed in brackets, have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or. Hydrogen may be written as a separate atom; when brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1 by the sign + for a positive charge or by - for a negative charge. For example, for ammonium. If there is more than one charge, it is written as digit.