Drug discoveryFrom Wikipedia, the free encyclopediaJump to: navigation, search

In the fields of medicine, biotechnology and pharmacology, drug discovery is the process by which new candidate medications are discovered.

Historically, drugs were discovered through identifying the active ingredient from traditional remedies or by serendipitous discovery. Later chemical libraries of synthetic small molecules, natural products or extracts were screened in intact cells or whole organisms to identify substances that have a desirable therapeutic effect in a process known as classical pharmacology. Since sequencing of the human genome which allowed rapid cloning and synthesis of large quantities of purified proteins, it has become common practice to use high throughput screening of large compounds libraries against isolated biological targets which are hypothesized to be disease modifying in a process known as reverse pharmacology. Hits from these screens are then tested in cells and then in animals for efficacy. Even more recently, scientists have been able to understand the shape of biological molecules at the atomic level, and to use that knowledge to design (see drug design) drug candidates.

Modern drug discovery involves the identification of screening hits, medicinal chemistry and optimization of those hits to increase the affinity, selectivity (to reduce the potential of side effects), efficacy/potency, metabolic stability (to increase the half-life), and oral bioavailability. Once a compound that fulfills all of these requirements has been identified, it will begin the process of drug development prior to clinical trials. One or more of these steps may, but not necessarily, involve computer-aided drug design.

Despite advances in technology and understanding of biological systems, drug discovery is still a lengthy, "expensive, difficult, and inefficient process" with low rate of new therapeutic discovery.[1] Currently, the research and development cost of each new molecular entity (NME) is approximately US$1.8 billion.[2]

Contents

[hide]

1 Drug targets 2 Screening and design 3 Historical background 4 Nature as source of drugs

o 4.1 Plant-derived o 4.2 Microbial metabolites o 4.3 Marine invertebrates

5 Chemical diversity of natural products 6 Natural product drug discovery

o 6.1 Screening o 6.2 Structural elucidation

7 See also 8 References 9 Further reading 10 External links

[edit] Drug targets

The definition of "target" itself is something argued within the pharmaceutical industry. Generally, the "target" is the naturally existing cellular or molecular structure involved in the pathology of interest that the drug-in-development is meant to act on. However, the distinction between a "new" and "established" target can be made without a full understanding of just what a "target" is. This distinction is typically made by pharmaceutical companies engaged in discovery and development of therapeutics. In an estimate from 2011, 435 human genome products were identified as therapeutic drug targets of FDA-approved drugs.[3]

"Established targets" are those for which there is a good scientific understanding, supported by a lengthy publication history, of both how the target functions in normal physiology and how it is involved in human pathology. This does not imply that the mechanism of action of drugs that are thought to act through a particular established targets is fully understood. Rather, "established" relates directly to the amount of background information available on a target, in particular functional information. The more such information is available, the less investment is (generally) required to develop a therapeutic directed against the target. The process of gathering such functional information is called "target validation" in pharmaceutical industry parlance. Established targets also include those that the pharmaceutical industry has had experience mounting drug discovery campaigns against in the past; such a history provides information on the chemical feasibility of developing a small molecular therapeutic against the target and can provide licensing opportunities and freedom-to-operate indicators with respect to small-molecule therapeutic candidates.

In general, "new targets" are all those targets that are not "established targets" but which have been or are the subject of drug discovery campaigns. These typically include newly discovered proteins, or proteins whose function has now become clear as a result of basic scientific research.

The majority of targets currently selected for drug discovery efforts are proteins. Two classes predominate: G-protein-coupled receptors (or GPCRs) and protein kinases.

[edit] Screening and design

The process of finding a new drug against a chosen target for a particular disease usually involves high-throughput screening (HTS), wherein large libraries of chemicals are tested for their ability to modify the target. For example, if the target is a novel GPCR, compounds will be

screened for their ability to inhibit or stimulate that receptor (see antagonist and agonist): if the target is a protein kinase, the chemicals will be tested for their ability to inhibit that kinase.

Another important function of HTS is to show how selective the compounds are for the chosen target. The ideal is to find a molecule which will interfere with only the chosen target, but not other, related targets. To this end, other screening runs will be made to see whether the "hits" against the chosen target will interfere with other related targets - this is the process of cross-screening. Cross-screening is important, because the more unrelated targets a compound hits, the more likely that off-target toxicity will occur with that compound once it reaches the clinic.

It is very unlikely that a perfect drug candidate will emerge from these early screening runs. It is more often observed that several compounds are found to have some degree of activity, and if these compounds share common chemical features, one or more pharmacophores can then be developed. At this point, medicinal chemists will attempt to use structure-activity relationships (SAR) to improve certain features of the lead compound:

increase activity against the chosen target reduce activity against unrelated targets improve the druglikeness or ADME properties of the molecule.

This process will require several iterative screening runs, during which, it is hoped, the properties of the new molecular entities will improve, and allow the favoured compounds to go forward to in vitro and in vivo testing for activity in the disease model of choice.Amongst the physico-chemical properties associated with drug absorption include ionization (pKa), and solubility; permeability can be determined by PAMPA and Caco-2. PAMPA is attractive as an early screen due to the low consumption of drug and the low cost compared to tests such as Caco-2, gastrointestinal tract (GIT) and Blood–brain barrier (BBB) with which there is a high correlation.

A range of parameters can be used to assess the quality of a compound, or a series of compounds, as proposed in the Lipinski's Rule of Five. Such parameters include calculated properties such as cLogP to estimate lipophilicity, molecular weight, polar surface area and measured properties, such as potency, in-vitro measurement of enzymatic clearance etc. Some descriptors such as ligand efficiency [4] (LE) and lipophilic efficiency [5] [6] (LiPE) combine such parameters to assess druglikeness.

While HTS is a commonly used method for novel drug discovery, it is not the only method. It is often possible to start from a molecule which already has some of the desired properties. Such a molecule might be extracted from a natural product or even be a drug on the market which could be improved upon (so-called "me too" drugs). Other methods, such as virtual high throughput screening, where screening is done using computer-generated models and attempting to "dock" virtual libraries to a target, are also often used.

Another important method for drug discovery is drug design, whereby the biological and physical properties of the target are studied, and a prediction is made of the sorts of chemicals that might (e.g.) fit into an active site. One example is fragment-based lead discovery (FBLD).

Novel pharmacophores can emerge very rapidly from these exercises. In general, computer-aided drug design is often but not always used to try to improve the potency and properties of new drug leads.

Once a lead compound series has been established with sufficient target potency and selectivity and favourable drug-like properties, one or two compounds will then be proposed for drug development. The best of these is generally called the lead compound, while the other will be designated as the "backup".

[edit] Historical background

The idea that effect of drug in human body are mediated by specific interactions of the drug molecule with biological macromolecules, (proteins or nucleic acids in most cases) led scientists to the conclusion that individual chemicals are required for the biological activity of the drug. This made for the beginning of the modern era in pharmacology, as pure chemicals, instead of crude extracts, became the standard drugs. Examples of drug compounds isolated from crude preparations are morphine, the active agent in opium, and digoxin, a heart stimulant originating from Digitalis lanata. Organic chemistry also led to the synthesis of many of the cochemicals isolated from biological sources.

[edit] Nature as source of drugs

Despite the rise of combinatorial chemistry as an integral part of lead discovery process, natural products still play a major role as starting material for drug discovery.[7] A report was published in 2007,[8] covering years 1981-2006 details the contribution of biologically occurring chemicals in drug development. According to this report, of the 974 small molecule new chemical entities, 63% were natural derived or semisynthetic derivatives of natural products. For certain therapy areas, such as antimicrobials, antineoplastics, antihypertensive and anti-inflammatory drugs, the numbers were higher. In many cases, these products have been used traditionally for many years.

Natural products may be useful as a source of novel chemical structures for modern techniques of development of antibacterial therapies.[9]

Despite the implied potential, only a fraction of Earth’s living species has been tested for bioactivity.

[edit] Plant-derived

Prior to Paracelsus, the vast majority of traditionally used crude drugs in Western medicine were plant-derived extracts. This has resulted in a pool of information about the potential of plant species as an important source of starting material for drug discovery. A different set of metabolites is sometimes produced in the different anatomical parts of the plant (e.g. root, leaves and flower), and botanical knowledge is crucial also for the correct identification of bioactive plant materials.

[edit] Microbial metabolites

Main article: Medicinal molds

Microbes compete for living space and nutrients. To survive in these conditions, many microbes have developed abilities to prevent competing species from proliferating. Microbes are the main source of antimicrobial drugs. Streptomyces species have been a valuable source of antibiotics. The classical example of an antibiotic discovered as a defense mechanism against another microbe is the discovery of penicillin in bacterial cultures contaminated by Penicillium fungi in 1928.

[edit] Marine invertebrates

Marine environments are potential sources for new bioactive agents.[10] Arabinose nucleosides discovered from marine invertebrates in 1950s, demonstrating for the first time that sugar moieties other than ribose and deoxyribose can yield bioactive nucleoside structures. However, it was 2004 when the first marine-derived drug was approved. The cone snail toxin ziconotide, also known as Prialt, was approved by the Food and Drug Administration to treat severe neuropathic pain. Several other marine-derived agents are now in clinical trials for indications such as cancer, anti-inflammatory use and pain. One class of these agents are bryostatin-like compounds,under investigation as anti-cancer therapy.

[edit] Chemical diversity of natural products

As above mentioned, combinatorial chemistry was a key technology enabling the efficient generation of large screening libraries for the needs of high-throughput screening. However, now, after two decades of combinatorial chemistry, it has been pointed out that despite the increased efficiency in chemical synthesis, no increase in lead or drug candidates has been reached.[8] This has led to analysis of chemical characteristics of combinatorial chemistry products, compared to existing drugs or natural products. The chemoinformatics concept chemical diversity, depicted as distribution of compounds in the chemical space based on their physicochemical characteristics, is often used to describe the difference between the combinatorial chemistry libraries and natural products. The synthetic, combinatorial library compounds seem to cover only a limited and quite uniform chemical space, whereas existing drugs and particularly natural products, exhibit much greater chemical diversity, distributing more evenly to the chemical space.[7] The most prominent differences between natural products and compounds in combinatorial chemistry libraries is the number of chiral centers (much higher in natural compounds), structure rigidity (higher in natural compounds) and number of aromatic moieties (higher in combinatorial chemistry libraries). Other chemical differences between these two groups include the nature of heteroatoms (O and N enriched in natural products, and S and halogen atoms more often present in synthetic compounds), as well as level of non-aromatic unsaturation (higher in natural products). As both structure rigidity and chirality are both well-established factors in medicinal chemistry known to enhance compounds specificity and efficacy as a drug, it has been suggested that natural products compare favourable to today's combinatorial chemistry libraries as potential lead molecules.

[edit] Natural product drug discovery

[edit] Screening

Two main approaches exist for the finding of new bioactive chemical entities from natural sources.

The first is sometimes referred to as random collection and screening of material, but in fact the collection is often far from random in that biological (often botanical) knowledge is used about which families show promise, based on a number of factors, including past screening. This approach is based on the fact that only a small part of earth’s biodiversity has ever been tested for pharmaceutical activity. It is also based on the fact that organisms living in a species-rich environment need to evolve defensive and competitive mechanisms to survive, mechanisms which might usefully be exploited in the development of drugs that can cure diseases affecting humans. A collection of plant, animal and microbial samples from rich ecosystems can potentially give rise to novel biological activities worth exploiting in the drug development process. One example of a successful use of this strategy is the screening for antitumour agents by the National Cancer Institute, started in the 1960s. Paclitaxel was identified from Pacific yew tree Taxus brevifolia. Paclitaxel showed anti-tumour activity by a previously undescribed mechanism (stabilization of microtubules) and is now approved for clinical use for the treatment of lung, breast and ovarian cancer, as well as for Kaposi's sarcoma. Early in the 21st century, Cabazitaxel (made by Sanofi, a French firm), another relative of taxol has been shown effective against prostate cancer, also because it works by preventing the formation of microtubules, which pull the chromosomes apart in dividing cells (such as cancer cells). Still another examples are: 1. Camptotheca (Camptothecin · Topotecan · Irinotecan · Rubitecan · Belotecan); 2. Podophyllum (Etoposide · Teniposide); 3a. Anthracyclines (Aclarubicin · Daunorubicin · Doxorubicin · Epirubicin · Idarubicin · Amrubicin · Pirarubicin · Valrubicin · Zorubicin); 3b. Anthracenediones (Mitoxantrone · Pixantrone).

Nor do all drugs developed in this manner come from plants. Professor Louise Rollins-Smith of Vanderbilt University's Medical Center, for example, has developed from the skin of frogs a compound which blocks AIDS. Professor Rollins-Smith is aware of declining amphibian populations and has said: "We need to protect these species long enough for us to understand their medicinal cabinet."

The second main approach involves Ethnobotany, the study of the general use of plants in society, and ethnopharmacology, an area inside ethnobotany, which is focused specifically on medicinal uses.

Both of these two main approaches can be used in selecting starting materials for future drugs. Artemisinin, an antimalarial agent from sweet wormtree Artemisia annua, used in Chinese medicine since 200BC is one drug used as part of combination therapy for multiresistant Plasmodium falciparum.

[edit] Structural elucidation

The elucidation of the chemical structure is critical to avoid the re-discovery of a chemical agent that is already known for its structure and chemical activity. Mass spectrometry, often used to determine structure, is a method in which individual compounds are identified based on their mass/charge ratio, after ionization. Chemical compounds exist in nature as mixtures, so the combination of liquid chromatography and mass spectrometry (LC-MS) is often used to separate the individual chemicals. Databases of mass spectras for known compounds are available. Nuclear magnetic resonance spectroscopy is another important technique for determining chemical structures of natural products. NMR yields information about individual hydrogen and carbon atoms in the structure, allowing detailed reconstruction of the molecule’s architecture.

Drug developmentFrom Wikipedia, the free encyclopediaJump to: navigation, search"Drug research" redirects here. For the journal, see Drug Research (journal).

The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (February 2013)

Drug development is a blanket term used to define the process of bringing a new drug to the market once a lead compound has been identified through the process of drug discovery. It includes pre-clinical research (microorganisms/animals) and clinical trials (on humans) and may include the step of obtaining regulatory approval to market the drug.

Contents

[hide]

1 New chemical entity development o 1.1 Pre-clinical o 1.2 Clinical phase

2 Cost 3 Success rate 4 Novel initiatives to boost drug development 5 See also 6 References 7 External links

[edit] New chemical entity development

Regulation of therapeutic goods in the

United States

Prescription drugs

Over-the-counter drugs

Law[show]

Government agencies[show]

Process[show]

International coordination[show]

Non-governmental organizations[show]

v

t

e

Broadly, the process of drug development can be divided into pre-clinical and clinical work.

[edit] Pre-clinical

New chemical entities (NCEs, also known as new molecular entities or NMEs) are compounds which emerge from the process of drug discovery. These will have promising activity against a particular biological target thought to be important in disease; however, little will be known about the safety, toxicity, pharmacokinetics and metabolism of this NCE in humans. It is the function of drug development to assess all of these parameters prior to human clinical trials. A further major objective of drug development is to make a recommendation of the dose and schedule to be used the first time an NCE is used in a human clinical trial ("first-in-man" [FIM] or First Human Dose [FHD]).

In addition, drug development is required to establish the physicochemical properties of the NCE: its chemical makeup, stability, solubility. The process by which the chemical is made will be optimized so that from being made at the bench on a milligram scale by a medicinal chemist, it can be manufactured on the kilogram and then on the ton scale. It will be further examined for its suitability to be made into capsules, tablets, aeresol, intramuscular injectable, subcuteneous injectable, or intravenous formulations. Together these processes are known in preclinical development as Chemistry, Manufacturing and Control (CMC).

Many aspects of drug development are focused on satisfying the regulatory requirements of drug licensing authorities. These generally constitute a number of tests designed to determine the major toxicities of a novel compound prior to first use in man. It is a legal requirement that an assessment of major organ toxicity be performed (effects on the heart and lungs, brain, kidney, liver and digestive system), as well as effects on other parts of the body that might be affected by the drug (e.g. the skin if the new drug is to be delivered through the skin). While, increasingly, these tests can be made using in vitro methods (e.g. with isolated cells), many tests can only be made by using experimental animals, since it is only in an intact organism that the complex interplay of metabolism and drug exposure on toxicity can be examined.

The information gathered from this pre-clinical testing, as well as information on CMC, and is submitted to regulatory authorities (in the US, to the FDA), as an Investigational New Drug application or IND. If the IND is approved, development moves to the clinical phase.

[edit] Clinical phase

Clinical trials involves three steps:

Phase I trials, usually in healthy volunteers, determine safety and dosing. Phase II trials are used to get an initial reading of efficacy and further explore safety in

small numbers of sick patients. Phase III trials are large, pivotal trials to determine safety and efficacy in sufficiently

large numbers of patients.

The process of drug development does not stop once an NCE begins human clinical trials. In addition to the tests required to move a novel drug into the clinic for the first time it is also important to ensure that long-term or chronic toxicities are determined, as well as effects on systems not previously monitored (fertility, reproduction, immune system, etc.). The compound will also be tested for its capability to cause cancer (carcinogenicity testing).

If a compound emerges from these tests with an acceptable toxicity and safety profile, and it can further be demonstrated to have the desired effect in clinical trials, then it can be submitted for marketing approval in the various countries where it will be sold. In the US, this process is called a New Drug Application or NDA. Most NCEs, however, fail during drug development, either because they have some unacceptable toxicity, or because they simply do not work in clinical trials.

[edit] Cost

The full cost of bringing a new drug (i.e. a drug that is a new chemical entity) to market - from discovery through clinical trials to approval - is complex and controversial. One element of the complexity is that the much-publicized final numbers often do not include just the simple out-of-pocket expenses, but also include "capital costs", which are included to take into account the long time period (often at least ten years) during which the out-of-pocket costs are expended; additionally it is often not stated whether a given figure includes the capitalized cost or comprises only out-of-pocket expenses. Another element of complexity is that all estimates are based on confidential information owned by drug companies, released by them voluntarily. There is currently no way to validate these numbers. The numbers are controversial, as drug companies use them to justify the prices of their drugs and various advocates for lower drug prices have challenged them. The controversy is not only between "high" and "low" -- the numbers also vary greatly at the high end.

A study published by Steve Paul et al. in 2010 in Nature Reviews: Drug Discovery compares many of the studies, provides both capitalized and out-of-pocket costs for each, and lays out the assumptions each makes: see Supplemental Box 2.[1] The authors offer their own estimate of the capitalized cost as being ~$1.8B, with out-of-pocket costs of ~$870M.

Studies published by diMasi et al. in 2003, report an average pre-tax, capitalized cost of approximately $800 million to bring one of the drugs from the study to market. Also, this $800 million dollar figure includes opportunity costs of $400 million.[2] A study published in 2006 estimates that costs vary from around $500 million to $2 billion depending on the therapy or the developing firm.[3] A study published in 2010 in the journal Health Economics, including an author from the US Federal Trade Commission, was critical of the methods used by diMasi et al. but came up with a higher estimate of ~$1.2 billion.[4]

[edit] Success rate

Candidates for a new drug to treat a disease might theoretically include from 5,000 to 10,000 chemical compounds. On average about 250 of these will show sufficient promise for further evaluation using laboratory tests, mice and other test animals. Typically, about ten of these will qualify for tests on humans.[5] A study conducted by the Tufts Center for the Study of Drug Development covering the 1980s and 1990s found that only 21.5 percent of drugs that start phase I trials are eventually approved for marketing.[6] The high failure rates associated with pharmaceutical development are referred to as the "attrition rate" problem. Careful decision making during drug development is essential to avoid costly failures.[7] In many cases, intelligent programme and clinical trial design can prevent false negative results. Well designed dose-finding studies and comparisons against both a placebo and a gold-standard treatment arm play a major role in achieving reliable data.[8]

[edit] Novel initiatives to boost drug development

Novel initiaives include partnering between governmental organisations and industry. The worlds largest such initiative is the Innovative Medicines Initiative (IMI), and examples of major national initiatives are Top Institute Pharma in the Netherlands and Biopeople in Denmark.

Drug designFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

Not to be confused with Designer drug.

Drug design, sometimes referred to as rational drug design or more simply rational design, is the inventive process of finding new medications based on the knowledge of a biological target.[1] The drug is most commonly an organic small molecule that activates or inhibits the function of a biomolecule such as a protein, which in turn results in a therapeutic benefit to the patient. In the most basic sense, drug design involves the design of small molecules that are complementary in shape and charge to the biomolecular target with which they interact and therefore will bind to it. Drug design frequently but not necessarily relies on computer modeling techniques.[2] This type of modeling is often referred to as computer-aided drug design. Finally, drug design that relies on the knowledge of the three-dimensional structure of the biomolecular target is known as structure-based drug design.

The phrase "drug design" is to some extent a misnomer. What is really meant by drug design is ligand design (i.e., design of a small molecule that will bind tightly to its target).[3] Although modeling techniques for prediction of binding affinity are reasonably successful, there are many other properties, such as bioavailability, metabolic half-life, lack of side effects, etc., that first must be optimized before a ligand can become a safe and efficacious drug. These other characteristics are often difficult to optimize using rational drug design techniques.

Contents

[hide]

1 Background 2 Types

o 2.1 Ligand-based o 2.2 Structure-based

2.2.1 Active site identification 2.2.2 Ligand fragment link 2.2.3 Scoring method

3 Rational drug discovery 4 Computer-aided drug design 5 Examples 6 See also 7 References 8 External links

[edit] Background

Typically a drug target is a key molecule involved in a particular metabolic or signaling pathway that is specific to a disease condition or pathology or to the infectivity or survival of a microbial pathogen. Some approaches attempt to inhibit the functioning of the pathway in the diseased state by causing a key molecule to stop functioning. Drugs may be designed that bind to the active region and inhibit this key molecule. Another approach may be to enhance the normal pathway by promoting specific molecules in the normal pathways that may have been affected in the diseased state. In addition, these drugs should also be designed so as not to affect any other important "off-target" molecules or antitargets that may be similar in appearance to the target molecule, since drug interactions with off-target molecules may lead to undesirable side effects. Sequence homology is often used to identify such risks.

Most commonly, drugs are organic small molecules produced through chemical synthesis, but biopolymer-based drugs (also known as biologics) produced through biological processes are becoming increasingly more common. In addition, mRNA-based gene silencing technologies may have therapeutic applications.

[edit] Types

Flow charts of two strategies of structure-based drug design

There are two major types of drug design. The first is referred to as ligand-based drug design and the second, structure-based drug design.

[edit] Ligand-based

Ligand-based drug design (or indirect drug design) relies on knowledge of other molecules that bind to the biological target of interest. These other molecules may be used to derive a pharmacophore model that defines the minimum necessary structural characteristics a molecule must possess in order to bind to the target.[4] In other words, a model of the biological target may be built based on the knowledge of what binds to it, and this model in turn may be used to design new molecular entities that interact with the target. Alternatively, a quantitative structure-activity relationship (QSAR), in which a correlation between calculated properties of molecules and their experimentally determined biological activity, may be derived. These QSAR relationships in turn may be used to predict the activity of new analogs.

[edit] Structure-based

Structure-based drug design (or direct drug design) relies on knowledge of the three dimensional structure of the biological target obtained through methods such as x-ray crystallography or NMR spectroscopy.[5] If an experimental structure of a target is not available, it may be possible to create a homology model of the target based on the experimental structure of a related protein. Using the structure of the biological target, candidate drugs that are predicted to bind with high affinity and selectivity to the target may be designed using interactive graphics and the intuition of a medicinal chemist. Alternatively various automated computational procedures may be used to suggest new drug candidates.

As experimental methods such as X-ray crystallography and NMR develop, the amount of information concerning 3D structures of biomolecular targets has increased dramatically. In parallel, information about the structural dynamics and electronic properties about ligands has also increased. This has encouraged the rapid development of the structure-based drug design. Current methods for structure-based drug design can be divided roughly into two categories. The first category is about “finding” ligands for a given receptor, which is usually referred as database searching. In this case, a large number of potential ligand molecules are screened to find those fitting the binding pocket of the receptor. This method is usually referred as ligand-based drug design. The key advantage of database searching is that it saves synthetic effort to obtain new lead compounds. Another category of structure-based drug design methods is about “building” ligands, which is usually referred as receptor-based drug design. In this case, ligand molecules are built up within the constraints of the binding pocket by assembling small pieces in a stepwise manner. These pieces can be either individual atoms or molecular fragments. The key advantage of such a method is that novel structures, not contained in any database, can be suggested.[6][7][8]

[edit] Active site identification

Active site identification is the first step in this program. It analyzes the protein to find the binding pocket, derives key interaction sites within the binding pocket, and then prepares the

necessary data for Ligand fragment link. The basic inputs for this step are the 3D structure of the protein and a pre-docked ligand in PDB format, as well as their atomic properties. Both ligand and protein atoms need to be classified and their atomic properties should be defined, basically, into four atomic types:

hydrophobic atom: All carbons in hydrocarbon chains or in aromatic groups. H-bond donor: Oxygen and nitrogen atoms bonded to hydrogen atom(s). H-bond acceptor: Oxygen and sp2 or sp hybridized nitrogen atoms with lone electron pair(s). Polar atom: Oxygen and nitrogen atoms that are neither H-bond donor nor H-bond acceptor,

sulfur, phosphorus, halogen, metal, and carbon atoms bonded to hetero-atom(s).

The space inside the ligand binding region would be studied with virtual probe atoms of the four types above so the chemical environment of all spots in the ligand binding region can be known. Hence we are clear what kind of chemical fragments can be put into their corresponding spots in the ligand binding region of the receptor.

[edit] Ligand fragment link

Flow chart for structure-based drug design

When we want to plant “seeds” into different regions defined by the previous section, we need a fragments database to choose fragments from. The term “fragment” is used here to describe the

building blocks used in the construction process. The rationale of this algorithm lies in the fact that organic structures can be decomposed into basic chemical fragments. Although the diversity of organic structures is infinite, the number of basic fragments is rather limited.

Before the first fragment, i.e. the seed, is put into the binding pocket, and other fragments can be added one by one, it is useful to identify potential problems. First, the possibility for the fragment combinations is huge. A small perturbation of the previous fragment conformation would cause great difference in the following construction process. At the same time, in order to find the lowest binding energy on the Potential energy surface (PES) between planted fragments and receptor pocket, the scoring function calculation would be done for every step of conformation change of the fragments derived from every type of possible fragments combination. Since this requires a large amount of computation, one may think using other possible strategies to let the program works more efficiently. When a ligand is inserted into the pocket site of a receptor, conformation favor for these groups on the ligand that can bind tightly with receptor should be taken priority. Therefore it allows us to put several seeds at the same time into the regions that have significant interactions with the seeds and adjust their favorite conformation first, and then connect those seeds into a continuous ligand in a manner that make the rest part of the ligand having the lowest energy. The conformations of the pre-placed seeds ensuring the binding affinity decide the manner that ligand would be grown. This strategy reduces calculation burden for the fragment construction efficiently. On the other hand, it reduces the possibility of the combination of fragments, which reduces the number of possible ligands that can be derived from the program. These two strategies above are well used in most structure-based drug design programs. They are described as “Grow” and “Link”. The two strategies are always combined in order to make the construction result more reliable.[6][7][9]

[edit] Scoring methodMain article: Scoring functions for docking

Structure-based drug design attempts to use the structure of proteins as a basis for designing new ligands by applying accepted principles of molecular recognition. The basic assumption underlying structure-based drug design is that a good ligand molecule should bind tightly to its target. Thus, one of the most important principles for designing or obtaining potential new ligands is to predict the binding affinity of a certain ligand to its target and use it as a criterion for selection.

One early method was developed by Böhm[10] to develop a general-purposed empirical scoring function in order to describe the binding energy. The following “Master Equation” was derived:

where:

desolvation – enthalpic penalty for removing the ligand from solvent motion – entropic penalty for reducing the degrees of freedom when a ligand binds to its

receptor configuration – conformational strain energy required to put the ligand in its "active"

conformation interaction – enthalpic gain for "resolvating" the ligand with its receptor

The basic idea is that the overall binding free energy can be decomposed into independent components that are known to be important for the binding process. Each component reflects a certain kind of free energy alteration during the binding process between a ligand and its target receptor. The Master Equation is the linear combination of these components. According to Gibbs free energy equation, the relation between dissociation equilibrium constant, Kd, and the components of free energy was built.

Various computational methods are used to estimate each of the components of the master equation. For example, the change in polar surface area upon ligand binding can be used to estimate the desolvation energy. The number of rotatable bonds frozen upon ligand binding is proportional to the motion term. The configurational or strain energy can be estimated using molecular mechanics calculations. Finally the interaction energy can be estimated using methods such as the change in non polar surface, statistically derived potentials of mean force, the number of hydrogen bonds formed, etc. In practice, the components of the master equation are fit to experimental data using multiple linear regression. This can be done with a diverse training set including many types of ligands and receptors to produce a less accurate but more general "global" model or a more restricted set of ligands and receptors to produce a more accurate but less general "local" model.[11][12][13]

[edit] Rational drug discovery

In contrast to traditional methods of drug discovery, which rely on trial-and-error testing of chemical substances on cultured cells or animals, and matching the apparent effects to treatments, rational drug design begins with a hypothesis that modulation of a specific biological target may have therapeutic value. In order for a biomolecule to be selected as a drug target, two essential pieces of information are required. The first is evidence that modulation of the target will have therapeutic value. This knowledge may come from, for example, disease linkage studies that show an association between mutations in the biological target and certain disease states. The second is that the target is "drugable". This means that it is capable of binding to a small molecule and that its activity can be modulated by the small molecule.

Once a suitable target has been identified, the target is normally cloned and expressed. The expressed target is then used to establish a screening assay. In addition, the three-dimensional structure of the target may be determined.

The search for small molecules that bind to the target is begun by screening libraries of potential drug compounds. This may be done by using the screening assay (a "wet screen"). In addition, if the structure of the target is available, a virtual screen may be performed of candidate drugs. Ideally the candidate drug compounds should be "drug-like", that is they should possess

properties that are predicted to lead to oral bioavailability, adequate chemical and metabolic stability, and minimal toxic effects. Several methods are available to estimate druglikeness such as Lipinski's Rule of Five and a range of scoring methods such as Lipophilic efficiency. Several methods for predicting drug metabolism have been proposed in the scientific literature, and a recent example is SPORCalc.[14] Due to the complexity of the drug design process, two terms of interest are still serendipity and bounded rationality. Those challenges are caused by the large chemical space describing potential new drugs without side-effects.

[edit] Computer-aided drug design

Computer-aided drug design uses computational chemistry to discover, enhance, or study drugs and related biologically active molecules. The most fundamental goal is to predict whether a given molecule will bind to a target and if so how strongly. Molecular mechanics or molecular dynamics are most often used to predict the conformation of the small molecule and to model conformational changes in the biological target that may occur when the small molecule binds to it. Semi-empirical, ab initio quantum chemistry methods, or density functional theory are often used to provide optimized parameters for the molecular mechanics calculations and also provide an estimate of the electronic properties (electrostatic potential, polarizability, etc.) of the drug candidate that will influence binding affinity.

Molecular mechanics methods may also be used to provide semi-quantitative prediction of the binding affinity. Also, knowledge-based scoring function may be used to provide binding affinity estimates. These methods use linear regression, machine learning, neural nets or other statistical techniques to derive predictive binding affinity equations by fitting experimental affinities to computationally derived interaction energies between the small molecule and the target.[15][16]

Ideally the computational method should be able to predict affinity before a compound is synthesized and hence in theory only one compound needs to be synthesized. The reality however is that present computational methods are imperfect and provide at best only qualitatively accurate estimates of affinity. Therefore in practice it still takes several iterations of design, synthesis, and testing before an optimal molecule is discovered. On the other hand, computational methods have accelerated discovery by reducing the number of iterations required and in addition have often provided more novel small molecule structures.

Drug design with the help of computers may be used at any of the following stages of drug discovery:

1. hit identification using virtual screening (structure- or ligand-based design)2. hit-to-lead optimization of affinity and selectivity (structure-based design, QSAR, etc.)3. lead optimization optimization of other pharmaceutical properties while maintaining affinity

Flowchart of a Usual Clustering Analysis for Structure-Based Drug Design

In order to overcome the insufficient prediction of binding affinity calculated by recent scoring functions, the protein-ligand interaction and compound 3D structure information are used to analysis. For structure-based drug design, several post-screening analysis focusing on protein-ligand interaction has been developed for improving enrichment and effectively mining potential candidates:

Consensus scoring[17][18]

o Selecting candidates by voting of multiple scoring functionso May lose the relationship between protein-ligand structural information and scoring

criterion Geometric analysis

o Comparing protein-ligand interactions by visually inspecting individual structureso Becoming intractable when the number of complexes to be analyzed increasing

Cluster analysis[19][20]

o Represent and cluster candidates according to protein-ligand 3D informationo Needs meaningful representation of protein-ligand interactions.

[edit] Examples

A particular example of rational drug design involves the use of three-dimensional information about biomolecules obtained from such techniques as X-ray crystallography and NMR spectroscopy. Computer-aided drug design in particular becomes much more tractable when there's a high-resolution structure of a target protein bound to a potent ligand. This approach to drug discovery is sometimes referred to as structure-based drug design. The first unequivocal example of the application of structure-based drug design leading to an approved drug is the carbonic anhydrase inhibitor dorzolamide, which was approved in 1995.[21][22]

Another important case study in rational drug design is imatinib, a tyrosine kinase inhibitor designed specifically for the bcr-abl fusion protein that is characteristic for Philadelphia chromosome-positive leukemias (chronic myelogenous leukemia and occasionally acute lymphocytic leukemia). Imatinib is substantially different from previous drugs for cancer, as most agents of chemotherapy simply target rapidly dividing cells, not differentiating between cancer cells and other tissues.

Additional examples include:

Many of the atypical antipsychotics Cimetidine , the prototypical H2-receptor antagonist from which the later members of the class

were developed Selective COX-2 inhibitor NSAIDs Dorzolamide , a carbonic anhydrase inhibitor used to treat glaucoma Enfuvirtide , a peptide HIV entry inhibitor Nonbenzodiazepines like zolpidem and zopiclone Probenecid SSRIs (selective serotonin reuptake inhibitors), a class of antidepressants Zanamivir , an antiviral drug Isentress , HIV Integrase inhibitor[23]

Case studies

5-HT3 antagonists Acetylcholine receptor agonists Angiotensin receptor blockers Bcr-Abl tyrosine kinase inhibitors Cannabinoid receptor antagonists CCR5 receptor antagonists Cyclooxygenase 2 inhibitors Dipeptidyl peptidase-4 inhibitors HIV protease inhibitors NK1 receptor antagonists Non-nucleoside reverse transcriptase inhibitors Proton pump inibitors Triptans TRPV1 antagonists Renin inhibitors c-Met inhibitors

Oral rehydration therapyFrom Wikipedia, the free encyclopediaJump to: navigation, search

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Nurses encouraging a patient to drink an oral rehydration solution to combat dehydration caused by cholera.

Oral Rehydration Therapy (ORT) is a simple treatment for dehydration associated with diarrhea, particularly gastroenteritis or gastroenteropathy, such as that caused by cholera or rotavirus. ORT consists of a solution of salts and sugars that is taken by mouth. It is used around the world, most significantly in the developing world, where it saves millions of children a year from death due to diarrhea, the second leading cause of death (after pneumonia) in children under five.[1]

Contents

[hide]

1 History 2 Availability 3 Definition

o 3.1 WHO/UNICEF definition of ORS o 3.2 Basic solution o 3.3 Switch to reduced osmolarity ORS

4 Administration o 4.1 Food and supplements o 4.2 Treatment when malnourished

5 Physiological basis 6 References

[edit] History

Prescriptions from the ancient Indian physician Sushruta date back over 2500 years with treatment of acute diarrhea with rice water, coconut juice, and carrot soup.[2] However, this knowledge did not carry over to the Western world, as dehydration was found to be the major cause of death secondary to the 1829 cholera pandemic in Russia and Western Europe. In 1831, William Brooke O'Shaughnessy noted the loss of water and salt in the stool of cholera patients and prescribed intravenous fluid therapy (IV) to compensate. The results were remarkable, as patients who were on the brink of death from dehydration recovered. The mortality rate of cholera dropped from 70 percent to 40 percent with the use of hypertonic IV solutions.[3] IV fluid replacement became entrenched as the standard of care for moderate/severe dehydration for over a hundred years. ORT replaced it with the support of several independent key advocates that ultimately convinced the medical community of the efficacy of ORT.[4]

In the late 1950s, ORT was prescribed by Dr. Hemendra Nath Chatterjee in India for patients diagnosed with cholera.[5] Although his findings predate physiological studies, his results failed to gain credibility and recognition because they did not provide scientific controls and detailed analysis.[4] Credit for discovery that in the presence of glucose, sodium and chloride became absorbable during diarrhea (in cholera patients) is typically ascribed to Dr. Robert A. Phillips. However, early attempts to translate this observation into an effective oral rehydration solution failed, due to incorrect solution formula and inadequate methodology.[4]

In the early 1960s, biochemist Robert K. Crane discovered the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.[6] Around the same time, others showed that the intestinal mucosa was not disrupted in cholera, as previously thought. These findings were confirmed in human experiments, where it was first shown that a glucose-saline oral therapy solution administered in quantities matching measured diarrhea volumes was effective in significantly decreasing the necessity for IV fluids by 80 percent.[7] These results helped establish the physiological basis for the use of ORT in clinical medicine.[3]

The events surrounding the Bangladesh Liberation War in 1971 convinced the world of the effectiveness of ORT.[4] As medical teams ran out of intravenous fluids to treat the spreading cholera epidemic, Dr. Dilip Mahalanabis instructed his staff to distribute oral rehydration salts (ORS) to the 350,000 people in refugee camps. Over 3,000 patients with cholera were treated, and the death rate was only 3.6 percent, compared with the typical 30 percent seen in intravenous fluid therapy.[3] The fact that ORT was delivered primarily by family members instead of trained staff across such a large population in an emergency fashion was demonstrative proof of the utility of ORT against cholera.[4]

Between 1980 and 2006, ORT decreased the number of deaths that occurred worldwide from 5 million a year to 3 million a year.[8] Death from diarrhea was the leading cause of infant mortality in the developing world until ORT was introduced.[9] It is now the second leading cause of mortality for children under five, accounting for 17 percent of all deaths, second only to pneumonia, at 19 percent.[1] Its remarkable success has led to naming the discovery of its underlying physiological basis as "potentially the most important medical advance [of the 20th] century."[9] ORT is part of UNICEF's GOBI program, a low cost program to increase child survival in developing countries, including growth monitoring, ORT, breastfeeding, and immunization.[10] Despite the success and effectiveness of ORT, its uptake has recently slowed

and even reversed in some developing countries. This raises concerns for increased mortality from diarrhea and highlights the need for effective community-level behavioral change and global funding and policy.[11]

The individuals and organizations involved in the development of ORT have been recognized widely. The 2001 Gates Award for Global Health was awarded to the Centre for Health and Population Research, located in Dhaka, Bangladesh, for its role in the development of ORT.[12] In 2002, the first Pollin Prize for Pediatric Research was awarded to Dr. Norbert Hirschhorn, Dr. Dilip Mahalanabis, Dr. David Nalin, and Dr. Nathaniel F. Pierce for their contributions in the discovery and implementation of ORT.[13] For promoting the use of ORT, the 2006 Prince Mahidol Award was awarded to Dr. Richard A. Cash of Harvard School of Public Health, Dr. David Nalin, and Dr. Dilip Mahalanabis in the field of public health; and to Dr. Stanley G Schultz in the field of medicine.[14]

[edit] Availability

The World Health Organizations states that some home products can be used to treat and prevent dehydration. This includes salted rice water, salted yogurt drink, and salted vegetable or chicken soup. A home-made solution of one liter of plain water with 3 grams table salt (one level teaspoonful) and 18 grams common sugar (three tablespoons) can also be made. And a medium amount of salt can also be added to water in which cereal has been cooked, unsalted soup, green coconut water, unsweetened weak tea, and unsweetened fruit juice.[15] The homemade solution should have the "taste of tears."[16] If available, supplemental zinc and potassium can be added to or given with the homemade solution.[15]

ORT is available anywhere that adequate nutrition is available. ORS, on the other hand, is typically packaged in pre-measured sachets that are ready to be mixed in with water (generally 1L). These are available via commercial manufacturers[17] or supplied by local/regional governments or relief agencies such as UNICEF. In 1996, UNICEF distributed 500 million sachets of ORS to over 60 developing nations.[18] Commercial suppliers produce a variety of formulations, and there is no restriction as to what formulation can be marketed as ORS. As such, some vendors include extra sugar or other flavoring to make the product more palatable, popular examples in the US being the various flavors and formulations of Drip Drop ORS,Frutolyte and Pedialyte.

Scheduled to start in October 2011,[dated info] there is a pilot program in Zambia that will test to what extent piggybacking ORS and zinc on Coca-Cola's distribution channels could improve access to these medicines, particularly in rural areas. The project, called Colalife uses the empty spaces in crated bottles of Coke to supply wedge-shaped packets of medicine that contain oral rehydration salts.[19]

[edit] Definition

The definition of ORT has changed over time, broadening in scope and encompassing a definition of a specific therapy appropriate for rehydration. Initially, in the early 1980s, ORT was

defined only as the solution prescribed by the WHO/UNICEF. It was later changed in 1988 to encompass recommended home fluids, because the official preparation was not always readily available. It was amended once again in 1988 to include continued feeding as appropriate management. In 1991, the definition was changed to define ORT as any increase in administered hydrational fluids. The final change came in 1993, and is the definition used today, which states that ORT is an increase in administered fluids and continued feeding.[20][21]

[edit] WHO/UNICEF definition of ORS

updated recipe http://apps.who.int/medicinedocs/en/d/Js4950e/2.4.html#Js4950e.2.4

[edit] Basic solution

A basic oral rehydration therapy solution is comprised primarily of salt, sugar, and water using a standard ratio[21][23][24] - (e.g.

30 ml sugar: 2.5 ml salt : 1 liter water 2 tbl. sugar : 0.5 tsp. salt : 1 quart water

The WHO and UNICEF jointly maintain the official guidelines[25] for the contents of reduced osmolarity ORS packets. These guidelines are used by manufacturers of commercial ORS packets that are available for purchase and were last updated in 2006.[26] The reduced osmolarity ORS has a total osmolarity of 245 mmol/L.[22]

[edit] Switch to reduced osmolarity ORS

In 2003, WHO/UNICEF changed the ORS formula to a reduced osmolarity version from what it had been recommending for over two decades prior.[22] This change was in response to numerous studies that showed that the standard ORS formula was ineffective in reducing diarrheal stool output compared to other solutions, including rice water.[27][28][29][30][31] Additionally, further studies showed that a reduced osmolarity solution not only decreased stool output, but also resulted in less vomiting and fewer unscheduled intravenous therapy cases.[32][33][34] Although UNICEF certifies reduced osmolarity ORS for all forms of dehydration,[22] at least one study cautions that for high stool output cholera-based diarrhea, reduced osmolarity ORS may not sufficiently replenish electrolyte levels, leading to hyponatremia. Though the actual consequence of this appeared negligible, further study was recommended.[35][36]

The change reduced the osmolarity of the ORS from 311 mmol/L to 245 mmol/L. The ingredients reduced in concentration were glucose and sodium chloride. Potassium and citrate concentrations remained the same.[22] The benefits of the reduced osmolarity ORS are reducing

Concentrations of ingredients in reduced osmolarity ORS[22]

Ingredient g/L mmol/L

sodium chloride (NaCl) 2.6 45

glucose, anhydrous (C6H12O6) 13.5 75

potassium chloride (KCl) 1.5 20

trisodium citrate, dihydrateNa3C6H5O7·2H2O

2.9 10

stool volume by about 25 percent, reducing vomiting by nearly 30 percent,[37] and reducing the need for unscheduled intravenous therapy by 33 percent.[38]

Many Indian pharmacy manufacturers lobbied to oppose this change as the reduction in glucose and sodium chloride concentrations degraded the taste of the ORS solution and hence adversely affected their sales. In 1981 during his clinical practice Dr dinesh Kumar Tiwari, from Saharanpur Uttar Pradesh India, was informed by one lady police constable that whenever she gives her son oral re hydration solution the water loss in stools of his son increase and she suspected the role of ORS for this change. Dr Dinesh Kumar Tiwari investigated the matter and found the observation true. After realizing the role of high glucose content of ORS for producing high osmolar diarrhoea, Dr Dinesh Kumar Tiwari lead a decade long campaign against the old high osmolarity formula, he called "Sweet Killer". The recommendations were at last adopted by CDSCO.

[edit] Administration

This section contains instructions, advice, or how-to content. The purpose of Wikipedia is to present facts, not to train. Please help improve this article either by rewriting the how-to content or by moving it to Wikiversity or Wikibooks. (August 2011)

Pouring of an ORS sachet into a bottle

Current WHO/UNICEF guidelines,[39] recommend that ORT should begin at home with "home fluids" or a home-prepared "sugar and salt" solution at the first sign of diarrhea to prevent dehydration.[40] Feeding should be continued at all times.[20] After initial fluid volume has been restored, the regimen should be switched to official preparations of oral rehydration salts (ORS) at the appropriate dosing times to maintain adequate hydration and proper electrolyte balance.

During the home-prepared stage, care should be taken to select the proper type of fluid to administer. The fluids given must contain both sugar and salt in the proper amounts. Liquids without salt can lead to low body salt (hyponatremia) because the diarrheal stool contains salt that must be replenished. Additionally, sugar must also be present in the administered fluid because salt absorption is coupled with sugar in the intestine via the SGLT1 transporter.[40]

Appropriate drinks to administer during the home-prepared stage include official ORSs, salted rice water, salted yogurt-based drinks, and vegetable or chicken soup with salt. Clean water should always be used when preparing a solution. Drinks to be avoided include soft drinks, sweetened fruit drinks, sweetened tea, coffee, and medical tea infusions with diuretic effects due to high sugar content and/or caffeine. In addition, drinks with a high concentration (osmolarity) of sugar can worsen diarrhea as they draw water out of the body and into the intestine because of their hypertonicity.[40]

If dehydration ensues even when ORT is begun with a home-prepared solution, if available, a qualified health professional should manage further rehydration with ORS to ensure proper electrolyte balance and to facilitate rapid rehydration, and treatment of the underlying cause of dehydration if appropriate.[38]

[edit] Food and supplements

An adult or child with diarrhea should continue to eat, and infants should continue to breast-feed.[15][41] In a 2005 publication for doctors regarding the treatment of diarrhea, the World Health Organization states: "When food is given, sufficient nutrients are usually absorbed to support continued growth and weight gain. Continued feeding also speeds the recovery of normal intestinal function, including the ability to digest and absorb various nutrients. In contrast, children whose food is restricted or diluted lose weight, have diarrhea of longer duration, and recover intestinal function more slowly."[15]

Zinc supplementation[42] is recommended for the management of diarrheal disease in addition to ORS, particularly for pediatric patients. For children under five, zinc supplementation significantly reduces the severity and duration of diarrhea and is strongly recommended as a supplement with ORS for dehydrated children.[38] Preparations are available as a zinc sulfate solution for adults,[43] a modified solution for children,[44] and also a tablet form for children.[45]

[edit] Treatment when malnourished

The treatment of diarrhea and dehydration in a child or an adult who is also malnourished is somewhat different from standard treatment. Dehydration may be overestimated in a marasmic/wasted child and underestimated in a kwashiorkor/edematous child. The diagnosis is based instead on whether the person has been having diarrhea.[15][46] The standard reduced-osmolarity oral rehydration solution needs to be modified so that it will have somewhat less salt and somewhat more sugar and potassium than standard in order to produce what is called a Rehydration Solution for Malnutrition (ReSoMal). Or, if diarrhea is severe, the standard reduced-osmolarity solution can be used.[46] In addition, the World Health Organization recommends that all malnourished individuals with diarrhea be treated with a course of broad-spectrum antibiotics.[15] Supplemental zinc is still recommended, and a dehydrated person should still continue to be given food.[15]

[edit] Physiological basis

Fluid from the body is normally pumped into the intestinal lumen during digestion. This fluid is typically isosmotic with blood because it contains a high concentration of sodium (approx. 142 mEq/L). A healthy individual will secrete 20-30 grams of sodium per day via intestinal secretions. Nearly all of this is reabsorbed by the intestine, helping to maintain constant sodium levels in the body (homeostasis).[47]

Because there is so much sodium secreted by the intestine, without intervention, heavy continuous diarrhea can be a very dangerous and potentially life-threatening condition within hours. This is because liquid secreted into the intestinal lumen during diarrhea passes through the gut so quickly that very little sodium is reabsorbed, leading to very low sodium levels in the body (severe hyponatremia).[47] This is the motivation for sodium and water replenishment via ORT.

Sodium absorption via the intestine occurs in two stages. The first is at the outermost cells (intestinal epithelial cells) at the surface of the intestinal lumen. Sodium passes into these outermost cells by co-transport via the SGLT1 protein.[47] From there, sodium is pumped out of the cells (basal side) and into the extracellular space by active transport via the sodium potassium pump.[48][49]

The co-transport of sodium into the epithelial cells via the SGLT1 protein requires glucose or galactose. Two sodium ions and one molecule of glucose/galactose are transported together across the cell membrane through the SGLT1 protein. Without glucose or galactose present, intestinal sodium will not be absorbed. This is the reason glucose is included in ORSs. For each cycle of the transport, hundreds of water molecules move into the epithelial cell, slowly rehydrating the affected individual.[47]