en>David Eppstein: Reverted edits by Drsadeq (talk) to last version by 138.246.2.177

2013-09-13T05:33:58Z

Reverted edits by Drsadeq (talk) to last version by 138.246.2.177

← Older revision		Revision as of 07:33, 13 September 2013
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	~~Alyson Meagher~~ is the ~~title her parents gave her but she doesn~~'~~t like when people use her full name~~. ~~Office supervising~~ is ~~exactly where her primary income arrives from~~. ~~I am truly fond~~ of ~~handwriting but I~~ can'~~t make it my occupation really~~. ~~For many years he~~'s ~~been residing~~ in ~~Alaska~~ and ~~he doesn't strategy on changing it~~.<br><br>~~my blog - [http~~://~~cspl~~.~~postech~~.ac.~~kr/zboard/Membersonly/144571 cheap psychic readings~~] ~~readings~~ [~~http://cartoonkorea~~.~~com/ce002/1093612 online psychic chat~~], [http://~~www~~.~~khuplaza.com~~/~~dent~~/~~14869889 http:~~//~~www~~.~~khuplaza~~.~~com/dent/14869889~~],		In [[pharmacology]] (and more specifically [[pharmacokinetics]]), '''absorption''' is the movement of a drug into the bloodstream.

			Absorption involves several phases. First, the drug needs to be introduced via some [[route of administration]] ([[Mouth\|oral]], [[skin\|topical-dermal]], ''etc''.) and in a specific [[dosage form]] such as a [[Tablet (pharmacy)\|tablet]], [[Capsule (pharmacy)\|capsule]], [[Solution (pharmacy)\|solution]] and so on.

			In other situations, such as [[intravenous therapy]], [[intramuscular injection]], [[Nasogastric intubation\|enteral nutrition]] and others, absorption is even more straightforward and there is less variability in absorption and [[bioavailability]] is often near 100%. It is considered that [[intravascular]] administration (e.g. IV) does not involve absorption, and there is no loss of drug.<ref>Kaplan Pharmacology 2010, page6, Absorption</ref> The fastest route of absorption is [[inhalation]], and not as mistakenly considered the intravenous administration. <ref>Kaplan Pharmacology 2010, Video Lectues, Absorption chapter</ref>

			Absorption is a primary focus in [[drug development]] and [[medicinal chemistry]], since the drug must be absorbed before any medicinal effects can take place. Moreover, the drug's pharmacokinetic profile can be easily and significantly changed by adjusting factors that affect absorption.

			==Dissolution==
			In the most common situation, a tablet is ingested and passes through the [[esophagus]] to the [[stomach]].

			The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, the drug may be released more gradually over time with a longer duration of action. Having a longer duration of action may improve [[Compliance (medicine)\|compliance]] since the medication will not have to be taken as often. Additionally, slow-release dosage forms may maintain concentrations within an acceptable therapeutic range over a long period of time, as opposed is quick-release dosage forms which may result in sharper peaks and troughs in serum concentrations.

			The rate of dissolution is described by the [[Noyes–Whitney equation]] as shown below:

			:<math>\frac{dW}{dt} = \frac{DA(C_{s}-C)}{L}</math>

			Where:
			*<math>\frac{dW}{dt}</math> is the rate of dissolution.
			*A is the [[surface area]] of the solid.
			*C is the concentration of the solid in the bulk dissolution medium.
			*<math>C_{s}</math> is the concentration of the solid in the [[diffusion]] layer surrounding the solid.
			*D is the diffusion [[coefficient]].
			*L is the [[diffusion layer]] thickness.

			As can be inferred by the Noyes-Whitney equation, the rate of dissolution may be modified primarily by altering the surface area of the solid. The surface area may be adjusted by altering the particle size (e.g. [[micronization]]). For many drugs, reducing the particle size leads to a reduction in the dose that is required to achieve the same therapeutic effect. However, it should be noted that although the reduction of particle size increases the [[specific surface area]] and the dissolution rate, it does not affect solubility.

			The rate of dissolution may also be altered by choosing a suitable [[Polymorphism (materials science)\|polymorph]] of a compound. Different polymorphs exhibit different solubility and dissolution rate characteristics. Specifically, crystalline forms dissolve slower than amorphous forms, since crystalline forms require more energy to leave lattice during dissolution. The most stable crystalline polymorph has the lowest dissolution rate. Dissolution is also different for anhydrous and hydrous forms of a drug. Anhydrous often dissolve faster than hydrated; however, anhydrous forms sometimes exhibit lower solubility.

			Chemical modification by esterification is also used to control solubility. For example, searate and estolate esters of a drug have decreased solubility in gastric fluid. Later, esterases in the GIT wall and blood hydrolze these esters to release the parent drug.

			Also, coatings on a tablet or a pellet may act as a barrier to reduce the rate of dissolution. Coating may also be used to modify where dissolution takes place. For example, [[enteric coating]]s may be applied to a drug, so that the coating only dissolves in the [[Base (chemistry)\|basic]] environment of the intestines. This will prevent release of the drug before reaching the intestines.

			Since solutions are already dissolved, they do not need to undergo dissolution before being absorbed.
			Lipid-soluble drugs are less absorbed than water-soluble drugs, especially when they are enteral.

			==Ionization==
			The [[gastrointestinal tract]] is lined with [[Epithelium\|epithelial cells]]. Drugs must pass or permeate through these cells in order to be absorbed into the [[circulatory system]]. One particular cellular barrier that may prevent absorption of a given drug is the [[cell membrane]]. Cell membranes are essentially [[lipid bilayer]]s which form a [[semipermeable membrane]]. Pure lipid bilayers are generally permeable only to small, uncharged solutes. Hence, whether or not a molecule is [[ionization\|ionized]] will affect its absorption, since ionic molecules are charged. Solubility favors charged species, and permeability favors neutral species. Some molecules have special exchange proteins and channels to facilitate movement from the lumen into the circulation.

			The [[Henderson-Hasselbalch equation]] offers a way to determine the proportion of a substance that is ionized at a given pH. In the stomach, drugs that are [[weak acid]]s (such as [[aspirin]]) will be present mainly in their non-ionic form, and weak bases will be in their ionic form. Since non-ionic species diffuse more readily through [[cell membrane]]s, weak acids will have a higher absorption in the highly acidic [[stomach]].

			However, the reverse is true in the [[Base (chemistry)\|basic]] environment of the intestines-- weak bases (such as [[caffeine]]) will diffuse more readily since they will be non-ionic.

			This aspect of absorption has been targeted by medicinal chemistry. For example, a suitable [[Analog (chemistry)\|analog]] may be chosen so that the drug is more likely to be in a non-ionic form. Also, [[prodrug]]s of a compound may be developed by medicinal chemists-- these chemical variants may be more readily absorbed and then metabolized by the body into the active compound. However, changing the structure of a molecule is less predictable than altering dissolution properties, since changes in chemical structure may affect the [[pharmacodynamic]] properties of a drug.

			==Other factors==
			Other facts that affect absorption include, but are not limited to, bioactivity, resonance, the inductive effect, isosterism, bio-isosterism, and consideration.

			==References==
			{{Reflist}}

			==External links==
			* [http://howmed.net/pharmacology/absorption-of-drugs/ Absorption of Drugs]

			==Further reading==
			{{Refbegin}}
			* {{cite book \| author = Avdeef, Alex \| authorlink = \| editor = \| others = \| title = Absorption and Drug Development \| edition = \| language = \| publisher = Wiley-Interscience/J. Wiley \| location = Hoboken, N.J \| year = 2003 \| origyear = \| pages = \| quote = \| isbn = 0-471-42365-3 \| oclc = \| doi = \| url = \| accessdate = }}
			{{Refend}}

			[[Category:Pharmacokinetics]]

en>Arjun G. Menon at 20:59, 16 August 2012

2012-08-16T20:59:33Z

New page

Alyson Meagher is the title her parents gave her but she doesn't like when people use her full name. Office supervising is exactly where her primary income arrives from. I am truly fond of handwriting but I can't make it my occupation really. For many years he's been residing in Alaska and he doesn't strategy on changing it.<br><br>my blog - [http://cspl.postech.ac.kr/zboard/Membersonly/144571 cheap psychic readings] readings [http://cartoonkorea.com/ce002/1093612 online psychic chat], [http://www.khuplaza.com/dent/14869889 http://www.khuplaza.com/dent/14869889],

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en>David Eppstein: Reverted edits by Drsadeq (talk) to last version by 138.246.2.177

en>Arjun G. Menon at 20:59, 16 August 2012