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For other uses, see Half-life (disambiguation).

The biological half-life of a substance is the time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition. Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books

Biological half-life is an important pharmacokinetic parameter and is usually denoted by the abbreviation t1/2. Pharmacokinetics (in Greek: “pharmacon” meaning drug and “kinetikos” meaning putting in motion the study of time dependency sometimes abbreviated as “PK” is a [1]

While a radioactive isotope decays perfectly according to first order kinetics where the rate constant is fixed, the elimination of a substance from a living organism follows more complex kinetics. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. See the article rate equation. The rate law or rate equation for a Chemical reaction is an equation which links the Reaction rate with concentrations or pressures of reactants and constant

Contents

Examples of biological half-lives

Water

The biological half-life of water in a human is about 7 to 10 days. It can be altered by behavior. Drinking large amounts of alcohol will reduce the biological half-life of water in the body. In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon This has been used to decontaminate humans who are internally contaminated with tritiated water (tritium). Tritiated water is a form of water where the usual hydrogen atoms are replaced with Tritium. Tritium (ˈtɹɪtiəm symbol or, also known as Hydrogen-3) is a radioactive Isotope of Hydrogen. Drinking the same amount of water would have a similar effect, but many would find it difficult to drink a large volume of water. The basis of this decontamination method (used at Harwell) is to increase the rate at which the water in the body is replaced with new water.

Alcohol

The removal of ethanol (alcohol) through oxidation by alcohol dehydrogenase in the liver from the human body is limited. In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon Alcohol dehydrogenase (ADH is an enzyme discovered in the mid-1960s in Drosophila melanogaster. The liver is a vital organ in the human body and is present in Vertebrates and some other animals Hence the removal of a large concentration of alcohol from blood may follow zero-order kinetics. Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells such as nutrients and oxygen—and transports Waste products Also the rate-limiting steps for one substance may be in common with other substances. For instance, the blood alcohol concentration can be used to modify the biochemistry of methanol and ethylene glycol. Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a Chemical compound Ethylene glycol ( monoethylene glycol ( MEG) 12-ethanediol, IUPAC name: ethane-12-diol) is an Alcohol with two -OH In this way the oxidation of methanol to the toxic formaldehyde and formic acid in the human body can be prevented by giving an appropriate amount of ethanol to a person who has ingested methanol. Toxicity is the degree to which a substance is able to damage an exposed organism Formaldehyde is a Chemical compound with the formula H2CO It is the simplest Aldehyde —an organic compound containing a terminal Carbonyl Formic acid (systematically called methanoic acid) is the simplest Carboxylic acid. In general terms eating (formally ingestion) is the process of consuming Food to provide for the Nutritional needs of an Animal, particularly Note that methanol is very toxic and causes blindness and death. Blindness is the condition of lacking Visual perception due to Physiological or Neurological factors A person who has ingested ethylene glycol can be treated in the same way.

Prescription medications

Substance Half-life Notes
Amiodarone 25 days
Cisplatin 20 to 30 minutes
Chlorambucil 1. Amiodarone is an Antiarrhythmic agent (medication used for irregular heart beat used for various types of tachyarrhythmias (fast forms of irregular heart beat both ventricular Cisplatin, cisplatinum or cis -diamminedichloridoplatinum(II (CDDP is a Platinum -based Chemotherapy drug used to treat Chlorambucil (marketed as Leukeran by GlaxoSmithKline) is a Chemotherapy drug that has been mainly used in the treatment of Chronic lymphocytic 53 hours
Digoxin 24 to 36 hours
Fluoxetine 1 to 6 days

The active metabolite of fluoxetine is lipophilic and migrates slowly from the brain to the blood. Digoxin ( INN) (dɨˈdʒɒksɨn also known as Digitalis, is a purified Cardiac glycoside extracted from the Foxglove plant Digitalis Metabolomics is the "systematic study of the unique chemical fingerprints that specific cellular processes leave behind" - specifically the study of their small-molecule metabolite Lipophilicity, fat-liking, refers to the ability of a Chemical compound to dissolve in fats oils lipids and non-polar solvents such as Hexane or The metabolite has a biological half-life of 4 to 16 days.

Methadone 15 to 60 hours, in rare cases up to 190 hours. Methadone ( Dolophine Amidone Methadose Physeptone Heptadon and many others is a synthetic Opioid, used medically as an Analgesic, Antitussive [2]
Oxaliplatin 14 minutes[3]
Salbutamol 7 hours

Metals

The biological half-life of caesium in humans is between one and four months. Oxaliplatin is a Platinum -based Chemotherapy drug in the same family as Cisplatin and Carboplatin. Salbutamol ( INN) or albuterol ( USAN) is a short-acting β2-adrenergic receptor agonist used for the relief of Bronchospasm Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 This can be shortened by feeding the person prussian blue. Prussian blue is a very dark blue colorfast non-toxic Pigment – one of the first synthetic Dyes – which was discovered accidentally in Berlin in 1704 The prussian blue in the digestive system acts as a solid ion exchanger which absorbs the caesium while releasing potassium ions. Ion exchange is an exchange of Ions between two Electrolytes or between an electrolyte Solution and a complex. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39

For some substances, it is important to think of the human or animal body as being made up of several parts, each with their own affinity for the substance, and each part with a different biological half-life. Attempts to remove a substance from the whole organism may have the effect of increasing the burden present in one part of the organism. For instance, if a person who is contaminated with lead is given EDTA in a chelation therapy, then while the rate at which lead is lost from the body will be increased, the lead within the body tends to relocate into the brain where it can do the most harm. EDTA is a widely used abbreviation for the Chemical compound ethylenediaminetetraacetic acid (and many other names see table Chelation therapy is the administration of chelating agents to remove Heavy metals from the body The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain

Rate equations

Zero-order elimination

There are circumstances where the half-life varies with the concentration of the drug. For example, ethanol may be consumed in sufficient quantity to saturate the metabolic enzymes in the liver, and so is eliminated from the body at an approximately constant rate (zero-order elimination). Thus the half-life, under these circumstances, is proportional to the initial concentration of the drug A0 and inversely proportional to the zero-order rate constant k0 where:

t_{1/2} = \frac{0.5 A_{0}}{k_{0}} \,

First-order elimination

This process is usually a logarithmic process - that is, a constant proportion of the agent is eliminated per unit time (Birkett, 2002). Thus the fall in plasma concentration after the administration of a single dose is described by the following equation:

C_{t} = C_{0} e^{-kt} \,

The relationship between the elimination rate constant and half-life is given by the following equation:

k = \frac{\ln 2}{t_{1/2}} \,

Half-life is determined by clearance (CL) and volume of distribution (VD) and the relationship is described by the following equation:

t_{1/2} = \frac{{\ln 2}.{V_D}}{CL} \,

In clinical practice, this means that it takes just over 4. In Medicine, the clearance is a measurement of the renal Excretion ability The volume of distribution (VD, also known as apparent volume of distribution, is a pharmacological term used to quantify the distribution 7 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed. So, for example, digoxin has a half-life (or t½) of 24-36 hours; this means that a change in the dose will take the best part of a week to take full effect. For this reason, drugs with a long half-life (e. g. amiodarone, elimination t½ of about 90 days) are usually started with a loading dose to achieve their desired clinical effect more quickly. Amiodarone is an Antiarrhythmic agent (medication used for irregular heart beat used for various types of tachyarrhythmias (fast forms of irregular heart beat both ventricular In Pharmacokinetics, a loading dose refers to an initial higher dose of a drug that may be given at the beginning of a course of treatment before dropping down to a lower

See also

References

  1. ^ International Union of Pure and Applied Chemistry. The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization "biological half life". Compendium of Chemical Terminology Internet edition. Compendium of Chemical Terminology (ISBN 0-86542-684-8 is a book published by IUPAC containing internationally accepted definitions for terms in Chemistry.
  2. ^ Manfredonia, John (March 2005). Prescribing Methadone for Pain Management in End-of-Life Care. JAOA—The Journal of the American Osteopathic Association. Retrieved on 2007-01-29. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 904 - Sergius III comes out of retirement to take over the papacy from the deposed Antipope Christopher.
  3. ^ Ehrsson, Hans et al (Winter 2002). Pharmacokinetics of oxaliplatin in humans. Medical Oncology. Retrieved on 2007-03-28. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 37 - Roman Emperor Caligula accepts the titles of the Principate, entitled to him by the Senate.

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