Biological half-life

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Biological half-life

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

Medical Subject Headings

Medical Subject Headings is a huge controlled vocabulary for the purpose of index journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching....
definition.

Biological half-life is an important pharmacokinetic parameter and is usually denoted by the abbreviation t_1/2.

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

Kinetics

Kinetics, derived from the Greek language word ????s?? meaning movement or the act of moving, may refer to:...
.

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Medical Subject Headings

Kinetics

Kinetics, derived from the Greek language word ????s?? meaning movement or the act of moving, may refer to:...
. See the article rate equation

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 parameters ....
.

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

Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
will reduce the biological half-life of water in the body. This has been used to decontaminate humans who are internally contaminated with tritiated water

Tritiated water

Tritiated water is a form of Water where the usual hydrogen atoms are replaced with tritium. In its pure form it may be called tritium oxide or super-heavy water....
(tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The atomic nucleus of tritium contains one proton and two neutrons, whereas the nucleus of Hydrogen atom contains one proton and no neutrons....
). 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

Atomic Energy Research Establishment

The Atomic Energy Research Establishment near Harwell, Oxfordshire was the main centre for Nuclear energy research and development in the United Kingdom from the 1940s to the 1990s....
) is to increase the rate at which the water in the body is replaced with new water.

Alcohol

The removal of ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....
(alcohol

Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
) through oxidation by alcohol dehydrogenase

Alcohol dehydrogenase

Alcohol dehydrogenase is an enzyme discovered in the mid-1960s in Drosophila melanogaster. Since then, there has been extensive research on the enzyme....
in the liver

Liver

The liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, a few of which are detoxification, protein synthesis, and production of biochemicals necessary for digestion....
from the human body is limited. Hence the removal of a large concentration of alcohol from blood

Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
may follow zero-order kinetics. 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

Methanol

Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a chemical compound with chemical formula carbonhydrogen3oxygenhydrogen ....
and ethylene glycol

Ethylene glycol

Ethylene glycol is an alcohol with two -OH groups , a chemical compound widely used as an automobile antifreeze. In its pure form, it is an odorless, colorless, syrupy, sweet tasting, toxic liquid....
. In this way the oxidation of methanol to the toxic formaldehyde

Formaldehyde

Formaldehyde is a chemical compound with the chemical formula H2CO. It is the simplest aldehyde. Formaldehyde exists in several forms aside from H2CO: the cyclic trimer trioxane and the polymer Polyoxymethylene....
and formic acid

Formic acid

Formic acid is the simplest carboxylic acid. Its formula is hydrogencarbonoxygenOH or CH2O2. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stingers....
in the (human body) can be prevented by giving an appropriate amount of ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....
to a person who has ingested

Eating

In general terms, eating is the process of consuming food to provide for the nutritional needs of an animal, particularly their food energy requirements and to growth....
methanol. Note that methanol is very toxic and causes blindness

Blindness

Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define "blindness." Total blindness is the complete lack of form and visual light perception and is clinically recorded as "NLP," an abbreviation for "no ligh...
and death. A person who has ingested ethylene glycol can be treated in the same way.

Prescription medications

Substance	Half-life	Notes
Amiodarone Amiodarone Amiodarone is an antiarrhythmic agent used for various types of tachyarrhythmias , both ventricular and supraventricular arrhythmias. Discovered in 1961, it was not approved for use in the United States until 1985....	25 days
Cisplatin Cisplatin Cisplatin, cisplatinum or cis-diamminedichloridoplatinum is a platinum-based chemotherapy medication used to treat various types of cancers, including sarcomas, some carcinomas , lymphomas and germ cell tumors....	30 to 100 hours
Chlorambucil Chlorambucil Chlorambucil is a chemotherapy drug that has been mainly used in the treatment of chronic lymphocytic leukemia. It is a nitrogen mustard alkylating antineoplastic agent and can be given orally....	1.53 hours
Digoxin Digoxin Digoxin , also known as Digitalis, is a purified cardiac glycoside extracted from the foxglove plant, Digitalis lanata. Its corresponding aglycone is digoxigenin....	24 to 36 hours
Fluoxetine Fluoxetine Fluoxetine hydrochloride is an antidepressant of the selective serotonin reuptake inhibitor class. Fluoxetine is approved for the treatment of major depressive disorder , obsessive-compulsive disorder , bulimia nervosa, anorexia nervosa, panic disorder and premenstrual dysphoric disorder....	1 to 6 days	The active metabolite Metabolite Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction.... of fluoxetine is lipophilic Lipophilic Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene.... and migrates slowly from the brain to the blood. The metabolite has a biological half-life of 4 to 16 days.
Methadone Methadone Methadone is a synthetic opioid, used medically as an analgesic, antitussive and a maintenance drug addiction#Anti-addictive drugs for use in patients on opioids....	15 to 60 hours, in rare cases up to 190 hours.
Oxaliplatin Oxaliplatin Oxaliplatin is a platinum-based chemotherapy medication in the same family as cisplatin and carboplatin. It is typically administered in combination with fluorouracil and leucovorin in a combination known as FOLFOX for the treatment of colorectal cancer....	14 minutes
Salbutamol Salbutamol Salbutamol or albuterol is a short-acting beta2-adrenergic receptor agonist used for the relief of bronchospasm in conditions such as asthma and chronic obstructive pulmonary disease....	7 hours

Metals

The biological half-life of caesium

Caesium

Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only liquid metal that are liquid at or near room temperature....
in humans is between one and four months. This can be shortened by feeding the person prussian blue

Prussian blue

Prussian blue is a very dark blue, colorfast, non-toxic pigment ? one of the first synthetic pigments ? which was discovered accidentally in Berlin in 1704....
. The prussian blue in the digestive system acts as a solid ion exchange

Ion exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex . In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion exchangers'....
r which absorbs the caesium while releasing potassium

Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
ions.

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

EDTA

EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid . EDTA is a polyamino carboxylic acid with the chemical formula [CH2N2]2....
in a chelation therapy

Chelation therapy

Chelation therapy is the administration of chelations to remove heavy metals from the body. For the most common forms of heavy metal intoxication?those involving lead, arsenic or Mercury ?the standard of care in the USA dictates the use of dimercaptosuccinic acid ....
, 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

Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
where it can do the most harm.

Polonium
Polonium

Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Curie and Pierre Curie. A rare and highly radioactive metalloid, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores....
in the body has a biological half-life
Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
of about 30 to 50 days.
Caesium
Caesium

Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only liquid metal that are liquid at or near room temperature....
in the body has a biological half-life of about one to four months.
Lead
Lead

Lead is a main-group Chemical element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metal ....
in bone
Bone

Bones are rigid organ that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red blood cell and white blood cells and store minerals....
has a biological half-life of about ten years.
Cadmium
Cadmium

Cadmium is a chemical element with the symbol Cd and atomic number 48. A relatively abundant , soft, bluish-white, transition metal, cadmium is known to cause cancer and occurs with zinc ores....
in bone has a biological half-life of about 30 years.
Plutonium
Plutonium

Plutonium is a rare transuranic radioactive chemical element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when plutonium oxide....
in bone has a biological half-life of about 100 years.
Plutonium
Plutonium

Plutonium is a rare transuranic radioactive chemical element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when plutonium oxide....
in the liver has a biological half-life of about 40 years.

Rate equations

First-order elimination

There are circumstances where the half-life varies with the concentration of the drug. 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:

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

C_t is concentration after time t
C₀ is the initial concentration (t=0)
k is the elimination rate constant

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

Half-life is determined by clearance

Clearance (medicine)

In medicine, the clearance is a measurement of the renal excretion ability. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance....
(CL) and volume of distribution

Volume of distribution

The volume of distribution , also known as apparent volume of distribution, is a pharmacology term used to quantify the Distribution of a medication between Blood_plasma and the rest of the body after Wiktionary:oral or parenteral dosing....
(V_D) and the relationship is described by the following equation:

In clinical practice, this means that it takes just over 4.7 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 t_½) 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

Amiodarone

Amiodarone is an antiarrhythmic agent used for various types of tachyarrhythmias , both ventricular and supraventricular arrhythmias. Discovered in 1961, it was not approved for use in the United States until 1985....
, elimination t_½ of about 58 days) are usually started with a loading dose

Loading dose

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 maintenance dose....
to achieve their desired clinical effect more quickly.

Biological half-life

Examples of biological half-lives

Water

Alcohol

Prescription medications

Metals

Rate equations

First-order elimination

Sample values and equations

See also