Biological half-life

Biological half-life (elimination half-life, pharmacological half-life) is the time taken for concentration of a biological substance (such as a medication) to decrease from its maximum concentration (C_max) to half of C_max in the blood plasma.^[1]^[2]^[3]^[4]^[5] It is denoted by the abbreviation $t_{\frac {1}{2}}$ .^[2]^[4]

This is used to measure the removal of things such as metabolites, drugs, and signalling molecules from the body. Typically, the biological half-life refers to the body's natural detoxification (cleansing) through liver metabolism and through the excretion of the measured substance through the kidneys and intestines. This concept is used when the rate of removal is roughly exponential.^[6]

In a medical context, half-life explicitly describes the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state when circulating in the full blood of an organism. This measurement is useful in medicine, pharmacology and pharmacokinetics because it helps determine how much of a drug needs to be taken and how frequently it needs to be taken if a certain average amount is needed constantly. By contrast, the stability of a substance in plasma is described as plasma stability. This is essential to ensure accurate analysis of drugs in plasma and for drug discovery.

The relationship between the biological and plasma half-lives of a substance can be complex depending on the substance in question, due to factors including accumulation in tissues, protein binding, active metabolites, and receptor interactions.^[7]

^ "Elimination Half-Life". Pharmacology in one semester. Archived from the original on 22 October 2020. Retrieved 20 February 2020.
^ ^a ^b "Definition of Half-Life (t½)". AIDSinfo. 19 February 2020. Archived from the original on 20 February 2020. Retrieved 20 February 2020.
^ Curry, Stephen H. (1993). "PHARMACOKINETICS OF ANTIPSYCHOTIC DRUGS". Antipsychotic Drugs and their Side-Effects. Elsevier. pp. 127–144. doi:10.1016/b978-0-12-079035-7.50015-4. ISBN 978-0-12-079035-7. The elimination half-life measures the kinetics of loss of drug from the body as a whole once all distribution equilibria have been achieved.
^ ^a ^b Dasgupta, Amitava; Krasowski, Matthew D. (2020). "Pharmacokinetics and therapeutic drug monitoring". Therapeutic Drug Monitoring Data. Elsevier. pp. 1–17. doi:10.1016/b978-0-12-815849-4.00001-3. ISBN 978-0-12-815849-4. S2CID 209258489. The half-life of a drug is the time required for the serum concentration to be reduced by 50%. Once the half-life of the drug is known, the time required for clearance can be estimated. Approximately 97% of the drug is eliminated by 5 halflives, while ~99% is eliminated by 7 half-lives.
^ Toutain, P. L.; Bousquet-Melou, A. (2004). "Plasma terminal half-life" (PDF). Journal of Veterinary Pharmacology and Therapeutics. 27 (6): 427–439. doi:10.1111/j.1365-2885.2004.00600.x. PMID 15601438. Archived from the original (PDF) on 20 February 2020. Following i.v. administration, the terminal half-life is the time required for plasma/blood concentration to decrease by 50% after pseudo-equilibrium of distribution has been reached; then, terminal half-life is computed when the decrease in drug plasma concentration is due only to drug elimination, and the term 'elimination half-life' is applicable. Therefore, it is not the time necessary for the amount of the administered drug to fall by one half.
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Biological Half Life". doi:10.1351/goldbook.B00658
^ Lin VW; Cardenas DD (2003). Spinal Cord Medicine. Demos Medical Publishing, LLC. p. 251. ISBN 1-888799-61-7.

[1] "Elimination Half-Life". Pharmacology in one semester. Archived from the original on 22 October 2020. Retrieved 20 February 2020.

[AIDSinfo_2020-2] "Definition of Half-Life (t½)". AIDSinfo. 19 February 2020. Archived from the original on 20 February 2020. Retrieved 20 February 2020.

[Curry_1993_pp._127–144-3] Curry, Stephen H. (1993). "PHARMACOKINETICS OF ANTIPSYCHOTIC DRUGS". Antipsychotic Drugs and their Side-Effects. Elsevier. pp. 127–144. doi:10.1016/b978-0-12-079035-7.50015-4. ISBN 978-0-12-079035-7. The elimination half-life measures the kinetics of loss of drug from the body as a whole once all distribution equilibria have been achieved.

[Dasgupta_Krasowski_2020_pp._1–17-4] Dasgupta, Amitava; Krasowski, Matthew D. (2020). "Pharmacokinetics and therapeutic drug monitoring". Therapeutic Drug Monitoring Data. Elsevier. pp. 1–17. doi:10.1016/b978-0-12-815849-4.00001-3. ISBN 978-0-12-815849-4. S2CID 209258489. The half-life of a drug is the time required for the serum concentration to be reduced by 50%. Once the half-life of the drug is known, the time required for clearance can be estimated. Approximately 97% of the drug is eliminated by 5 halflives, while ~99% is eliminated by 7 half-lives.

[5] Toutain, P. L.; Bousquet-Melou, A. (2004). "Plasma terminal half-life" (PDF). Journal of Veterinary Pharmacology and Therapeutics. 27 (6): 427–439. doi:10.1111/j.1365-2885.2004.00600.x. PMID 15601438. Archived from the original (PDF) on 20 February 2020. Following i.v. administration, the terminal half-life is the time required for plasma/blood concentration to decrease by 50% after pseudo-equilibrium of distribution has been reached; then, terminal half-life is computed when the decrease in drug plasma concentration is due only to drug elimination, and the term 'elimination half-life' is applicable. Therefore, it is not the time necessary for the amount of the administered drug to fall by one half.

[6] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Biological Half Life". doi:10.1351/goldbook.B00658

[SCM-7] Lin VW; Cardenas DD (2003). Spinal Cord Medicine. Demos Medical Publishing, LLC. p. 251. ISBN 1-888799-61-7.

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