Biological half-life

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Elimination half-life
)

Graph of drug plasma concentrations over 96 hours
Time course of drug plasma concentrations over 96 hours following oral administrations every 24 hours (τ). Absorption half-life 1 h, elimination half-life 12 h.

Biological half-life (elimination half-life, pharmacological half-life) is the time taken for concentration of a

concentration (Cmax) to half of Cmax in the blood plasma.[1][2][3][4][5]
It is denoted by the abbreviation .[2][4]

This is used to measure the removal of things such as

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]

Examples

Water

The biological half-life of water in a human is about 7 to 14 days. It can be altered by behavior. Drinking large amounts of alcohol will reduce the biological half-life of water in the body.[8][9] This has been used to decontaminate patients who are internally contaminated with tritiated water. The basis of this decontamination method is to increase the rate at which the water in the body is replaced with new water.

Alcohol

The removal of

blindness and death. A person who has ingested ethylene glycol
can be treated in the same way. Half life is also relative to the subjective metabolic rate of the individual in question.

Common prescription medications

Substance Biological half-life
Adenosine Less than 10 seconds (estimate)[10]
Norepinephrine
 2 minutes[11]
Oxaliplatin 14 minutes[12]
Zaleplon 1 hour[13]
Morphine 1.5–4.5 hours[14]
Flurazepam 2.3 hours[15]

Active metabolite (

N-desalkylflurazepam): 47–100 hours[15]

Methotrexate 3–10 hours (lower doses),

8–15 hours (higher doses)[16]

Methadone 15–72 hours

in rare cases up to 8 days[17]

Diazepam 20–50 hours[18]

Active metabolite (nordazepam): 30–200 hours[18]

Phenytoin 20–60 hours[19]
Buprenorphine 28–35 hours[20]
Clonazepam 30–40 hours[21]
Donepezil 3 days (70 hours)[22]
Fluoxetine 4–6 days (under continuous administration)[23]

Active lipophilic metabolite (

norfluoxetine): 4–16 days[23]

Amiodarone 14–107 days[24]
Vandetanib 19 days[25]
Dutasteride 21–35 days (under continuous administration)[26]
Bedaquiline 165 days[27]

Metals

The biological half-life of caesium in humans is between one and four months. This can be shortened by feeding the person prussian blue. The prussian blue in the digestive system acts as a solid ion exchanger which absorbs the caesium while releasing potassium 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 (

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.[28]

  • Polonium in the body has a biological half-life of about 30 to 50 days.
  • Caesium in the body has a biological half-life of about one to four months.
  • Mercury (as methylmercury) in the body has a half-life of about 65 days.
  • Lead in the blood has a half life of 28–36 days.[29][30]
  • Lead in bone has a biological half-life of about ten years.
  • Cadmium in bone has a biological half-life of about 30 years.
  • Plutonium in bone has a biological half-life of about 100 years.
  • Plutonium in the liver has a biological half-life of about 40 years.

Peripheral half-life

Some substances may have different half-lives in different parts of the body. For example,

psychoactive effects in humans.[32][33] In addition, unlike the case of peripheral administration, intranasal oxytocin has a central duration of at least 2.25 hours and as long as 4 hours.[34][35] In likely relation to this fact, endogenous oxytocin concentrations in the brain have been found to be as much as 1000-fold higher than peripheral levels.[31]

Rate equations

Main article: Rate equation
See also: Pharmacokinetics § Metrics

First-order elimination

Timeline of an exponential decay process[36][37][38]
Time (t) Percent of initial value Percent completion
t½ 50% 50%
t½ × 2 25% 75%
t½ × 3 12.5% 87.5%
t½ × 3.322 10.00% 90.00%
t½ × 4 6.25% 93.75%
t½ × 4.322 5.00% 95.00%
t½ × 5 3.125% 96.875%
t½ × 6 1.5625% 98.4375%
t½ × 7 0.78125% 99.21875%
t½ × 10 ~0.09766% ~99.90234%

Half-times apply to processes where the elimination rate is exponential. If is the concentration of a substance at time , its time dependence is given by

where k is the reaction rate constant. Such a decay rate arises from a first-order reaction where the rate of elimination is proportional to the amount of the substance:[39]

The half-life for this process is[39]

Alternatively, half-life is given by

where λz is the slope of the terminal phase of the time–concentration curve for the substance on a semilogarithmic scale.[40][41]

Half-life is determined by

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

In clinical practice, this means that it takes 4 to 5 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 h; 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 58 days) are usually started with a loading dose to achieve their desired clinical effect more quickly.

Biphasic half-life

Many drugs follow a biphasic elimination curve — first a steep slope then a shallow slope:

STEEP (initial) part of curve —> initial distribution of the drug in the body.
SHALLOW part of curve —> ultimate excretion of drug, which is dependent on the release of the drug from tissue compartments into the blood.

The longer half-life is called the terminal half-life and the half-life of the largest component is called the dominant half-life.[39] For a more detailed description see Pharmacokinetics § Multi-compartmental models.

See also

References

  1. ^ "Elimination Half-Life". Pharmacology in one semester. Archived from the original on 22 October 2020. Retrieved 20 February 2020.
  2. ^ a b "Definition of Half-Life (t½)". AIDSinfo. 19 February 2020. Archived from the original on 20 February 2020. Retrieved 20 February 2020.
  3. 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.
  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. 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. doi:10.1351/goldbook.B00658
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  10. ^ Haberfeld H, ed. (2020). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Adenosin Baxter3 mg/ml Injektionslösung.
  11. ^ Haberfeld H, ed. (2020). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Noradrenalin Orpha 1 mg/ml Konzentrat zur Herstellung einer Infusionslösung.
  12. S2CID 1068099. Archived from the original
    on 28 September 2007. Retrieved 28 March 2007.
  13. ^ Zaleplon Monograph. Accessed 15 April 2021.
  14. ^ Morphine Monograph. Accessed 15 April 2021.
  15. ^ a b Flurazepam Monograph. Accessed 15 April 2021.
  16. ^ "Trexall, Otrexup (methotrexate) dosing, indications, interactions, adverse effects, and more". reference.medscape.com.
  17. PMID 18154194. Archived from the original
    on 20 May 2007. Retrieved 29 January 2007.
  18. ^ a b Diazepam Monograph. Accessed 15 April 2021.
  19. ^ Haberfeld H, ed. (2020). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Epilan D 100 mg-Tabletten.
  20. ^ Buprenorphine Monograph. Accessed 15 April 2021.
  21. ^ "Klonopin (clonazepam) Prescribing Guide" (PDF). Genetech USA, Inc. October 2017. Retrieved 20 January 2019.
  22. PMID 22469221
    . Plasma donepezil concentrations decline with a half-life of approximately 70 h. Sex, race, and smoking history have no clinically significant influence on plasma concentrations of donepezil [46–51].
  23. ^ a b Fluoxetine Monograph. Accessed 15 April 2021.
  24. ^ Haberfeld H, ed. (2020). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Sedacoron 200 mg-Tabletten.
  25. ^ "Caprelsa (vandetanib) Tablets, for Oral Use. Full Prescribing Information" (PDF). Sanofi Genzyme, Cambridge, MA, Dec 2016. Retrieved 24 February 2020.
  26. ^ Haberfeld H, ed. (2020). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Avodart 0,5 mg Weichkapseln.
  27. ^ "Sirturo (bedaquiline) Tablets. Full Prescribing Information" (PDF). Janssen Products, Dec 2012. Retrieved 24 February 2020.
  28. PMID 16192916
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  29. ^ Griffin et al. 1975 as cited in ATSDR 2005
  30. ^ Rabinowitz et al. 1976 as cited in ATSDR 2005
  31. ^
    PMID 26441508
    .
  32. ISBN 9780071481274
    . Oxytocin can be delivered to humans via nasal spray following which it crosses the blood–brain barrier. ... In a double-blind experiment, oxytocin spray increased trusting behavior compared to a placebo spray in a monetary game with real money at stake.
  33. PMID 18475254
    . Recent studies also highlight remarkable anxiolytic and prosocial effects of intranasally administered OT in humans, including increased 'trust', decreased amygdala activation towards fear-inducing stimuli, improved recognition of social cues and increased gaze directed towards the eye regions of others (Kirsch et al., 2005; Kosfeld et al., 2005; Domes et al., 2006; Guastella et al., 2008)
  34. S2CID 25253083
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  35. PMID 22467107
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  36. ISBN 978-0-08-091922-5
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  37. S2CID 57512164
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  38. ISBN 978-1-4614-8947-4
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  39. ^
    ISBN 9780971176744
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  40. PMID 15601438
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  41. ISBN 978-0-306-46820-9
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