# عمر النصف الحيوي

(تم التحويل من نصف عمر التخلص)
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عمر النصف الحيوي Biological half-life هو الوقت الذي تفقد خلاله مادة فعالة حيوياً نصف نشاطها الدوائي أو الفيزيولوجي أو الشعاعي في الجسم، وهو من المعالم الهامة في الحركيات الدوائية.

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## أمثلة

### الأدوية الموصوفة الشائعة

Substance Biological half-life
Adenosine Less than 10 seconds (estimate)[1]
Norepinephrine 2 minutes[2]
Oxaliplatin 14 minutes[3]
Zaleplon 1 hour[4]
Morphine 1.5–4.5 hours[5]
Flurazepam 2.3 hours[6]

Active metabolite (N-desalkylflurazepam): 47–100 hours[6]

Methotrexate 3–10 hours (lower doses),

8–15 hours (higher doses)[7]

in rare cases up to 8 days[8]

Diazepam 20–50 hours[9]

Active metabolite (nordazepam): 30–200 hours[9]

Phenytoin 20–60 hours[10]
Buprenorphine 28–35 hours[11]
Clonazepam 30–40 hours[12]
Donepezil 3 days (70 hours)[13]
Fluoxetine 4–6 days (under continuous administration)[14]

Active lipophilic metabolite (norfluoxetine): 4–16 days[14]

Amiodarone 14–107 days[15]
Vandetanib 19 days[16]
Dutasteride 21–35 days (under continuous administration)[17]
Bedaquiline 165 days[18]

### الفلزات

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 (physiologically-based pharmacokinetic modelling). 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.[19]

• 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.[20][21]
• 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.

### عمر النصف الطرفي

Some substances may have different half-lives in different parts of the body. For example, oxytocin has a half-life of typically about three minutes in the blood when given intravenously. Peripherally administered (e.g. intravenous) peptides like oxytocin cross the blood-brain-barrier very poorly, although very small amounts (< 1%) do appear to enter the central nervous system in humans when given via this route.[22] In contrast to peripheral administration, when administered intranasally via a nasal spray, oxytocin reliably crosses the blood–brain barrier and exhibits psychoactive effects in humans.[23][24] In addition, also unlike the case of peripheral administration, intranasal oxytocin has a central duration of at least 2.25 hours and as long as 4 hours.[25][26] 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.[22]

## معادلات المعدل

### التخلص من الدرجة الأولى

Timeline of an exponential decay process[27][28][29]
الوقت (t) النسبة من القيمة الأولية نسبة الاكتمال
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 0.781% 99.219%
t½ × 10 0.098% 99.902%

Half-times apply to processes where the elimination rate is exponential. If ${\displaystyle C(t)}$  is the concentration of a substance at time ${\displaystyle t}$ , its time dependence is given by

${\displaystyle C(t)=C(0)e^{-kt}\,}$

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:[30]

${\displaystyle {\frac {dC}{dt}}=-kC.}$

The half-life for this process is[30]

${\displaystyle t_{\frac {1}{2}}={\frac {\ln 2}{k}}.\,}$

Alternatively, half-life is given by

${\displaystyle t_{\frac {1}{2}}={\frac {\ln 2}{\lambda _{z}}}\,}$

where λz is the slope of the terminal phase of the time–concentration curve for the substance on a semilogarithmic scale.[31][32]

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

${\displaystyle t_{\frac {1}{2}}={\frac {{\ln 2}\cdot {V_{D}}}{CL}}\,}$

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.

### عمر النصف ثنائي الطور

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.[30] For a more detailed description see Pharmacokinetics § Multi-compartmental models.

## انظر أيضاً

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## الهامش

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3. ^ Ehrsson, Hans; et al. (Winter 2002). "Pharmacokinetics of oxaliplatin in humans". Medical Oncology. 19 (4): 261–5. doi:10.1385/MO:19:4:261. PMID 12512920. S2CID 1068099. Archived from the original on 2007-09-28. Retrieved 2007-03-28.
4. ^ Zaleplon Monograph. Accessed 15 April 2021.
5. ^ Morphine Monograph. Accessed 15 April 2021.
6. ^ أ ب Flurazepam Monograph. Accessed 15 April 2021.
7. ^ "Trexall, Otrexup (methotrexate) dosing, indications, interactions, adverse effects, and more". reference.medscape.com.
8. ^ Manfredonia, John (March 2005). "Prescribing Methadone for Pain Management in End-of-Life Care". Journal of the American Osteopathic Association. 105 (3 supplement): S18-21. PMID 18154194. Retrieved 2007-01-29.
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13. ^ Asiri, Yousif A.; Mostafa, Gamal A.E. (2010). "Donepezil". Profiles of Drug Substances, Excipients and Related Methodology. 35. Elsevier. pp. 117–150. doi:10.1016/s1871-5125(10)35003-5. ISBN 978-0-12-380884-4. ISSN 1871-5125. 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].
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20. ^ Griffin et al. 1975 as cited in ATSDR 2005
21. ^ Rabinowitz et al. 1976 as cited in ATSDR 2005
22. ^ أ ب Baribeau, Danielle A; Anagnostou, Evdokia (2015). "Oxytocin and vasopressin: linking pituitary neuropeptides and their receptors to social neurocircuits". Frontiers in Neuroscience. 9: 335. doi:10.3389/fnins.2015.00335. ISSN 1662-453X. PMC 4585313. PMID 26441508.
23. ^ Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 7: Neuropeptides". In Sydor A, Brown RY (eds.). Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. p. 195. 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.
24. ^ McGregor IS, Callaghan PD, Hunt GE (May 2008). "From ultrasocial to antisocial: a role for oxytocin in the acute reinforcing effects and long-term adverse consequences of drug use?". British Journal of Pharmacology. 154 (2): 358–68. doi:10.1038/bjp.2008.132. PMC 2442436. 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)
25. ^ Weisman O, Zagoory-Sharon O, Feldman R (2012). "Intranasal oxytocin administration is reflected in human saliva". Psychoneuroendocrinology. 37 (9): 1582–6. doi:10.1016/j.psyneuen.2012.02.014. PMID 22436536. S2CID 25253083.
26. ^ Huffmeijer R, Alink LR, Tops M, Grewen KM, Light KC, Bakermans-Kranenburg MJ, Ijzendoorn MH (2012). "Salivary levels of oxytocin remain elevated for more than two hours after intranasal oxytocin administration". Neuro Endocrinology Letters. 33 (1): 21–5. PMID 22467107.
27. ^ Miles Hacker; William S. Messer; Kenneth A. Bachmann (19 June 2009). Pharmacology: Principles and Practice. Academic Press. p. 205. ISBN 978-0-08-091922-5.
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30. ^ أ ب ت Bonate, Peter L.; Howard, Danny R. (2004). Clinical study design and analysis. Arlington, VA: AAPS Press. pp. 237–239. ISBN 9780971176744.
31. ^ Toutain, P. L.; Bousquet-Melou, A. (2004). "Plasma terminal half-life". Journal of Veterinary Pharmacology and Therapeutics. 27 (6): 427–439. doi:10.1111/j.1365-2885.2004.00600.x. ISSN 0140-7783.
32. ^ Younggil Kwon (8 May 2007). Handbook of Essential Pharmacokinetics, Pharmacodynamics and Drug Metabolism for Industrial Scientists. Springer Science & Business Media. pp. 24–. ISBN 978-0-306-46820-9.