tmax and t1/2 or peak plasma time and elimination half-life of a drug often cause confusion. Frequently these terms tmax and t1/2 are used to understand many equations or topics in biopharmaceutics. Differences between tmax and t1/2 are:
1. Definition of Peak plasma time or time for peak plasma concentration or time of peak plasma concentration or tmax and t1/2
The t max (time of peak plasma concentration) is the time required to reach maximum drug concentration in the plasma after drug administration. t max is peak plasma time. Simply, t max is the time to reach C max. C max is the maximum (peak) plasma drug concentration attained after the oral administration of the drug. Also, at t max, the drug absorption rate exactly equals the drug elimination rate.
On the other hand, t 1/2 (elimination half-life) is the time required to decrease the drug concentration within the body by one-half during elimination. In other words, t ½ (elimination half-life) is the time required for the amount or concentration of a drug to decrease by one-half [1]. In pharmacokinetic, elimination half-life, t 1/2 refers to the time required for half the dose of drug administered to be removed from the body.
2. The t max is dependent on the elimination rate (k) and absorption rate constant (ka). while t 1/2 is dependent on the elimination rate (k) and volume of distribution (Vd).
3. At tmax, the concentration of a drug is in peak. While at t 1/2, the concentration of a drug may or may not in peak.
4. Equations for tmax and t1/2
The equation for calculating t max (peak plasma time):
𝑡 𝑚𝑎𝑥 = ln (𝑘𝑎 / 𝑘) / (𝑘𝑎−𝑘)
= 2.303 log (𝑘𝑎 / 𝑘)/ (𝑘𝑎−𝑘)
On the other hand, the equation for calculating t½:
t½ = 0.693 × (Vd /CL)
Where, Vd = Volume of distribution and CL = Clearance.
5. t max is peak plasma time. On the other hand, t 1/2 is the elimination half-life.
6. t max (peak plasma time) is a rough marker of the average rate of drug absorption. On the other hand, t 1/2 is an indicator of how fast a drug is eliminated from the body.
Summary of the difference between tmax and t1/2
| tmax | t1/2 | |
| 1. | The time of peak plasma concentration is the time required to reach maximum drug concentration in the plasma after drug administration. | t 1/2 (elimination half-life) is the time required to decrease the concentration of a drug within the body by one-half during elimination. |
| 2. | The t max is dependent on the absorption rate constant (ka) and elimination rate (k). | t 1/2 is dependent on the volume of distribution (Vd) and elimination rate (k). |
| 3. | At t max, the drug concentration is in peak. | At t 1/2, drug concentration may or may not in peak. |
| 4. | Equation for calculating t max:
𝑡 𝑚𝑎𝑥 = ln (𝑘𝑎 / 𝑘) / (𝑘𝑎−𝑘) = 2.303 log (𝑘𝑎 / 𝑘)/ (𝑘𝑎−𝑘) | Equation for calculating t ½:
t½ = 0.693 × (Vd /CL) |
| 5. | It is Peak plasma time. | It is an elimination half-life. |
| 6. | It is a rough marker of the average rate of drug absorption. | The half-life is a useful indicator of how fast a drug is removed from the body. |
| 7. |  |  |
This article may provide the answer to the following questions:
- First of all, the difference between tmax and t1/2.
- Second, the difference between the time of peak plasma concentration (tmax) and elimination half-life (t1/2).
- Third, what is the difference between peak plasma time and elimination half-life?
- Finally, what is peak plasma time or time for peak plasma concentration?
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References
- Shargel, L., & McGraw-Hill Education. (2016). Applied biopharmaceutics & pharmacokinetics. New York: McGraw-Hill Education.