The plasma concentration-time profile

The plasma concentration-time profile following extravascular administration (e.g., oral or intramuscular administration) typically includes several key pharmacokinetic (PK) and pharmacodynamic (PD) phases and parameters.

Here is a neat plasma concentration-time profile following extravascular administration. The graph highlights key pharmacokinetic parameters like the absorption phase, C_max, T_max, and elimination phase, along with important markers such as the therapeutic window between MEC and MTC.

 

Description of the Profile:

1. Absorption Phase:

   • The drug concentration rises as it enters the bloodstream from the site of administration. The rate of absorption affects the shape and steepness of this phase.

2. C_max (Maximum Concentration):

   • The peak plasma concentration after the drug is administered. It represents the highest concentration the drug reaches in the plasma.

3. T_max (Time to Maximum Concentration):

   • The time at which the peak concentration (C_max) is achieved. It gives an idea of how quickly the drug is absorbed.

4. Distribution Phase:

   • After absorption, the drug is distributed to tissues and organs. The plasma concentration may decrease slightly as the drug is redistributed.

5. Elimination Phase:

   • The drug concentration starts declining as the drug is metabolized or excreted. This phase follows first-order kinetics for most drugs, where a constant proportion of the drug is eliminated per unit of time.

6. Half-Life (T_1/2):

   • The time required for the plasma concentration to decrease by half. This is critical for determining dosing intervals.

7. Area Under the Curve (AUC):

   • A measure of the total exposure to the drug over time. It reflects the extent of absorption and is proportional to the bioavailability of the drug.

8. MEC (Minimum Effective Concentration):

   • The minimum plasma concentration required for the drug to have a therapeutic effect.

9. MTC (Maximum Tolerable Concentration):

   • The highest concentration that does not result in unacceptable side effects or toxicity.

10. Therapeutic Window:

    • The concentration range between the MEC and MTC, where the drug is effective without causing toxicity.

Pharmacokinetic (PK) Parameters:

• Absorption Rate Constant (Ka): Describes the rate at which the drug enters the bloodstream.
• Elimination Rate Constant (Ke): Describes the rate at which the drug is removed from the body.

Pharmacodynamic (PD) Parameters:

• E_max: Maximum drug effect.
• EC_50: The concentration of the drug at which 50% of the maximum effect is observed.