Plasma Drug Concentration vs. Time Curve

Cmax (Peak Plasma Concentration)

The highest concentration of drug in the blood.

• Board Concept: It represents both the rate and the extent of absorption. If you increase the dose, $C_{max}$ increases.

tmax (Time of Peak Concentration)

The time it takes to reach Cmax.

• Board Concept: It represents the rate of absorption. A smaller $t_{max}$ means faster absorption (e.g., a liquid solution has a smaller $t_{max}$ than a solid tablet). At $t_{max}$, the rate of absorption exactly equals the rate of elimination.

AUC (Area Under the Curve)

The total integrated area under the plasma concentration-time profile.

• Board Concept: It represents the extent of systemic exposure (total amount of intact drug that reached systemic circulation). It is the primary metric used to calculate bioavailability.

MEC (Minimum Effective Concentration)

The baseline plasma concentration required to produce the desired pharmacological effect. Anything below this line is subtherapeutic.

Therapeutic Window (or Therapeutic Range)

The vertical distance between the MEC and MTC. Drugs with a narrow therapeutic window (e.g., Digoxin, Lithium, Warfarin, Theophylline) require strict therapeutic drug monitoring (TDM).

Onset of Action

The exact time (t) it takes for the plasma curve to cross the MEC line and start working.

Duration of Action

The total time the drug concentration remains above the MEC line

Intensity of Action

The vertical distance between Cmax and the MEC. Higher intensity usually correlates with a stronger clinical effect

The Three Phases of the Oral Curve

_{^{When you take an oral tablet, the curve naturally divides into three distinct slopes based on the competing rates of absorption (ka) and elimination (ke).}}

Absorption Phase

Peak (Cmax)

Elimination Phase

Absorption Phase

Slope Direction: Ascending (Upward)

Mathematical Relationship: Rate of Absorption > Rate of Elimination

What is happening?: The drug is rapidly moving from the GI tract into the systemic circulation.

Peak (Cmax)

Slope Direction: Flat (Zero slope)

Mathematical Relationship: Rate of Absorption = Rate of Elimination

What is happening?: Maximum blood concentration. Absorption is complete or perfectly balanced by elimination.

Elimination Phase

Slope Direction: Descending (Downward)

Mathematical Relationship: Rate of Elimination > Rate of Absorption

What is happening?: The liver and kidneys are clearing the drug faster than it is entering the blood.

Crucial Distinction for IV Drugs

An Intravenous (IV) bolus dose does not have an absorption phase. The curve immediately starts at its absolute peak ($C_0$ or $C_{max}$) at $t=0$, and only consists of a declining elimination phase.

Absolute Bioavailability (F)

Compares the systemic availability of an oral dosage form to an Intravenous (IV) reference dose (which is 100% bioavailable).

F = AUC oral / AUV IV X Dose IV / Dose oral

Relative Bioavailability

Compares the bioavailability of one oral dosage form (like a generic test drug) to a recognized standard (the brand-name innovator drug).

Relative F = AUC test / AUC standard X Dose standard / Dose test

Volume of Distribution ($V_d$)

Relates the amount of drug in the body ($X_0$ or Dose) to the initial plasma concentration ($C_0$).

Vd = X0 / C0

Clearance ($Cl$)

The volume of plasma completely cleared of drug per unit time. It ties together the elimination rate constant ($k_e$) and the volume of distribution ($V_d$).

Cl = ke x Vd

Alternatively, using the entire curve:

Cl = Dose IV / AUC

Three Pharmacokinetic (PK) Parameters

Peak Plasma Concentration

Time of Peak Concentration

Area Under the Curve

Pharmacodynamic (PD) Parameters

MEC (Minimum Effective Concentration)

MTC (Minimum Toxic Concentration)

Therapeutic Window (or Therapeutic Range)

Onset of Action

Duration of Action

Intensity of Action