How does body weight influence PK parameter scaling?

• A. Allometric scaling: CL ∝ BW^0.75; Vd ∝ BW^1.0 (though exponents may vary)
• B. Allometric scaling: CL ∝ BW^1.25; Vd ∝ BW^0.5
• C. Allometric scaling: CL ∝ BW^0.5; Vd ∝ BW^0.5
• D. Allometric scaling: CL ∝ BW^1.0; Vd ∝ BW^1.5

Answer: (A)

Allometric scaling shows how PK parameters change with body size using power-law relationships. Clearance, which reflects metabolic capacity and organ function, tends to increase with body weight but not in direct proportion; the established relationship is CL ∝ BW^0.75. Volume of distribution, representing the space the drug can occupy in the body, generally scales with body size more directly, so Vd ∝ BW^1.0. This means that when comparing individuals of different weights, clearance grows with weight to the 0.75 power, while the distribution volume grows roughly linearly with weight.

So, scaling from one body weight to another, you’d adjust clearance by the factor (BW2/BW1)^0.75 and volume by (BW2/BW1)^1.0. The option with CL ∝ BW^0.75 and Vd ∝ BW^1.0 matches these standard allometric relationships.

The other exponent patterns don’t align with typical physiological scaling: a higher exponent for clearance would overstate how quickly clearance rises with weight, a 0.5 exponent for both would understate how clearance scales, and a Vd exponent of 1.5 implies volumes enlarge more than body size, which isn’t commonly observed for most drugs.