How is allometric scaling used in pharmacokinetics?

• A. Cl ∝ weight^1.0; Vd ∝ weight^0.75
• B. Cl ∝ weight^0.5; Vd ∝ weight^1.5
• C. Cl ∝ weight^0.75; Vd ∝ weight^1.0
• D. Cl ∝ weight^1.25; Vd ∝ weight^0.5

Answer: (C)

Allometric scaling uses power-law relationships to relate PK parameters to body size. Clearance, which reflects how quickly the body removes a drug, tends to scale with body weight to the 3/4 power because metabolic capacity and organ function increase with size in a sublinear way. Volume of distribution, which describes how extensively a drug spreads into body tissues, generally scales roughly linearly with body weight since larger bodies have proportionally more tissue and body water to accommodate distribution. Putting these together gives Cl ∝ weight^0.75 and Vd ∝ weight^1.0. This framework lets us translate PK across sizes, such as from animal data to humans or across pediatric to adult patients, by applying the same weight-based exponents. The other exponent combos don’t fit the observed scaling patterns: they either over- or under-estimate how clearance or distribution space changes with size, leading to inaccurate predictions.