Allometric basis of enrofloxacin scaling in green iguanas

When body size varies greatly, drug disposition can best be described as an allometric function of body weight. Therefore, the allometry of standard metabolic rate (SMR; 3/4 power) and body surface area (BSA; 2/3 power) have been advocated as surrogate markers for the prediction of key pharmacokinetic parameters. The goal of the present study was to examine the allometric basis of pharmacokinetic scaling within a species, green iguanas. Enrofloxacin was administered intravenously to 20 green iguanas (322–3824 g), and noncompartmental analysis was used to calculate standard pharmacokinetic parameters, which were log₁₀ transformed and regressed against log₁₀ body weight. The slopes of significant regressions were compared with the values of unity, 3/4, and 2/3. The slope of enrofloxacin total body clearance (Cl) was also compared with the slopes relating SMR and renal Cl of ^99mTc-diethylenetriamine penta-acetic acid (^99mDTPA) to body weight in iguanas. Enrofloxacin Cl depended allometrically on body weight with the power of 0.32. The slope of enrofloxacin Cl was significantly less than those of SMR, Cl of ^99mDTPA, and the 2/3 value. Therefore, the relationship between enrofloxacin Cl and body weight does not directly depend on the allometry of BSA, SMR, or renal Cl of ^99mDTPA in iguanas.