The bioavailability and disposition kinetics of genistein in cats

The absorption and disposition kinetics of the soy isoflavone genistein were determined in cats (n = 6). An oral dose of 100 mg/kg was administered, which has previously been demonstrated to be the minimum oral estrogenic dose, and was administered intravenously at a dose of 20 mg/kg, being the largest practical dose that could be safely administered. Plasma free, and total (conjugated + free) genistein concentrations were determined by HPLC following organic extraction. Noncompartmental analysis revealed a half-life of 21.67 ± 7.9 h (free) and 9.95 ± 2.7 h (conjugated), volume of distribution 31.94 ± 10.38 L/kg (free) and 11.82 ± 3.96 L/kg (conjugated) following intravenous administration. Following oral administration the half-lives were determined to be 17 ± 4.8 h (free) and 8.56 ± 4.65 h (conjugated), with tmax = 4.4 ± 0.6 h (free) and 4.42 ± 0.99 h (conjugated), and Cmax = 0.276 ± 0.1 μg/mL (free) and 6.24 ± 6.58 μg/mL (conjugated). Oral bioavailabilities were 1.379 ± 0.9% (free) and 29.85 ± 22.61% (conjugated). The ratio of total:free genistein ranged from 25.9 to 5.5. Poor oral absorption and efficient conjugation explain the low bioavailability of free genistein. Accumulation of genistein in peripheral lipophilic compartments may occur.