Bioassay of amoxicillin in rats

Few reports are available about tissue concentration of amoxicillin. The techniques used to measure tissue concentration usually require rupture and are expensive. The objective of the present study is to assess the utility of an animal model to predict tissue concentration of amoxicillin using induced granulomatous tissue. We used 160 rats with four polyurethane sponges previously implanted in their backs. At 7, 14, 21 and 28 d after sponge introduction, groups of eight animals each received 3.5, 7.0, 40.0 or 80.0 mg/kg of amoxicillin (p.o.) or 1 ml of 0.9% NaCl solution (control group). One hour after drug administration, 10 microl of serum and granulomatous tissue were obtained. Tissue and serum were placed on different plates containing Mueller Hinton agar inoculated with 10(8) cfu (colony forming unit) of Staphylococcus aureus (ATCC 25923), and the diameters of the inhibition zones were measured after 18 h of incubation. Analysis of variance showed no statistically significant differences (p>0.05) among time periods for the same dose of amoxicillin. These results suggest that the pharmacokinetics of amoxicillin did not change in relation to the development of granulomatous tissue; therefore this method is valid to measure the tissue concentration of amoxicillin.