Pharmacokinetics of sulfamethoxazole and trimethoprim association in hens

A pharmacokinetic study of sulfamethoxazole (SMZ) and trimethoprim (TMP) was carried out on Warren hens after intravenous (iv) dosage of 4:1, and oral administration of dosages of 5:1, and 2.5:1. The SMZ and TMP half-lives were 8.2 and 2.4 hr, respectively. The ratio between SMZ and TMP serum levels (S/T ratio), after iv administration of the 4:1 dosage, was approximately 20 and remained constant for only 1 hr, while after 5:1 and 2.5:1 oral dosages, the S/T ratio was 40 and 20, respectively, and remained invariable for 12 hr. The oral bioavailability of SMZ and TMP was .46 and .36, respectively. The apparently slower elimination of TMP from serum following oral administration should provide an extended duration of the synergistic antibacterial effect when this route of administration is employed. Finally, the 2.5:1 combination, after oral administration, resulted in an appropriate synergistic ratio (S/T ratio = 20:1) of the drugs in the serum.