Aditoprim (AP) is a new dihydrofolate reductase inhibitor, which is structurally related to trimethoprim (TMP). The pharmacokinetics of AP (10 mg/kg) and TMP (20 mg/kg) were assessed in healthy dwarf goats. Therapeutic efficacy against rickettsial infections was tested in tick-borne fever (TBF) infected goats. The animals were given TMP (n = 5) or AP (n = 5) by i.v. injection, and subsequently the drugs were administered orally (same groups, similar doses). Finally, both groups were infected with TBF and the i.v. experiment was repeated. Plasma concentration-time curves for both drugs followed first-order two-compartment decay. For TMP, mean t1/2 beta +/- SEM (h) was 0.84 +/- 0.06 (i.v. control) and 0.90 +/- 0.06 (i.v. infected), respectively, whereas for AP values of 8.00 +/- 0.31 (i.v. control) and 10.28 +/- 0.67 (i.v. infected) were obtained (P less than 0.05). Mean Vd beta +/- SEM values (l/kg) were 3.84 +/- 0.27 (i.v. control) and 4.07 +/- 0.85 (i.v. infected) for TMP (NS) and 7.02 +/- 0.63 vs 9.29 +/- 0.21 (P less than 0.05) for AP. After i.v. injection, rumen fluid concentrations of AP were significantly (P less than 0.05) higher and more persistent than those of TMP. For AP, the plasma and rumen fluid concentrations at 3 h were 1.20 +/- 0.06 micrograms/ml and 0.85 +/- 0.17 microgram/ml, respectively. After oral administration of TMP, Cmax in plasma was 0.12 +/- 0.01 microgram/ml and the maximum was reached after 1.2 +/- 0.16 h; systemic bioavailability (F) was 10.3% (relative to AUC i.v.). Oral treatment with AP resulted in a Cmax value of 0.21 +/- 0.02 microgram/ml with Tmax of 22.5 +/- 1.65 h and a F value of 71%. Based on WBC, serum ALP and rectal temperature responses, it was concluded that both TMP and AP were inactive against Ehrlichia phagocytophila.

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