The pharmacokinetics of a single oral or rectal dose of concurrently administered isoniazid, rifampin, pyrazinamide, and ethambutol in Asian elephants (Elephas maximus)

Tuberculosis, caused by Mycobacterium tuberculosis, is a disease of concern in captive Asian elephants (Elephas maximus). Treatment for tuberculosis in elephants utilizes multidrug protocols combining isoniazid, rifampin, pyrazinamide, and/or ethambutol. In this study, a single, coformulated dose of isoniazid 5 mg/kg, rifampin 10 mg/kg, pyrazinamide 30 mg/kg, and ethambutol 30 mg/kg was administered orally to six Asian elephants, and rectally to five elephants using a cross-over design. Blood samples were collected serially over 24 h. Pyrazinamide and ethambutol concentrations were determined using validated gas chromatography assays. Isoniazid and rifampin concentrations were determined using validated high-performance liquid chromatography assays. Rectal isoniazid produced an earlier Tmax compared with oral administration. Oral isoniazid resulted in a comparatively lower Cmax , but higher AUC values compared with rectal isoniazid. Oral rifampin and oral ethambutol were well absorbed while rectal rifampin was not. Oral pyrazinamide produced comparatively higher Cmax and AUC values compared with rectal pyrazinamide. Results of this study indicate that currently recommended therapeutic monitoring sample collection times for rectal isoniazid and oral rifampin do not provide an accurate assessment of exposure for these drugs. This study demonstrates notable individual variability, indicating that dosing of these medications requires individual monitoring and provides additional information to guide the clinician when treating elephants.