Oral administration of an extended-release formulation of nitrofurantoin results in high concentrations in the urine of dogs

OBJECTIVE Sporadic bacterial cystitis in both dogs and humans is often caused by Escherichia coli. In humans, nitrofurantoin is a first-line antimicrobial for the treatment of bacterial cystitis but in dogs a lack of available data may be part of the reason it is only recommended as a second-line treatment. The objective of this preliminary study was to determine the plasma pharmacokinetics and urine concentrations of nitrofurantoin monohydrate-macrocrystalline in dogs. ANIMALS 8 healthy female hound dogs. PROCEDURES From July 26 to July 28, 2021, dogs received a single oral dose of nitrofurantoin monohydrate-macrocrystalline 100 mg with food. Blood and urine were collected at predetermined times. Nitrofurantoin concentrations were assayed by UPLC-MS/MS and plasma data were analyzed using noncompartmental methods. RESULTS Plasma concentrations were low for all dogs with a mean +/- SD maximum concentration (C-max) of 0.242 +/- 0.098 mu g/mL (range, 0.14 to 0.42 mu g/mL) occurring between 2 and 24 hours. Urine concentrations were manyfold higher than for plasma. C-max in urine was 134 +/- 54 mu g/mL (range, 49.1 to 218 mu g/mL) occurring between 6 and 36 hours. As seen in other species, nitrofurantoin concentrated in urine with concentrations being 500 times higher than the concentration in plasma.