The pharmacokinetics of methocarbamol and guaifenesin after single intravenous and multiple-dose oral administration of methocarbamol in the horse

A simple LC/MSMS method has been developed and fully validated to determine concentrations and characterize the concentration vs. time course of methocarbamol (MCBL) and guaifenesin (GGE) in plasma after a single intravenous dose and multiple oral dose administrations of MCBL to conditioned Thoroughbred horses. The plasma concentration–time profiles for MCBL after a single intravenous dose of 15 mg/kg of MCBL were best described by a three‐compartment model. Mean extrapolated peak (C0) plasma concentrations were 23.2 (±5.93) μg/mL. Terminal half‐life, volume of distribution at steady‐state, mean residence time, and systemic clearance were characterized by a median (range) of 2.96 (2.46–4.71) h, 1.05 (0.943–1.21) L/kg, 1.98 (1.45–2.51) h, and 8.99 (6.68–10.8) mL/min/kg, respectively. Oral dose of MCBL was characterized by a median (range) terminal half‐life, mean transit time, mean absorption time, and apparent oral clearance of 2.89 (2.21–4.88) h, 2.67 (1.80–2.87) h, 0.410 (0.350–0.770) h, and 16.5 (13.0–20) mL/min/kg. Bioavailability of orally administered MCBL was characterized by a median (range) of 54.4 (43.2–72.8)%. Guaifenesin plasma concentrations were below the limit of detection in all samples collected after the single intravenous dose of MCBL whereas they were detected for up to 24 h after the last dose of the multiple‐dose oral regimen. This difference may be attributed to first‐pass metabolism of MCBL to GGE after oral administration and may provide a means of differentiating the two routes of administration.