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

The research article presents an investigation into the pharmacokinetics, the study of how drugs are absorbed, distributed, metabolised, and eliminated by the body, of two drugs – methocarbamol (MCBL) and guaifenesin (GGE) in horses. The study analysed the concentration and behavior over time of these drugs in plasma, both after a single intravenous dose and multiple oral doses of MCBL to conditioned Thoroughbred horses.

Methodology

• The researchers developed a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to measure the concentration of MCBL and GGE in the plasma of the horses. This method was fully validated for accuracy and reliability.
• The study referred to MCBL’s plasma concentration-time profiles which was best described by a three-compartment model, which is a model used in pharmacokinetics to represent the drug distribution in the body.
• The horses were observed after a single intravenous dose of 15 mg/kg of MCBL, as well as after multiple oral doses.

Results

• The mean peak plasma concentrations were found to be 23.2 (± 5.93) μg/mL.
• The pharmacokinetic parameters like terminal half-life, volume of distribution at steady-state, mean residence time, and systemic clearance were measured. These parameters help to understand how the body absorbs, distributes, metabolises and eliminates the drug.
• The values for the terminal half-life, mean transit time, mean absorption time, and apparent oral clearance were obtained and specified for both the intravenous and oral administration.
• The bioavailability of MCBL when administered orally was determined to be between 43.2% and 72.8%.
• The GGE plasma concentrations were below detection limit after the single dose of MCBL intravenously but was detected for up to 24 hours after the last oral dose.

Conclusion

• The researchers attributed the difference in GGE detection between the two methods of administration to first-pass metabolism of MCBL to GGE, which occurs when a drug is absorbed from the gut and distributed to the liver to be metabolized before it reached systemic circulation.
• This finding may provide a way to differentiate the two modes of administration in future studies or practical applications, as each has its own benefits and drawbacks.