Human pharmacokinetics of the muscle relaxant, eperisone hydrochloride by liquid chromatography-electrospray tandem mass spectrometry.

Eperisone hydrochloride (4'-ethyl-2-methyl-3-piperidinopropiophenone hydrochloride) is a muscle relaxant agent, widely used in the treatment of patients with muscular contractures, low back pain or spasticity. Because of its mechanism of action (inhibition of gamma-efferent firing and local vasodilatation activity), side effects on central nervous system are rarely observed. A sensitive liquid chromatography-electrospray ionization-mass spectrometry method for determination of eperisone in human plasma has been developed, with a lower limit of quantification of 0.01 ng/mL. The method was applied to a pharmacokinetic study in 12 healthy volunteers given eperisone 100 mg as single dose on day 1 and three times daily on days 2 to 4. Eperisone was rapidly absorbed after oral administration (T (max) = 1.6 h) as it was expected by its fast-onset relaxant activity. Moreover, eperisone underwent a rapid elimination from the body (biological half-life 1.87 h), which was not modified during the repeated dosing as suggested by the C (max) cumulation observed, not different from that expected for a t (1/2) of 1.87 h as suggested by the similar and negligible plasma concentration values (0.063 and 0.067 ng/mL) measured on day 4 before the morning dose and 12 h after evening dose, thus ruling out any potential risk for drug accumulation. Thus, the pharmacokinetic characteristics of eperisone provide further justification for its tolerability in patients with low back pain or spastic palsy, in which the drug is given for periods ranging from few days to several months, respectively.