Single- and repeated-dose pharmacokinetics of intramuscular thiocolchicoside in healthy volunteers

The aim of this study was to investigate the pharmacokinetics and the accumulation and stationarity of thiocolchicoside after repeated intramuscular administration.The pharmacokinetics of thiocolchicoside were studied in 6 healthy male volunteers after one single dose and repeated intramuscular doses of 4 mg twice a day for seven days. Plasma and urine samples were assayed for thiocolchicoside levels by a radioimmunoassay (RIA) using a cross-reacting colchicine-specific polyclonal antibody. The pharmacokinetic parameters between the first and the last days were compared using Student's t-test.Thiocolchicoside pharmacokinetic parameters, calculated after the single dose using non-compartmental analysis, were in good agreement with those obtained in previous studies. Following the repeated-dose regimen, the terminal half-life was not significantly different (2.7 (0.3) h) from that predicted from a single-dose (2.8 (0.2) h). The accumulation ratio, based on the repeated-dose/single-dose ratio of AUCtau was approximately 1.25. A decrease of CLT/f was found between day 1 (24.1 (5.2) l/h) and day 7 (19.9 (3.4) l/h), suggesting that moderate time-related alterations occur in the pharmacokinetics of thiocolchicoside, which may be due to a change in its CL(NR) (CL(R) was constant) or to the extent of bioavailability, explained by enterohepatic recirculation.Serum thiocolchicoside concentrations accumulated to steady-state when the drug was given twice a day for seven days and the pharmacokinetics were modified. But no adjustments of dose or dosing interval were necessary because the accumulation did not lead to marked change in the plasma levels.