PHARMACOKINETICS OF THIOCOLCHICOSIDE IN HUMANS USING A SPECIFIC RADIOIMMUNOASSAY

The myorelaxant thiocolchicoside (TC), an analogue of colchicine (COL) , was assayed in plasma and urine by a radioimmunoassay (RIA) using a cross-reacting COL-specific polyclonal antibody. Cross-reactivity was 56% for TC, giving a limit of quantification of 0.5 ng/ml and a linear response from 0.5 to 100 ng/ml, Specificity was checked by cross-reac tivity studies with COL analogues and by using liquid chromatography a nd RIA in tandem on urine samples. Two immunoreactive peaks were detec ted, but the nonspecific peak represented <2% of the total urine conce ntration of TC. Pharmacokinetics of TC following infusion of 4 mg in t wo subjects revealed a moderate distribution (Vss from 31 to 35 L) and mainly extrarenal elimination (75% of the total body clearance). Term inal half-lives ranged from 2.4 to 2.7 h in plasma and from 3.2 to 3.7 h in urine.