Relationship between plasma concentration and effect of dantrolene sodium in man.

Dantrolen sodium is a muscle relaxant, which is used in the treatment of spasticity. Although it is given chronically, little is known about its pharmacokinetic behaviour. The relationship between the effect of a single oral dose of dantrolene sodium and its plasma concentration in healthy volunteers was studied by measuring the effect on the twitch tension, and in spastic patients on the decrease in muscle hypertonia. On the twitch tension dantrolene gave a depression of 49.1 +/- 9.4% (+/- DS) within 1.15 and 3.45 h after ingestion of 100 mg. The mean maximal plasma concentration was 1.24 +/- 0.32 microgram/ml (+/- SD). The effect and the plasma concentration were correlated. No relationship between the plasma concentration of dantrolene sodium and its effect could be established in patients, although definite activity in 6 out of 7 patients was observed after a single oral dose of 100 mg, and plasma concentration of dantrolene sodium greater than 0.3 microgram/ml were consistenly associated with better results than placebo treatment in 6 out of 7 patients.