PHARMACOKINETICS OF RITODRINE ADMINISTERED INTRAVENOUSLY - RECOMMENDATIONS FOR CHANGES IN THE CURRENT REGIMEN

We define the pharmacokinetics of ritodrine in 13 pregnant women who received the drug intravenously. With constant infusion of 50 μg/minute, steady state ritodrine concentrations reached 28 ± 11 ng/ml (SD) with a range of 15 to 45 ng/ml. This wide variation is a result of differences in plasma clearance, which ranged from 1.0 to 3.3 L/min, mean 1.94 ± 0.71 L/min. The apparent volume of distribution was 6.95 ± 3.54 L/kg, indicating that ritodrine is extensively bound to extravascular tissue. When an infusion of ritodrine is stopped, plasma concentrations fall rapidly initially with a distribution half-life of 5.9 ± 6.0 minutes. After the initial rapid fall, plasma concentrations decrease more slowly with a mean disposition half-life of 156 ± 51 minutes. On the basis of the pharmacokinetic parameters defined, we recommend that the current infusion regimen for ritodrine be changed. The infusion rate of ritodrine should start at 50 μg/minute rather than 100 μg/minute. The maximal infusion rate of 350 μg/minute should be increased and once labor is inhibited, the infusion rate should be reduced. © 1990.