CURRENTLY RECOMMENDED ORAL REGIMENS FOR RITODRINE TOCOLYSIS RESULT INEXTREMELY LOW PLASMA-LEVELS

OBJECTIVE: Our aim was to compare plasma drug levels in patients recei ving ritodrine intravenously with those in patients receiving ritodrin e orally at recommended dosages. STUDY DESIGN: Plasma samples from 20 pregnant patients treated with intravenous ritodrine (50 to 300 mug/mi n), 9 patients treated with oral ritodrine only (60 to 120 mg per 24 h ours), and 9 patients treated first with intravenous and subsequently with oral ritodrine were analyzed for ritodrine concentration with the use of high-performance liquid chromatography. RESULTS: Average plasm a ritodrine levels of patients receiving different intravenous dosages ranged from 27.8 +/- 3.5 to 113.3 +/- 38.8 ng/ml. Levels during oral therapy ranged between 9.8 +/- 3.2 and 13.8 +/- 4.4 ng/ml. In both mod es of drug delivery, concentrations were significantly correlated with doses. In patients treated first with intravenous ritodrine and subse quently with the oral form, plasma concentrations during oral therapy averaged 27.7% +/- 18.8% of those obtained during intravenous infusion . CONCLUSION: Subtherapeutic plasma concentrations might be responsibl e for the failure to demonstrate clinical benefits of oral ritodrine i n prevention of recurrent preterm labor. A twofold to threefold increa se in the maximum recommended oral dosage of ritodrine should be consi dered, especially for patients who had previously required relatively high intravenous infusion rates (> 100 mug/min).