• Birgitte Buur Rasmussen, Lotte Seiding Larsen, and Thomas Senderovitz.
• Department of Experimental Medicine, Ferring Pharmaceuticals A/S, Copenhagen, Denmark.
• BJOG. 2005 Nov 1;112(11):1492-9.

ObjectivesIn two separate trials, we studied the concomitant administration of atosiban with labetalol and betamethasone to determine any possibility of a clinically relevant pharmacokinetic interaction.DesignStudy 1 was an open-label, single dose atosiban, multiple dose labetalol, interaction study. Study 2 was an open-label, randomised, three-period crossover pharmacokinetic study.SettingThe studies were carried out at the Clinical Pharmacology Unit of AAI Deutschland GmbH & Co KG, Neu-Ulm, Germany.PopulationThe study population consisted of healthy female volunteers.MethodsIn Study 1, 14 healthy female volunteers participated. On study day 1, a 12-hour intravenous infusion of 114.75 mg atosiban was administered; on days 2-4, participants received labetalol orally (100 mg twice daily), and on study day 5 they received the combined treatment. In Study 2, a total of 18 healthy female volunteers received, on three separate occasions, a 12-hour intravenous infusion of 114.75 mg atosiban, a single intramuscular injection of 12 mg betamethasone or the two drugs in combination.Main Outcome MeasureFor Study 1, the outcome parameter for atosiban was area under the plasma concentration-time curve (AUC); the study parameters for labetalol were AUC, maximum plasma concentration (C(max)) and time to C(max) (t(max)). In Study 2, AUC, C(max) and time to C(max) (t(max)) were assessed for atosiban and betamethasone.ResultsLabetalol had no clinically relevant influence on the bioavailability (AUC) of atosiban. For labetalol, the co-administration with atosiban did not affect the extent of bioavailability, however, C(max) decreased by 36% and t(max) increased by 45 minutes. The C(min) was not affected by atosiban. The betamethasone and atosiban combination led to similar mean plasma concentration-time curves as the administration of each substance alone. Pharmacokinetic parameters (AUC, C(max), t(max)) did not differ markedly between treatments and all 90% CIs for ratios between treatments were fully within limits (80-125%). The co-administration of atosiban with labetalol or betamethasone resulted in similar tolerability to each substance alone.ConclusionThe co-administration of atosiban with betamethasone or labetalol had no clinically relevant influence on their bioavailability or tolerability.

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