Multidose misoprostol pharmacokinetics and its effect on the fecal microbiome in healthy, adult horses

OBJECTIVE To compare the pharmacokinetics between repeated doses and to characterize changes in the fecal microbiome after oral and rectal multidose misoprostol administration. ANIMALS 6 healthy university-owned geldings. PROCEDURES In a randomized, crossover study, misoprostol (5 μg/kg) was administered orally or rectally every 8 hours for 10 doses, or not administered (control), with a 21-day washout between treatments. Concentration-versus-time data for dose 1 and dose 10 were subject to noncompartmental analysis. For microbiota analysis using 16S rRNA amplicon sequencing, manure was collected 7 days before study onset, immediately before dose 1, and 6 hours, 7 days, and 14 days after dose 10, with time-matched points in controls. RESULTS Repeated dosing-related differences in pharmacokinetic parameters were not detected for either administration route. The area under the concentration-versus-time curve was greater (P <.04) after oral versus rectal administra-tion. The relative bioavailability of rectal administration was 4 to 86% of that of oral administration. Microbial com-position, richness, and β-diversity differed among subjects (P <.001 all) while only composition differed between treatments (P ≤.01). Richness was decreased 6 hours after dose 10 and at the control-matched time point (P =.0109) in all subjects. No other differences for time points, treatments, or their interactions were observed. CLINICAL RELEVANCE Differences in systemic exposure were associated with the route of administration but were not detected after repeated administration of misoprostol. Differences in microbiota parameters were primarily associated with inter-individual variation and management rather than misoprostol administration.