Pharmacokinetics and ex vivo anti-inflammatory effects of oral misoprostol in horses

Current Topics in Medical Colic

The list of medical causes of acute or chronic colic in horses is extensive. The purpose of this article is to review 4 medical causes of equine colic with a focus on newer trends in treatment. The 4 topics selected include gastric impaction, gastric glandular disease, colon displacement, and inflammatory bowel disease.

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 administration. The relative bioavailability of rectal administration was 4 to 86% of that of oral administration. Microbial composition, 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 interindividual variation and management rather than misoprostol administration.

Oral misoprostol does not hasten oviductal transport of day-5 horse embryos

In horses, prostaglandin E₂ (PGE₂) is produced by embryos around Day 5 post-ovulation; PGE₂ functions directly at the oviduct promoting embryo transport into the uterus. Non-surgical collection of horse embryos for cryopreservation is recommended at Day 6.5-7 post-ovulation. It was proposed that misoprostol administered orally will hasten oviductal transport of horse embryos. In Experiment 1 (n = 15) there was comparison of time of embryo recovery (Day 6 and 6.5 post-ovulation) from mares administered misoprostol (Day 5 and 5.5) orally to that of untreated mares. On Day 6, embryo collections were attempted; if no embryo was collected, there was a second attempt on Day 6.5. In Experiment 2, (n = 16) misoprostol treatment was initiated on Day 4.5; there was the first embryo collection attempt on Day 5.5, followed by Day 6 and 6.5 if no embryo was collected. Blood samples were collected at 12 h intervals on Day 4.5 or 5, to Day 6.5. In Experiment 1, on days 6 and 6.5, respectively, there was collection of seven and one of a total of eight embryos detected at the time of collection per group (P = 1). In Experiment 2, 12 embryos were collected during 15 cycles with there being a total of three, two, and one collected from mares of both groups on Day 5.5, 6, and 6.5 post-ovulation, respectively (P = 1). Serum progesterone concentrations were not different (P ≥ 0.05). In conclusion, misoprostol, when administered orally, does not hasten oviductal transport of horse embryos.

Single-dose pharmacokinetics of orally and rectally administered misoprostol in adult horses

OBJECTIVE

To characterize the pharmacokinetics of a clinically relevant dose of misoprostol administered PO or per rectum (PR) to horses.

ANIMALS

8 healthy adult horses.

PROCEDURES

In a randomized 3-way crossover design, horses received a single dose of misoprostol (5 μg/kg) administered PO (with horses fed and unfed) and PR, with a minimum 3-week washout period separating the experimental conditions. Blood samples were obtained before and at various points after drug administration (total, 24 hours), and plasma concentrations of misoprostol free acid were measured.

RESULTS

Mean maximum plasma concentration of misoprostol was significantly higher in the PR condition (mean ± SD, 967 ± 492 pg/mL) and unfed PO condition (655 ± 259 pg/mL) than in the fed PO condition (352 ± 109 pg/mL). Mean area under the concentration-versus-time curve was significantly lower in the PR condition (219 ± 131 pg•h/mL) than in the unfed (1,072 ± 360 pg•h/mL) and fed (518 ± 301 pg•h/mL) PO conditions. Mean time to maximum concentration was ≤ 30 minutes for all conditions. Mean disappearance half-life was shortest in the PR condition (21 ± 29 minutes), compared with values for the unfed (170 ± 129 minutes) and fed (119 ± 51 minutes) PO conditions. No adverse effects were noted.

CONCLUSIONS AND CLINICAL RELEVANCE

Misoprostol was rapidly absorbed and eliminated regardless of whether administered PO or PR to horses. Rectal administration may be a viable alternative for horses that cannot receive misoprostol PO, but this route may require more frequent administration to maintain therapeutic drug concentrations.