Peripheral α 2 -adrenoceptor antagonism affects the absorption of intramuscularly coadministered drugs.

OBJECTIVE: We determined the possible effects of a peripherally acting α2 -adrenoceptor antagonist, MK-467, on the absorption of intramuscularly (IM) coadministered medetomidine, butorphanol and midazolam.

STUDY DESIGN: Randomized, experimental, blinded crossover study.

ANIMALS: Six healthy Beagle dogs.

METHODS: Two IM treatments were administered: 1) medetomidine hydrochloride (20 μg kg-1 ) + butorphanol (100 μg kg-1 ) + midazolam (200 μg kg-1 ; MBM) and 2) MBM + MK-467 hydrochloride (500 μg kg-1 ; MBM-MK), mixed in a syringe. Heart rate was recorded at regular intervals. Sedation was assessed with visual analog scales (0-100 mm). Drug concentrations in plasma were analyzed with liquid chromatography-tandem mass spectrometry, with chiral separation of dex- and levomedetomidine. Maximum drug concentrations in plasma (Cmax ) and time to Cmax (Tmax ) were determined. Paired t-tests, with Bonferroni correction when appropriate, were used for comparisons between the treatments.

RESULTS: Data from five dogs were analyzed. Heart rate was significantly higher from 20 to 90 minutes after MBM-MK. The Tmax values for midazolam and levomedetomidine (mean ± standard deviation) were approximately halved with coadministration of MK-467, from 23 ± 9 to 11 ± 6 minutes (p = 0.049) for midazolam and from 32 ± 15 to 18 ± 6 minutes for levomedetomidine (p = 0.036), respectively.

CONCLUSIONS AND CLINICAL RELEVANCE: MK-467 accelerated the absorption of IM coadministered drugs. This is clinically relevant as it may hasten the onset of peak sedative effects.