Pharmacokinetic Profile of Fentanyl in the Koala (Phascolarctos cinereus) after Intravenous Administration, and Absorption via a Transdermal Patch

Abstract

Fentanyl was administered as a single intravenous bolus injection at 5 µg/kg to five koalas and fentanyl plasma concentrations for a minimum of 2 h were quantified by an enzyme-linked immunosorbent assay (ELISA). The median (range) fentanyl elimination half-life and clearance were 0.53 (0.38-0.91) h, and 10.01 (7.03-11.69) L/kg/h, respectively. Assuming an analgesic therapeutic plasma concentration of 0.23 ng/mL (extrapolated from human studies), an intravenous constant infusion rate was estimated at approximately between 1.7 to 2.7 µg/kg/h (using the clearance 95% confidence intervals). A transdermal fentanyl patch was applied to the antebrachium of an additional two koalas for 72 h. Fentanyl plasma concentrations were determined during the patch application and after patch removal at 80 h. The fentanyl plasma concentration was greater than 0.23 ng/mL after 12 to 16 h. While the patch was applied, the maximum fentanyl concentration was approximately 0.7 ng/mL from 32 to 72 h. Fentanyl plasma concentrations increased to 0.89 ng/mL 1 h after the patch was removed, and then decreased to a mean of 0.47 ng/mL at 80 h. The transdermal fentanyl patch is likely to provide some level of analgesia but should be initially co-administered with another faster acting analgesic for the first 12 h.

Keywords: Phascolarctos cinereus; analgesia; fentanyl; koala; opioid; transdermal patch.

Figure 1

Mean ± S.D heart rate on administration of fentanyl 5 µg/kg as a single i.v. bolus over the first hour. Normal koala heart rate (bpm) is within dotted lines. There was only a significant difference in mean heart rate between 0 and 2 min (p = 0.04).

Figure 2

Change in fentanyl plasma concentration over time in minutes. Koala 1 dosed at 10 μg/kg the other five koalas were dosed at 5 µg/kg. Dotted lines at 0.23 and 1.0 ng/mL denote suggested analgesic range (see Discussion). Additional dotted line at 0.1 ng/mL denotes assay LLOQ.

Figure 3

Fentanyl concentration vs time after transdermal fentanyl patch (TFP), (with a delivery rate of 25 µg/h), application and removal for both koalas. Dotted lines at 0.23 ng/mL denote suggested minimal analgesic range in human patient controlled analgesia models (see Discussion).