Pharmacokinetic profile of injectable tramadol in the koala (Phascolarctos cinereus) and prediction of its analgesic efficacy

Abstract

Tramadol is used as an analgesic in humans and some animal species. When tramadol is administered to most species it undergoes metabolism to its main metabolites M1 or O-desmethyltramadol, and M2 or N-desmethyltramadol, and many other metabolites. This study describes the pharmacokinetic profile of tramadol when a single subcutaneous bolus of 2 mg/kg was initially administered to two koalas. Based on the results of these two koalas, subsequently 4 mg/kg as a single subcutaneous injection, was administered to an additional four koalas. M1 is recognised as an active metabolite and has greater analgesic activity than tramadol, while M2 is considered inactive. A liquid chromatography assay to quantify tramadol, M1 and M2 in koala plasma was developed and validated. Liquid chromatography-mass spectrometry confirmed that M1 had been identified. Additionally, the metabolite didesmethyltramadol was identified in chromatograms of two of the male koalas. When 4 mg/kg tramadol was administered, the median half-life of tramadol and M1 were 2.89 h and 24.69 h, respectively. The M1 plasma concentration remained well above the minimally effective M1 plasma concentration in humans (approximately 36 ng/mL) over 12 hours. The M1 plasma concentration, when tramadol was administered at 2 mg/kg, did not exceed 36 ng/mL at any time-point. When tramadol was administered at 2 mg/kg and 4 mg/kg the area under the curve M1: tramadol ratios were 0.33 and 0.50, respectively. Tramadol and M1 binding to plasma protein were determined using thawed, frozen koala plasma and the mean binding was 20% and 75%, respectively. It is concluded that when tramadol is administered at 4 mg/kg as a subcutaneous injection to the koala, it is predicted to have some analgesic activity.

Fig 1. Chromatograms of tramadol and its metabolites in koala plasma.

A) blank koala plasma; B) blank plasma spiked with tramadol, M1, and M2 (all at a concentration of 62.5 ng/mL); C) male koala plasma 2h after 4 mg/kg tramadol administration; D) female koala plasma 2 h after 4 mg/kg tramadol administration. Additional unidentified metabolites can be seen in C and D.

Fig 2. Median (with error) tramadol plasma concentrations vs. time curves when administered at 2 mg/kg, s.c. (n = 2) and 4 mg/kg, s.c. (n = 4).

Dotted line signifies suggested tramadol minimally effective plasma concentration in humans of 100 ng/mL [24]. The graph of the 4 mg/kg tramadol dose dips slightly below the 2 mg/kg tramadol dose–this may be due to individual variability between koalas.

Fig 3. Median (with error) semi-log tramadol plasma concentration versus time when administered at 2 mg/kg, s.c. (n = 2) and 4 mg/kg, s.c. (n = 4).

Dotted line signifies tramadol minimally effective plasma concentration of 100 ng/mL [24]. The graph of the 4 mg/kg tramadol dose dips slightly below the 2 mg/kg tramadol dose–this may be due to individual variability between koalas. The tramadol concentrations of the 4 mg/kg tramadol dose at not provided at 12 h as they are below the LLOQ = 15.63 ng/mL.

Fig 4. Median (with error) semi-log M1 plasma concentrations vs. time curves when administered at 2 mg/kg, s.c. (n = 2) and 4 mg/kg, s.c. (n = 4).

Dotted line signifies suggested M1 minimally effective plasma concentration in humans of 36 ng/mL [24].