Single-dose pharmacokinetics of oxytetracycline and penicillin G in tammar wallabies (Macropus eugenii) D. J. MC LELLAND* ,   I. K. BARKER* G. CRAWSHAW   L. A. HINDS à L. SPILSBURY § & R. JOHNSON* *Ontario Veterinary College, University of Guelph, Guelph, ON, Canada;   Toronto Zoo, 361a Old Finch Ave, Scarborough, ON, Canada; à CSIRO, Entomology, Canberra, Australia; § Laboratory Services Division, University of Guelph, Guelph, ON, Canada McLelland, D. J., Barker, I. K., Crawshaw, G., Hinds, L. A., Spilsbury, L., Johnson, R. Single-dose pharmacokinetics of oxytetracycline and penicillin G in tammar wallabies (Macropus eugenii). J. vet. Pharmacol. Therap. 34, 160–167. The pharmacokinetics of oxytetracycline and penicillin G was investigated in tammar wallabies (Macropus eugenii). Groups of eight healthy tammar wallabies were administered i.v. oxytetracycline hydrochloride (40 mg ⁄ kg), i.m. long- acting-oxytetracycline (20 mg ⁄ kg), i.v. sodium penicillin G (30 mg ⁄ kg), or i.m. procaine ⁄ benzathine penicillin G (30 mg ⁄ kg). Plasma concentrations of oxytetracycline were determined using high-performance liquid chromatogra- phy. Pharmacokinetic parameters were comparable to those reported for eutherians of equivalent size and suggest that the practice of adjusting allometrically scaled doses to account for the lower metabolic rate of marsupials may not be valid. Long-acting oxytetracycline and penicillin G both demon- strated depot effects. However, the plasma concentrations achieved question the therapeutic efficacy of the long-acting preparations. (Paper received 10 September 2009; accepted for publication 4 June 2010) David McLelland, Zoos South Australia, Frome Road, Adelaide, SA, 5000, Australia. E-mail: dmclelland@zoossa.com.au INTRODUCTION Antibiotics such as oxytetracycline and penicillin G are fre- quently used in marsupials, both in the rehabilitation of sick and injured wildlife and in the treatment of animals kept in captivity in zoos, in research institutions, and as pets. There are many references to the use of antibiotics for treating a range of conditions in marsupials (Post Graduate Foundation in Veteri- nary Science, 1999; Vogelnest & Woods, 2008). Dosage regimens have been largely inferred from domestic animal literature and ⁄ or are based on anecdotal reports of clinical response to treatment. In addition to body weight, the low metabolic rate of marsupials relative to eutherians (placental mammals) is accounted for in a commonly used allometric scaling method to extrapolate dosages and dose frequencies to a new species (Sedgwick, 1993). This assumes that the physiologic processes influencing pharmacokinetics vary proportionately with meta- bolic rate. The validity of metabolic scaling of dosage regimens across species has been questioned (Hunter & Isaza, 2008), and current information on marsupial physiology does not appear to support the assumptions inherent in the metabolic scaling of dosage regimens. Despite lower metabolic rates, some marsupials have been shown to be more efficient than eutherians in eliminating certain xenobiotics – e.g. terpenes in folivorous marsupials (Boyle et al., 1999), paracetamol and warfarin in brushtail possums (Eason et al., 1999), and sodium fluoroacetate in Western Australian marsupials (King et al., 1978). Gastrointestinal physiology appears to be an important factor in some trends in biotransformation ability across species. Folivorous marsupials employ very efficient oxidation systems compared to other species (Boyle et al., 1999), and the microsomal enzyme systems of grazing marsupials, such as macropods, appear to resemble those of grazing eutherians more than those of folivorous marsupials (Pass & Brown, 1990). Many pharmaceuticals are eliminated via the kidneys, either directly or following biotransformation (Riviere, 1999). Glo- merular function of a number of marsupials is comparable to that of eutherian mammals (Denny & Dawson, 1977). However, a number of macropods, notably arid-zone species, are capable of reductions in glomerular filtration rate (GFR) in response to dehydration that are substantially greater than those of eutherians, primarily through reductions in renal blood flow (Kinnear et al., 1968; Denny & Dawson, 1977; Bradshaw et al., 2001). Variation in water conservation physiology between two arid-zone macropods indicates that renal function is not even consistent among the Macropodidae (Bradshaw et al., 2001). Oxytetracycline is a broad spectrum antibiotic. In eutherians, 18 – 75% of administered drug is bound to plasma proteins depending on the species (Papich & Riviere, 2009a). Approxi- mately 60% of circulating drug is eliminated unchanged via glomerular filtration, and the remainder is excreted via the J. vet. Pharmacol. Therap. 34, 160–167. doi: 10.1111/j.1365-2885.2010.01216.x. 160 Ó 2010 Blackwell Publishing Ltd