Intestinal and hepatic contributions to the pharmacokinetic
interaction between gamithromycin and rifampicin after
single-dose and multiple-dose administration in healthy foals
, S. WALLSTABE
, E. SCHEUCH
, S. OSWALD
, M. HASAN
, D. WEGNER
, M. GRUBE
, M. VENNER
and W. SIEGMUND
Department of Clinical Pharmacology, Center of Drug Absorption and Transport (C_DAT), University Medicine of Greifswald, Greifswald, Germany
Lewitz Stud, Neustadt-Glewe, Germany
Department of General Pharmacology, Center of Drug Absorption and Transport (C_DAT), University Medicine of Greifswald, Greifswald, Germany
Veterinary Clinic for Horses, Destedt, Germany
PRIMACYT Cell Culture Technology GmbH, Schwerin, Germany.
*Correspondence email: firstname.lastname@example.org; Received: 17.04.17; Accepted: 02.12.17
Background: Standard treatment of foals with severe abscessing lung infection caused by Rhodococcus equi using rifampicin and a macrolide
antibiotic can be compromised by extensive inhibition and/or induction of drug metabolising enzymes (e.g. CYP3A4) and transport proteins (e.g. P-
glycoprotein), as has been shown for rifampicin and clarithromycin. The combination of rifampicin with the new, poorly metabolised gamithromycin, a
long-acting analogue of azithromycin and tulathromycin with lower pharmacokinetic interaction potential, might be a suitable alternative.
Objectives: To evaluate the pharmacokinetic interactions and pulmonary distribution of rifampicin and gamithromycin in healthy foals, and to
investigate the cellular uptake of gamithromycin in vitro.
Study design: Controlled, four-period, consecutive, single-dose and multiple-dose study.
Methods: Pharmacokinetics and lung distribution of rifampicin (10 mg/kg) and gamithromycin (6 mg/kg) were measured in nine healthy foals using LC-
MS/MS. Enzyme induction was conﬁrmed using the 4b-OH-cholesterol/cholesterol ratio. Afﬁnity of gamithromycin to drug transport proteins was
evaluated in vitro using equine hepatocytes and MDCKII-cells stably transfected with human OATP1B1, OATP1B3 and OATP2B1.
Results: Rifampicin signiﬁcantly (P<0.05) increased the plasma exposure of gamithromycin (16.2 Æ 4.77 vs. 8.57 Æ 3.10 lg 9 h/mL) by decreasing the
total body clearance. Otherwise, gamithromycin signiﬁcantly lowered plasma exposure of single- and multiple-dose rifampicin (83.8 Æ 35.3 and
112 Æ 43.1 vs. 164 Æ 96.7 lg 9 h/mL) without a change in metabolic ratio and half-life. Gamithromycin was identiﬁed as an inhibitor of human
OATP1B1, OATP1B3 and OATP2B1 and as a substrate of OATP2B1. In addition, it was extracted by equine hepatocytes via a mechanism which could be
inhibited by rifampicin.
Main limitations: Inﬂuence of gamithromycin on pulmonary distribution of rifampicin was not evaluated.
Conclusion: The plasma exposure of gamithromycin is signiﬁcantly increased by co-administration of rifampicin which is most likely caused by
inhibition of hepatic elimination.
Keywords: horse; rifampicin; gamithromycin; drug-drug interaction; lung distribution; healthy foals; Rhodococcus equi
The treatment of pulmonary infections caused by Rhodococcus equi
(R. equi) in foals follows the ‘watch-and-wait’ strategy. In mild to
moderate cases, antimicrobial treatment is only initiated after life-
threatening deterioration of clinical signs (e.g. abscess score ≥10 cm,
leucocyte count ≥13.000 cells/lL) . Macrolide antibiotics are a major
part of the standard therapy  as they widely distribute into pulmonary
compartments, which are the survival and reproduction site of R. equi
. The combination of macrolides with rifampicin is recommended to
achieve substantial synergism in efﬁcacy . However, the
pharmacodynamic beneﬁts can be limited by adverse pharmacokinetic
drug-drug interactions (DDIs), as has recently been shown for
DDIs between macrolide antibiotics and rifampicin can result from the
mutual inﬂuence of drug metabolising enzymes and transporters involved
in drug disposition. Some macrolides are substrates of the efﬂux
transporters P-glycoprotein (P-gp) and the multidrug resistance-associated
protein 2 (MRP2)  and of the organic anion transporting polypeptides
(OATP) 1A2, 1B1 and 1B3 [8,9]. The analogue of human OATP1B1 and 1B3
in horses is OATP1B4. Furthermore, macrolides are metabolised by
cytochrome P450 (CYP) 3A4 (analogue in horses: CYP3A89) and inhibit P-
gp, OATPs [10,11] and CYP3A4 . Otherwise, rifampicin is a substrate
and inhibitor of P-gp and OATPs [13,14], and a ligand of the nuclear
pregnane X receptor (PXR) which regulates gene transcription of P-gp,
MRP2 and CYP3A4 . Therefore, rifampicin should preferably be
combined with macrolides with low pharmacokinetic interaction potential
such as the novel 15-membered macrolactone gamithromycin which is
currently approved for the treatment of swine and bovine respiratory
Gamithromycin is rapidly and widely distributed (V
~ 25 L/kg) and
penetrates into the epithelial lining ﬂuid (ELF) and bronchoalveolar lavage
cells (BALCs) from which it is slowly eliminated with half-lives of 63.6 h
(ELF) and 70.3 h (BALCs) respectively . The major route of systemic
elimination of gamithromycin in different species (e.g. rats, dogs, pigs and
cattle)isbybiliaryexcretionoftheparentcompound(≤90%) [18,19]. So far,
little is known about drug transport proteins involved in the distribution
and hepatic elimination of gamithromycin. Studies investigating the
potential pharmacokinetic interactions between rifampicin and
gamithromycin in foals are also missing so far.
To evaluate the interaction potential, we investigated the cellular uptake
of gamithromycin into primary equine hepatocytes and measured its
afﬁnity to human OATPs. Finally, we conducted a clinical study in healthy
foals to evaluate potential DDIs with rifampicin.
Equine Veterinary Journal 50 (2018) 525–531 © 2017 EVJ Ltd
Equine Veterinary Journal ISSN 0425-1644