Background-Ciclosporin (CSA) is approved for the treatment of canine atopic dermatitis. Ciclosporin is metab-olized by liver cytochrome P450 enzymes, a process inhibited by ketoconazole (KTZ). Hypothesis ⁄ Objectives-The aims of this study were to determine skin and blood CSA concentrations when CSA was administered alone at 5.0 (Treatment 1) or 2.5 mg ⁄ kg (Treatment 2) and when CSA was administered at 2.5 mg ⁄ kg concurrently with KTZ at 5 (Treatment 3) or 2.5 mg ⁄ kg (Treatment 4). We hypothesized that skin and blood CSA concentrations in Treatment 1 would not differ from those obtained with T3 or T4. Animals-In a randomized cross-over study, six healthy research dogs received each of the treatments (Treat-ment 1, 2, 3 and 4) once daily for 7 days. Methods-After the first, fourth and seventh dose for each treatment, a peak and trough skin punch biopsy sample and whole blood sample were collected and analysed with high-performance liquid chromatography-tandem mass spectrometry. Data were analysed using a repeated measures approach with PROC MIXED in SAS. Pairwise comparisons were performed with least squares means and Tukey-Kramer adjustment for multiple comparisons. Results-Mean blood CSA concentrations in Treatment 1 were not different from those in Treatment 2 or 4, but were less than in Treatment 3. Mean skin CSA concentrations in Treatment 1 were greater than in Treatment 2, not different from those in Treatment 4, and less than those in Treatment 3. Conclusions and clinical importance-Administration of CSA and KTZ concurrently at 2.5 mg ⁄ kg each may be as effective as CSA alone at 5.0 mg ⁄ kg for treatment of canine atopic dermatitis.

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