Pharmacokinetics of a cytosine arabinoside subcutaneous protocol in dogs with meningoencephalomyelitis of unknown aetiology

Cytosine arabinoside (CA) is a commonly used treatment for dogs with meningoencephalomyelitis of unknown aetiology (MUE) with various proposed protocols, many requiring 24 hours (h) of hospitalization or two visits within 24 h. This is a unidirectional study evaluating the pharmacokinetics of a CA subcutaneous (SC) protocol and a standard constant rate infusion (CRI) protocol in 8 dogs with MUE. Dogs received the CRI (200 mg/m² IV over 24 h), followed by a SC protocol (50 mg/m² every 2 h for 4 treatments) four weeks later. Plasma CA concentrations were measured by high-pressure liquid chromatography-tandem mass spectrometry (HPLC-MS). Median peak CA concentration for the SC protocol (3.40 µg/ml, range 1.60-9.70 µg/ml) was significantly higher than the CRI (1.09 µg/ml, range 0.77-1.67 µg/ml; p = .02). Median concentration at 1h and 8h following initiation of treatment was significantly higher for the SC protocol (CA₁ 2.28 µg/ml, range 0.97-2.67; CA₈ 1.83 µg/ml, range 0.77-2.84) compared to the CRI (CA1 0.01 µg/ml, range 0-0.45; CA₈ 0.74 µg/ml, range 0.67-1.11; p = .01). While the PK properties of CA when administered as a CRI has been previously investigated, this study demonstrated that CA when administered via repeated 50 mg/m² injections every 2 h over an 8-h period, provided sustained plasma levels above its therapeutic target and for a significantly longer duration of time than did a standard CRI protocol.