Pharmacokinetic profile of the selective 5-HT receptor antagonist ondansetron in the rat: an original study and a minireview of the behavioural pharmacological literature in the rat.

The availability of agonists and antagonists to modulate the activity of the 5-hydroxytryptamine (5-HT) type 3 (5-HT) receptor has renewed interest in its role as a therapeutic target. Ondansetron is a highly selective 5-HT receptor antagonist that is well tolerated as an anti-emetic for patients undergoing chemotherapy. Preclinical studies in rat have shown the effects of small doses of ondansetron on cognition, behavioural sensitisation, and epilepsy. However, the pharmacokinetic (PK) profile of ondansetron in rat has not been described, which limits the translational relevance of these findings. Here, we aim to determine, in the rat, the PK profile of ondansetron in the plasma and to determine associated brain levels. The plasma PK profile was determined following acute subcutaneous administration of ondansetron (0.1, 1, and 10 μg/kg). Brain levels were measured following subcutaneous administration of ondansetron at 1 μg/kg. Plasma and brain levels of ondansetron were determined using high-performance liquid chromatography - tandem mass spectrometry. Following administration of all three doses, measured ondansetron plasma levels (≈30-3000 pg/mL) were below levels achieved with doses usually administered in the clinic, with a rapid absorption phase and a short half-life (≈30-40 min). We also found that brain levels of ondansetron at 1 μg/kg were significantly lower than plasma levels, with brain to plasma ratios of 0.45 and 0.46 in the motor and pre-frontal cortices. We discuss our findings in the context of a minireview of the literature. We hope that our study will be helpful to the design of preclinical studies with therapeutic end-points.