The Pharmacokinetics and Pharmacogenetics of the Antiemetic Cyclizine in Palliative Care Patients

ContextCyclizine, an antihistaminic antiemetic, is commonly used in palliative care. Its pharmacokinetics have been poorly studied, and its metabolic pathway is unknown but may involve the genetically controlled cytochrome P450 2D6 (CYP2D6). If this is the case, the metabolic ratio of cyclizine to norcyclizine and efficacy/adverse effects may vary between patients according to their CYP2D6 genotype.ObjectivesTo deduce the pharmacokinetics and antiemetic/sedative effects of cyclizine and relate these and its metabolic ratio to the CYP2D6 genotype in palliative care patients.MethodsPalliative care patients initiated on continuous cyclizine subcutaneous (SC) infusions had blood samples taken and efficacy/toxicity scores measured during the approach to steady state. Another group of patients at steady state receiving oral cyclizine had a single blood sample taken. Samples were analyzed to elucidate pharmacokinetic parameters and CYP2D6 genetics.ResultsSC dosing group: The median (interquartile range) cyclizine half-life, volume of distribution, and clearance were 13 (7–48) hours, 23 (12–30) L/kg, and 15 (11–26) mL/min/kg, respectively. Nausea and sedation scores were 3.0 (1.2–5.7) and 5.0 (2.6–8.1), respectively, overall and did not vary with genotype (P = 0.76 and 0.11, respectively). The median overall metabolic ratio at steady state was 4.9 (3.8–9.2) and did vary with CYP2D6 genotype (P = 0.02). Oral dosing group: The median metabolic ratio was 2.1 (1.5–2.9) and did not vary with CYP2D6 genotype (P = 0.37).ConclusionPalliative care patients have similar cyclizine pharmacokinetics to those reported in other patient groups. Cyclizine metabolism to norcyclizine may include CYP2D6 as the metabolic ratio varied with CYP2D6 genotype in the SC group.