Characterization and validation of a pharmacokinetic model for controlled-release oxycodone

1. Oxycodone is a strong opioid agonist that is currently available in immediate-release (IR) formulations for the treatment of moderate to severe pain. Recently, controlled-release (CR) oxycodone tablets were developed to provide the benefits of twice-a-day dosing to patients treated with oxycodone. The purpose of this investigation was to develop and validate a pharmacokinetic model for CR oxycodone tablets in comparison with IR oxycodone solution. 2. Twenty-four normal male volunteers were enrolled in a single-dose, randomized, analytically blinded, two-way crossover study designed to compare the pharmacokinetics of two 10 mg CR oxycodone tablets with 20 mg IR oxycodone oral solution. Pharmacokinetic models describing the oxycodone plasma concentration vs time profiles of CR tablets and IR solution were derived using NONMEM version IV. The predictive performance of the models was assessed by comparison of predicted oxycodone plasma concentrations with actual oxycodone plasma concentrations observed in a separate group of 21 volunteers who received repeated doses of IR and CR oxycodone for 4 days. 3. The unit impulse disposition function of oxycodone was best described by a one-compartment model. Absorption rate of the IR solution was best described by a mono-exponential model with a lag time, whereas absorption rate of the CR tablet was best described using a bi-exponential model. The absorption profile of the CR tablets was characterized by a rapid absorption component (t1/2abs = 37 min) accounting for 38% of the available dose and a slow absorption phase (t1/2abs = 6.2 h) accounting for 62% of the available dose. Two 10 mg tablets of oral CR oxycodone hydrochloride were 102.7% bioavailable relative to 20 mg of IR oxycodone hydrochloride oral solution. The population model derived after administration of a single dose accurately predicted both the mean and range of oxycodone concentrations observed during 4 days of repeated dosing. The mean prediction error was 2.7% with a coefficient of variation of 54%. 4. The absorption characteristics of CR oxycodone tablets should allow effective plasma concentrations of oxycodone to be reached quickly and for effective concentrations to be maintained for a longer period after dosing compared with the IR oral solution. The CR dosage form has pharmacokinetic characteristics that permit 12 hourly dosing.