Population pharmacokinetic-pharmacodynamic modelling of liquid and controlled-release formulations of oxycodone in healthy volunteers

Oral controlled-release formulations are playing an ever-increasing role in opioid therapy; however, little is known about their influence on the relationship between pharmacokinetics and pharmacodynamics. The study aim was to characterize the pharmacokinetic-pharmacodynamics of two controlled-release tablet formulations and a liquid formulation of oxycodone in healthy, opioid-naïve volunteers, which can serve as a reference for future patient studies. A semi-double-blinded, three-way crossover study was conducted, with fifteen healthy volunteers receiving two differently designed 20 mg monophasic controlled-release oxycodone tablets and 10 mg oral solution oxycodone in a randomized order. Venous plasma concentrations and pupil diameter were determined pre-dose and 0.25, 0.5, 0.75, 1, 1.5, 2, 2.33, 2.66, 3, 3.33, 3.66, 4, 5, 6, 8, 12 and 24 hour post-dose. Oxycodone pharmacokinetics was best described by a two-compartment model with first-order absorption. The controlled-release formulations had an absorption lag of 0.23 hour and a slower absorption rate constant (k_aCR = 0.19 hour^-1) compared to the oral solution (k_aSOL = 0.94 hour^-1). Effects on pupil diameter were delayed relative to plasma (14 minutes half-life) for all formulations and were best described by a proportional E_max model. The plasma concentration of oxycodone at half-maximum effect was lower in males (31.1 μg/L) compared to females (52.8 μg/L; P <.001). The absorption profile of controlled-release oxycodone formulations provided a prolonged onset and offset of action compared to oral solution oxycodone. The controlled-release formulations showed no differences in pharmacokinetic and pharmacodynamic parameters suggesting that both may be used interchangeably in human beings with normal gastrointestinal function.