Is saliva a valid substitute for plasma in pharmacokinetic studies of oxycodone and its metabolites in patients with cancer?

PURPOSE: Little is known about the pharmacokinetics (PKs) of oxycodone in patients with advanced cancer. There is considerable reluctance to subject these patients to non-essential tests including repeated venipuncture that has been necessary in PK studies to date. We investigated the possibility of using saliva sampling as a simple non-invasive test to investigate opioid PKs.
METHODS: Patients with malignant disease receiving oral sustained release (SR) oxycodone at any dose were asked to provide saliva samples at the same time as blood samples. Samples were not taken within 6 h of a dose of immediate release oxycodone. Plasma and saliva oxycodone and metabolite concentrations were measured using HPLC coupled with tandem mass spectrometric detection.
RESULTS: One hundred and thirty-nine paired plasma/saliva samples were collected from 43 cancer patients who had been taking SR oxycodone for more than 5 days at doses ranging from 10 to 600 mg/day (median 40 mg/day). Plasma concentrations of oxycodone and noroxycodone ranged from 1.0 to 256.0 and 0.9-269.4 μg/L, respectively. Salivary concentrations of oxycodone (range 0.93-3,620, mean 336 μg/L) were much higher than plasma concentrations (mean 38.2 μg/L). There was a poor correlation between concentrations of both oxycodone and noroxycodone in plasma and saliva over a range of times following dosing (r (2) = 0.4641 and 0.3891, respectively). No correlation was shown between salivary pH and oxycodone or noroxycodone concentrations. The majority of patients questioned chose saliva sampling over plasma sampling as the preferred method.
CONCLUSION: High levels of both oxycodone and its major metabolite are present in saliva, but this does not provide a valid substitute for plasma when monitoring oxycodone levels for PK studies or therapeutic monitoring.