PLASMA-CONCENTRATIONS OF FENTANYL WITH SUBCUTANEOUS INFUSION IN PALLIATIVE CARE PATIENTS

1 Plasma concentrations of fentanyl were measured by g.c. in 20 patien ts (median age: 75 years and range: 54-86 years; eight females) in pal liative care receiving the drug by continuous s.c. infusion (median ra te: 1200 mu g day(-1) and range: 100-5000 mu g day(-1)). 2 The infusio n rate was significantly related to the duration of therapy (Spearman p=0.56, P<0.05). The total steady-state plasma concentrations of fenta nyl ranged between 0.1. and 9 ng ml(-1), with a median of 1 ng ml(-1). The unbound fraction of fentanyl in the plasma ranged from 17.8 to 44 .4%, with a median value of 33.6%. Infusion rates and both total and u nbound plasma concentrations of fentanyl were correlated (Spearman p=0 .92, P<0.05 in each case). Even with standardization for dosage, there was an eightfold variation in total plasma concentrations and 3.5-fol d variation in unbound plasma concentrations of fentanyl. 3 There is c onsiderable inter-patient variability in the pharmacokinetics of fenta nyl with s.c. infusion in the palliative care setting, which necessita tes careful titration of dosage according to individual clinical respo nse.