title: Effect of ritonavir-induced cytochrome P450 3A4 inhibition on plasma fentanyl concentrations during patient-controlled epidural labor analgesia: a pharmacokinetic simulation.
abstract: BACKGROUND
  Ritonavir inhibition of cytochrome P450 3A4 decreases the elimination clearance of fentanyl by 67%. We used a pharmacokinetic model developed from published data to simulate the effect of sample patient-controlled epidural labor analgesic regimens on plasma fentanyl concentrations in the absence and presence of ritonavir-induced cytochrome P450 3A4 inhibition.
  
  
  METHODS
  Fentanyl absorption from the epidural space was modeled using tanks-in-series delay elements. Systemic fentanyl disposition was described using a three-compartment pharmacokinetic model. Parameters for epidural drug absorption were estimated by fitting the model to reported plasma fentanyl concentrations measured after epidural administration. The validity of the model was assessed by comparing predicted plasma concentrations after epidural administration to published data. The effect of ritonavir was modeled as a 67% decrease in fentanyl elimination clearance. Plasma fentanyl concentrations were simulated for six sample patient-controlled epidural labor analgesic regimens over 24 h using ritonavir and control models. Simulated data were analyzed to determine if plasma fentanyl concentrations producing a 50% decrease in minute ventilation (6.1 ng/mL) were achieved.
  
  
  RESULTS
  Simulated plasma fentanyl concentrations in the ritonavir group were higher than those in the control group for all sample labor analgesic regimens. Maximum plasma fentanyl concentrations were 1.8 ng/mL and 3.4 ng/mL for the normal and ritonavir simulations, respectively, and did not reach concentrations associated with 50% decrease in minute ventilation.
  
  
  CONCLUSIONS
  Our model predicts that even with maximal clinical dosing regimens of epidural fentanyl over 24 h, ritonavir-induced cytochrome P450 3A4 inhibition is unlikely to produce plasma fentanyl concentrations associated with a decrease in minute ventilation.
authors: Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, IL, USA. Electronic address: c-cambic@md.northwestern.edu.
authors: C R
authors: CR
authors: Cambic
authors: Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.
authors: M J
authors: MJ
authors: Avram
authors: Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.
authors: D K
authors: DK
authors: Gupta
authors: Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.
authors: C A
authors: CA
authors: Wong
mesh_terms: D015360:Analgesia, Epidural / Q000379:methods*; D016058:Analgesia, Patient-Controlled / Q000379:methods*; D000701:Analgesics, Opioid / Q000097:blood* / Q000493:pharmacokinetics; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism; D065692:Cytochrome P-450 CYP3A Inhibitors; D004347:Drug Interactions; D005260:Female; D005283:Fentanyl / Q000097:blood* / Q000493:pharmacokinetics*; D017320:HIV Protease Inhibitors / Q000494:pharmacology; D006801:Humans; D011247:Pregnancy; D019438:Ritonavir / Q000494:pharmacology*
chemical_list: D000701:Analgesics, Opioid; D065692:Cytochrome P-450 CYP3A Inhibitors; D017320:HIV Protease Inhibitors; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D019438:Ritonavir; D005283:Fentanyl
keywords: Analgesic techniques; Extradural; Obstetric Analgesia; Pharmacokinetics fentanyl; Ritonavir