title: Active apical secretory efflux of the HIV protease inhibitors saquinavir and ritonavir in Caco-2 cell monolayers.
abstract: OBJECTIVE
  To investigate in vitro the mechanisms involved in the gastro-intestinal absorption of the HIV protease inhibitor, saquinavir mesylate (Invirase), whose oral bioavailability is low, variable, and significantly increased by co-administration with ritonavir, also an HIV protease inhibitor but with higher oral bioavailability.
  
  
  METHODS
  Confluent epithelial layers of human Caco-2 cells mimicking the intestinal barrier.
  
  
  RESULTS
  Both saquinavir and ritonavir showed polarized transport through Caco-2 cell monolayers in the basolateral to apical direction (secretory pathway), exceeding apical to basolateral transport (absorptive pathway) by factors of 50-70 and 15-25, respectively. Active efflux was temperature dependent, saturable and inhibited by verapamil and cyclosporin A. Saquinavir and ritonavir decreased each other's secretory permeability and hence elevated their net transport by the absorptive pathway.
  
  
  CONCLUSIONS
  Saquinavir and ritonavir are both substrates for an efflux mechanism in the gut, most likely P-glycoprotein, which acts as a counter-transporter for both drugs. Together with sensitivity to gutwall metabolism by cytochrome P-450 3A, this may partially account for the low and variable oral bioavailability of saquinavir in clinical studies and for its increased bioavailability after co-administration with ritonavir.
authors.affiliation: Preclinical Research Department, F. Hoffman-La Roche Ltd, Basel, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Alsenz
authors.forename: H
authors.initials: H
authors.lastname: Steffen
authors.forename: R
authors.initials: R
authors.lastname: Alex
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors*; D000972:Antineoplastic Agents, Phytogenic / Q000378:metabolism; D001682:Biological Availability; D001692:Biological Transport / Q000187:drug effects; D018938:Caco-2 Cells / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000378:metabolism; D016572:Cyclosporine / Q000378:metabolism; D017320:HIV Protease Inhibitors / Q000378:metabolism* / Q000493:pharmacokinetics; D006801:Humans; D007166:Immunosuppressive Agents / Q000378:metabolism; D007408:Intestinal Absorption; D019438:Ritonavir / Q000378:metabolism* / Q000493:pharmacokinetics; D019258:Saquinavir / Q000378:metabolism* / Q000493:pharmacokinetics; D014700:Verapamil / Q000378:metabolism; D014747:Vinblastine / Q000378:metabolism
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000972:Antineoplastic Agents, Phytogenic; D002121:Calcium Channel Blockers; D017320:HIV Protease Inhibitors; D007166:Immunosuppressive Agents; D014747:Vinblastine; D016572:Cyclosporine; D014700:Verapamil; D019258:Saquinavir; D019438:Ritonavir