title: Cellular uptake kinetics of bortezomib in relation to efficacy in myeloma cells and the influence of drug transporters.
abstract: OBJECTIVE
  Despite overall successful application to multiple myeloma patients, clinical efficacy of the proteasome inhibitor bortezomib is typically challenged by primary and secondary resistance of unknown origin. So far, the potential impact of intracellular concentrations on drug efficacy of bortezomib and the influence of drug transporters are unknown.
  
  
  METHODS
  We determined cellular bortezomib kinetics in nine myeloma cell lines using ultrahigh-performance liquid chromatography coupled to tandem mass spectrometry. The potential influence of drug transporters on the uptake kinetics observed in these cell lines was investigated by testing substrate characteristics of bortezomib for several transporters in over-expressing model cells. Additionally, transporter mRNA expression was quantified in myeloma cell lines by real-time polymerase chain reaction (RT-PCR).
  
  
  RESULTS
  All myeloma cells revealed an extensive intracellular bortezomib accumulation (47.5-183 ng/ml) exceeding extracellular concentrations (0.04-0.17 ng/ml) by more than factor 1,000. Only organic anion-transporting polypeptide 1B1 facilitated the uptake in over-expressing cells, however, to a negligible extent (factor 1.36). Bortezomib efflux via P-glycoprotein was confirmed by demonstrating reduced sensitivity (IC50 11.6 vs. 2.8 ng/ml) and intracellular concentrations (-56.1%) in over-expressing cells compared to controls. RT-PCR revealed a varying but overall weak transporter expression in the studied myeloma cells without any correlation to intracellular concentrations. Although principally valid as demonstrated in the P-glycoprotein over-expressing cell model, there was no significant correlation between intracellular concentrations and bortezomib efficacy in myeloma cell lines.
  
  
  CONCLUSIONS
  Differences in intracellular concentrations in myeloma cell lines neither result from variable transporter expression nor represent the main factor determining bortezomib efficacy in vitro.
authors.affiliation: Department of Clinical Pharmacology and Pharmacoepidemiology, University Hospital Heidelberg, Im Neuenheimer Feld 410, 69120, Heidelberg, Germany.
authors.forename: Jannick
authors.initials: J
authors.lastname: Clemens
authors.forename: Anja
authors.initials: A
authors.lastname: Seckinger
authors.forename: Dirk
authors.initials: D
authors.lastname: Hose
authors.forename: Dirk
authors.initials: D
authors.lastname: Theile
authors.forename: Magdalena
authors.initials: M
authors.lastname: Longo
authors.forename: Walter Emil
authors.initials: WE
authors.lastname: Haefeli
authors.forename: Jürgen
authors.initials: J
authors.lastname: Burhenne
authors.forename: Johanna
authors.initials: J
authors.lastname: Weiss
mesh_terms: D000970:Antineoplastic Agents / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D001897:Boronic Acids / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D000069286:Bortezomib; D002352:Carrier Proteins / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D006801:Humans; D009101:Multiple Myeloma / Q000188:drug therapy* / Q000378:metabolism*; D011719:Pyrazines / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics
chemical_list: D000970:Antineoplastic Agents; D001897:Boronic Acids; D002352:Carrier Proteins; D011719:Pyrazines; D012333:RNA, Messenger; D000069286:Bortezomib