title: Measurement of cremophor EL following taxol: plasma levels sufficient to reverse drug exclusion mediated by the multidrug-resistant phenotype.
abstract: BACKGROUND
  Paclitaxel (Taxol) is the first of a new class of cytotoxic agents with activity against tumors resistant to other drugs. For clinical use, paclitaxel is currently formulated in a vehicle of 50% ethanol and 50% polyethoxylated surfactant Cremophor EL (Cremophor). We have previously shown that Cremophor will block the P-glycoprotein drug efflux pump responsible for the multidrug-resistant phenotype. Overexpression of P-glycoprotein is one mechanism of in vitro resistance to a number of currently used cytotoxic agents including paclitaxel.
  
  
  OBJECTIVE
  Our aim was to develop a bioassay to measure plasma levels of Cremophor and to determine whether or not plasma levels of Cremophor achieved during paclitaxel therapy are sufficient to inhibit the activity of the P-glycoprotein.
  
  
  METHODS
  All patients studied had histologically proven, advanced ovarian carcinoma with measurable or evaluable disease and had received at least one prior platinum-containing regimen. The bioassay used flow cytometry to measure the increase in equilibrium intracellular daunorubicin levels in multidrug-resistant human T-cell leukemia cells (CEM/VLB100) in the presence of a series of concentrations of Cremophor. Levels of Cremophor were measured in plasma from 21 patients after a 3-hour infusion of 135 or 175 mg/m2 paclitaxel. Both dose levels were given following premedication with oral dexamethasone, intravenous promethazine hydrochloride, and intravenous cimetidine. The Cremophor bioassay involved incubation of CEM/VLB100 cells (5 x 10(5)) for 1 hour with 2 micrograms/mL daunorubicin in 0.5 mL HL-1 medium plus 0.5 mL plasma prior to flow cytometric analysis. Pretreatment plasma was used to derive a standard curve for the effect of Cremophor on equilibrium daunorubicin levels. All measurements were done in triplicate.
  
  
  RESULTS
  In vitro experiments indicated that, for maximal inhibition of P-glycoprotein activity, concentrations of Cremophor of 0.1% (vol/vol) were required. At the end of a 3-hour infusion of paclitaxel, plasma levels of Cremophor in 19 of 21 patients were 0.1% or higher and 0.09% in the remaining two. Concentrations of 5-20 microM paclitaxel dissolved in ethanol without Cremophor did not inhibit P-glycoprotein in this assay.
  
  
  CONCLUSIONS
  The concentrations of Cremophor measured in plasma drawn from patients after a 3-hour infusion of paclitaxel at 135 or 175 mg/m2 were found to be sufficient to inhibit P-glycoprotein activity in vitro.
  
  
  CONCLUSIONS
  The efficacy of paclitaxel against some tumors may be aided by its administration in a vehicle solution containing Cremophor in quantities that reach concentrations in the plasma sufficient to reverse multidrug resistance of neoplastic cells.
authors.affiliation: Peter MacCallum Cancer Institute, Melbourne, Victoria, Australia.
authors.forename: L
authors.initials: L
authors.lastname: Webster
authors.forename: M
authors.initials: M
authors.lastname: Linsenmeyer
authors.forename: M
authors.initials: M
authors.lastname: Millward
authors.forename: C
authors.initials: C
authors.lastname: Morton
authors.forename: J
authors.initials: J
authors.lastname: Bishop
authors.forename: D
authors.initials: D
authors.lastname: Woodcock
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000328:Adult; D000368:Aged; D001681:Biological Assay; D002352:Carrier Proteins / Q000037:antagonists & inhibitors; D004351:Drug Resistance; D005260:Female; D005434:Flow Cytometry; D005990:Glycerol / Q000031:analogs & derivatives* / Q000097:blood / Q000494:pharmacology; D006801:Humans; D008562:Membrane Glycoproteins / Q000037:antagonists & inhibitors; D008875:Middle Aged; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors; D010051:Ovarian Neoplasms / Q000188:drug therapy / Q000378:metabolism*; D017239:Paclitaxel / Q000627:therapeutic use*; D014677:Pharmaceutical Vehicles / Q000493:pharmacokinetics* / Q000494:pharmacology; D010641:Phenotype; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D009363:Neoplasm Proteins; D014677:Pharmaceutical Vehicles; C000515:cremophor EL; D017239:Paclitaxel; D005990:Glycerol