title: In vitro and in vivo metabolism of paclitaxel poliglumex: identification of metabolites and active proteases.
abstract: BACKGROUND
  The efficacy and tolerability of paclitaxel is limited by its low solubility, high systemic exposure, and a lack of selective tumor uptake. Paclitaxel poliglumex (PPX; XYOTAX) is a macromolecular drug conjugate that was developed to overcome these limitations; the 2' hydroxyl group of paclitaxel is linked to a biodegradable polymer, poly-L: -glutamic acid, to form an inactive polymeric conjugate. PPX was previously shown to accumulate in tumor tissue, presumably by taking advantage of the hyperpermeable tumor vasculature and suppressed lymphatic clearance in tumor tissue.
  
  
  METHODS
  Because anti-tumor activity requires the release of paclitaxel from the polymer-drug conjugate, the current report characterizes PPX biodegradation and release of paclitaxel as determined by quantitative HPLC/mass spectral analysis.
  
  
  RESULTS
  The identification of monoglutamyl-paclitaxel metabolites in tumor tissue confirmed the in vivo metabolism of PPX in a panel of mouse tumor models. In vitro characterization of the metabolic pathway suggests that PPX can enter tumor cells, and is metabolized to form both mono- and diglutamyl-paclitaxel cleavage products. The intracellular formation of these intermediate metabolites is at least partially dependent on the proteolytic activity of the lysosomal enzyme cathepsin B; PPX metabolism is inhibited by a highly selective inhibitor of cathepsin B, CA-074. Reduced metabolism of PPX in livers and spleens from cathepsin B deficient mice confirms that cathepsin B is an important mediator of PPX metabolism in vivo; however, other proteolytic enzymes may contribute as well.
  
  
  CONCLUSIONS
  The cathepsin B-mediated release of paclitaxel may have therapeutic implications as cathepsin B is upregulated in malignant cells, particularly during tumor progression.
authors.affiliation: Cell Therapeutics, Inc., 501 Elliott Avenue West, Seattle, WA 98119, USA.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Shaffer
authors.forename: Cassie
authors.initials: C
authors.lastname: Baker-Lee
authors.forename: Jacob
authors.initials: J
authors.lastname: Kennedy
authors.forename: Man Shun
authors.initials: MS
authors.lastname: Lai
authors.forename: Peter
authors.initials: P
authors.lastname: de Vries
authors.forename: Kent
authors.initials: K
authors.lastname: Buhler
authors.forename: Jack W
authors.initials: JW
authors.lastname: Singer
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000378:metabolism*; D002401:Cathepsin B / Q000502:physiology*; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D005260:Female; D008297:Male; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017239:Paclitaxel / Q000031:analogs & derivatives* / Q000378:metabolism; D011099:Polyglutamic Acid / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D011099:Polyglutamic Acid; D002401:Cathepsin B; D017239:Paclitaxel; C509139:paclitaxel poliglumex