title: Targeted radiotherapy of prostate cancer with a gastrin-releasing peptide receptor antagonist is effective as monotherapy and in combination with rapamycin.
abstract: The gastrin-releasing peptide receptor (GRPr) is overexpressed in prostate cancer and is an attractive target for radionuclide therapy. In addition, inhibition of the protein kinase mammalian target of rapamycin (mTOR) has been shown to sensitize various cancer cells to the effects of radiotherapy.
  
  
  METHODS
  To determine the effect of treatment with rapamycin and radiotherapy with a novel (177)Lu-labeled GRPr antagonist ((177)Lu-RM2, BAY 1017858) alone and in combination, in vitro and in vivo studies were performed using the human PC-3 prostate cancer cell line. PC-3 cell proliferation and (177)Lu-RM2 uptake after treatment with rapamycin were assessed in vitro. To determine the influence of rapamycin on (177)Lu-RM2 tumor uptake, in vivo small-animal PET studies with (68)Ga-RM2 were performed after treatment with rapamycin. To study the efficacy of (177)Lu-RM2 in vivo, mice with subcutaneous PC-3 tumors were treated with (177)Lu-RM2 alone or after pretreatment with rapamycin.
  
  
  RESULTS
  Stable expression of GRPr was maintained after rapamycin treatment with doses up to 4 mg/kg in vivo. Monotherapy with (177)Lu-RM2 at higher doses (72 and 144 MBq) was effective in inducing complete tumor remission in 60% of treated mice. Treatment with 37 MBq of (177)Lu-RM2 and rapamycin in combination led to significantly longer survival than with either agent alone. No treatment-related toxicity was observed.
  
  
  CONCLUSIONS
  Radiotherapy using a (177)Lu-labeled GRPr antagonist alone or in combination with rapamycin was efficacious in inhibiting in vivo tumor growth and may be a promising strategy for treatment of prostate cancer.
authors.affiliation: Department of Nuclear Medicine, University of Freiburg, Freiburg, Germany. rdumont@ucla.edu
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Dumont
authors.forename: MariaLuisa
authors.initials: M
authors.lastname: Tamma
authors.forename: Friederike
authors.initials: F
authors.lastname: Braun
authors.forename: Sandra
authors.initials: S
authors.lastname: Borkowski
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
authors.forename: Helmut
authors.initials: H
authors.lastname: Maecke
authors.forename: Wolfgang A
authors.initials: WA
authors.lastname: Weber
authors.forename: Rosalba
authors.initials: R
authors.lastname: Mansi
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D003131:Combined Modality Therapy; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D008187:Lutetium / Q000627:therapeutic use; D008297:Male; D051379:Mice; D058990:Molecular Targeted Therapy / Q000379:methods*; D009842:Oligopeptides / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology / Q000627:therapeutic use*; D011471:Prostatic Neoplasms / Q000188:drug therapy* / Q000473:pathology / Q000532:radiotherapy*; D011868:Radioisotopes / Q000627:therapeutic use; D018004:Receptors, Bombesin / Q000037:antagonists & inhibitors* / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology / Q000627:therapeutic use*
chemical_list: D009842:Oligopeptides; D011868:Radioisotopes; D018004:Receptors, Bombesin; D008187:Lutetium; D020123:Sirolimus
keywords: bombesin antagonist; combination therapy; prostate cancer; radiopeptide therapy