title: Modulation of O6-alkylguanine alkyltransferase-directed DNA repair in metastatic colon cancers.
abstract: OBJECTIVE
  Carmustine (BCNU) resistance has been correlated with tumor expression of the DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AT). It has been shown that streptozotocin will deplete AT activity of human colon cancer cells in vitro and potentiate BCNU cytotoxicity. This clinical trial was conducted to determine whether streptozotocin can be used as a modulator of AT in metastatic colorectal cancers and thereby overcome clinical resistance to BCNU.
  
  
  METHODS
  Fifteen patients with fluorouracil-resistant metastatic colon or rectal cancers were treated sequentially with 2 g/m2 of streptozotocin followed 5 1/2 hours later by BCNU. Sequential biopsies of metastases before and after streptozotocin were conducted to determine whether streptozotocin depletes tumor AT. Peripheral-blood mononuclear cells (PBMCs) were evaluated as a surrogate tissue for prediction of baseline AT levels and streptozotocin posttreatment modulation of the AT in metastases.
  
  
  RESULTS
  Streptozotocin treatment led to a 78% (range, 69% to 89%) decrease in the AT levels in colon cancer metastases; however, myelosuppression and hepatic toxicity limited the BCNU dose to 130 mg/m2. A similar decrease in AT levels of PBMCs was found; however, the absolute levels of AT in PBMCs at baseline and following streptozotocin were not predictive of the levels expressed in metastases from the same patient. Despite the decrease in tumor levels of AT, no clinical responses were observed.
  
  
  CONCLUSIONS
  Streptozotocin decreases but does not fully deplete AT activity in metastatic colorectal cancers and the residual AT level in metastases is sufficient to maintain clinical resistance to BCNU. We have also demonstrated that sequential computed tomography (CT)-directed biopsies of colorectal cancer metastases can be used to evaluate strategies to investigate modulators of AT-directed repair. AT levels of PBMCs do not predict for the AT level or degree of modulation achieved in the metastatic tumor.
authors.affiliation: Department of Medicine, Case Western Reserve/University Ireland Cancer Center, University Hospitals of Cleveland, OH 44106-4937, USA.
authors.forename: J K
authors.initials: JK
authors.lastname: Willson
authors.forename: J R
authors.initials: JR
authors.lastname: Haaga
authors.forename: J E
authors.initials: JE
authors.lastname: Trey
authors.forename: T A
authors.initials: TA
authors.lastname: Stellato
authors.forename: N H
authors.initials: NH
authors.lastname: Gordon
authors.forename: S L
authors.initials: SL
authors.lastname: Gerson
mesh_terms: D000328:Adult; D000368:Aged; D002330:Carmustine / Q000627:therapeutic use*; D015179:Colorectal Neoplasms / Q000188:drug therapy / Q000201:enzymology* / Q000473:pathology; D004260:DNA Repair; D004351:Drug Resistance; D004359:Drug Therapy, Combination; D005260:Female; D006801:Humans; D007963:Leukocytes, Mononuclear / Q000201:enzymology; D008297:Male; D008780:Methyltransferases / Q000378:metabolism*; D008875:Middle Aged; D009362:Neoplasm Metastasis; D019853:O(6)-Methylguanine-DNA Methyltransferase; D012044:Regression Analysis; D013311:Streptozocin / Q000627:therapeutic use*; D014057:Tomography, X-Ray Computed
chemical_list: D013311:Streptozocin; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D002330:Carmustine