title: Measurement of cetuximab and panitumumab-unbound serum EGFR extracellular domain using an assay based on slow off-rate modified aptamer (SOMAmer) reagents.
abstract: BACKGROUND
  Response to cetuximab (Erbitux®) and panitumumab (Vectibix®) varies among individuals, and even those who show response ultimately gain drug resistance. One possible etiologic factor is differential interaction between the drug and target. We describe the development of an assay based on Slow Off-rate Modified Aptamer (SOMAmer(™)) reagents that can distinguish drug-bound from unbound epidermal growth factor receptor (EGFR).
  
  
  METHODS
  This quantitative assay uses a SOMAmer reagent specific for EGFR extracellular domain (ECD) as a capturing reagent. Captured SOMAmer is quantitated using PCR. Linearity and accuracy (recovery) of the assay were assessed using normal sera and purified EGFR ECD.
  
  
  RESULTS
  This EGFR ECD assay showed linearity between 2.5 and 600 ng/mL. Average recovery was 101%. The assay detected EGFR but showed little cross-reactivity to other ErbB proteins: 0.4% for ErbB2, 6.9% for ErbB3, and 1.3% for ErbB4. Preincubation of normal serum with either cetuximab or panitumumab resulted in a dose-dependent decrease in EGFR ECD levels measured using the SOMAmer assay; preincubation did not affect measurement with an ELISA.
  
  
  CONCLUSIONS
  This SOMAmer-based serum EGFR ECD assay accurately and specifically measures EGFR in serum. Detection of significant amounts of drug-unbound EGFR in patients undergoing cetuximab or panitumumab treatment could be an indicator of poor drug response. Further studies are needed to evaluate the utility of the assay as an indicator of drug efficacy or as a guide to dosing.
authors.affiliation: Quest Diagnostics Nichols Institute San Juan Capistrano, San Juan Capistrano, California, United States of America.
authors.forename: Noh Jin
authors.initials: NJ
authors.lastname: Park
authors.forename: Xiuqiang
authors.initials: X
authors.lastname: Wang
authors.forename: Angelica
authors.initials: A
authors.lastname: Diaz
authors.forename: Dana M
authors.initials: DM
authors.lastname: Goos-Root
authors.forename: Christopher
authors.initials: C
authors.lastname: Bock
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Vaught
authors.forename: Weimin
authors.initials: W
authors.lastname: Sun
authors.forename: Charles M
authors.initials: CM
authors.lastname: Strom
mesh_terms: D000911:Antibodies, Monoclonal / Q000008:administration & dosage / Q000378:metabolism*; D061067:Antibodies, Monoclonal, Humanized / Q000008:administration & dosage / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000008:administration & dosage / Q000378:metabolism; D052157:Aptamers, Nucleotide / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D015415:Biomarkers / Q000097:blood; D000068818:Cetuximab; D019411:Clinical Laboratory Techniques / Q000379:methods*; D004797:Enzyme-Linked Immunosorbent Assay; D066246:ErbB Receptors / Q000037:antagonists & inhibitors / Q000097:blood* / Q000378:metabolism; D006801:Humans; D000077544:Panitumumab; D016133:Polymerase Chain Reaction; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000037:antagonists & inhibitors / Q000097:blood / Q000378:metabolism; D020893:Receptor, ErbB-3 / Q000037:antagonists & inhibitors / Q000097:blood / Q000378:metabolism; D066247:Receptor, ErbB-4; D015203:Reproducibility of Results
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D052157:Aptamers, Nucleotide; D015415:Biomarkers; D000068818:Cetuximab; D066246:ErbB Receptors; D000077544:Panitumumab; D018719:Erb-b2 Receptor Tyrosine Kinases; D020893:Receptor, ErbB-3; D066247:Receptor, ErbB-4; C508053:ERBB2 protein, human; C581292:ERBB3 protein, human; C587689:ERBB4 protein, human