title: Simultaneous detection of intracellular target and off-target binding of small molecule cancer drugs at nanomolar concentrations.
abstract: OBJECTIVE
  In vitro assays that determine activities of drug candidates with isolated targets have only limited predictive value for activities in cellular assays. Poor membrane permeability and off-target binding are major reasons for such discrepancies. However, it still difficult to directly analyse off-target binding at the same time as target binding, on a subcellular level. Here, we present a combination of fluorescence correlation spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS) as a solution to this problem.
  
  
  METHODS
  The well-established dihydrofolate reductase inhibitor methotrexate and the kinase inhibitors PD173956 and purvalanol B were conjugated via polyethylene glycol linkers with the fluorophore Cy5. The cellular uptake and subcellular distribution of these compounds in single human cancer-derived cells were investigated by confocal laser scanning microscopy. In addition, molecular interactions inside the cell with the respective target proteins and off-target binding were detected simultaneously in the nanomolar range by FCCS and FCS, respectively, using cells expressing green fluorescent protein fusion proteins of dihydrofolate reductase and Abelson kinase 1.
  
  
  RESULTS
  Large differences in the interaction patterns were found for these compounds. For methotrexate-Cy5, drug-target interactions could be detected and dissociation constants determined. In contrast, PD173956-Cy5 showed strong interactions with intracellular high-molecular weight structures, other than its target.
  
  
  CONCLUSIONS
  The combination of FCS and FCCS provides a powerful means to assess subcellular pharmacokinetics and dynamics of drug candidates at nanomolar concentrations.
authors.affiliation: Department of Biochemistry, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.
authors.forename: Heike
authors.initials: H
authors.lastname: Glauner
authors.forename: Ivo R
authors.initials: IR
authors.lastname: Ruttekolk
authors.forename: Kerrin
authors.initials: K
authors.lastname: Hansen
authors.forename: Ben
authors.initials: B
authors.lastname: Steemers
authors.forename: Yi-Da
authors.initials: YD
authors.lastname: Chung
authors.forename: Frank
authors.initials: F
authors.lastname: Becker
authors.forename: Stefan
authors.initials: S
authors.lastname: Hannus
authors.forename: Roland
authors.initials: R
authors.lastname: Brock
mesh_terms: D000042:Absorption; D000225:Adenine / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D000970:Antineoplastic Agents / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics; D002460:Cell Line; D004354:Drug Screening Assays, Antitumor / Q000379:methods*; D005260:Female; D005456:Fluorescent Dyes / Q000737:chemistry; D005493:Folic Acid Antagonists / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D047569:Laser Scanning Cytometry; D008727:Methotrexate / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D036641:Microscopy, Fluorescence, Multiphoton / Q000379:methods; D011092:Polyethylene Glycols / Q000737:chemistry; D047428:Protein Kinase Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D016315:Proto-Oncogene Proteins c-abl / Q000235:genetics / Q000378:metabolism; D011728:Pyridones / Q000378:metabolism / Q000493:pharmacokinetics; D011743:Pyrimidines / Q000378:metabolism / Q000493:pharmacokinetics; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D005456:Fluorescent Dyes; D005493:Folic Acid Antagonists; C553170:PD 173956; D047428:Protein Kinase Inhibitors; D011728:Pyridones; D011743:Pyrimidines; D011993:Recombinant Fusion Proteins; C113584:purvalanol B; D049452:Green Fluorescent Proteins; D011092:Polyethylene Glycols; D013762:Tetrahydrofolate Dehydrogenase; D016315:Proto-Oncogene Proteins c-abl; D000225:Adenine; D008727:Methotrexate