title: Quantification of internalization of EGFR-binding Affibody molecules: Methodological aspects.
abstract: Tumor cell internalization of targeting agents is of interest, since internalization influences the local retention time of a radionuclide and thereby imaging quality in PET and SPECT and effects of radionuclide therapy. In cases where nuclear methods are not applicable at the cellular level, quantitative fluorescent techniques are useful as described in this article. Two fluorescence-based methods to study cellular internalization were applied: the CypHer and the Alexa488-quenching methods, both utilized in fluorescence microscopy and flow cytometry. Two EGFR-binding Affibody molecules were analyzed in A431 cells: the monomer Z1907 and the dimer (Z1907)2. EGF, cetuximab and non-specific Affibody molecules were used as controls. For comparison, internalization of 111In-labeled Z1907 was studied with the acid wash internalization assay. The Cypher method is straightforward, but requires equal labeling of all compounds for accurate quantification. The Alexa488-quenching method is preferable since it is independent of the dye-to-protein ratio. According to this method, about 45% of EGF and 19-24% of the bound Affibody molecules and cetuximab were internalized within one hour. Similar results were seen with 111In-Z1907 in the acid wash method, while (Z1907)2 was not removed by acid and thus could not be studied this way. The fluorescence-based Alexa488-quenching method is well suited to quantitatively analyze internalization of targeting agents, also those that resist acid wash. The internalized fraction showed that both the monomeric and dimeric Affibody molecules are expected to give good uptake and thereby good retention of metallic radionuclides which will render good tumor to background values.
authors.affiliation: Biomedical Radiation Sciences, Department of Oncology, Radiology and Clinical Immunology, Rudbeck Laboratory, Uppsala University, SE-751 85 Uppsala, Sweden. lovisa.gostring@bms.uu.se
authors.forename: Lovisa
authors.initials: L
authors.lastname: Göstring
authors.forename: Ming Tsuey
authors.initials: MT
authors.lastname: Chew
authors.forename: Anna
authors.initials: A
authors.lastname: Orlova
authors.forename: Ingmarie
authors.initials: I
authors.lastname: Höidén-Guthenberg
authors.forename: Anders
authors.initials: A
authors.lastname: Wennborg
authors.forename: Jörgen
authors.initials: J
authors.lastname: Carlsson
authors.forename: Fredrik Y
authors.initials: FY
authors.lastname: Frejd
mesh_terms: D001692:Biological Transport; D002232:Carbocyanines; D045744:Cell Line, Tumor; D004705:Endocytosis; D066246:ErbB Receptors / Q000378:metabolism*; D005434:Flow Cytometry / Q000379:methods; D005456:Fluorescent Dyes; D006801:Humans; D007205:Indium Radioisotopes; D007700:Kinetics; D008024:Ligands; D018613:Microscopy, Confocal / Q000379:methods; D008856:Microscopy, Fluorescence / Q000379:methods; D011993:Recombinant Fusion Proteins / Q000378:metabolism*; D013388:Succinimides
chemical_list: C492694:Alexa Fluor 488 carboxylic acid succinimidyl ester; D002232:Carbocyanines; C474471:CypHer 5 NHS ester; D005456:Fluorescent Dyes; D007205:Indium Radioisotopes; D008024:Ligands; D011993:Recombinant Fusion Proteins; D013388:Succinimides; C512478:EGFR protein, human; D066246:ErbB Receptors