title: Preclinical Comparison of Albumin-Binding Radiofolates: Impact of Linker Entities on the in Vitro and in Vivo Properties.
abstract: Tumor targeting with folic acid radioconjugates has been proposed as a promising strategy for radionuclide therapy of folate receptor α (FR)-positive cancer. Recently, it was shown that modification of radiofolates with an albumin-binding entity increased the tumor-to-kidney ratios of accumulated radioactivity in mice. The goal of this study was to evaluate the lead compound cm10 and compare it with new albumin-binding folate conjugates. Compound cm12 was designed with a long spacer consisting of a PEG-11 entity, and compound cm13 contained a short alkane chain between the albumin-binding moiety and folic acid. All of the derivatives were labeled with177Lu (t1/2= 6.65 days, Eβ-,average= 134 keV; Eγ= 113 keV, 208 keV), a clinically established radionuclide for therapeutic purposes. The evaluation revealed that all of the albumin-binding radiofolates exhibited increased in vitro stability compared with the reference compound (177Lu-cm14) without albumin binder. Serum protein binding, determined with an ultrafiltration assay, was high (>88%) for the derivatives with albumin-binding entities. The FR-binding affinity was in the same range (KD= 4.0-7.5 nM) for all of the radiofolates, independent of the albumin-binding entity and spacer length. FR-specific uptake was proven in vitro using FR-positive KB tumor cells. In vivo studies with KB-tumor-bearing mice were performed in order to assess the tissue distribution profile of the novel radiofolates.177Lu-cm13 showed high tumor uptake at late time points (13.3 ± 2.94% IA/g, 48 h p.i.) and tumor-to-kidney ratios (0.59 ± 0.03, 48 h p.i.) in the same range as177Lu-cm10 (0.55 ± 0.07, 48 h p.i.). However, the tumor-to-kidney ratio of177Lu-cm12 (0.28 ± 0.07, 48 h p.i.) was reduced compared with177Lu-cm10 and177Lu-cm13. The results of this study indicate that the spacer entity between folic acid and the albumin binder is of critical importance with regard to the tissue distribution profile of the radiofolate. The PEG spacer compromised the beneficial effects of the lead compound, but the design with a short alkane spacer appeared to be promising. Future studies will focus on the design of radiofolates with lipophilic and more rigid spacer entities, which may allow a further improvement of their tissue distribution profiles.
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.
authors.forename: Klaudia
authors.initials: K
authors.lastname: Siwowska
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Haller
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.
authors.forename: Francesca
authors.initials: F
authors.lastname: Bortoli
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.|Department of Chemistry and Applied Biosciences, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Martina
authors.initials: M
authors.lastname: Benešová
authors.affiliation: Merck & Cie Schaffhausen , 8200 Schaffhausen, Switzerland.
authors.forename: Viola
authors.initials: V
authors.lastname: Groehn
authors.affiliation: Department of Radiation Physics, The Sahlgrenska Academy, University of Gothenburg , SE-413 45 Gothenburg, Sweden.|Department of Medical Physics and Medical Bioengineering, Sahlgrenska University Hospital , SE-413 45 Gothenburg, Sweden.
authors.forename: Peter
authors.initials: P
authors.lastname: Bernhardt
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.|Department of Chemistry and Applied Biosciences, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Roger
authors.initials: R
authors.lastname: Schibli
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institut , 5232 Villigen-PSI, Switzerland.|Department of Chemistry and Applied Biosciences, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Cristina
authors.identifier: 0000-0001-9357-9688
authors.initials: C
authors.lastname: Müller
mesh_terms: D000418:Albumins / Q000737:chemistry* / Q000378:metabolism; D000818:Animals; D045744:Cell Line, Tumor; D005260:Female; D058975:Folate Receptor 1 / Q000378:metabolism; D005492:Folic Acid / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D007624:KB Cells; D051379:Mice; D008819:Mice, Nude; D011868:Radioisotopes / Q000737:chemistry* / Q000378:metabolism; D019275:Radiopharmaceuticals / Q000737:chemistry* / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D000418:Albumins; D058975:Folate Receptor 1; D011868:Radioisotopes; D019275:Radiopharmaceuticals; D005492:Folic Acid
keywords: 177Lu; PEG spacer; albumin-binding entity; alkane spacer; folate radioconjugate; linker; radionuclide therapy; spacer