Design and Evaluation of Novel Albumin-Binding Folate Radioconjugates: Systematic Approach of Varying the Linker Entities.

Tumor targeting using folate radioconjugates is a promising strategy for theragnostics of folate receptor-positive tumors. The aim of this study was to investigate the impact of structural modifications of folate radioconjugates on their pharmacokinetic properties. Four novel folate radioconjugates ([ 177 Lu]Lu-OxFol-2, [ 177 Lu]Lu-OxFol-3, [ 177 Lu]Lu-OxFol-4, and [ 177 Lu]Lu-OxFol-5), modified with a lipophilic or hydrophilic linker entity in close proximity to the albumin-binding 4-( p -iodophenyl)butanoate entity or the DOTA chelator, respectively, were designed and evaluated for comparison with the previously developed [ 177 Lu]Lu-OxFol-1. A hydrophobic 4-(aminomethyl)benzoic acid linker, incorporated in close proximity to the 4-( p -iodophenyl)butanoate entity, enhanced the albumin-binding properties (relative affinity 7.3) of [ 177 Lu]Lu-OxFol-3 as compared to those of [ 177 Lu]Lu-OxFol-1 (relative affinity set as 1.0). On the other hand, a hydrophilic d-glutamic acid (d-Glu) linker entity used in [ 177 Lu]Lu-OxFol-2 compromised the albumin-binding properties. [ 177 Lu]Lu-OxFol-4 and [ 177 Lu]Lu-OxFol-5, in which the respective linker entities were incorporated adjacent to the DOTA chelator, showed similar albumin-binding properties (0.6 and 1.0, respectively) as [ 177 Lu]Lu-OxFol-1. Biodistribution studies in KB tumor-bearing nude mice revealed twofold higher tumor-to-kidney ratios at 4 h and 24 h after injection of [ 177 Lu]Lu-OxFol-3 (∼1.2) than after injection of [ 177 Lu]Lu-OxFol-1 (∼0.6). The tumor-to-kidney ratios of [ 177 Lu]Lu-OxFol-2 were, however, much lower (∼0.2) due to the high kidney retention of this radioconjugate. The tumor-to-kidney ratios of [ 177 Lu]Lu-OxFol-5 were only slightly increased (∼0.9), and the ratios for [ 177 Lu]Lu-OxFol-4 (∼0.7) were in the same range as for [ 177 Lu]Lu-OxFol-1. SPECT/CT imaging studies demonstrated similar tumor uptake of all radioconjugates but a clearly improved tumor-to-kidney ratio for [ 177 Lu]Lu-OxFol-3 as compared to that for [ 177 Lu]Lu-OxFol-1. Based on these data, it can be concluded that the linker entity in close proximity to the 4-( p -iodophenyl)butanoate entity affects the radioconjugate's pharmacokinetic profile considerably due to the altered affinity to albumin. Changes in the linker entity, which connects the DOTA chelator with the folate molecule, do not have a major impact on the radioconjugate's tissue distribution profile, however. As a result of these findings, [ 177 Lu]Lu-OxFol-3 had a comparable therapeutic effect to that of [ 177 Lu]Lu-OxFol-1 but appeared advantageous in preventing kidney damage. Provided that the kidneys will present the dose-limiting organs in patients, [ 177 Lu]Lu-OxFol-3 would be the preferred candidate for a clinical translation.