Radioiodinated Folic Acid Conjugates: Evaluation of a Valuable Concept To Improve Tumor-to-Background Contrast

Folic acid radioconjugates can be used for targeting folate receptor positive (FR+) tumors. However, the high renal uptake of radiofolates is a drawback of this strategy, particularly with respect to a therapeutic application due to the risk of damage to the kidneys by particle radiation. The goal of this study was to develop and evaluate radioiodinated folate conjugates as a novel class of folate-based radiopharmaceuticals potentially suitable for therapeutic application. Two different folic acid conjugates, tyrosine-folate (1) and tyrosine-click-folate (3), were synthesized and radioiodinated using the Iodogen method resulting in [125I]-2 and [125/131I]-4. Both radiofolates were highly stable in mouse and human plasma. Determination of FR binding affinities using 3H-folic acid and FR+ KB tumor cells revealed affinities in the nanomolar range for 2 and 4. The cell uptake of [125I]-2 and [125/131I]-4 proved to be FR specific as it was blocked by the coincubation of folic acid. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) in vitro assays were employed for the determination of tumor cell viability upon exposure to [131I]-4. Compared to untreated control cells, significantly reduced cell viability was observed for FR+ cancer cells (KB, IGROV-1, SKOV-3), while FR– cells (PC-3) were not affected. Biodistribution studies performed in tumor bearing nude mice showed the specific accumulation of both radiofolates in KB tumor xenografts ([125I]-2: 3.43 ± 0.28% ID/g; [125I]-4: 2.28 ± 0.46% ID/g, 4 h p.i.) and increasing tumor-to-kidney ratios over time. The further improvement of the tumor-to-background contrast was achieved by preinjection of the mice with pemetrexed allowing excellent imaging via single-photon emission computed tomography (SPECT/CT). These findings confirmed the hypothesis that the application of radioiodinated folate conjugates may be a valuable concept to improve tumor-to-background contrast. The inhibitory effect of [131I]-4 on FR+ cancer cells in vitro indicates the potential of this class of radiofolates for therapeutic application.