Head-to-head comparison of different classes of FAP radioligands designed to increase tumor residence time: monomer, dimer, albumin binders, and small molecules vs peptides

Abstract

Purpose Fibroblast activation protein-α (FAP)-targeting radioligands have recently demonstrated high diagnostic potential. However, their therapeutic value is impaired by the short tumor residence time. Several strategies have been tested to overcome this limitation, but a head-to-head comparison has never been done. With the aim to identify strengths and limitations of the suggested strategies, we compared the monomer FAPI-46 versus (a) its dimer (FAPI-46-F1D), (b) two albumin binders conjugates (FAPI-46-Ibu (ibuprofen) and FAPI-46-EB (Evans Blue)), and (c) cyclic peptide FAP-2286. Methods 177Lu-labeled ligands were evaluated in vitro in cell lines with low (HT-1080.hFAP) and high (HEK-293.hFAP) humanFAP expression. SPECT/CT imaging and biodistribution studies were conducted in HT-1080.hFAP and HEK-293.hFAP xenografts. The areas under the curve (AUC) of the tumor uptake and tumor-to-critical-organs ratios and the absorbed doses were estimated. Results Radioligands showed IC50 in the picomolar range. Striking differences were observed in vivo regarding tumor uptake, residence, specificity, and total body distribution. All [177Lu]Lu-FAPI-46-based radioligands showed similar uptake between the two tumor models. [177Lu]Lu-FAP-2286 showed higher uptake in HEK-293.hFAP and the least background. The AUC of the tumor uptake and absorbed dose was higher for [177Lu]Lu-FAPI-46-F1D and the two albumin binder conjugates, [177Lu]Lu-FAPI-46-Ibu and [177Lu]Lu-FAPI-46-EB, in HT1080.hFAP xenografts and for [177Lu]Lu-FAPI-46-EB and [177Lu]Lu-FAP-2286 in HEK293.hFAP xenografts. The tumor-to-critical-organs AUC values and the absorbed doses were in favor of [177Lu]Lu-FAP-2286, but tumor-to-kidneys. Conclusion The study indicated dimerization and cyclic peptide structures as promising strategies for prolonging tumor residence time, sparing healthy tissues. Albumin binding strategy outcome depended on the albumin binding moiety. The peptide showed advantages in terms of tumor-to-background ratios, besides tumor-to-kidneys, but its tumor uptake was FAP expression–dependent.

Rational Design and Comparison of Novel 99mTc-Labeled FAPI Dimers for Visualization of Multiple Tumor Types

“…18,21,28,35,36 In a head-to-head comparison, dimerization of FAPI has emerged as a highly promising strategy for enhancing tumor uptake. 37 An overview presents the current status of FAP-targeted dimers in Table 1.…”

“…Concurrently, the ligand’s interaction with a single receptor significantly boosts its localized concentration near the receptor, which consequently intensifies the binding avidity. Enhancements in binding avidity and a decreased receptor off-rate collectively contribute to a significant elevation in tumor uptake, thereby underscoring the pivotal role of dimerization interactions in optimizing the efficacy of targeted therapeutic agents. ,,,, In a head-to-head comparison, dimerization of FAPI has emerged as a highly promising strategy for enhancing tumor uptake . An overview presents the current status of FAP-targeted dimers in Table .…”

Rational Design and Pharmacomodulation of 18F-Labeled Biotin/FAPI-Conjugated Heterodimers

“…Similar to FAP, BR also represents a broad-spectrum target, presenting advantages for application in tumors where cancer cells exhibit low or no FAP expression. Preclinical studies often utilize artificially transfected tumor cells (HT1080-FAP) or tumor cells with high FAP expression like U87-MG, , which may not accurately reflect the clinical scenario where most cancer cells within tumors exhibit either low or no expression of FAP. , This mismatch can reduce the sensitivity of radioligands, particularly in cases where the TME has not formed or FAP expression on CAFs is not elevated . In this study, A549 cells were employed to address this discrepancy.…”

Design and Preclinical Evaluation of a Novel Prostate-Specific Membrane Antigen Radioligand Modified with a Transthyretin Binder

“…Similarly, we observed that the hydrophilic properties of the transthyretin binder (TB-01) enabled to maintain the hydrophilic properties of the resultant radioligand visible by comparable logD values of [ 177 Lu]Lu-PSMA-TB-01 (logD: −3.4) and [ 177 Lu]Lu-PSMA-617 (logD: −4.4 [14]), while the use of the p-iodophenyl and p-tolyl entities ([ 177 Lu]Lu-PSMA-ALB-53/56, logD values: −2.7 to −2.9) commonly enhanced the lipophilicity of the resultant radioligands. The increased lipophilicity upon the modification with an albumin binder has also been observed using other tumor targeting agents, which resulted in higher background activity [33,34]. Hydrophilic properties of radiopharmaceuticals are commonly desirable to favor renal excretion and prevent background activity in the liver and intestinal tract.…”