Synthesis and Preclinical Evaluation of Novel 68Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Shreya Bendre; Hsiou-Ting Kuo; Helen Merkens; Zhengxing Zhang; Antonio A W L Wong; François Bénard; Kuo-Shyan Lin

doi:10.3390/ph16060798

Synthesis and Preclinical Evaluation of Novel ⁶⁸Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Pharmaceuticals (Basel). 2023 May 28;16(6):798. doi: 10.3390/ph16060798.

Authors

Shreya Bendre¹, Hsiou-Ting Kuo¹, Helen Merkens¹, Zhengxing Zhang¹, Antonio A W L Wong¹, François Bénard^{1

2

3}, Kuo-Shyan Lin^{1

2

3}

Affiliations

¹ Department of Molecular Oncology, BC Cancer Research Institute, Vancouver, BC V5Z 1L3, Canada.
² Department of Functional Imaging, BC Cancer Research Institute, Vancouver, BC V5Z 4E6, Canada.
³ Department of Radiology, University of British Columbia, Vancouver, BC V5Z 1M9, Canada.

Abstract

Fibroblast activation protein (FAP) is a membrane-tethered serine protease overexpressed in the reactive stromal fibroblasts of >90% human carcinomas, which makes it a promising target for developing radiopharmaceuticals for the imaging and therapy of carcinomas. Here, we synthesized two novel (R)-pyrrolidin-2-yl-boronic acid-based FAP-targeted ligands: SB02055 (DOTA-conjugated (R)-(1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)glycyl)pyrrolidin-2-yl)boronic acid) and SB04028 (DOTA-conjugated ((R)-1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)-D-alanyl)pyrrolidin-2-yl)boronic acid). ^natGa- and ⁶⁸Ga-complexes of both ligands were evaluated in preclinical studies and compared to previously reported ^natGa/⁶⁸Ga-complexed PNT6555. Enzymatic assays showed that FAP binding affinities (IC₅₀) of ^natGa-SB02055, ^natGa-SB04028 and ^natGa-PNT6555 were 0.41 ± 0.06, 13.9 ± 1.29 and 78.1 ± 4.59 nM, respectively. PET imaging and biodistribution studies in HEK293T:hFAP tumor-bearing mice showed that while [⁶⁸Ga]Ga-SB02055 presented with a nominal tumor uptake (1.08 ± 0.37 %ID/g), [⁶⁸Ga]Ga-SB04028 demonstrated clear tumor visualization with ~1.5-fold higher tumor uptake (10.1 ± 0.42 %ID/g) compared to [⁶⁸Ga]Ga-PNT6555 (6.38 ± 0.45 %ID/g). High accumulation in the bladder indicated renal excretion of all three tracers. [⁶⁸Ga]Ga-SB04028 displayed a low background level uptake in most normal organs, and comparable to [⁶⁸Ga]Ga-PNT6555. However, since its tumor uptake was considerably higher than [⁶⁸Ga]Ga-PNT6555, the corresponding tumor-to-organ uptake ratios for [⁶⁸Ga]Ga-SB04028 were also significantly greater than [⁶⁸Ga]Ga-PNT6555. Our data demonstrate that (R)-(((quinoline-4-carbonyl)-d-alanyl)pyrrolidin-2-yl)boronic acid is a promising pharmacophore for the design of FAP-targeted radiopharmaceuticals for cancer imaging and radioligand therapy.

Keywords: (R)-pyrrolidin-2-yl-boronic acid-based radiopharmaceuticals; FAP inhibitors (FAPIs); PET imaging; cancer-associated fibroblasts (CAFs); fibroblast activation protein α (FAP-α); gallium-68.

Grants and funding

PJT-180300/CAPMC/ CIHR/Canada