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A Trifunctional Theranostic Ligand Targeting Fibroblast Activation Protein-α (FAPα)

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Abstract

Purpose

Fibroblast activation protein-α (FAPα) is uniquely expressed in activated fibroblasts, including cancer-associated fibroblasts that populate tumor stroma and contribute to proliferation and immunosuppression. Radiolabeled FAPα inhibitors enable imaging of multiple human cancers, but time-dependent clearance from tumors currently limits their utility as FAPα-targeted radiotherapeutics. We sought to increase the area under the curve (AUC) by constructing a trifunctional ligand that binds FAPα with high affinity and also binds albumin and theranostic radiometals.

Procedures

RPS-309 comprised a FAPα-targeting moiety, an albumin-binding group, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Inhibition of recombinant human FAPα (rhFAPα) was determined by colorimetric assay. Affinity for human serum albumin (HSA) was determined by high-performance affinity chromatography. The tissue distribution of [68Ga]Ga-RPS-309 in SW872 tumor xenograft-bearing mice was imaged by microPET/CT and quantified by biodistribution studies performed from 30 min to 3 h post injection (p.i.). The biodistribution of [177Lu]Lu-RPS-309 was determined at 4, 24, and 96 h p.i.

Results

RPS-309 inhibits rhFAPα with IC50 = 7.3 ± 1.4 nM. [68Ga]Ga-RPS-309 is taken up specifically by FAPα-expressing cells and binds HSA with Kd = 4.6 ± 0.1 μM. Uptake of the radiolabeled ligand in tumors was evident from 30 min p.i. (> 5 %ID/g) and was significantly reduced by co-injection of RPS-309. Specific skeletal uptake was also observed. Activity in tumors was constant through 4 h p.i., but cleared significantly by 24 h. The AUC in this period was 127 (%ID/g) × h.

Conclusions

RPS-309 is a high-affinity FAPα inhibitor with prolonged plasma residence. Introduction of the albumin-binding group did not compromise FAPα binding. Although initial tumor uptake was high and FAPα-specific, RPS-309 also progressively cleared from tumors. Nevertheless, RPS-309 incorporates multiple sites in which structural diversity can be introduced, and therefore serves as a platform for future structure-activity relationship studies.

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Acknowledgments

The authors wish to thank Prof. Dr. Uwe Haberkorn for providing the HT-1080-FAP and HT-1080 cells and to acknowledge Dr. J. David Warren for assistance with compound purification and analysis.

Author information

Author notes

  1. Shashikanth Ponnala

    Present address: Angion Biomedica Corp., Uniondale, NY, 11553, USA

  2. Anastasia Nikolopoulou

    Present address: The Janssen Pharmaceutical Companies of Johnson & Johnson, Spring House, PA, 19477, USA

Authors and Affiliations

  1. Molecular Imaging Innovations Institute (MI3), Department of Radiology, Weill Cornell Medicine, New York, NY, 10065, USA

    James M. Kelly, Thomas M. Jeitner, Shashikanth Ponnala, Clarence Williams Jr, Anastasia Nikolopoulou & John W. Babich

  2. Citigroup Biomedical Imaging Center, Weill Cornell Medicine, New York, NY, 10021, USA

    Anastasia Nikolopoulou & John W. Babich

  3. Departments of Pharmaceutical Sciences and Chemistry, UIC College of Pharmacy, Chicago, IL, USA

    Stephen G. DiMagno

  4. Sandra and Edward Meyer Cancer Center, Weill Cornell Medicine, New York, NY, 10065, USA

    John W. Babich

  5. Department of Radiology, Weill Cornell Medicine, Belfer Research Building, Room 1600, 413 E 69th St, New York, NY, 10021, USA

    John W. Babich

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Correspondence to John W. Babich.

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Conflict of Interest

James M. Kelly, Shashikanth Ponnala, and John W. Babich hold equity in Noria Therapeutics, Inc, which has licensed intellectual property rights related to compounds described in this manuscript.

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Kelly, J.M., Jeitner, T.M., Ponnala, S. et al. A Trifunctional Theranostic Ligand Targeting Fibroblast Activation Protein-α (FAPα). Mol Imaging Biol 23, 686–696 (2021). https://doi.org/10.1007/s11307-021-01593-1

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