Protein-based Radiopharmaceuticals that target fibroblast activation protein alpha: a review of current progress

Abstract

Background: Fibroblast activation protein alpha (FAP) is a serine protease that is expressed at basal levels in benign tissues but is overexpressed in a variety of pathologies, including cancer. Consequently, significant research efforts have been expended to develop diagnostic radiopharmaceuticals and effective radiotherapies that target this protein. The aim of this review is to summarize the current progress on the development of protein-based radiopharmaceuticals that target FAP.

Main body: A literature survey spanning nearly 40 years was conducted to assess the historical development and current progress in protein-based radiopharmaceuticals that target FAP. To date, more than 20 publications have been introduced that describe these agents in preclinical and clinical settings. This review summarizes the development and evaluation of radiopharmaceuticals involving antibodies, antibody fragments, and single domain antibodies.

Conclusion: The results of this literature review demonstrate that while significant research efforts have been expended on peptide-based radiopharmaceuticals and small molecule FAP inhibitors, the development of protein-based radiopharmaceuticals that target FAP remains an active research area that has yet to reach its full potential.

Keywords: Antibody; Antibody fragment; Cancer; Fibroblast activation protein; Positron emission tomography; Radiotherapy; Single domain antibody; Single photon emission computed tomography; Theranostics.

Fig. 1

Microenvironmental regulation of primary tumor progression and metastasis The evolving tumor microenvironment (TME) during all stages of cancer progression is depicted with key representative cell types shown. The TME includes diverse immune cells, cancer-associated fibroblasts (CAFs), endothelial cells, and the extracellular matrix (ECM), among others. These components may vary by tissue type and co-evolve with the tumor as it progresses. The normal tissue microenvironment can constrain cancer outgrowth through the suppressive functions of immune cells, fibroblasts, and the ECM. However, for cancer to advance, it must evade these functions and instead influence cells in the TME to become tumor promoting, resulting in increased proliferation, invasion, and intravasation at the primary site. Cells and factors of the TME also play a vital role in preparing the premetastatic niche, regulating cancer cell survival in the circulation, and promoting extravasation. During the metastatic stages, the TME helps to control metastatic cell dormancy, emergence from this state, and subsequent metastatic outgrowth. This research was originally published in Cancer Cell. de Visser et al. The evolving tumor microenvironment: From cancer initiation to metastatic outgrowth. Cancer Cell 2023; 41:374-403. Copyright Cell Press

Fig. 2

Human and Camelid antibody fragments. Structures of human antibody fragments discussed in this review. Heavy chain is colored in blue and light chain in red. Antibody domains are labelled with their appropriate names used for molecular imaging. Heavy chain is colored in blue and light chain in red. Antibody do-(CH—constant heavy; VH—variable heavy; VL—variable light; VHH—variable heavy of heavy-chain-only antibodies). Size and circulation half-lives are mentioned below the structures. This research was originally published in Biomolecules. Berland, l. et al. Nanobodies for Medical Imaging: About Ready for Prime Time? Biomolecules 2021; 11:637. Copyright MDPI

Fig. 3

Cerenkov luminescence image of FAP⁺U87MG tumors-bearing mice 72 h p.i. using [⁸⁹Zr] Zr-DFO-Bz-F19. Experiments indicated increased luminescence intensity among the FAP ⁺ tumors. FAP^- tissues such as muscle displayed an average radiance not exceeding background levels. Ex vivo organ imaging confirmed the in vivo results. Based on ROI analysis, FAP ⁺ tumors had an average radiance of 8.5 x_ 10³ _ ± 1.5 _x 10³ p/s/cm²/sr. This research was originally published in Molecules. Pandya, D. et al. Imaging of Fibroblast Activation Protein Alpha Expression in a Preclinical Mouse Model of Glioma Using Positron Emission Tomography. Molecules 2020; 25:3672. Copyright MDPI

Fig. 4

Small animal PET/CT imaging of CIA mice imaged with [⁸⁹Zr] Zr-28H1. A PET/CT scans (3D) of mouse with CIA, injected with 5 MBq of [⁸⁹Zr] Zr-28H1 and scanned at 72 h after injection. Joint scores were 1.75, 1.5, 0.25, and 2 (front right, front left, hind right, and hind left, respectively). B PET/CT scan of mouse with CIA, injected with 10 MBq of [¹⁸F] F-FDG and scanned at 1 h after injection. Joint scores were 0, 0.25, 1.25, and 0.25 (front right, front left, hind right, and hind left, respectively). This research was originally published in the Journal of Nuclear Medicine. Laverman, P. et al. Immuno-PET and Immuno-SPECT of Rheumatoid Arthritis with Radiolabeled Anti-Fibroblast Activation Protein Antibody Correlates with Severity of Arthritis. The Journal of Nuclear Medicine 2015; 56: 778–83. Copyright Society of Nuclear Medicine and Molecular Imaging, Inc

Fig. 5

Biodistribution of ^99mTc- and ⁶⁸Ga-labeled anti-FAP sdAbs. A Maximum intensity projections (MIPs) corresponding to small-animal SPECT/CT images obtained 1 h after administration of ^99mTc-labeled sdAbs in hFAP-positive U87-MG tumor–xenografted mice (n = 3). B MIPs corresponding to small-animal PET/CT image 1 h after administration of [⁶⁸Ga]Ga-DOTA-4AH29 in hFAP-positive U87-MG tumor–xenografted mice (n = 3). %ID/CC = percentage injected activity per cubic centimeter; B = bladder; K = kidney; T = tumor. C Ex vivo biodistribution obtained after dissections 1.5 h after administration of ^99mTc-labeled sdAbs. This research was originally published in The Journal of Nuclear Medicine. Dekempeer, Y. et al. Preclinical Evaluation of a Radiotheranostic Single-Domain Antibody Against Fibroblast Activation Protein α. The Journal of Nuclear Medicine 2023; 64: 1941–1948. Copyright Society of Nuclear Medicine and Molecular Imaging

Fig. 6

Therapeutic efficacy of [¹³¹I]I-GMIB-4AH29 and [²²⁵Ac]Ac-DOTA-4AH29. A Therapeutic potential of [¹³¹I]I-GMIB-4AH29 and [²²⁵Ac]Ac-DOTA-4AH29 was evaluated in U87-MG tumor–bearing mice (n = 10). B Subcutaneous tumor development over time for each treatment group. C Kaplan–Meier survival curve obtained after administration of [¹³¹I]I-GMIB-4AH29 or [²²⁵Ac]Ac-DOTA-4AH29. MS = median survival; s.c. = subcutaneous. This research was originally published in The Journal of Nuclear Medicine. Dekempeer, Y. et al. Preclinical Evaluation of a Radiotheranostic Single-Domain Antibody Against Fibroblast Activation Protein α. The Journal of Nuclear Medicine 2023; 64: 1941–1948. Copyright Society of Nuclear Medicine and Molecular Imaging

Fig. 7

Therapy study in PDX2494-P5 with schematic overview of therapeutic schedule and dosimetry. (A and B) Tumor growth control (A) and Kaplan–Meier plot of survival probability (B) as function of time. Mice were euthanized when tumor volume was 2,000 mm³. Survival data reflect progression of primary tumors, because no adverse effects led to anticipated animal euthanasia. n.d. = not determined; Tz = tetrazine. This research was originally published in The Journal of Nuclear Medicine. Poty, S. et al. Optimizing the Therapeutic Index of sdAb-Based Radiopharmaceuticals Using Pretargeting. The Journal of Nuclear Medicine 2024; 65: 1564–1570. Copyright Society of Nuclear Medicine and Molecular Imaging