What is FAP-α ?
Fibroblast activation protein-α (type II) (also known as "Seprase" or "Prolyl endopeptidase") is a trans-membrane serine protease (760 amino acids).
Extracellular domain consists of 7-bladed β-propeller (99-499 amino acids) and catalytic αβ-hydrolase (500-761 amino acids, including Ser624, Asp702, His734 triad) (1-2).
Glycosylation is necessary for its enzymatic (protease) activities and is proteolytically active in the form of homodimer or heterodimer (such as FAP-α with DPPIV (Dipeptidyl peptidase IV)) (Figure-1A).
Enzymatic activity of FAP-α results in either dipeptidyl peptidase (cleaves two amino acids post proline near N-terminus) or endopeptidase (cleaves more than two amino acids post proline near N-terminus)
Not expressed in normal adult tissues but upregulated expression is observed in activated fibroblasts (also known as myofibroblasts) in response to wounds and reactive stroma due to epithelial carcinoma.
To target FAP-α, several approaches using either prodrugs comprising of FAP-α-substrates (peptides) coupled to cytotoxic drugs (such as doxorubicin) (Figure-1B) or selective FAP-α-enzyme inhibitors have been reported (Figure-1C).
After cleavage of an enzyme specific sequence (post proline) of peptide in prodrug, cytotoxic agent is released and activated yo react on tumor cell (Figure-1B).
When protease inhibitors (FAP-α specific and selective) bind to the active site of catalytic domain, the enzymatic activity of FAP-α is blocked ----- Its like "Arresting the tumor proliferation" (Figure-1C).
Functions: Tissue remodeling, inflammation, fibrosis, and tumor growth
Figure-1: A) FAP-α Activity, B) In presence of prodrugs, C) In presence of inhibitors
What is activated Stroma ?
The term "Activated stroma" (also known as "Desmoplasia") attributes to the precancerous/cancerous state of stromal cells (1). For details, see the figure below (Figure-12:
Figure-2: Stromal changes in tumor microenvironment
What better than FDG ?
As described above (Figure-1), the role of activated fibroblasts (CAFs or myofibroblasts) is very significant for tumor metastasis. In addition, the expression of FAP-α is significantly enhanced in epithelial carcinomas compared to normal cells which makes it a promising target for many cancer types. The first PET study of [68Ga]Ga-DOTA-FAPI-04 (a potent FAP-α inhibitor) in 28 different cancer types was reported by Giesel and Haberkorn group 2019 (Figure-3).
Figure-3: [68Ga]Ga-DOTA-FAPI-04 PET/CT images in patients reflecting 15 different cancer types. (abbreviations: Ca = cancer; CCC = cholangiocellular carcinoma; CUP = carcinoma of unknown primary; MTC = medullary thyroid cancer; NET = neuroendocrine tumor). (Images Courtesy: JNM, For non-commercial and web-use only: Kratochwil et al., J Nucl Med. 2019; 60 (6): 801–805. (4))
In comparison with [18F]FDG in 6 different cancer types (pancreatic, esophageal, head and neck, thyroid, non-small cell lung cancer), [68Ga]Ga-DOTA-FAPI-04 showed no uptake in pharyngeal mucosa and liver, providing lower background noise and easy identification of primary or metastatic lesions (5).
In recent study, [68Ga]Ga-DOTA-FAPI-04 showed far better uptake in nodular type lesions in a pancreatic cancer patient (Figure-4) compared to [18F]FDG which showed negative uptake in these lesions (such as in liver capsule, mesenterium, and pelvis) (6).
Figure-4: PET/CT scan in pancreatic cancer patient (upper row, lesions in the liver capsule, dotted arrow; middle row, lesions in the mesenterium, solid arrow; lower row, lesions in the pelvis, solid arrow), A) Abnormal [68Ga]Ga-DOTA-FAPI-04 activity in the upper part of the body, located in the sternum, thickened pleura, and enlarged internal mammary lymph nodes, B) Negative / minimal uptake of [18F]FDG PET/CT images in these nodular-type lesions (Images license by Springer Nature, for non-commercial and web-use only: Zhao et al., Eur J Nucl Med Mol Imag. 2021 (6)).
Besides showing better tumor / background ratio than [18F]FDG, [68Ga]Ga-DOTA-FAPI-04 is preferred for several other reasons such as no need of fasting, feasibility in diabetic patients too, rapid tumor uptake, prolonged tumor retention time, reduced injected radioactivity, and minimal uptake time (10 min). The high demand of [68Ga]Ga-DOTA-FAPI-04 has also raised the clinical importance of [68Ga] which can be produced by both generators (68Ge/68Ga) and cyclotrons (< 15 MeV). To meet the clinical demands, the recent developments in [68Ga] production via solid target from cyclotron have increased its market value as cyclotron can produce 100 times more radioactivity (ca. 300 GBq/per 2h irradiation/batch) than produced by generators (ca. 1.5 GBq/batch) (7).
Hamson EJ, Keane FM, Tholen S, et al. Understanding fibroblast activation protein (FAP): substrates, activities, expression and targeting for cancer therapy. Proteomics Clin Appl., 2014; 8 (5-6): 454-63.[CrossRef]
Kelly T. Fibroblast activation protein-α and dipeptidyl peptidase IV (CD26): Cell-surface proteases that activate cell signaling and are potential targets for cancer therapy. Drug Resis Updates, 2005; 8 (1–2): 51-58. [CrossRef]
Mueller MM, Fusenig. Friends or foes - Bipolar effects of the tumour stroma in cancer. Nat Rev Can., 2004; 839–849. [CrossRef]
Kratochwil C, Flechsig P, Lindner T, et al. 68Ga-FAPI PET/CT: Tracer uptake in 28 different kinds of cancer. Nucl Med., 2019; 60 (6): 801-805. [CrossRef]
Giesel FL, Kratochwil C, Lindner T, et al. 68Ga-FAPI PET/CT: Biodistribution and preliminary dosimetry estimate of 2 DOTA-containing FAP-targeting agents in patients with various cancers. J Nucl Med., 2019; 60 (3): 386-392. [CrossRef]
Zhao L, Pang Y, Luo Z, et al. Role of [68Ga]Ga-DOTA-FAPI-04 PET/CT in the evaluation of peritoneal carcinomatosis and comparison with [ 18 F]-FDG PET/CT. Eur J Nucl Med Mol Imag., 2021; doi: 10.1007/s00259-020-05146-6. [CrossRef]
Thisgaard H, Kumlin J, Langkjær N et al. Multi-curie production of gallium-68 on a biomedical cyclotron and automated radiolabelling of PSMA-11 and DOTATATE. EJNMMI Radiopharm Chem., 2021; 6, 1. https://doi.org/10.1186/s41181-020-00114-9. [CrossRef]