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MR Molecular Imaging of Prostate Cancer with a Small Molecular CLT1 Peptide Targeted Contrast Agent
Published on: September 3, 2013
Hetero-bivalent agents targeting FAP and PSMA
Srikanth Boinapally1, Alla Lisok1, Gabriela Lofland1
1Russell H. Morgan Department of Radiology and Radiological Science, Baltimore, MD, USA.
Purpose:
We developed a theranostic radiopharmaceutical that engages two key cell surface proteases, fibroblast activation protein alpha (FAP) and prostate-specific membrane antigen (PSMA), each frequently overexpressed within the tumor microenvironment (TME). The latter is also expressed in most prostate tumor epithelium. To engage a broader spectrum of cancers for imaging and therapy, we conjugated small-molecule FAP and PSMA-targeting moieties using an optimized linker to provide 64Cu-labeled compounds.
Methods:
We synthesized FP-L1 and FP-L2 using two linker constructs attaching the FAP and PSMA-binding pharmacophores. We determined in vitro inhibition constants (Ki) for FAP and PSMA. Cell uptake assays and flow cytometry were conducted in human glioma (U87), melanoma (SK-MEL-24), prostate cancer (PSMA + PC3 PIP and PSMA - PC3 flu), and clear cell renal cell carcinoma lines (PSMA + /PSMA - 786-O). Quantitative positron emission tomography/computed tomography (PET/CT) and tissue biodistribution studies were performed using U87, SK-MEL-24, PSMA + PC3 PIP, and PSMA + 786-O experimental xenograft models and the KPC genetically engineered mouse model of pancreatic cancer.
Results:
64Cu-FP-L1 and 64Cu-FP-L2 were produced in high radiochemical yields (> 98%) and molar activities (> 19 MBq/nmol). Ki values were in the nanomolar range for both FAP and PSMA. PET imaging and biodistribution studies revealed high and specific targeting of 64Cu-FP-L1 and 64Cu-FP-L2 for FAP and PSMA. 64Cu-FP-L1 displayed more favorable pharmacokinetics than 64Cu-FP-L2. In the U87 tumor model at 2 h post-injection, tumor uptake of 64Cu-FP-L1 (10.83 ± 1.02%ID/g) was comparable to 64Cu-FAPI-04 (9.53 ± 2.55%ID/g). 64Cu-FP-L1 demonstrated high retention 5.34 ± 0.29%ID/g at 48 h in U87 tumor. Additionally, 64Cu-FP-L1 showed high retention in PSMA + PC3 PIP tumor (12.06 ± 0.78%ID/g at 2 h and 10.51 ± 1.82%ID/g at 24 h).
Conclusions:
64Cu-FP-L1 demonstrated high and specific tumor targeting of FAP and PSMA. This compound should enable imaging of lesions expressing FAP, PSMA, or both on the tumor cell surface or within the TME. FP-L1 can readily be converted into a theranostic for the management of heterogeneous tumors.
Insights
A novel theranostic radiopharmaceutical, 64Cu-FP-L1, targets both fibroblast activation protein alpha (FAP) and prostate-specific membrane antigen (PSMA). This dual-targeting agent shows high tumor uptake and retention, enabling precise imaging for diverse cancers.
Area of Science:
- Radiopharmaceutical chemistry
- Molecular imaging
- Cancer theranostics
Background:
- Fibroblast activation protein alpha (FAP) and prostate-specific membrane antigen (PSMA) are key proteases overexpressed in the tumor microenvironment (TME) and in certain tumor epithelia.
- Targeting these proteases offers a strategy for broader cancer imaging and therapy.
- Developing theranostic agents that engage both FAP and PSMA is crucial for managing heterogeneous tumors.
Purpose of the Study:
- To develop and evaluate a novel theranostic radiopharmaceutical targeting both FAP and PSMA for broader cancer applications.
- To synthesize and characterize 64Cu-labeled compounds (FP-L1 and FP-L2) with optimized linker constructs.
- To assess the in vitro and in vivo performance of these agents for imaging FAP- and PSMA-expressing tumors.
Main Methods:
- Synthesis of FP-L1 and FP-L2 using small-molecule FAP and PSMA-targeting moieties conjugated via optimized linkers.
- In vitro determination of inhibition constants (Ki) for FAP and PSMA.
- Cell uptake assays, flow cytometry, quantitative positron emission tomography/computed tomography (PET/CT), and tissue biodistribution studies in various cancer models (glioma, melanoma, prostate, renal, pancreatic).
Main Results:
- High radiochemical yields (>98%) and molar activities (>19 MBq/nmol) achieved for 64Cu-FP-L1 and 64Cu-FP-L2.
- Nanomolar Ki values confirmed potent inhibition of both FAP and PSMA.
- PET/CT and biodistribution studies demonstrated high and specific tumor targeting, with 64Cu-FP-L1 showing favorable pharmacokinetics and high retention in FAP/PSMA-expressing tumors.
Conclusions:
- 64Cu-FP-L1 exhibits high and specific tumor targeting of both FAP and PSMA.
- This dual-targeting agent enables imaging of lesions expressing FAP, PSMA, or both.
- FP-L1 holds significant potential for conversion into a theranostic agent for managing heterogeneous tumors.
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