Investigation of Tumor Cells and Receptor-Ligand Simulation Models for the Development of PET Imaging Probes Targeting PSMA and GRPR and a Possible Crosstalk between the Two Receptors

Advances in structure-based drug design targeting membrane protein markers in prostate cancer

Prostate cancer (PCa) is one of the leading cancers in men and the lack of suitable biomarkers or their modulators results in poor prognosis. Membrane proteins (MPs) have a crucial role in the development and progression of PCa and can be attractive therapeutic targets. However, experimental limitations in targeting MPs hinder effective biomarker and inhibitor discovery. To overcome this barrier, computational methods can yield structural insights and screen large libraries of compounds, accelerating lead identification and optimization. In this review, we examine current breakthroughs in computer-aided drug design (CADD), with emphasis on structure-based approaches targeting the most relevant membrane-bound PCa biomarkers.

Computer-Assisted Design of Peptide-Based Radiotracers

In medical imaging, techniques such as magnetic resonance imaging, contrast-enhanced computerized tomography, positron emission tomography (PET), and single-photon emission computed tomography (SPECT) are extensively available and routinely used for disease diagnosis. PET probes with peptide-based targeting are typically composed of small peptides especially developed to have high affinity and specificity for a range of cellular and tissue targets. These probes’ key benefits include being less expensive than traditional antibody-based PET tracers and having an effective chemical modification process that allows them to be radiolabeled with almost any radionuclide, making them highly appealing for clinical usage. Currently, as with every pharmaceutical design, the use of in silico strategies is steadily growing in this field, even though it is not part of the standard toolkit used during radiopharmaceutical design. This review describes the recent applications of computational design approaches in the design of novel peptide-based radiopharmaceuticals.

Up-Regulation of PSMA Expression In Vitro as Potential Application in Prostate Cancer Therapy

Possibilities to improve the therapeutic efficacy of Lu-177-PSMA-617 radionuclide therapy by modulation of target expression are being investigated. Knowledge on regulatory factors that promote prostate cancer (PCa) progression may contribute to targeting prostate cancer more effectively. We aimed at the stimulation of PCa cell lines using the substances 5-aza-2'-deoxycitidine (5-aza-dC) and valproic acid (VPA) to achieve increased prostate-specific membrane antigen (PSMA) expression. PC3, PC3-PSMA, and LNCaP cells were incubated with varying concentrations of 5-aza-dC and VPA to investigate the cell-bound activity of Lu-177-PSMA-617. Stimulation effects on both the genetically modified cell line PC3-PSMA and the endogenously PSMA-expressing LNCaP cells were demonstrated by increased cellular uptake of the radioligand. For PC3-PSMA cells, the fraction of cell-bound radioactivity was enhanced by about 20-fold compared to that of the unstimulated cells. Our study reveals an increased radioligand uptake mediated by stimulation for both PC3-PSMA and LNCaP cell lines. In perspective of an enhanced PSMA expression, the present study might contribute to advanced radionuclide therapy approaches that improve the therapeutic efficacy, as well as combined treatment options.

Hetero-bivalent agents targeting FAP and PSMA

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 (K_i) 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). K_i 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.

Synthesis and in vitro proof-of-concept studies on bispecific iron oxide magnetic nanoparticles targeting PSMA and GRP receptors for PET/MR imaging of prostate cancer

Prostate cancer (PCa) is the most common malignancy worldwide in men. This is a proof-of-concept study describing the development of ⁶⁸Ga-magnetic iron oxide nanoparticles (mNP) targeting prostate specific membrane antigen (PSMA) and gastrin releasing peptide (GRPR) receptors as potential tools for diagnosis of PCa with PET/MRI. Two pharmacophores targeting PSMA, 1, and GRPR, 2, were coupled to mNPs carrying -SH (mNP-S1/2) or -NH₂ (mNP-N1/2) groups. The mNP-S1/2 and mNP-N1/2 were characterized for their size, zeta potential, structure, and efficiency of functionalization using dynamic light scattering (DLS), FT-IR and RP-HPLC. A direct ⁶⁸Ga-labelling procedure was followed, where ⁶⁸Ga-mNP-N1/2 proved superior to ⁶⁸Ga-mNP-S1/2 regarding radiolabelling efficiency, and thus were further evaluated in vitro. Toxicity studies in PCa cells (LNCaP, PC-3) showed low toxicity, and minimal hemolysis of red blood cells. In vitro assays in cells expressing PSMA (LNCaP), and GRPR (PC-3), showed specific time-dependent binding (40 min to plateau), high avidity (PC-3: K_d = 28.27 nM, LNCaP: K_d = 11.49 nM) and high internalization rates for ⁶⁸Ga-mNP-N1/2 in both cell lines.