PSMA Targeted Molecular Imaging and Radioligand Therapy for Prostate Cancer: Optimal Patient and Treatment Issues

Baseline Imaging Derived Factors of Response Following [225Ac]Ac-J591 Therapy in Metastatic Castration-Resistant Prostate Cancer: A Lesion Level Analysis

PURPOSE

Actinium-225 labeled prostate-specific membrane antigen (PSMA) targeted radionuclide therapy has emerged as a potential treatment option in the management of men with metastatic castrate-resistant prostate cancer (mCRPC). This study investigated molecular imaging-derived parameters and compared imaging response of lesions categorized by tumor site.

METHODS

Men with mCRPC treated with [225Ac]Ac-J591 from 2017 to 2022 at our center on two prospective trials (NCT03276572 and NCT04506567) with pre- and post-treatment [68Ga]Ga-PSMA-11 Positron Emission Tomography (PET) imaging studies available were included. SUVpeak of the 3 most- and 3 least-avid lesions of the tumor sites were manually assessed. The median change of the SUVpeak from pre- to post-treatment per tumor site was evaluated using the paired Wilcox test. An objective response (OR) in the follow-up image was defined as complete or partial response using PET Response Criteria in Solid Tumors (PERCIST) 1.0.

RESULTS

A total of 46 cases met the criteria for image review; most of them (n = 25, 54.3%) had more than one tumor site category. In total, 445 PSMA PET-positive lesions were assessed: 220 osseous, 163 nodal, 41 visceral, and 21 prostatic lesions. After treatment with [225Ac]Ac-J591, absolute SUVpeak values per tumor site declined significantly (p < 0.05) except for prostatic lesions (p = 1). The PERCIST-OR rate for osseous, nodal, visceral, and prostatic lesions was 53%, 28%, 56%, and 38%, respectively.

CONCLUSION

[225Ac]Ac-J591 is an active treatment in men with mCRPC. Tumor distribution patterns may influence treatment response and potentially prognosis. Our findings warrant further validation in a larger cohort but may be considered in treatment planning and trial design.

A Prostate-Specific Membrane Antigen-Targeting Small Molecule-Drug Conjugate Strategy to Overcome the Hematological Toxicity of Olaparib

PARP inhibitors have gained attention in the treatment of metastatic castration-resistant prostate cancer, but approximately half of patients have to abort treatment due to severe hematological toxicity. Herein, we proposed a prostate-specific membrane antigen (PSMA)-targeting small molecule-drug conjugate (SMDC) strategy to address this issue. This led to CQ-16, which achieved its targeting to prostate tumor cells through binding to PSMA. Also, CQ-16 retained the PARP inhibitory activity and exhibited highly selective antiproliferative activities between PSMA-positive and PSMA-negative prostate cells. Moreover, the hematological toxicity observed in Olaparib was not showing in the group of CQ-16 even at a high dose of 390 mg/kg. Moreover, oral administration of CQ-16 exerted significant tumor growth inhibition in the 22Rv1 xenograft mouse model. These above findings not only highlight the potential of CQ-16 to overcome the hematological toxicity associated with PARP inhibitors but also provide a strategy to develop an SMDC with enhanced safety profiles.

Combining PET and Compton imaging with edge-on CZT detectors for enhanced diagnostic capabilities

The key metrics for positron emission tomography (PET) imaging devices include the capability to capture the maximum available amount of annihilation photon information while generating high-quality images of the radiation distribution. This capability carries clinical implications by reducing scanning time for imaging, thus reducing radiation exposure for patients. However, imaging quality is degraded by positron range effects and the non-collinearity of positron annihilation photons. Utilizing an edge-on configuration of cadmium zinc telluride (CZT) detector crystals offers a potential solution to increase PET sensitivity. The high cross-section of CZT and its capacity to detect both 511 keV annihilation gammas and high-energy prompt gammas, along with multiple photon interaction events, contribute to this increased sensitivity. In this study, we propose a dual-panel edge-on CZT detector system comprised of 4 × 4 × 0.5 cm3 CZT detectors, with panel dimensions of 20 × 15 cm2 and a thickness of 4 cm. In this study, we demonstrate the increased sensitivity of our imaging system due to the detection of the Compton kinematics of high-energy gammas originating from prompt-gamma-emitting isotopes. This was achieved using Monte Carlo simulations of a prompt-gamma-emitting isotope,72As, with mean positron ranges >3 mm. Our system's dynamic energy range, capable of detecting gammas up to 1.2 MeV, allows it to operate in a dual-mode fashion as both a Compton camera (CC) and standard PET. By presenting reconstructions of 72As, we highlight the absence of positron range effects in CC reconstructions compared to PET reconstructions. In addition, we evaluate the system's increased sensitivity resulting from its ability to detect high-energy prompt gammas.

A Synergistic Strategy Combining Chemotherapy and Photodynamic Therapy to Eradicate Prostate Cancer

Prostate cancer is the most prevalent cancer among men in the United States and is a leading cause of cancer-related death. Prostate specific membrane antigen (PSMA) has been established as a biomarker for prostate cancer diagnosis and treatment. This study aimed to develop a novel theranostic agent, PSMA-1-MMAE-Pc413, which integrates a PSMA-targeting ligand, the photosensitizer Pc413, and the microtubular inhibitor monomethyl auristatin E (MMAE) for synergistic therapeutic efficacy. In vitro uptake studies revealed that PSMA-1-MMAE-Pc413 demonstrated selective and specific uptake in PSMA-positive PC3pip cells but not in PSMA-negative PC3flu cells, with the uptake in PC3pip cells being approximately three times higher. In vitro cytotoxicity assays showed that, when exposed to light, PSMA-1-MMAE-Pc413 had a synergistic effect, leading to significantly greater cytotoxicity in PSMA-positive cells (IC50 = 2.2 nM) compared to PSMA-1-Pc413 with light irradiation (IC50 = 164.9 nM) or PSMA-1-MMAE-Pc413 without light irradiation (IC50 = 12.6 nM). In vivo imaging studies further demonstrated the selective uptake of PSMA-1-MMAE-Pc413 in PC3pip tumors. In in vivo studies, PSMA-1-MMAE-Pc413 dramatically improves the therapeutic outcome for prostate cancer by providing a synergistic effect that surpasses the efficacy of each treatment modality alone in PC3pip tumors. These findings suggest that PSMA-1-MMAE-Pc413 has strong potential for clinical application in improving prostate cancer treatment.

Two Novel [68Ga]Ga-Labeled Radiotracers Based on Metabolically Stable [Sar11]RM26 Antagonistic Peptide for Diagnostic Positron Emission Tomography Imaging of GRPR-Positive Prostate Cancer

Gastrin releasing peptide receptor (GRPR) is overexpressed in prostate cancer (PC-3) and can be used for diagnostic purposes. We herein present the design and preclinical evaluation of two novel NOTA/NODAGA-containing peptides suitable for labeling with the positron emission tomography (PET) radionuclide Ga-68. These analogs are based on the previously reported GRPR-antagonist DOTAGA-PEG2-[Sar11]RM26, developed for targeted radiotheraostic applications. Both NOTA-PEG2-[Sar11]RM26 and NODAGA-PEG2-[Sar11]RM26 were successfully labeled with Ga-68 and evaluated in vitro and in vivo using PC-3 cell models. Both, [68Ga]Ga-NOTA-PEG2-[Sar11]RM26 and [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 displayed high metal-chelate stability in phosphate buffered saline and against the EDTA-challenge. The two [68Ga]Ga-labeled conjugates demonstrated highly GRPR-mediated uptake in vitro and in vivo and exhibited a slow internalization over time, typical for radioantagonistis. The [natGa]Ga-loaded peptides displayed affinity in the low nanomole range for GRPR in competition binding experiments. The new radiotracers demonstrated biodistribution profiles suitable for diagnostic imaging shortly after administration with fast background clearance. Their high tumor uptake (13 ± 1 and 15 ± 3% IA/g for NOTA and NODAGA conjugates, respectively) and high tumor-to-blood ratios (60 ± 10 and 220 ± 70, respectively) 3 h pi renders them promising PET tracers for use in patients. Tumor-to-normal organ ratios were higher for [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 than for the NOTA-containing counterpart. The performance of the two radiopeptides was further supported with the PET/CT images. In conclusion, [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 is a promising PET imaging tracer for visualization of GRPR-expressing lesions with high imaging contrast shortly after administration.

Structure-activity relationship study of mesyl and busyl phosphoramidate antisense oligonucleotides for unaided and PSMA-mediated uptake into prostate cancer cells

Phosphorothioate (PS) group is a key component of a majority of FDA approved oligonucleotide drugs that increase stability to nucleases whilst maintaining interactions with many proteins, including RNase H in the case of antisense oligonucleotides (ASOs). At the same time, uniform PS modification increases nonspecific protein binding that can trigger toxicity and pro-inflammatory effects, so discovery and characterization of alternative phosphate mimics for RNA therapeutics is an actual task. Here we evaluated the effects of the introduction of several N-alkane sulfonyl phosphoramidate groups such as mesyl (methanesulfonyl) or busyl (1-butanesulfonyl) phosphoramidates into gapmer ASOs on the efficiency and pattern of RNase H cleavage, cellular uptake in vitro, and intracellular localization. Using Malat1 lncRNA as a target, we have identified patterns of mesyl or busyl modifications in the ASOs for optimal knockdown in vitro. Combination of the PSMA ligand-mediated delivery with optimized mesyl and busyl ASOs resulted in the efficient target depletion in the prostate cancer cells. Our study demonstrated that other N-alkanesulfonyl phosphoramidate groups apart from a known mesyl phosphoramidate can serve as an essential component of mixed backbone gapmer ASOs to reduce drawbacks of uniformly PS-modified gapmers, and deserve further investigation in RNA therapeutics.

The Current Landscape of Prostate-Specific Membrane Antigen (PSMA) Imaging Biomarkers for Aggressive Prostate Cancer

The review examines the vital role of prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) in the diagnosis, staging, and treatment of prostate cancer (PCa). It focuses on the superior diagnostic abilities of PSMA PET/CT for identifying both nodal and distant PCa, and its potential as a prognostic indicator for biochemical recurrence and overall survival. Additionally, we focused on the variability of PSMA's expression and its impact on personalised treatment, particularly the use of [177Lu] Lu-PSMA-617 radioligand therapy. This review emphasises the essential role of PSMA PET/CT in enhancing treatment approaches, improving patient outcomes, and reducing unnecessary interventions, positioning it as a key element in personalised PCa management.