Translational Molecular Imaging of Prostate Cancer

RAGE Inhibitors for Targeted Therapy of Cancer: A Comprehensive Review

The receptor for advanced glycation end products (RAGE) is a member of the immunoglobulin family that is overexpressed in several cancers. RAGE is highly expressed in the lung, and its expression increases proportionally at the site of inflammation. This receptor can bind a variety of ligands, including advanced glycation end products, high mobility group box 1, S100 proteins, adhesion molecules, complement components, advanced lipoxidation end products, lipopolysaccharides, and other molecules that mediate cellular responses related to acute and chronic inflammation. RAGE serves as an important node for the initiation and stimulation of cell stress and growth signaling mechanisms that promote carcinogenesis, tumor propagation, and metastatic potential. In this review, we discuss different aspects of RAGE and its prominent ligands implicated in cancer pathogenesis and describe current findings that provide insights into the significant role played by RAGE in cancer. Cancer development can be hindered by inhibiting the interaction of RAGE with its ligands, and this could provide an effective strategy for cancer treatment.

A Feasibility Study Showing [68Ga]Citrate PET Detects Prostate Cancer

PURPOSE

The management of advanced or recurrent prostate cancer is limited in part by the lack of effective imaging agents. Metabolic changes in prostate cancer have previously been exploited for imaging, culminating in the recent US FDA approval of [¹¹C]choline for the detection of subclinical recurrent disease after definitive local therapy. Despite this milestone, production of [¹¹C]choline requires an on-site cyclotron, limiting the scope of medical centers at which this scan can be offered. In this pilot study, we tested whether prostate cancer could be imaged with positron emission tomography (PET) using [⁶⁸Ga]citrate, a radiotracer that targets iron metabolism but is produced without a cyclotron.

PROCEDURES

Eight patients with castrate-resistant prostate cancer were enrolled in this single-center feasibility study. All patients had evidence of metastatic disease by standard of care imaging [X-ray computed tomography (CT), bone scan, or magnetic resonance imaging (MRI)] prior to PET with [⁶⁸Ga]citrate. Patients were intravenously injected with increasing doses of [⁶⁸Ga]citrate (136.9 to a maximum of 259 MBq). Uptake time was steadily increased from 1 h to approximately 3.5 h for the final 4 patients, and all patients were imaged with a PET/MRI. Qualitative and semi-quantitative (maximum standardized uptake value (SUV_max)) assessment of the metastatic lesions was performed and compared to the standard of care imaging.

RESULTS

At 1- and 2-h imaging times post injection, there were no detectable lesions with [⁶⁸Ga]citrate PET. At 3- to 4-h uptake time, there were a total of 71 [⁶⁸Ga]citrate-positive lesions (67 osseous, 1 liver, and 3 lymph node). Of these, 65 lesions were visible on the standard of care imaging (CT and/or bone scan). One PET-avid osseous vertebral body metastasis was not apparent on either CT or bone scan. Twenty-five lesions were not PET-avid but seen on CT and bone scan (17 bone, 6 lymph node, 1 pleural, and 1 liver). The average of the maximum SUVs for bone or soft tissue metastases for patients treated at higher doses and uptake time was statistically higher than the corresponding parameter in normal liver, muscle, and bone. Visually obvious blood pool activity was observed even 3-4 h post injection, suggesting that further optimization of the [⁶⁸Ga]citrate imaging protocol is required to maximize signal-to-background ratios.

CONCLUSIONS

Our preliminary results support that PET with [⁶⁸Ga]citrate may be a novel tool for imaging prostate cancer. Future studies are needed to determine the optimal imaging protocol, the clinical significance of [⁶⁸Ga]citrate uptake, and its role in therapeutic decisions.

Prostate-Specific Membrane Antigen-Targeted Radiohalogenated PET and Therapeutic Agents for Prostate Cancer

Radiohalogenated agents are often the first line of pursuit in the development of new radiopharmaceuticals-whether antibodies, peptides, or small molecules-because of their ease of synthesis, lack of substantial steric perturbation of the original affinity agent (in some cases, providing enhanced affinity), and capacity to be transformed into therapeutics (in some cases, with a mere switch of an isotope). They often provide proof of a principle before optimization for pharmacokinetics or generation of radiometallated agents, when the latter are necessary. In particular, ¹⁸F has been well integrated into normal clinical work flow in the form of ¹⁸F-FDG for oncologic imaging, with reliable daily production and distribution to sites for immediate use, without the need for on-site preparation. Here we discuss radiohalogenated versions of imaging and therapeutic agents targeting the prostate-specific membrane antigen (PSMA); these were among the first such agents to be synthesized and used clinically. PSMA is highly expressed on prostate cancer epithelial cells and is currently being extensively investigated around the world as a target for imaging and therapy of prostate cancer. Additionally, the presence of PSMA on nonprostate tumor neovasculature has opened the possibility of PSMA-targeted molecules as generalizable cancer imaging and therapy agents. We focus on ¹⁸F-labeled agents for PET, as they begin to redefine-along with the corresponding ⁶⁸Ga-labeled agents discussed elsewhere in this supplement to The Journal of Nuclear Medicine-the management of prostate cancer across a variety of clinical contexts.

Radiolabeled enzyme inhibitors and binding agents targeting PSMA: Effective theranostic tools for imaging and therapy of prostate cancer

Because of the broad incidence, morbidity and mortality associated with prostate-derived cancer, the development of more effective new technologies continues to be an important goal for the accurate detection and treatment of localized prostate cancer, lymphatic involvement and metastases. Prostate-specific membrane antigen (PSMA; Glycoprotein II) is expressed in high levels on prostate-derived cells and is an important target for visualization and treatment of prostate cancer. Radiolabeled peptide targeting technologies have rapidly evolved over the last decade and have focused on the successful development of radiolabeled small molecules that act as inhibitors to the binding of the N-acetyl-l-aspartyl-l-glutamate (NAAG) substrate to the PSMA molecule. A number of radiolabeled PSMA inhibitors have been described in the literature and labeled with SPECT, PET and therapeutic radionuclides. Clinical studies with these agents have demonstrated the improved potential of PSMA-targeted PET imaging agents to detect metastatic prostate cancer in comparison with conventional imaging technologies. Although many of these agents have been evaluated in humans, by far the most extensive clinical literature has described use of the ⁶⁸Ga and ¹⁷⁷Lu agents. This review describes the design and development of these agents, with a focus on the broad clinical introduction of PSMA targeting motifs labeled with ⁶⁸Ga for PET-CT imaging and ¹⁷⁷Lu for therapy. In particular, because of availability from the long-lived ⁶⁸Ge (T_1/2=270days)/⁶⁸Ga (T_1/2=68min) generator system and increasing availability of PET-CT, the ⁶⁸Ga-labeled PSMA targeted agent is receiving widespread interest and is one of the fastest growing radiopharmaceuticals for PET-CT imaging.

S100A16 promotes cell proliferation and metastasis via AKT and ERK cell signaling pathways in human prostate cancer

S100A16 is a member of the S100 calcium-binding protein family. It is overexpressed in many types of tumors and associated with proliferation, migration, and invasion; however, its function in human prostate cancer is unresolved. Our objective was to determine its effects and the underlying pathways of S100A16 in prostate cancer tissues and cells. We measured S100A16 expression by quantitative real-time polymerase and Western blotting in eight matched prostate cancer and adjacent normal tissues, and in three prostate cancer cell lines, DU-145, LNCaP, and PC-3, compared to a normal prostate epithelial cell line PrEC. DU-145 cells stably overexpressing S100A16 and PC-3 cells with S100A16 knockdown were established by transfection with S100A16 overexpression plasmid or shRNAs. Invasion, migration, and proliferation were analyzed by transwell assay, wound healing, and colony formation assays, respectively. Western blotting and invasion assays were performed to determine expressions and activation of AKT, ERK, p21, and p27. S100A16 was significantly overexpressed in both prostate cancer tissues and cells lines compared to normal controls (P < 0.05). Overexpression of S100A16 significantly promoted invasion, migration, and proliferation in prostate cancer cells in vitro, whereas silencing S100A16 showed the converse effects (P < 0.05). Furthermore, overexpression of S100A16 activated cell signaling proteins AKT and ERK and downregulated tumor suppressors p21 and p27. Specific inhibitors, LY294002 and PD98059, suppressed activation of AKT and ERK, which attenuated DU-145 cell clone formation and invasion induced by S100A16 overexpression. S100A16 may promote human prostate cancer progression via signaling pathways involving AKT, ERK, p21, and p27 downstream effectors. Our findings suggest that S100A16 may serve as a novel therapeutic or diagnostic target in human prostate cancer.

Automated synthesis of [(18)F]DCFPyL via direct radiofluorination and validation in preclinical prostate cancer models

Background

Prostate-specific membrane antigen (PSMA) is frequently overexpressed and upregulated in prostate cancer. To date, various ¹⁸F- and ⁶⁸Ga-labeled urea-based radiotracers for PET imaging of PSMA have been developed and entered clinical trials. Here, we describe an automated synthesis of [¹⁸F]DCFPyL via direct radiofluorination and validation in preclinical models of prostate cancer.

Methods

[¹⁸F]DCFPyL was synthesized via direct nucleophilic heteroaromatic substitution reaction in a single reactor TRACERlab FX_FN automated synthesis unit. Radiopharmacological evaluation of [¹⁸F]DCFPyL involved internalization experiments, dynamic PET imaging in LNCaP (PSMA+) and PC3 (PSMA−) tumor-bearing BALB/c nude mice, biodistribution studies, and metabolic profiling. In addition, reversible two-tissue compartmental model analysis was used to quantify pharmacokinetics of [¹⁸F]DCFPyL in LNCaP and PC3 tumor models.

Results

Automated radiosynthesis afforded radiotracer [¹⁸F]DCFPyL in decay-corrected radiochemical yields of 23 ± 5 % (n = 10) within 55 min, including HPLC purification. Dynamic PET analysis revealed rapid and high uptake of radioactivity (SUV_5min 0.95) in LNCaP tumors which increased over time (SUV_60min 1.1). Radioactivity uptake in LNCaP tumors was blocked in the presence of nonradioactive DCFPyL (SUV_60min 0.22). The muscle as reference tissue showed rapid and continuous clearance over time (SUV_60min 0.06). Fast blood clearance of radioactivity resulted in tumor-blood ratios of 1.0 after 10 min and 8.3 after 60 min. PC3 tumors also showed continuous clearance of radioactivity over time (SUV_60min 0.11). Kinetic analysis of PET data revealed the two-tissue compartmental model as best fit with K₁ = 0.12, k₂ = 0.18, k₃ = 0.08, and k₄ = 0.004 min⁻¹, confirming molecular trapping of [¹⁸F]DCFPyL in PSMA+ LNCaP cells.

Conclusions

[¹⁸F]DCFPyL can be prepared for clinical applications simply and in good radiochemical yields via a direct radiofluorination synthesis route in a single reactor automated synthesis unit. Radiopharmacological evaluation of [¹⁸F]DCFPyL confirmed high PSMA-mediated tumor uptake combined with superior clearance parameters. Compartmental model analysis points to a two-step molecular trapping mechanism based on PSMA binding and subsequent internalization leading to retention of radioactivity in PSMA+ LNCaP tumors.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0195-6) contains supplementary material, which is available to authorized users.

Multidisciplinary intervention of early, lethal metastatic prostate cancer: Report from the 2015 Coffey-Holden Prostate Cancer Academy Meeting

BACKGROUND

The 2015 Coffey-Holden Prostate Cancer Academy Meeting, themed: "Multidisciplinary Intervention of Early, Lethal Metastatic Prostate Cancer," was held in La Jolla, California from June 25 to 28, 2015.

METHODS

The Prostate Cancer Foundation (PCF) sponsors an annual, invitation-only, action-tank-structured meeting on a critical topic concerning lethal prostate cancer. The 2015 meeting was attended by 71 basic, translational, and clinical investigators who discussed the current state of the field, major unmet needs, and ideas for addressing earlier diagnosis and treatment of men with lethal prostate cancer for the purpose of extending lives and making progress toward a cure.

RESULTS

The questions addressed at the meeting included: cellular and molecular mechanisms of tumorigenesis, evaluating, and targeting the microenvironment in the primary tumor, advancing biomarkers for clinical integration, new molecular imaging technologies, clinical trials, and clinical trial design in localized high-risk and oligometastatic settings, targeting the primary tumor in advanced disease, and instituting multi-modal care of high risk and oligometastatic patients.

DISCUSSION

This article highlights the current status, greatest unmet needs, and anticipated field changes that were discussed at the meeting toward the goal of optimizing earlier interventions to potentiate cures in high-risk and oligometastatic prostate cancer patients.

¹⁸F-DCFBC PET/CT for PSMA-Based Detection and Characterization of Primary Prostate Cancer

We previously demonstrated the ability to detect metastatic prostate cancer using N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-4-(18)F-fluorobenzyl-L-cysteine ((18)F-DCFBC), a low-molecular-weight radiotracer that targets the prostate-specific membrane antigen (PSMA). PSMA has been shown to be associated with higher Gleason grade and more aggressive disease. An imaging biomarker able to detect clinically significant high-grade primary prostate cancer reliably would address an unmet clinical need by allowing for risk-adapted patient management.

METHODS

We enrolled 13 patients with primary prostate cancer who were imaged with (18)F-DCFBC PET before scheduled prostatectomy, with 12 of these patients also undergoing pelvic prostate MR imaging. Prostate (18)F-DCFBC PET was correlated with MR imaging and histologic and immunohistochemical analysis on a prostate-segment (12 regions) and dominant-lesion basis. There were no incidental extraprostatic findings on PET suggestive of metastatic disease.

RESULTS

MR imaging was more sensitive than (18)F-DCFBC PET for detection of primary prostate cancer on a per-segment (sensitivities of up to 0.17 and 0.39 for PET and MR imaging, respectively) and per-dominant-lesion analysis (sensitivities of 0.46 and 0.92 for PET and MR imaging, respectively). However, (18)F-DCFBC PET was more specific than MR imaging by per-segment analysis (specificities of 0.96 and 0.89 for PET and MR imaging for corresponding sensitivity, respectively) and specific for detection of high-grade lesions (Gleason 8 and 9) greater than 1.0 mL in size (4/4 of these patients positive by PET). (18)F-DCFBC uptake in tumors was positively correlated with Gleason score (ρ = 0.64; PSMA expression, ρ = 0.47; and prostate-specific antigen, ρ = 0.52). There was significantly lower (18)F-DCFBC uptake in benign prostatic hypertrophy than primary tumors (median maximum standardized uptake value, 2.2 vs. 3.5; P = 0.004).

CONCLUSION

Although the sensitivity of (18)F-DCFBC for primary prostate cancer was less than MR imaging, (18)F-DCFBC PET was able to detect the more clinically significant high-grade and larger-volume tumors (Gleason score 8 and 9) with higher specificity than MR imaging. In particular, there was relatively low (18)F-DCFBC PET uptake in benign prostatic hypertrophy lesions, compared with cancer in the prostate, which may allow for more specific detection of primary prostate cancer by (18)F-DCFBC PET. This study demonstrates the utility of PSMA-based PET, which may be used in conjunction with MR imaging to identify clinically significant prostate cancer.

⁸⁹Zr-huJ591 immuno-PET imaging in patients with advanced metastatic prostate cancer

PURPOSE

Given the bone tropism of prostate cancer, conventional imaging modalities poorly identify or quantify metastatic disease. (89)Zr-huJ591 positron emission tomography (PET) imaging was performed in patients with metastatic prostate cancer to analyze and validate this as an imaging biomarker for metastatic disease. The purpose of this initial study was to assess safety, biodistribution, normal organ dosimetry, and optimal imaging time post-injection for lesion detection.

METHODS

Ten patients with metastatic prostate cancer received 5 mCi of (89)Zr-huJ591. Four whole-body scans with multiple whole-body count rate measurements and serum activity concentration measurements were obtained in all patients. Biodistribution, clearance, and lesion uptake by (89)Zr-huJ591 immuno-PET imaging was analyzed and dosimetry was estimated using MIRD techniques. Initial assessment of lesion targeting of (89)Zr-huJ591 was done. Optimal time for imaging post-injection was determined.

RESULTS

The dose was well tolerated with mild chills and rigors seen in two patients. The clearance of (89)Zr-huJ591 from serum was bi-exponential with biological half-lives of 7 ± 4.5 h (range 1.1-14 h) and 62 ± 13 h (range 51-89 h) for initial rapid and later slow phase. Whole-body biological clearance was 219 ± 48 h (range 153-317 h). The mean whole-body and liver residence time was 78.7 and 25.6 h, respectively. Dosimetric estimates to critical organs included liver 7.7 ± 1.5 cGy/mCi, renal cortex 3.5 ± 0.4 cGy/mCi, and bone marrow 1.2 ± 0.2 cGy/mCi. Optimal time for patient imaging after injection was 7 ± 1 days. Lesion targeting of bone or soft tissue was seen in all patients. Biopsies were performed in 8 patients for a total 12 lesions, all of which were histologically confirmed as metastatic prostate cancer. One biopsy-proven lesion was not positive on (89)Zr-huJ591, while the remaining 11 lesions were (89)Zr-huJ591 positive. Two biopsy-positive nodal lesions were noted only on (89)Zr-huJ591 study, while the conventional imaging modality was negative.

CONCLUSION

(89)Zr-huJ591 PET imaging of prostate-specific membrane antigen expression is safe and shows good localization of disease in prostate cancer patients. Liver is the critical organ for dosimetry, and 7 ± 1 days is the optimal imaging time. A larger study is underway to determine lesion detection in an expanded cohort of patients with metastatic prostate cancer.