Exploring the role of combined external beam radiotherapy and targeted radioligand therapy with [177Lu]Lu-PSMA-617 for prostate cancer - from bench to bedside.

Management of prostate cancer (PC) might be improved by combining external beam radiotherapy (EBRT) and prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) with lutetium-177 (177Lu)-labeled PSMA inhibitors. We hypothesized a higher efficacy of the combination due to augmentation of the radiation dose to the tumor and interactions of EBRT with PSMA expression potentially increasing radiopharmaceutical uptake. Therefore, this study analyzed the influence of radiation on PSMA expression levels in vitro. The results were translated to evaluate the efficacy of the combination of photon EBRT and [177Lu]Lu-PSMA-617 in a murine PC xenograft model. Finally, a clinical case report on a combined elective field EBRT with RLT dose escalation illustrates a proof-of-concept. Methods: PSMA gene and protein expression were assessed in human PSMA-overexpressing LNCaP cells after irradiation using reverse transcription quantitative polymerase chain reaction (RT-qPCR), flow cytometry and On-Cell Western assays. In the in vivo therapy study, LNCaP tumor-bearing BALB/c nu/nu mice were irradiated once with 2 Gy X-ray EBRT and injected with 40 MBq [177Lu]Lu-PSMA-617 after 4 h or received single or no treatment (n = 10 each). Tumor-absorbed doses by [177Lu]Lu-PSMA-617 were calculated according to the Medical Internal Radiation Dosimetry (MIRD) formalism after deriving time-activity curves using a gamma probe. An exemplified patient case is demonstrated where fractionated EBRT (54 Gy to prostate; 45 Gy to pelvic lymphatics) and three cycles of [177Lu]Lu-PSMA-617 (3.4-6.0 GBq per cycle) were sequentially combined under concurrent androgen deprivation for treating locally advanced PC. Results: At 4 h following irradiation with 2-8 Gy, LNCaP cells displayed a PSMA protein upregulation by around 18% relative to non-irradiated cells, and a stronger upregulation on mRNA level (up to 2.6-fold). This effect was reversed by 24 h when PSMA protein levels were downregulated by up to 22%. Mice treated with the combination therapy showed significantly improved outcomes regarding tumor control and median survival (p < 0.0001) as compared to single or no treatment. Relative to monotherapy with PSMA-RLT or EBRT, the tumor doubling time was prolonged 1.7- or 2.7-fold and the median survival was extended by 24% or 60% with the combination, respectively. Additionally, tumors treated with EBRT exhibited a 14% higher uptake of the radiopharmaceutical as evident from the calculated tumor-absorbed dose, albeit with high variability in the data. Concerning the patient case, the tri-modality treatment was well tolerated and the patient responded with a long-lasting complete biochemical remission for five years following end of PSMA-RLT. The patient then developed a biochemical relapse with oligo-recurrent disease on follow-up imaging. Conclusion: The present preclinical and clinical data demonstrate that the combination of EBRT with dose escalation by PSMA-RLT improves tumor control and potentially prolongs survival. This may pave the way for further clinical investigations of this approach to explore the curative potential of the combination therapy.

Reserpine-induced rat model for depression: Behavioral, physiological and PET-based dopamine receptor availability validation.

BACKGROUND: Reserpine (RES), a Vesicular Monoamine Transporter 2 (VMAT2) inhibitor agent, has been used in preclinical research for many years to create animal models for depression and to test experimental antidepressant strategies. Nevertheless, evidence of the potential use and validity of RES as a chronic pharmacological model for depression is lacking, and there are no comprehensive studies of the behavioral effects in conjunction with molecular outcomes. METHODS: Experiment 1. Following baseline behavior testing sensitive to depression-like phenotype and locomotion (Phase 1), 27 Sprague-Dawley (SD) rats received i.p. either vehicle solution (0.0 mg/kg), low (0.2 mg/kg) or high (0.8 mg/kg) RES dose for 20 days using a pre-determined schedule and reassessed for behavioral phenotypes (Phase 2). After 10 days washout period, and a final behavioral assessment (Phase 3), the brains were collected 16 days after the last injection for mRNA-expression assessment. Experiment 2. In a similar timetable as in Experiment 1 but without the behavioral testing, 12 SD rats underwent repetitive dopamine D2/3 receptor PET scanning with [18F]DMFP following each Phase. The binding potential (BPND) of [18F]DMFP was quantified by kinetic analysis as a marker of striatal D2/3R availability. Weight and welfare were monitored throughout the study. RESULTS: Significant, dose-dependent weight loss and behavioral deficits including both motor (hypo-locomotion) and non-motor behavior (anhedonia, mild anxiety and reduced exploration) were found for both the low and high dose groups with significant decrease in D2R mRNA expression in the accumbal region for the low RES group after Phase 3. Both RES treated groups showed substantial increase in [18F]DMFP BPND (in line with dopamine depletion) during Phase 2 and 3 compared to baseline and Controls. CONCLUSIONS: The longitudinal design of the study demonstrated that chronic RES administration induced striatal dopamine depletion that persisted even after the wash-out period. However, the behavior phenotype observed were transient. The data suggest that RES administration can induce a rodent model for depression with mild face validity.

Development and Preclinical Evaluation of [211At]PSAt-3-Ga: An Inhibitor for Targeted α-Therapy of Prostate Cancer.

The application of prostate-specific membrane antigen (PSMA)-targeted α-therapy is a promising alternative to ß--particle-based treatments. 211At is among the potential α-emitters that are favorable for this concept. Herein, 211At-based PSMA radiopharmaceuticals were designed, developed, and evaluated. Methods: To identify a 211At-labeled lead, a surrogate strategy was applied. Because astatine does not exist as a stable nuclide, it is commonly replaced with iodine to mimic the pharmacokinetic behavior of the corresponding 211At-labeled compounds. To facilitate the process of structural design, iodine-based candidates were radiolabeled with the PET radionuclide 68Ga to study their preliminary in vitro and in vivo properties before the desired 211At-labeled lead compound was formed. The most promising candidate from this evaluation was chosen to be 211At-labeled and tested in biodistribution studies. Results: All 68Ga-labeled surrogates displayed affinities in the nanomolar range and specific internalization in PSMA-positive LNCaP cells. PET imaging of these compounds identified [68Ga]PSGa-3 as the lead compound. Subsequently, [211At]PSAt-3-Ga was synthesized in a radiochemical yield of 35% and showed tumor uptake of 19 ± 8 percentage injected dose per gram of tissue (%ID/g) at 1 h after injection and 7.6 ± 2.9 %ID/g after 24 h. Uptake in off-target tissues such as the thyroid (2.0 ± 1.1 %ID/g), spleen (3.0 ± 0.6 %ID/g), or stomach (2.0 ± 0.4 %ID/g) was low, indicating low in vivo deastatination of [211At]PSAt-3-Ga. Conclusion: The reported findings support the use of iodine-based and 68Ga-labeled variants as a convenient strategy for developing astatinated compounds and confirm [211At]PSAt-3 as a promising radiopharmaceutical for targeted α-therapy.

Towards Improving the Efficacy of PSMA-Targeting Radionuclide Therapy for Late-Stage Prostate Cancer-Combination Strategies.

PURPOSE OF REVIEW: [177Lu]Lu-PSMA-617 is a radiopharmaceutical that emits beta-minus radiation and targets prostate-specific membrane antigen (PSMA)-positive prostate cancer. Despite its clinical success, there are still patients not showing sufficient response rates. This review compiles latest studies aiming at therapy improvement in [177Lu]Lu-PSMA-617-naïve and -resistant patients by alternative or combination treatments. RECENT FINDINGS: A variety of agents to combine with [177Lu]Lu-PSMA-617 are currently under investigation including alpha radiation-emitting pharmaceuticals, radiosensitizers, taxane chemotherapeutics, androgen receptor pathway inhibitors, immune checkpoint inhibitors, and external beam radiation. Actinium-225 (225Ac)-labeled PSMA-targeting inhibitors are the most studied pharmaceuticals for combination therapy or as an alternative for treatment after progression under [177Lu]Lu-PSMA-617 therapy. Alpha emitters seem to have a potential of achieving a response to PSMA-targeting radionuclide therapy in both initial non-responders or responders to [177Lu]Lu-PSMA-617 later developing treatment resistance. Emerging evidence for immunostimulatory effects of radiopharmaceuticals and first prospective studies support the combination of [177Lu]Lu-PSMA-617 and immune checkpoint inhibition for late-stage prostate cancer.

Prognostic Value of Tumor Volume Assessment on PSMA PET After 177Lu-PSMA Radioligand Therapy Evaluated by PSMA PET/CT Consensus Statement and RECIP 1.0.

Quantitative evaluation of prostate-specific membrane antigen (PSMA)-targeting PET/CT remains challenging but is urgently needed for the use of standardized PET-based response criteria, such as the PSMA PET/CT consensus statement or Response Evaluation Criteria in PSMA PET/CT (RECIP 1.0). A recent study evaluated the prognostic value of whole-body tumor volume using a semiautomatic method relying on a 50% threshold of lesion SUVmax (PSMATV50). In the present study, we analyzed the suitability of this approach comparing 18F-PSMA-1007 with 68Ga-PSMA-11 PET/CT scans and the potential of PSMATV50 for the prediction of overall survival (OS) in patients before 177Lu-PSMA radioligand therapy (RLT). Moreover, PSMATV50 was integrated into the PSMA PET/CT consensus statement as well as RECIP 1.0, and the prognostic value of these response classification systems was compared. Methods: This retrospective study included 70 patients with metastatic castration-resistant prostate cancer undergoing PSMA RLT. Thirty-three patients were monitored by 68Ga-PSMA-11 PET/CT, and 37 patients by 18F-PSMA-1007 PET/CT. PET/CT scans before (baseline) and at the end of PSMA RLT after 2-4 cycles (follow-up) were separately analyzed by 2 readers. PSMATV50 at baseline and its change at the time of follow-up (ΔPSMATV50, expressed as a ratio) were correlated with OS using Cox proportional-hazards regression. The results of both subgroups were compared. The integration of ΔPSMATV50 in existing response classification systems was evaluated. To assess and compare the discriminatory strength of these classification systems, Gönen and Heller concordance probability estimates were calculated. Results: PSMATV50 determination was technically feasible in all examinations. A higher PSMATV50 at baseline and a higher ΔPSMATV50 were strongly associated with a shorter OS for both 68Ga-PSMA-11 (PSMATV50: hazard ratio [HR] of 1.29 [95% CI, 1.05-1.55], P = 0.009; ΔPSMATV50: HR of 1.83 [95% CI, 1.08-3.09], P = 0.024) and 18F-PSMA-1007 (PSMATV50: HR of 1.84 [95% CI, 1.13-2.99], P = 0.014; ΔPSMATV50: HR of 1.23 [95% CI, 1.04-1.51], P = 0.03). Response assessment provided high discriminatory power for OS for the PSMA PET/CT consensus statement (concordance probability estimate, 0.73) as well as RECIP 1.0 (concordance probability estimate, 0.74). Conclusion: PSMATV50 and ΔPSMATV50 proved to be predictive of OS not only for 68Ga-PSMA-11 but also for 18F-PSMA-1007 PET/CT scans. Subsequent integration of ΔPSMATV50 into the PSMA PET/CT consensus statement and RECIP 1.0 provided equally high prognostic value for both classification systems.

A New Class of PSMA-617-Based Hybrid Molecules for Preoperative Imaging and Intraoperative Fluorescence Navigation of Prostate Cancer.

The development of PSMA-targeting low-molecular-weight hybrid molecules aims at advancing preoperative imaging and accurate intraoperative fluorescence guidance for improved diagnosis and therapy of prostate cancer. In hybrid probe design, the major challenge is the introduction of a bulky dye to peptidomimetic core structures without affecting tumor-targeting properties and pharmacokinetic profiles. This study developed a novel class of PSMA-targeting hybrid molecules based on the clinically established theranostic agent PSMA-617. The fluorescent dye-bearing candidates of the strategically designed molecule library were evaluated in in vitro assays based on their PSMA-binding affinity and internalization properties to identify the most favorable hybrid molecule composition for the installation of a bulky dye. The library's best candidate was realized with IRDye800CW providing the lead compound. Glu-urea-Lys-2-Nal-Chx-Lys(IRDye800CW)-DOTA (PSMA-927) was investigated in an in vivo proof-of-concept study, with compelling performance in organ distribution studies, PET/MRI and optical imaging, and with a strong PSMA-specific tumor uptake comparable to that of PSMA-617. This study provides valuable insights about the design of PSMA-targeting low-molecular-weight hybrid molecules, which enable further advances in the field of peptidomimetic hybrid molecule development.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives.

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Rational Linker Design to Accelerate Excretion and Reduce Background Uptake of Peptidomimetic PSMA-Targeting Hybrid Molecules.

The evolution of peptidomimetic hybrid molecules for preoperative imaging and guided surgery targeting the prostate-specific membrane antigen (PSMA) significantly progressed over the past few years, and some approaches are currently being evaluated for further clinical translation. However, accumulation in nonmalignant tissue such as kidney, bladder, spleen, or liver might limit tumor-to-background contrast for precise lesion delineation, particularly in a surgical setting. To overcome these limitations, a rational linker design aims at the development of a second generation of PSMA-11-based hybrid molecules with an enhanced pharmacokinetic profile and improved imaging contrast. Methods: A selection of rationally designed linkers was introduced to the PSMA-targeting hybrid molecule Glu-urea-Lys-HBED-CC-IRDye800CW, resulting in a second-generation peptidomimetic hybrid molecule library. The biologic properties were investigated in cell-based assays. In a preclinical proof-of-concept study with the radionuclide 68Ga, the impact of the modifications was evaluated by determination of specific tumor uptake, pharmacokinetics, and fluorescence imaging in tumor-bearing mice. Results: The modified hybrid molecules carrying various selected linkers revealed high PSMA-specific binding affinity and effective internalization. The highest tumor-to-background contrast of all modifications investigated was identified for the introduction of a histidine- (H) and glutamic acid (E)-containing linker ((HE)3-linker) between the PSMA-binding motif and the chelator. In comparison to the parental core structure, uptake in nonmalignant tissue was significantly reduced to a minimum, as exemplified by an 11-fold reduced spleen uptake from 38.12 ± 14.62 percentage injected dose (%ID)/g to 3.47 ± 1.39 %ID/g (1 h after injection). The specific tumor uptake of this compound (7.59 ± 0.95 %ID/g, 1 h after injection) was detected to be significantly higher than that of the parental tracer PSMA-11. These findings confirmed by PET and fluorescence imaging are accompanied by an enhanced pharmacokinetic profile with accelerated background clearance at early time points after injection. Conclusion: The novel generation of PSMA-targeting hybrid molecules reveals fast elimination, reduced background organ enrichment, and high PSMA-specific tumor uptake meeting the key demands for potent tracers in nuclear medicine and fluorescence-guided surgery. The approach's efficacy in improving the pharmacokinetic profile highlights the strengths of rational linker design as a powerful tool in strategic hybrid-molecule development.

Cytoplasmic Localization of Prostate-Specific Membrane Antigen Inhibitors May Confer Advantages for Targeted Cancer Therapies.

Targeted imaging and therapy approaches based on novel prostate-specific membrane antigen (PSMA) inhibitors have fundamentally changed the treatment regimen of prostate cancer. However, the exact mechanism of PSMA inhibitor internalization has not yet been studied, and the inhibitors' subcellular fate remains elusive. Here, we investigated the intracellular distribution of peptidomimetic PSMA inhibitors and of PSMA itself by stimulated emission depletion (STED) nanoscopy, applying a novel nonstandard live cell staining protocol. Imaging analysis confirmed PSMA cluster formation at the cell surface of prostate cancer cells and clathrin-dependent endocytosis of PSMA inhibitors. Following the endosomal pathway, PSMA inhibitors accumulated in prostate cancer cells at clinically relevant time points. In contrast with PSMA itself, PSMA inhibitors were found to eventually distribute homogeneously in the cytoplasm, a molecular condition that promises benefits for treatment as cytoplasmic and in particular perinuclear enrichment of the radionuclide carriers may better facilitate the radiation-mediated damage of cancerous cells. This study is the first to reveal the subcellular fate of PSMA/PSMA inhibitor complexes at the nanoscale and aims to inspire the development of new approaches in the field of prostate cancer research, diagnostics, and therapeutics. SIGNIFICANCE: This study uses STED fluorescence microscopy to reveal the subcellular fate of PSMA/PSMA inhibitor complexes near the molecular level, providing insights of great clinical interest and suggestive of advantageous targeted therapies. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/2234/F1.large.jpg.

Library and post-translational modifications of peptide-based display systems.

Innovative biotechnological methods empower the successful identification of new drug candidates. Phage, ribosome and mRNA display represent high throughput screenings, allowing fast and efficient progress in the field of targeted drug discovery. The identification range comprises low molecular weight peptides up to whole antibodies. However, a major challenge poses the stability and affinity in particular of peptides. Chemical modifications e.g. the introduction of unnatural amino acids or cyclization, have been proven to be essential tools to overcome these limitations. This review article particularly focuses on available methods for the targeted chemical modification of peptides and peptide libraries in selected display approaches.