Human ABC and SLC Transporters: The Culprit Responsible for Unspecific PSMA-617 Uptake?

[177Lu]Lu-PSMA-617 has recently been successfully approved by the FDA, the MHRA, Health Canada and the EMA as Pluvicto®. However, salivary gland (SG) and kidney toxicities account for its main dose-limiting side-effects, while its corresponding uptake and retention mechanisms still remain elusive. Recently, the presence of different ATP-binding cassette (ABC) transporters, such as human breast cancer resistance proteins (BCRP), multidrug resistance proteins (MDR1), multidrug-resistance-related proteins (MRP1, MRP4) and solute cassette (SLC) transporters, such as multidrug and toxin extrusion proteins (MATE1, MATE2-K), organic anion transporters (OAT1, OAT2v1, OAT3, OAT4) and peptide transporters (PEPT2), has been verified at different abundances in human SGs and kidneys. Therefore, our aim was to assess whether [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are substrates of these ABC and SLC transporters. For in vitro studies, the novel isotopologue ([α,β-3H]Nal)Lu-PSMA-617 was used in cell lines or vesicles expressing the aforementioned human ABC and SLC transporters for inhibition and uptake studies, respectively. The corresponding probe substrates and reference inhibitors were used as controls. Our results indicate that [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are neither inhibitors nor substrates of the examined transporters. Therefore, our results show that human ABC and SLC transporters play no central role in the uptake and retention of [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 in the SGs and kidneys nor in the observed toxicities.

Implementing Ac-225 labelled radiopharmaceuticals: practical considerations and (pre-)clinical perspectives

BACKGROUND

In the past years, there has been a notable increase in interest regarding targeted alpha therapy using Ac-225, driven by the observed promising clinical anti-tumor effects. As the production and technology has advanced, the availability of Ac-225 is expected to increase in the near future, making the treatment available to patients worldwide.

MAIN BODY

Ac-225 can be labelled to different biological vectors, whereby the success of developing a radiopharmaceutical depends heavily on the labelling conditions, purity of the radionuclide source, chelator, and type of quenchers used to avoid radiolysis. Multiple (methodological) challenges need to be overcome when working with Ac-225; as alpha-emission detection is time consuming and highly geometry dependent, a gamma co-emission is used, but has to be in equilibrium with the mother-nuclide. Because of the high impact of alpha emitters in vivo it is highly recommended to cross-calibrate the Ac-225 measurements for used quality control (QC) techniques (radio-TLC, HPLC, HP-Ge detector, and gamma counter). More strict health physics regulations apply, as Ac-225 has a high toxicity, thereby limiting practical handling and quantities used for QC analysis.

CONCLUSION

This overview focuses specifically on the practical and methodological challenges when working with Ac-225 labelled radiopharmaceuticals, and underlines the required infrastructure and (detection) methods for the (pre-)clinical application.

α-Labeling of J591, an Antibody Targeting Prostate-Specific Membrane Antigen: The Technique and Considerations from the First Dedicated Production Lab at an Academic Institution in the United States

The protein expression of the prostate-specific membrane antigen correlates with unfavorable or aggressive histologic features of prostate cancer, resulting in use as a diagnostic PET imaging radiotracer and therapeutic target. Here, we discuss the methods to develop 225Ac-DOTA-J591, an α-labeled compound targeting an extracellular epitope of prostate-specific membrane antigen, which is currently being studied in early clinical trials. In addition, we review quality control, radiation safety measures, and clinical considerations before administration of this radioimmunotherapeutic agent.

Innovation in Radionuclide Therapy for the Treatment of Prostate Cancers: Radiochemical Perspective and Recent Therapeutic Practices

Prostate cancer represents the second cause of death by cancer in males in western countries. While early-stage diseases are accessible to surgery and/or external radiotherapy, advanced metastatic prostate cancers are primarily treated with androgen deprivation therapy, to which new generation androgen receptor antagonists or taxane-based chemotherapies are added in the case of tumor relapse. Nevertheless, patients become invariably resistant to castration with a median survival that rarely exceeds 3 years. This fostered the search for alternative strategies, independent of the androgen receptor signaling pathway. In this line, radionuclide therapies may represent an interesting option as they could target either the microenvironment of sclerotic bone metastases with the use of radiopharmaceuticals containing samarium-153, strontium-89 or radium-223 or tumor cells expressing the prostate-specific membrane antigen (PSMA), a protein found at the surface of prostate cancer cells. This review gives highlights the chemical properties of radioligands targeting prostate cancer cells and recapitulates the clinical trials evaluating the efficacy of radionuclide therapies, alone or in combination with other approved treatments, in patients with castration-resistant prostate tumors. It discusses some of the encouraging results obtained, especially the benefit on overall survival that was reported with [¹⁷⁷Lu]-PSMA-617. It also addresses the specific requirements for the use of this particular class of drugs, both in terms of medical staff coordination and adapted infrastructures for efficient radioprotection.

Challenges and opportunities in developing Actinium-225 radiopharmaceuticals

Actinium-225 (225Ac) has emerged as a promising therapeutic radioisotope for targeted alpha therapy. It emits net four alpha particles during its decay to stable daughter bismuth-209, rightly called an in-vivo nano-generator. Compared to the worldwide demand of 225Ac, the amount produced via depleted thorium-229 sources is minimal, making it an expensive radionuclide. However, many research groups are working on optimizing the parameters for the production of 225Ac via different routes, including cyclotrons, reactors and high-energy linear accelerators. The present review article focuses on the various aspects associated with the development of 225Ac radiopharmaceuticals. It includes the challenges and opportunities associated with the production methods, labeling chemistry, in-vivo kinetics and dosimetry of 225Ac radiopharmaceuticals. A brief description is also given about the 225Ac radiopharmaceuticals at preclinical stages, clinical trials and used routinely.

Advances in PSMA theranostics

•

PSMA is an appealing target for theranostic because it is a transmembrane protein with a known substrate that is overexpessed on prostate cancer cells and internalizes upon ligand binding.

•

There are a number of PSMA theranostic ligands in clinical evaluation, clinical trial, or clinically approved.

•

PSMA theranostic ligands increase progression-free survival, overall survival, and pain in patients with metastatic castration resistant prostate cancer.

•

A major obstacle to PSMA-targeted radioligand therapy is off-target toxicity in salivary glands.

The validation of prostate specific membrane antigen (PSMA) as a molecular target in metastatic castration-resistant prostate cancer has stimulated the development of multiple classes of theranostic ligands that specifically target PSMA. Theranostic ligands are used to image disease or selectively deliver cytotoxic radioactivity to cells expressing PSMA according to the radioisotope conjugated to the ligand. PSMA theranostics is a rapidly advancing field that is now integrating into clinical management of prostate cancer patients. In this review we summarize published research describing the biological role(s) and activity of PSMA, highlight the most clinically advanced PSMA targeting molecules and biomacromolecules, and identify next generation PSMA ligands that aim to further improve treatment efficacy. The goal of this review is to provide a comprehensive assessment of the current state-of-play and a roadmap to achieving further advances in PSMA theranostics.

A suitable time point for quantifying the radiochemical purity of 225Ac-labeled radiopharmaceuticals

BACKGROUND

As 225Ac-labeled radiopharmaceuticals continue to show promise as targeted alpha therapeutics, there is a growing need to standardize quality control (QC) testing procedures. The determination of radiochemical purity (RCP) is an essential QC test. A significant obstacle to RCP testing is the disruption of the secular equilibrium between actinium-225 and its daughter radionuclides during labeling and QC testing. In order to accelerate translation of actinium-225 targeted alpha therapy, we aimed to determine the earliest time point at which the RCP of an 225Ac-labeled radiopharmaceutical can be accurately quantified.

RESULTS

Six ligands were conjugated to macrocyclic metal chelators and labeled with actinium-225 under conditions designed to generate diverse incorporation yields. RCP was determined by radio thin layer chromatography (radioTLC) followed by exposure of the TLC plate on a phosphor screen either 0.5, 2, 3.5, 5, 6.5, or 26 h after the plate was developed. The dataset was used to create models for predicting the true RCP for any pre-equilibrium measurement taken at an early time point. The 585 TLC measurements span RCP values of 1.8-99.5%. The statistical model created from these data predicted an independent data set with high accuracy. Predictions made at 0.5 h are more uncertain than predictions made at later time points. This is primarily due to the decay of bismuth-213. A measurement of RCP > 90% at 2 h predicts a true RCP > 97% and guarantees that RCP will exceed 90% after secular equilibrium is reached. These findings were independently validated using NaI(Tl) scintillation counting and high resolution gamma spectroscopy on a smaller set of samples with 10% ≤ RCP ≤ 100%.

CONCLUSIONS

RCP of 225Ac-labeled radiopharmaceuticals can be quantified with acceptable accuracy at least 2 h after radioTLC using various methods of quantifying particle emissions. This time point best balances the need to accurately quantify RCP with the need to safely release the batch as quickly as possible.

Antibodies targeting Prostate-Specific Membrane Antigen positive prostate cancer: from diagnostic imaging to theranostics

PURPOSE OF REVIEW

Targeting Prostate-Specific Membrane Antigen (PSMA) has paved the way for personalized medicine in prostate cancer (PCa) patients. This review aims to highlight the role of PSMA targeting antibodies in PCa, for diagnostic and therapeutic purposes.

RECENT FINDINGS

PSMA Positron Emission Tomography/Computed Tomography has been a game changer in the diagnosis of PCa in the recent decade. Two anti-PSMA monoclonal antibodies have been studied in PCa: 7E11-C35 (limited use) and J591. J591 antibody was used for diagnostic purposes coupled with different radionuclides. Most importantly, it was combined to numerous therapeutic radionuclides such as Lutetium-177 (177Lu), Yttrium-90 (90Y), Indium-111 (111In), and Actinium-225 (225Ac). It was also conjugated to drugs forming antibody-drug conjugates (e.g. MLN2704 and PSMA-ADC). These compounds were tested in recent phase I/II clinical trials.

SUMMARY

PSMA targeting antibodies are very promising for further clinical investigation and continue to be a momentous research area, for both imaging and therapeutic settings. Although some clinical trials resulted in unfavorably safety profiles for some antibodies, they validated PSMA as a crucial immunoconjugate target.

Signal to Noise Ratio as a Cross-Platform Metric for Intraoperative Fluorescence Imaging

Real-time molecular imaging to guide curative cancer surgeries is critical to ensure removal of all tumor cells; however, visualization of microscopic tumor foci remains challenging. Wide variation in both imager instrumentation and molecular labeling agents demands a common metric conveying the ability of a system to identify tumor cells. Microscopic disease, comprised of a small number of tumor cells, has a signal on par with the background, making the use of signal (or tumor) to background ratio inapplicable in this critical regime. Therefore, a metric that incorporates the ability to subtract out background, evaluating the signal itself relative to the sources of uncertainty, or noise is required. Here we introduce the signal to noise ratio (SNR) to characterize the ultimate sensitivity of an imaging system and optimize factors such as pixel size. Variation in the background (noise) is due to electronic sources, optical sources, and spatial sources (heterogeneity in tumor marker expression, fluorophore binding, and diffusion). Here, we investigate the impact of these noise sources and ways to limit its effect on SNR. We use empirical tumor and noise measurements to procedurally generate tumor images and run a Monte Carlo simulation of microscopic disease imaging to optimize parameters such as pixel size.

Prostate-Specific Membrane Antigen Uptake and Survival in Metastatic Castration-Resistant Prostate Cancer

Background

Prostate-specific membrane antigen (PSMA) imaging has been suggested as highly sensitive modality for detection of metastases in patients with biochemically recurrent or advanced prostate cancer (PCa). PSMA expression is associated with grade and stage and has a relationship with androgen receptor signaling. The aim of this study was to evaluate the prognostic utility of radiographic PSMA expression in men with metastatic castration-resistant prostate cancer (mCRPC).

Methods

Patients with mCRPC and available baseline PSMA imaging were studied. Images by planar/single-photon emission computed tomography (SPECT) or positron emission tomography (PET)/CT were reviewed. Planar/SPECT images were scored semi-quantitatively and PET/CT scored quantitatively with comparison of tumor uptake to liver uptake on a scale of 0–4 in order to determine an imaging score (IS). The IS (high: 2–4 versus low: 0–1), subsequent receipt of life-prolonging systemic therapies (taxane chemotherapy, potent androgen receptor pathway inhibitors, sipuleucel-T, and radium-223), and the CALGB prognostic risk stratification of patients were analyzed according to overall survival (OS) in univariate and multivariate Cox’s proportional hazards models.

Results

High PSMA expression (IS 2–4) was found in 179 (75.21%) patients, and 59 (24.79%) patients had low PSMA uptake. The median OS of the entire cohort was 16.8 (95%CI: 14.9–19.3) months. Patients with a high IS had a significantly shorter OS of 15.8 (95%CI 13.0–18.1) months compared to those with low expression [22.7 (95%CI: 17.7–30.7) months, p = 0.002]. After accounting for use of life-prolonging therapies (p<0.001) and CALGB prognostic groups (p = 0.001), high PSMA IS emerged as an independent prognostic factor for OS [HR(95%CI): 1.7 (1.2–2.2); p = 0.003].

Conclusion

Presence of high radiographic PSMA expression on SPECT or PET/CT may portend a poor prognosis in patients with mCRPC treated with standard systemic therapies. This provides implications for therapeutic targeting of PSMA-avid disease as a means to improve outcomes.

Preclinical and Clinical Status of PSMA-Targeted Alpha Therapy for Metastatic Castration-Resistant Prostate Cancer

Bone, lymph node, and visceral metastases are frequent in castrate-resistant prostate cancer patients. Since such patients have only a few months' survival benefit from standard therapies, there is an urgent need for new personalized therapies. The prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer and is a molecular target for imaging diagnostics and targeted radionuclide therapy (theragnostics). PSMA-targeted α therapies (PSMA-TAT) may deliver potent and local radiation more selectively to cancer cells than PSMA-targeted β- therapies. In this review, we summarize both the recent preclinical and clinical advances made in the development of PSMA-TAT, as well as the availability of therapeutic α-emitting radionuclides, the development of small molecules and antibodies targeting PSMA. Lastly, we discuss the potentials, limitations, and future perspectives of PSMA-TAT.

Calculation of radium-223 and actinium-225 α-emitter radiopharmaceuticals dose rates in treatment of metastatic castration-resistant prostate cancer

Aim of Study

There is limited information regarding the α-emitter radiopharmaceuticals dose calculation used in the setting of men with prostate cancer (PCa). The present study investigates the α-emitter radiopharmaceuticals absorbed dose distribution in the body organs.

Materials and Methods

The α-emitter radiopharmaceuticals dose coefficient and absorbed doses biokinetics distribution, which are used for the treatment of PCa in all over the world, were performed using the "Internal Dose Assessed by Computer" (IDAC-Dose 2.1) program. The results of absorbed dose distribution in any organ of the body, were compared in studied α-emitter radiopharmaceuticals.

Results

The absorbed dose value of ²²³Ra radiopharmaceutical in the prostate organ was evaluated 9.47E-9 Gy/Bq. The maximum and minimum absorbed doses due to biokinetics distribution of ²²³Ra were found in the thymus (9.53E-8 Gy/Bq) and eye lenses (1.30E-10 Gy/Bq) organs, respectively. Furthermore, the ²²⁵Ac absorbed dose in the prostate organ was obtained 1.91E-9 Gy/Bq, where this value is 1% of total body dose. While the absorbed dose distribution of ²²⁵Ac in body organs shows the highest concentration in the spleen (1.47E-8 Gy/Bq) and lowest in the eye lenses (7.93E-12 Gy/Bq).

Conclusion

The absorbed dose in the body organs due to ²²³Ra and ²²⁵Ac α-emitter radiopharmaceuticals which are used in metastasized castration-resistant prostate cancer (mCRPC), calculated in this study. The results of this study will assist in evaluating and analyzing human body organ doses from application of ²²³Ra and ²²⁵Ac that used in mCRPC patients.

Glypican-3-Targeted Alpha Particle Therapy for Hepatocellular Carcinoma

Glypican-3 (GPC3) is expressed in 75% of hepatocellular carcinoma (HCC), but not normal liver, making it a promising HCC therapeutic target. GC33 is a full-length humanized monoclonal IgG1 specific to GPC3 that can localize to HCC in vivo. GC33 alone failed to demonstrate therapeutic efficacy when evaluated in patients with HCC; however, we posit that cytotoxic functionalization of the antibody with therapeutic radionuclides, may be warranted. Alpha particles, which are emitted by radioisotopes such as Actinium-225 (Ac-225) exhibit high linear energy transfer and short pathlength that, when targeted to tumors, can effectively kill cancer and limit bystander cytotoxicity. Macropa, an 18-member heterocyclic crown ether, can stably chelate Ac-225 at room temperature. Here, we synthesized and evaluated the efficacy of [²²⁵Ac]Ac–Macropa–GC33 in mice engrafted with the GPC3-expressing human liver cancer cell line HepG2. Following a pilot dose-finding study, mice (n = 10 per group) were treated with (1) PBS, (2) mass-equivalent unmodified GC33, (3) 18.5 kBq [²²⁵Ac]Ac–Macropa–IgG1 (isotype control), (4) 9.25 kBq [²²⁵Ac]Ac–Macropa–GC33, and (5) 18.5 kBq [²²⁵Ac]Ac–Macropa–GC33. While significant toxicity was observed in all groups receiving radioconjugates, the 9.25 kBq [²²⁵Ac]Ac–Macropa–GC33 group demonstrated a modest survival advantage compared to PBS (p = 0.0012) and 18.5 kBq [²²⁵Ac]Ac–IgG1 (p = 0.0412). Hematological analysis demonstrated a marked, rapid reduction in white blood cells in all radioconjugate-treated groups compared to the PBS and unmodified GC33 control groups. Our studies highlight a significant disadvantage of using directly-labeled biomolecules with long blood circulation times for TAT. Strategies to mitigate such treatment toxicity include dose fractionation, pretargeting, and using smaller targeting ligands.

Toward the Discovery and Development of PSMA Targeted Inhibitors for Nuclear Medicine Applications

BACKGROUND

The rising incidence rate of prostate cancer (PCa) has promoted the development of new diagnostic and therapeutic radiopharmaceuticals during the last decades. Promising improvements have been achieved in clinical practice using prostate specific membrane antigen (PSMA) labeled agents, including specific antibodies and small molecular weight inhibitors. Focusing on molecular docking studies, this review aims to highlight the progress in the design of PSMA targeted agents for a potential use in nuclear medicine.

RESULTS

Although the first development of radiopharmaceuticals able to specifically recognize PSMA was exclusively oriented to macromolecule protein structure such as radiolabeled monoclonal antibodies and derivatives, the isolation of the crystal structure of PSMA served as the trigger for the synthesis and the further evaluation of a variety of low molecular weight inhibitors. Among the nuclear imaging probes and radiotherapeutics that have been developed and tested till today, labeled Glutamate-ureido inhibitors are the most prevalent PSMA-targeting agents for nuclear medicine applications.

CONCLUSION

PSMA represents for researchers the most attractive target for the detection and treatment of patients affected by PCa using nuclear medicine modalities. [99mTc]MIP-1404 is considered the tracer of choice for SPECT imaging and [68Ga]PSMA-11 is the leading diagnostic for PET imaging by general consensus. [18F]DCFPyL and [18F]PSMA-1007 are clearly the emerging PET PSMA candidates for their great potential for a widespread commercial distribution. After paving the way with new imaging tools, academic and industrial R&Ds are now focusing on the development of PSMA inhibitors labeled with alpha or beta minus emitters for a theragnostic application.

Emerging treatments for metastatic castration-resistant prostate cancer: Immunotherapy, PARP inhibitors, and PSMA-targeted approaches

Recently there has been an explosion of new agents being investigated for the treatment of prostate cancer. These modalities represent new therapies aimed at old targets, and new therapies addressing new targets. This review will highlight three novel and emerging areas of treatment that have the potential to significantly impact the management of metastatic castration-resistant prostate cancer (mCRPC) in the near future: immunotherapy, poly ADP-ribose polymerase (PARP) inhibitors, and prostate-specific membrane antigen (PSMA)-targeted modalities. Immunotherapy, particularly immune checkpoint blockers, PARP inhibitors, and PSMA-targeted therapies are all increasingly being studied for the treatment of mCRPC although none are currently FDA-approved specifically for prostate cancer. Together these three classes of treatments may drastically change the future landscape of mCRPC. This review will cover what is currently known about the utility of these agents for the treatment of mCRPC, the areas of active research, and how these agents may be useful for patients in the future. It will also emphasize the notion of biomarker selection to help inform which patients are more likely to respond to these therapies.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.