Novel Bispecific PSMA/GRPr Targeting Radioligands with Optimized Pharmacokinetics for Improved PET Imaging of Prostate Cancer

Biodistribution and radiation dosimetry of [99mTc]Tc-N4-BTG in patients with biochemical recurrence of prostate cancer

BACKGROUND

In patients with prostate cancer (PCa), imaging with gastrin-releasing peptide receptor (GRPR) ligands is an alternative to PSMA-targeted tracers, particularly if PSMA expression is low or absent. [99mTc]Tc-N4-BTG is a newly developed GRPR-directed probe for conventional scintigraphy and single photon emission computed tomography (SPECT) imaging. The current study aims to investigate the safety, biodistribution and dosimetry of [99mTc]Tc-N4-BTG in patients with biochemical recurrence (BCR) of PCa.

RESULTS

No adverse pharmacologic effects were observed. Injection of [99mTc]Tc-N4-BTG resulted in an effective dose of 0.0027 ± 0.0002 mSv/MBq. The urinary bladder was the critical organ with the highest mean absorbed dose of 0.028 ± 0.001 mGy/MBq, followed by the pancreas with 0.0043 ± 0.0015 mGy/MBq, osteogenic cells with 0.0039 ± 0.0005 mGy/MBq, the kidneys with 0.0034 ± 0.0003 mGy/MBq, and the liver with 0.0019 ± 0.0004 mGy/MBq, respectively. No focal tracer uptake suggestive of PCa recurrence could be revealed for any of the patients.

CONCLUSION

[99mTc]Tc-N4-BTG appears to be a safe diagnostic agent. Compared to GRPR-targeted PET tracers, this 99mTc-labelled SPECT agent could contribute to a broader application and better availability of this novel approach. Further research to assess its clinical value is warranted.

Elevating theranostics: The emergence and promise of radiopharmaceutical cell-targeting heterodimers in human cancers

Optimal therapeutic and diagnostic efficacy is essential for healthcare's global mission of advancing oncologic drug development. Accurate diagnosis and detection are crucial prerequisites for effective risk stratification and personalized patient care in clinical oncology. A paradigm shift is emerging with the promise of multi-receptor-targeting compounds. While existing detection and staging methods have demonstrated some success, the traditional approach of monotherapy is being reevaluated to enhance therapeutic effectiveness. Heterodimeric site-specific agents are a versatile solution by targeting two distinct biomarkers with a single theranostic agent. This review describes the innovation of dual-targeting compounds, examining their design strategies, therapeutic implications, and the promising path they present for addressing complex diseases.

Towards the Magic Radioactive Bullet: Improving Targeted Radionuclide Therapy by Reducing the Renal Retention of Radioligands

Targeted radionuclide therapy (TRT) is an emerging field and has the potential to become a major pillar in effective cancer treatment. Several pharmaceuticals are already in routine use for treating cancer, and there is still a high potential for new compounds for this application. But, a major issue for many radiolabeled low-to-moderate-molecular-weight molecules is their clearance via the kidneys and their subsequent reuptake. High renal accumulation of radioactive compounds may lead to nephrotoxicity, and therefore, the kidneys are often the dose-limiting organs in TRT with these radioligands. Over the years, different strategies have been developed aiming for reduced kidney retention and enhanced therapeutic efficacy of radioligands. In this review, we will give an overview of the efforts and achievements of the used strategies, with focus on the therapeutic potential of low-to-moderate-molecular-weight molecules. Among the strategies discussed here is coadministration of compounds that compete for binding to the endocytic receptors in the proximal tubuli. In addition, the influence of altering the molecular design of radiolabeled ligands on pharmacokinetics is discussed, which includes changes in their physicochemical properties and implementation of cleavable linkers or albumin-binding moieties. Furthermore, we discuss the influence of chelator and radionuclide choice on reabsorption of radioligands by the kidneys.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Preclinical Characterisation of PSMA/GRPR-Targeting Heterodimer [68Ga]Ga-BQ7812 for PET Diagnostic Imaging of Prostate Cancer: A Step towards Clinical Translation

The development of radioligands targeting prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) has shown promising results for the imaging and therapy of prostate cancer. However, studies have shown that tumors and metastases can express such targets heterogeneously. To overcome this issue and to improve protein binding, radioligands with the ability to bind both PSMA and GRPR have been developed. Herein, we present the preclinical characterization of [68Ga]Ga-BQ7812; a PSMA/GRPR-targeting radioligand for the diagnostic PET imaging of prostate cancer. This study aimed to evaluate [68Ga]Ga-BQ7812 to promote the translation of such imaging probes into the clinic. [68Ga]Ga-BQ7812 demonstrated rapid and specific binding to both targets in a PSMA/GRPR-expressing PC3-pip cell line. Results from the biodistribution study in PC3-pip xenografted mice showed specific binding to both targets, with the highest activity uptake at 1 h pi in tumor (PSMA+/GRPR+, 10.4 ± 1.0% IA/g), kidneys (PSMA+, 45 ± 16% IA/g), and pancreas (GRPR+, 5.6 ± 0.7% IA/g). At 3h pi, increased tumour-to-organ ratios could be seen due to higher retention in the tumor compared with other PSMA or GRPR-expressing organs. These results, together with low toxicity and an acceptable estimated dosimetry profile (total effective dose = 0.0083 mSv/MBq), support the clinical translation of [68Ga]Ga-BQ7812 and represent a step towards its first clinical trial.

Radiometal-theranostics: the first 20 years*

This review describes the basic principles of radiometal-theranostics and its dawn based on the development of the positron-emitting 86Y and 86Y-labeled radiopharmaceuticals to quantify biodistribution and dosimetry of 90Y-labeled analogue therapeutics. The nuclear and inorganic development of 86Y (including nuclear and cross section data, irradiation, radiochemical separation and recovery) led to preclinical and clinical evaluation of 86Y-labeled citrate and EDTMP complexes and yielded organ radiation doses in terms of mGy/MBq 90Y. The approach was extended to [86/90Y]Y-DOTA-TOC, yielding again yielded organ radiation doses in terms of mGy/MBq 90Y. The review further discusses the consequences of this early development in terms of further radiometals that were used (68Ga, 177Lu etc.), more chelators that were developed, new biological targets that were addressed (SSTR, PSMA, FAP, etc.) and subsequent generations of radiometal-theranostics that resulted out of that.

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.

Radiosynthesis and biological evaluation of 18F-labeled bispecific heterodimer targeted dual gastrin-releasing peptide receptor and prostate-specific membrane antigen for prostate cancer imaging

OBJECTIVE

Approximately 5% of prostatic primary tumors and 15% of metastatic tumors were found to be prostate-specific membrane antigen (PSMA)-negative. Targeting gastrin-releasing peptide receptor (GRPR) has been shown to complement patients with PSMA-negative prostate cancer (PCa). Based on previous findings, simultaneously targeting PSMA and GRPR imaging may improve the diagnosis of PCa. In this study, we report the radiosynthesis and biological evaluation of a bispecific heterodimer of NOTA-GRPR-PSMA that targeted both PSMA and GRPR for extended PCa imaging.

METHODS

NOTA-GRPR-PSMA was labeled using the Al18F-chelating one-step method. The competitive combination experiment and specific binding assay were performed in vitro using 22Rv1 (PSMA+) and PC-3 (GRPR+) cells. To determine the distribution and specificity in vivo, biodistribution and micro-PET/computed tomography of [18F]AlF-GRPR-PSMA were performed on mice bearing 22Rv1 or PC-3 tumors.

RESULTS

[18F]AlF-GRPR-PSMA had a radiochemical purity of over 98% and demonstrated high stability in vivo and in vitro, with a LogD of -1.2 ± 0.05. Cell uptake and inhibition studies of [18F]AlF-GRPR-PSMA in 22Rv1 and PC-3 cells revealed bispecific GRPR and PSMA bindings. According to the biodistribution study and PET imaging, [18F]AlF-GRPR-PSMA was mainly excreted through the kidney. Tumor uptake was high in 22Rv1 tumor (10.1 ± 0.4 %ID/g) and moderate in PC-3 tumor (2.1 ± 0.6 %ID/g) 2 h p.i., whereas blocking studies significantly decreased the tumor uptake of 22Rv1 and PC-3.

CONCLUSION

[18F]AlF-GRPR-PSMA has the potential to simultaneously target PSMA and GRPR for PCa imaging.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

PSMA-targeted low-molecular double conjugates for diagnostics and therapy

This review presents data on dual conjugates of therapeutic and diagnostic action for targeted delivery to prostate cancer cells. The works of the last ten years on this topic were analyzed. The mail attention focuses on low-molecular-weight conjugates directed to the prostate-specific membrane antigen (PSMA); the comparison of high and low molecular weight PSMA-targeted conjugates was made. The considered conjugates were divided in the review into two main classes: diagnostic bimodal conjugates (which are containing two fragments for different types of diagnostics), theranostic conjugates (containing both therapeutic and diagnostic agents); also bimodal high molecular weight therapeutic conjugates containing two therapeutic agents are briefly discussed. The data of in vitro and in vivo studies for PSMA-targeted double conjugates available by the beginning of 2021 have been analyzed.

Radiolabeled Bombesin Analogs

Simple Summary

Recent medical advancements have strived for a personalized medicine approach to patients, aimed at optimizing therapy outcomes with minimum toxicity. In this respect, nuclear medicine methodologies have been playing increasingly important roles. For example, the overexpression of peptide receptors, such as the gastrin-releasing peptide receptor (GRPR), on tumor cells as opposed to their lack of expression in healthy surrounding tissues can be elegantly exploited with the aid of “smart” peptide carriers, such as the analogs of the amphibian 14-peptide bombesin (BBN). These molecules can bring clinically attractive radionuclides to malignant lesions in prostate, breast, and other human cancers, sparing healthy tissues. Depending upon the radionuclide in question, diagnostic imaging with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) has been pursued, identifying patients who are eligible for peptide radionuclide receptor therapy (PRRT) in an integrated “theranostic” approach. In the present review, we (i) discuss the major steps taken in the development of anti-GRPR theranostic radioligands, with a focus on those selected for clinical testing; (ii) comment on the present status in this field of research; and (iii) reflect on the current limitations as well as on new opportunities for their broader and more successful clinical applications.

Abstract

The gastrin-releasing peptide receptor (GRPR) is expressed in high numbers in a variety of human tumors, including the frequently occurring prostate and breast cancers, and therefore provides the rationale for directing diagnostic or therapeutic radionuclides on cancer lesions after administration of anti-GRPR peptide analogs. This concept has been initially explored with analogs of the frog 14-peptide bombesin, suitably modified at the N-terminus with a number of radiometal chelates. Radiotracers that were selected for clinical testing revealed inherent problems associated with these GRPR agonists, related to low metabolic stability, unfavorable abdominal accumulation, and adverse effects. A shift toward GRPR antagonists soon followed, with safer analogs becoming available, whereby, metabolic stability and background clearance issues were gradually improved. Clinical testing of three main major antagonist types led to promising outcomes, but at the same time brought to light several limitations of this concept, partly related to the variation of GRPR expression levels across cancer types, stages, previous treatments, and other factors. Currently, these parameters are being rigorously addressed by cell biologists, chemists, nuclear medicine physicians, and other discipline practitioners in a common effort to make available more effective and safe state-of-the-art molecular tools to combat GRPR-positive tumors. In the present review, we present the background, current status, and future perspectives of this endeavor.

Evolution of Peptide-Based Prostate-Specific Membrane Antigen (PSMA) Inhibitors: An Approach to Novel Prostate Cancer Therapeutics

BACKGROUND

Prostate cancer is one of the most common cancers worldwide, with approximately 1.1 million cases diagnosed annually. The rapid development of molecular imaging has facilitated greater structural understanding, which can help formulate novel combinations of therapeutic regimens and more accurate diagnosis, avoiding unnecessary prostate biopsies. This accumulated knowledge also provides a greater understanding of the aggressive stages of the disease and tumor recurrence. Recently, much progress has been made on developing peptidomimetic-based inhibitors as promising candidates to effectively bind to the prostate- specific membrane antigen (PSMA), which is expressed by prostate cancer cells.

OBJECTIVE

In this review, recent advances covering small-molecule and peptide-based PSMA inhibitors will be extensively reviewed, providing a base for the rational design of future PSMA inhibitors.

METHOD

Herein, the literature on selected PSMA inhibitors that have been developed from 1996 to 2020 were reviewed, emphasizing recent synthetic advances and chemical strategies whilst highlighting therapeutic potential and drawbacks of each inhibitor.

RESULTS

Synthesized inhibitors presented in this review demonstrate the clinical application of certain PSMA inhibitors, exhibited in vitro and in vivo.

CONCLUSION

This review highlights the clinical potential of PSMA inhibitors, analyzing the advantages and setbacks of the chemical synthetic methodologies utilized, setting precedence for the discovery of novel PSMA inhibitors for future clinical applications.

Synthesis and preclinical evaluation of an Al18F radiofluorinated bivalent PSMA ligand

Prostate-specific membrane antigen (PSMA) has become as an outstanding prostate cancer-related target for diagnostic imaging and targeted radiotherapy. Clinical studies on a few PSMA radiotracers are currently underway to determine their efficacy as imaging agents to detect prostate cancer. To improve tumor retention and tumor-to-normal tissue contrast, we herein report the synthesis and preclinical evaluation of an Al¹⁸F-labeled bivalent PSMA ligand (¹⁸F-Bi-PSMA). ¹⁸F-Bi-PSMA was successful automated preparation and in vitro evaluation showed that ¹⁸F-Bi-PSMA was potent binding affinity, high specificity, and rapid internalization in PSMA-expressing cells. Biodistribution studies revealed a high and specific tumor uptake of 20.5 ± 3.5 %ID/g in 22Rv1 tumor-bearing mice. Furthermore, compared to the clinically used monomeric PSMA-targeting tracers, ⁶⁸Ga-PSMA-11 and ¹⁸F-PSMA-1007, ¹⁸F-Bi-PSMA exhibited improved pharmacokinetics and higher tumor uptake, as well as better tumor-to-normal tissue contrast, resulting in considerably high imaging quality. Our findings indicated that the bivalent PSMA radioligand, ¹⁸F-Bi-PSMA, was successfully synthesized and ideal imaging properties.

Drug and molecular radiotherapy combinations for metastatic castration resistant prostate cancer

Metastatic castration resistant prostate cancer (mCRPC) is a highly lethal disease. Several novel therapies have been assessed in the past years. Targeting DNA damage response (DDR) pathways in prostate cancer became a promising treatment strategy and olaparib and rucaparib, Poly(ADP-ribose) polymerase (PARP) inhibitors, have been approved for patients carrying mutations in homologous recombination (HR) repair pathways. Other DDR inhibitor targets, such as ATM, ATR, CHK1, CHK2, and WEE1 are under extensive investigation. Additionally, molecular radiotherapy (MRT) including [¹⁷⁷Lu]Lu-PSMA, [²²⁵Ac]Ac-PSMA, [²²³Ra]Ra-dichloride, [¹⁵³Sm]-EDTMP, [¹⁸⁸Re]Re-HDMP and GRPR-targeted MRT treat cancer through internal ionizing radiation causing DNA damage and demonstrate promising efficacy in clinical trials. In the field of immunotherapy, checkpoint inhibition as well as sipuleucel-T and PROSTVAC demonstrated only limited efficacy in mCRPC when used as monotherapy. This review discusses recent therapeutic strategies for mCRPC highlighting the need for rational combination of treatment options.

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 years and some approaches are currently evaluated for further clinical translation. However, accumulation in non-malignant 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 enhanced pharmacokinetic profile and improved imaging contrasts. Methods: A selection of rational 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 biological 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 PSMA-binding motif and chelator. In comparison to the parental core structure, uptake in non-malignant tissue was significantly reduced to a minimum exemplified by an 11-fold reduced spleen uptake from 38.12±14.62 %ID/g to 3.47±1.39 %ID/g (1 h p.i.). The specific tumor uptake of this compound (7.59±0.95 %ID/g, 1 h p.i.) was detected to be significantly higher as compared to 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 post injection. Conclusion: The novel generation of PSMA-targeting hybrid molecules reveal 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 of improving the pharmacokinetic profile highlights the strengths of rational linker design as a powerful tool in strategic hybrid molecule development.

Biodistribution and Radiation Dosimetric Analysis of [68Ga]Ga-RM2: A Potent GRPR Antagonist in Prostate Carcinoma Patients

[68Ga]Ga-RM2 is a promising innovative positron emission tomography (PET) tracer for patients with primary or metastatic prostate carcinoma. This study aims to analyze the biodistribution and radiation dosimetry of [68Ga]Ga-RM2 in five prostate cancer patients. The percentages of injected activity in the source organs and blood samples were determined. Bone marrow residence time was calculated using an indirect blood-based method. OLINDA/EXM version 2.0 (Hermes Medical Solutions, Stockholm, Sweden) was used to determine residence times, organ absorbed and effective doses. Physiological uptake was seen in kidneys, urinary bladder, pancreas, stomach, spleen and liver. Blood clearance was fast and followed by rapid clearance of activity from kidneys resulting in high activity concentrations in the urinary bladder. The urinary bladder wall was the most irradiated organ with highest mean organ absorbed dose (0.470 mSv/MBq) followed by pancreas (0.124 mSv/MBq), stomach wall (0.063 mSv/MBq), kidneys (0.049 mSv/MBq) and red marrow (0.010 mSv/MBq). The effective dose was found to be 0.038 mSv/MBq. Organ absorbed doses were found to be comparable to other gallium-68 labelled GRPR antagonists and lower than [68Ga]Ga-PSMA with the exception of the urinary bladder, pancreas and stomach wall. Remarkable interindividual differences were observed for the organ absorbed doses. Therefore, [68Ga]Ga-RM2 is a safe diagnostic agent with a significantly lower kidney dose but higher pancreas and urinary bladder doses as compared to [68Ga]Ga-PSMA.

Light-Emitting Probes for Labeling Peptides

Peptides are versatile biopolymers composed of 2-100 amino acid residues that present a wide range of biological functions and constitute potential therapies for numerous diseases, partly due to their ability to penetrate cell membranes. However, their mechanisms of action have not been fully elucidated due to the lack of appropriate tools. Existing light-emitting probes are limited by their cytotoxicity and large size, which can alter peptide structure and function. Here, we describe the available fluorescent, bioluminescent, and chemiluminescent probes for labeling peptides, with a focus on minimalistic options.

New Frontiers in Molecular Imaging Using Peptide-Based Radiopharmaceuticals for Prostate Cancer

The high incidence of prostate cancer (PCa) increases the need for progress in its diagnosis, staging, and precise treatment. The overexpression of tumor-specific receptors for peptides in human cancer cells, such as gastrin-releasing peptide receptor, natriuretic peptide receptor, and somatostatin receptor, has indicated the ideal molecular basis for targeted imaging and therapy. Targeting these receptors using radiolabeled peptides and analogs have been an essential topic on the current forefront of PCa studies. Radiolabeled peptides have been used to target receptors for molecular imaging in human PCa with high affinity and specificity. The radiolabeled peptides enable optimal quick elimination from blood and normal tissues, producing high contrast for positron emission computed tomography and single-photon emission computed tomography imaging with high tumor-to-normal tissue uptake ratios. Owing to their successful application in visualization, peptide derivatives with therapeutic radionuclides for peptide receptor radionuclide therapy in PCa have been explored in recent years. These developments offer the promise of personalized, molecular medicine for individual patients. Hence, we review the preclinical and clinical literature in the past 20 years and focus on the newer developments of peptide-based radiopharmaceuticals for the imaging and therapy of PCa.

Current State of Radiolabeled Heterobivalent Peptidic Ligands in Tumor Imaging and Therapy

Over the past few years, an approach emerged that combines different receptor-specific peptide radioligands able to bind different target structures on tumor cells concomitantly or separately. The reason for the growing interest in this special field of radiopharmaceutical development is rooted in the fact that bispecific peptide heterodimers can exhibit a strongly increased target cell avidity and specificity compared to their corresponding monospecific counterparts by being able to bind to two different target structures that are overexpressed on the cell surface of several malignancies. This increase of avidity is most pronounced in the case of concomitant binding of both peptides to their respective targets but is also observed in cases of heterogeneously expressed receptors within a tumor entity. Furthermore, the application of a radiolabeled heterobivalent agent can solve the ubiquitous problem of limited tumor visualization sensitivity caused by differential receptor expression on different tumor lesions. In this article, the concept of heterobivalent targeting and the general advantages of using radiolabeled bispecific peptidic ligands for tumor imaging or therapy as well as the influence of molecular design and the receptors on the tumor cell surface are explained, and an overview is given of the radiolabeled heterobivalent peptides described thus far.

Heterodimeric Radiotracer Targeting PSMA and GRPR for Imaging of Prostate Cancer-Optimization of the Affinity towards PSMA by Linker Modification in Murine Model

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [¹¹¹In]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [¹¹¹In]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.

Pharmacokinetic Properties of 68Ga-labelled Folic Acid Conjugates: Improvement Using HEHE Tag

The folate receptor (FR) is a promising cell membrane-associated target for molecular imaging and radionuclide therapy of cancer (FR-α) and potentially also inflammatory diseases (FR-β) through use of folic acid-based radioconjugate. FR is often overexpressed by cells of epithelial tumors, including tumors of ovary, cervix, endometrium, lungs, kidneys, etc. In healthy tissues, FR can be found in small numbers by the epithelial cells, mainly in the kidneys. Extremely high undesired accumulation of the folate radioconjugates in the renal tissue is a main drawback of FR-targeting concept. In the course of this work, we aimed to reduce the undesirable accumulation of folate radioconjugates in the kidneys by introducing a histidine/glutamic acid tag into their structure. Two folic acid based compounds were synthesized: NODAGA-1,4-butanediamine-folic acid (FA-I, as control) and NODAGA-[Lys-(HE)₂]-folic acid (FA-II) which contains a (His-Glu)₂ fragment. In vitro studies with FR (+) cells (KB and others) showed that both compounds have specificity for FR. Introduction of (HE)₂-tag does not affect FR binding ability of the conjugates. In vivo biodistribution studies with normal laboratory animals, as well as with KB tumor bearing animals, were carried out. The results showed that introduction of the (HE)₂ tag into the structure of folate radioconjugates can significantly reduce the accumulation of these compounds in non-target tissues and important organs (the accumulation in the kidneys is reduced 2-4 times), leaving the accumulation in tumor at least at the same level, and even increasing it.

Multi-Modal PET and MR Imaging in the Hen's Egg Test-Chorioallantoic Membrane (HET-CAM) Model for Initial in Vivo Testing of Target-Specific Radioligands

The validation of novel target-specific radioligands requires animal experiments mostly using mice with xenografts. A pre-selection based on a simpler in vivo model would allow to reduce the number of animal experiments, in accordance with the 3Rs principles (reduction, replacement, refinement). In this respect, the chick embryo or hen’s egg test–chorioallantoic membrane (HET-CAM) model is of special interest, as it is not considered an animal until day 17. Thus, we evaluated the feasibility of quantitative analysis of target-specific radiotracer accumulation in xenografts using the HET-CAM model and combined positron emission tomography (PET) and magnetic resonance imaging (MRI). For proof-of-principle we used established prostate-specific membrane antigen (PSMA)-positive and PSMA-negative prostate cancer xenografts and the clinically widely used PSMA-specific PET-tracer [⁶⁸Ga]Ga-PSMA-11. Tracer accumulation was quantified by PET and tumor volumes measured with MRI (n = 42). Moreover, gamma-counter analysis of radiotracer accumulation was done ex-vivo. A three- to five-fold higher ligand accumulation in the PSMA-positive tumors compared to the PSMA-negative tumors was demonstrated. This proof-of-principle study shows the general feasibility of the HET-CAM xenograft model for target-specific imaging with PET and MRI. The ultimate value for characterization of novel target-specific radioligands now has to be validated in comparison to mouse xenograft experiments.

Synthesis, characterization and evaluation of 68Ga labelled monomeric and dimeric quinazoline derivatives of the HBED-CC chelator targeting the epidermal growth factor receptor

Tyrosine kinase (TK) receptors including epidermal growth factor receptors (EGFRs) are known to be overexpressed in a wide variety of solid tumors associated with poor prognosis. The HBED-CC chelator N,N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid 1 was coupled via one or both its propionic acid moieties with the quinazoline EGFR-TK inhibiting pharmacophore 4-amino-N-(4-((3-bromophenyl)amino)quinazolin-6-yl)butanamide 3 resulting in either a monomeric 4 or a dimeric 5 species. Ligands 4 and 5 reacted with Ga³⁺ generating the corresponding complexes Ga4 and Ga5. Both ligands and complexes were characterized with mass spectrometry and NMR spectroscopy and evaluated in vitro with MTT assays in A431 cells, where they showed IC₅₀ values in the range 51.6 to 68.8 μM. Labeling of ligands 4 and 5 with the PET radionuclide ⁶⁸Ga was quantitative and resulted in tracers [⁶⁸Ga]Ga4 and [⁶⁸Ga]Ga5 with radiochemical purities greater than 98%, which were also characterised by comparative RP-HPLC studies with Ga4 and Ga5 respectively. Radiotracers [⁶⁸Ga]Ga4 and [⁶⁸Ga]Ga5 were stable (intact tracer over 98%) in the reaction mixture (120 min) and in human serum (30 min). Both tracers were evaluated in vivo with biodistribution experiments in SCID mice bearing A431 tumors presenting tumor uptake of 1.34 for [⁶⁸Ga]Ga4 and 1.01 %ID/g for [⁶⁸Ga]Ga5 at 5 min, which was slightly decreased at 60 min p.i. and then remained stable until 120 min p.i. To the best of our knowledge, this is the first report of monomeric and dimeric quinazoline conjugates with the chelator HBED-CC, which can serve as a basis for further development of EGFR-TKI targeting tracers.

The Future of PSMA-Targeted Radionuclide Therapy: An Overview of Recent Preclinical Research

Prostate specific membrane antigen (PSMA) has become a major focus point in the research and development of prostate cancer (PCa) imaging and therapeutic strategies using radiolabeled tracers. PSMA has shown to be an excellent target for PCa theranostics because of its high expression on the membrane of PCa cells and the increase in expression during disease progression. Therefore, numerous PSMA-targeting tracers have been developed and (pre)clinically studied with promising results. However, many of these PSMA-targeting tracers show uptake in healthy organs such as the salivary glands, causing radiotoxicity. Furthermore, not all patients respond to PSMA-targeted radionuclide therapy (TRT). This created the necessity of additional preclinical research studies in which existing tracers are reevaluated and new tracers are developed in order to improve PSMA-TRT by protecting the (PSMA-expressing) healthy organs and improving tumor uptake. In this review we will give an overview of the recent preclinical research projects regarding PCa-TRT using PSMA-specific radiotracers, which will give an indication of where the PSMA-TRT research movement is going and what we can expect in future clinical trials.

Bispecific GRPR-Antagonistic Anti-PSMA/GRPR Heterodimer for PET and SPECT Diagnostic Imaging of Prostate Cancer

Simultaneous targeting of the prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) could improve the diagnostic accuracy in prostate cancer (PCa). The aim of this study was to develop a PSMA/GRPR-targeting bispecific heterodimer for SPECT and positron emission tomography (PET) diagnostic imaging of PCa. The heterodimer NOTA-DUPA-RM26 was produced by manual solid-phase peptide synthesis. NOTA-DUPA-RM26 was labeled with ¹¹¹In and ⁶⁸Ga, with yields >98%, and demonstrated a high stability and binding specificity to PSMA and GRPR. IC₅₀ values for ^natIn-NOTA-DUPA-RM26 were 4 ± 1 nM towards GRPR and 824 ± 230 nM towards PSMA. An in vivo binding specificity 1 h pi of ¹¹¹In-NOTA-DUPA-RM26 in PC3-PIP-xenografted mice demonstrated partially blockable tumor uptake when co-injected with an excess of PSMA- or GRPR-targeting agents. Simultaneous co-injection of both agents induced pronounced blocking. The biodistribution of ¹¹¹In-NOTA-DUPA-RM26 and ⁶⁸Ga-NOTA-DUPA-RM26 revealed fast activity clearance from the blood and normal organs via the kidneys. Tumor uptake exceeded normal organ uptake for both analogs 1 h pi. ⁶⁸Ga-NOTA-DUPA-RM26 had a significantly lower tumor uptake (8 ± 2%ID/g) compared to ¹¹¹In-NOTA-DUPA-RM26 (12 ± 2%ID/g) 1 h pi. Tumor-to-organ ratios increased 3 h pi, but decreased 24 h pi, for ¹¹¹In-NOTA-DUPA-RM26. MicroPET/CT and microSPECT/CT scans confirmed biodistribution data, suggesting that ⁶⁸Ga-NOTA-DUPA-RM26 and ¹¹¹In-NOTA-DUPA-RM26 are suitable candidates for the imaging of GRPR and PSMA expression in PCa shortly after administration.

A theranostic PSMA ligand for PET imaging and retargeting of T cells expressing the universal chimeric antigen receptor UniCAR

Chimeric antigen receptor (CAR) T cells have shown impressive therapeutic potential. Due to the lack of direct control mechanisms, therapy-related adverse reactions including cytokine release- and tumor lysis syndrome can even become life-threatening. In case of target antigen expression on non-malignant cells, CAR T cells can also attack healthy tissues. To overcome such side effects, we have established a modular CAR platform termed UniCAR: UniCAR T cells per se are inert as they recognize a peptide epitope (UniCAR epitope) that is not accessible on the surface of living cells. Bifunctional adapter molecules termed target modules (TM) can cross-link UniCAR T cells with target cells. In the absence of TMs, UniCAR T cells automatically turn off. Until now, all UniCAR TMs were constructed by fusion of the UniCAR epitope to an antibody domain. To open up the wide field of low-molecular-weight compounds for retargeting of UniCAR T cells to tumor cells, and to follow in parallel the progress of UniCAR T cell therapy by PET imaging we challenged the idea to convert a PET tracer into a UniCAR-TM. For proof of concept, we selected the clinically used PET tracer PSMA-11, which binds to the prostate-specific membrane antigen overexpressed in prostate carcinoma. Here we show that fusion of the UniCAR epitope to PSMA-11 results in a low-molecular-weight theranostic compound that can be used for both retargeting of UniCAR T cells to tumor cells, and for non-invasive PET imaging and thus represents a member of a novel class of theranostics.

Synthesis and Preclinical Evaluation of Radio-Iodinated GRPR/PSMA Bispecific Heterodimers for the Theranostics Application in Prostate Cancer

Gastrin-releasing peptide receptor (GRPR) and prostate-specific membrane antigen (PSMA) are overexpressed in most prostate cancers. GRPR expression is higher in early stages while PSMA expression increases with progression. The possibility of targeting both markers with a single theranostics radiotracer could improve patient management. Three GRPR/PSMA-targeting bispecific heterodimers (urea derivative PSMA-617 and bombesin-based antagonist RM26 linked via X-triazolyl-Tyr-PEG2, X = PEG2 (BO530), (CH2)8 (BO535), none (BO536)) were synthesized by solid-phase peptide synthesis. Peptides were radio-iodinated and evaluated in vitro for binding specificity, cellular retention, and affinity. In vivo specificity for all heterodimers was studied in PC-3 (GRPR-positive) and LNCaP (PSMA-positive) xenografts. [125I]I-BO530 was evaluated in PC-3pip (GRPR/PSMA-positive) xenografts. Micro single-photon emission computed tomography/computed tomography (microSPECT/CT) scans were acquired. The heterodimers were radiolabeled with high radiochemical yields, bound specifically to both targets, and demonstrated high degree of activity retention in PC-3pip cells. Only [125I]I-BO530 demonstrated in vivo specificity to both targets. A biodistribution study of [125I]I-BO530 in PC-3pip xenografted mice showed high tumor activity uptake (30%-35%ID/g at 3 h post injection (pi)). Activity uptake in tumors was stable and exceeded all other organs 24 h pi. Activity uptake decreased only two-fold 72 h pi. The GRPR/PSMA-targeting heterodimer [125I]I-BO530 is a promising agent for theranostics application in prostate cancer.

Synthesis and preclinical evaluation of the 177Lu-DOTA-PSMA(inhibitor)-Lys3-bombesin heterodimer designed as a radiotheranostic probe for prostate cancer

BACKROUND

Human tumors show intrinsic heterogeneity and changes in phenotype during disease progression, which implies different expression levels of cell surface receptors. The research on new heterodimeric lutetium-177 (Lu)-radiopharmaceuticals interacting with two different targets on tumor cells is a strategy for improvement of radiotheranostic performance. This study aimed to synthesize and characterize the Lu-DOTA-PSMA(inhibitor)-Lys-bombesin (Lu-DOTA-iPSMA-Lys-BN) heterodimer and to evaluate its potential to target prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPr) overexpressed in prostate cancer.

METHODS

The heterodimeric conjugate was synthesized and characterized by infrarred, mass, and H-NMR spectroscopies. The ligand was labeled with Lu and the radiochemical purity was assessed by radio-high-performance liquid chromatography. PSMA/GRPr affinity and the heterobivalent effect on cell viability were evaluated in LNCaP and PC3 prostate cancer cell lines. The biodistribution profile (3 and 96 h) was assessed in athymic mice with induced prostate tumors. Using pulmonary LNCaP (PSMA-positive) and PC3 (GRPr-negative) micrometastasis models, the influence of heterobivalency and affinity on tumor uptake was quantified (micro-SPECT/CT).

RESULTS

Lu-iPSMA-BN (radiochemical purity>98%) showed specific recognition for PSMA and GRPr (IC50=5.62 and 3.49 nmol/l, respectively) with a significant decrease in cell viability (10.15% of cell viability in LNCaP and 40.10% in PC3 at 48 h), as well as high LNCaP and PC3 tumor uptake (5.21 and 3.21% ID/g at 96 h, respectively). Micro-SPECT/CT imaging showed the heterodimer ability to target the tumors (SUVmax of 1.93±0.30 and 1.76±0.10 in LNCaP and PC3, respectively), possibly influenced by the heterobivalent effect. Lu-DOTA-iPSMA-Lys-BN showed suitable affinity for PSMA and GRPr.

CONCLUSION

The results warrant further preclinical studies to establish the Lu-radiotracer theranostic efficacy.

Monomeric and Dimeric 68Ga-Labeled Bombesin Analogues for Positron Emission Tomography (PET) Imaging of Tumors Expressing Gastrin-Releasing Peptide Receptors (GRPrs)

The GRPr, highly expressed in prostate PCa and breast cancer BCa, is a promising target for the development of new PET radiotracers. The chelator HBED-CC ( N, N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine- N, N'-diacetic acid) was coupled to the bombesin peptides: HBED-C-BN(2-14) 1, HBED-CC-PEG₂-[d-Tyr⁶,β-Ala¹¹,Thi¹³,Nle¹⁴]-BN(6-14) 2, HBED-CC-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14) (Y = 4-amino-1-carboxymethylpiperidine) 3, and HBED-CC-{PEG₂-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14)}₂ 4 (homodimer). Compounds 1-4 presented high binding affinities for GRPr (T47D, 0.56-3.51 nM; PC-3, 2.12-4.68 nM). In PC-3 and T47D cells, agonists [⁶⁸Ga]1 and [⁶⁸Ga]2 were mainly internalized while antagonists [⁶⁸Ga]3 and [⁶⁸Ga]4 were surface bound. Cell-related radioactivity reached a maximum after 45 min, while tracer levels followed GRPr expression (PC-3 > T47D > LNCaP > MDA-MB-231). [⁶⁸Ga]4 showed the highest cell-bound radioactivity (PC-3 and T47D). In vivo, tumor (PC-3) targeting for [⁶⁸Ga]3 and [⁶⁸Ga]4 increased over time, with dynamic μPET showing clearer tumors images at later time points. [⁶⁸Ga]3 and [⁶⁸Ga]4 can be considered suitable PET tracers for imaging PCa and BCa expressing GRPr.

Fluoroquinolones as imaging agents for bacterial infection

Diagnosis of deep-seated bacterial infection is difficult, as neither standard anatomical imaging nor radiolabeled, autologous leukocytes distinguish sterile inflammation from infection. Two recent imaging efforts are receiving attention: (1) radioactive derivatives of sorbitol show good specificity with Gram-negative bacterial infections, and (2) success in combining anatomical and functional imaging for cancer diagnosis has rekindled interest in 99mTc-fluoroquinolone-based imaging. With the latter, computed tomography (CT) would be combined with single-photon-emission-computed tomography (SPECT) to detect 99mTc-fluoroquinolone-bacterial interactions. The present minireview provides a framework for advancing fluoroquinolone-based imaging by identifying gaps in our understanding of the process. One issue is the reliance of 99mTc labeling on the reduction of sodium pertechnetate, which can lead to colloid formation and loss of specificity. Specificity problems may be reduced by altering the quinolone structure (for example, switching from ciprofloxacin to sitafloxacin). Another issue is the uncharacterized nature of 99mTc-ciprofloxacin binding to, or sequestration in, bacteria: specific interactions with DNA gyrase, an intracellular fluoroquinolone target, are unlikely. Labeling with 68Ga rather than 99mTc enables detection by positron emission tomography, but with similar biological uncertainties. Replacing the C6-F of the fluoroquinolone with 18F provides an alternative to pertechnetate and gallium that may lead to imaging based on drug interactions with gyrase. Gyrase-based imaging requires knowledge of fluoroquinolone action, which we update. We conclude that quinolone-based probes show promise for the diagnosis of infection, but improvements in specificity and sensitivity are needed. These improvements include the optimization of the quinolone structure; such chemistry efforts can be accelerated by refining microbiological assays.

Improving the Imaging Contrast of 68Ga-PSMA-11 by Targeted Linker Design: Charged Spacer Moieties Enhance the Pharmacokinetic Properties

⁶⁸Ga-Glu-urea-Lys-(Ahx)-HBED-CC (⁶⁸Ga-PSMA-11) represents a successful radiopharmaceutical for PET/CT imaging of prostate cancer. Further optimization of the tumor-to-background contrast might significantly enhance the sensitivity of PET/CT imaging and the probability of detecting recurrent prostate cancer at low PSA values. This study describes the advantage of histidine (H)/glutamic acid (E) and tryptophan (W)/glutamic acid (E) containing linkers on the pharmacokinetic properties of ⁶⁸Ga-PSMA-11. The tracers were obtained by a combination of standard Fmoc-based solid-phase synthesis and copper(I)-catalyzed azide-alkyne cycloaddition. Their ⁶⁸Ga complexes were compared to the clinical reference ⁶⁸Ga-PSMA-11 with respect to cell binding, effective internalization, and tumor targeting properties in LNCaP-bearing balb/c nu/nu mice. The introduction of (HE)_i (i = 1-3) or (WE)_i (i = 1-3) into PSMA-11 resulted in a significantly changed biodistribution profile. The uptake values in kidneys, spleen, liver, and other background organs were reduced for (HE)₃ while the tumor uptake was not affected. For (HE)₁ the tumor uptake was significantly increased. The introduction of tryptophan-containing linkers also modulated the organ distribution profile. The results clearly indicate that histidine is of essential impact in order to improve the tumor-to-organ contrast. Hence, the histidine/glutamic acid linker modifications considerably improved the pharmacokinetic properties of ⁶⁸Ga-PSMA-11 leading to a reduced uptake in dose limiting organs and a significantly enhanced tumor-to-background contrast. Glu-urea-Lys-(HE)₃-HBED-CC represents a promising ⁶⁸Ga complex ligand for PET/CT-imaging of prostate cancer.

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.

Ultrasmall inorganic nanoparticles: State-of-the-art and perspectives for biomedical applications

Ultrasmall nanoparticulate materials with core sizes in the 1-3nm range bridge the gap between single molecules and classical, larger-sized nanomaterials, not only in terms of spatial dimension, but also as regards physicochemical and pharmacokinetic properties. Due to these unique properties, ultrasmall nanoparticles appear to be promising materials for nanomedicinal applications. This review overviews the different synthetic methods of inorganic ultrasmall nanoparticles as well as their properties, characterization, surface modification and toxicity. We moreover summarize the current state of knowledge regarding pharmacokinetics, biodistribution and targeting of nanoscale materials. Aside from addressing the issue of biomolecular corona formation and elaborating on the interactions of ultrasmall nanoparticles with individual cells, we discuss the potential diagnostic, therapeutic and theranostic applications of ultrasmall nanoparticles in the emerging field of nanomedicine in the final part of this review.