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

177Lu-Labeled Bivalent Ligands of Prostate-Specific Membrane Antigen for Endoradiotherapy of Prostate Cancer

Recently, we developed a bivalent prostate-specific membrane antigen (PSMA) radioligand ([18F]AlF-Bi-PSMA), which showed higher tumor uptake and retention in PSMA-positive mouse models than the clinically used radioligands, [68Ga]Ga-PSMA-11 and [18F]PSMA-1007. Here, we developed two 177Lu-labeled bivalent PSMA ligands with (DOTA-Alb-Bi-PSMA) or without an albumin-binding motif (DOTA-Bi-PSMA) to enhance radiotherapeutic efficacy with minimal toxicity. The results demonstrated that both 177Lu-labeled bivalent radioligands showed good stability, high binding affinity, and PSMA-targeting specificity in vitro. Compared with [177Lu]Lu-PSMA-617, both [177Lu]Lu-Bi-PSMA and [177Lu]Lu-Alb-Bi-PSMA showed a higher area under the curve (AUC) of tumor accumulation and superior therapeutic efficacy. However, [177Lu]Lu-Alb-Bi-PSMA exhibited a dose-dependent increase in acute damage to kidneys. In terms of the radionuclide therapy efficacy and side effects, [177Lu]Lu-Bi-PSMA exhibited well-balanced action with high tumor-to-organs AUC ratios, resulting in remarkable therapeutic efficacy and negligible side effects. These promising results warrant further investigations to achieve the clinical translation of [177Lu]Lu-Bi-PSMA.

Carrier systems of radiopharmaceuticals and the application in cancer therapy

Radiopharmaceuticals play a vital role in cancer therapy. The carrier of radiopharmaceuticals can precisely locate and guide radionuclides to the target, where radionuclides kill surrounding tumor cells. Effective application of radiopharmaceuticals depends on the selection of an appropriate carrier. Herein, different types of carriers of radiopharmaceuticals and the characteristics are briefly described. Subsequently, we review radiolabeled monoclonal antibodies (mAbs) and their derivatives, and novel strategies of radiolabeled mAbs and their derivatives in the treatment of lymphoma and colorectal cancer. Furthermore, this review outlines radiolabeled peptides, and novel strategies of radiolabeled peptides in the treatment of neuroendocrine neoplasms, prostate cancer, and gliomas. The emphasis is given to heterodimers, bicyclic peptides, and peptide-modified nanoparticles. Last, the latest developments and applications of radiolabeled nucleic acids and small molecules in cancer therapy are discussed. Thus, this review will contribute to a better understanding of the carrier of radiopharmaceuticals and the application in cancer therapy.

GRPr Theranostics: Current Status of Imaging and Therapy using GRPr Targeting Radiopharmaceuticals

Addressing molecular targets, that are overexpressed by various tumor entities, using radiolabeled molecules for a combined diagnostic and therapeutic (theranostic) approach is of increasing interest in oncology. The gastrin-releasing peptide receptor (GRPr), which is part of the bombesin family, has shown to be overexpressed in a variety of tumors, therefore, serving as a promising target for those theranostic applications. A large amount of differently radiolabeled bombesin derivatives addressing the GRPr have been evaluated in the preclinical as well as clinical setting showing fast blood clearance and urinary excretion with selective GRPr-binding. Most of the available studies on GRPr-targeted imaging and therapy have evaluated the theranostic approach in prostate and breast cancer applying bombesin derivatives tagged with the predominantly used theranostic pair of ⁶⁸Ga/¹⁷⁷Lu which is the focus of this review.

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

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.

More Than Meets the Eye: Scientific Rationale behind Molecular Imaging and Therapeutic Targeting of Prostate-Specific Membrane Antigen (PSMA) in Metastatic Prostate Cancer and Beyond

Simple Summary

Prostate-specific membrane antigen (PSMA) is a transmembrane protein that is overexpressed in prostate cancer and correlates with the aggressiveness of the disease. PSMA is a promising target for imaging and therapeutics in prostate cancer patients validated in prospective trials. However, the role of PSMA in prostate cancer progression is poorly understood. In this review, we discuss the biology and scientific rationale behind the use of PSMA and other targets in the detection and theranostics of metastatic prostate cancer.

Abstract

Prostate cancer is the second most common cancer type in men globally. Although the prognosis for localized prostate cancer is good, no curative treatments are available for metastatic disease. Better diagnostic methods could help target therapies and improve the outcome. Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein that is overexpressed on malignant prostate tumor cells and correlates with the aggressiveness of the disease. PSMA is a clinically validated target for positron emission tomography (PET) imaging-based diagnostics in prostate cancer, and during recent years several therapeutics have been developed based on PSMA expression and activity. The expression of PSMA in prostate cancer can be very heterogeneous and some metastases are negative for PSMA. Determinants that dictate clinical responses to PSMA-targeting therapeutics are not well known. Moreover, it is not clear how to manipulate PSMA expression for therapeutic purposes and develop rational treatment combinations. A deeper understanding of the biology behind the use of PSMA would help the development of theranostics with radiolabeled compounds and other PSMA-based therapeutic approaches. Along with PSMA several other targets have also been evaluated or are currently under investigation in preclinical or clinical settings in prostate cancer. Here we critically elaborate the biology and scientific rationale behind the use of PSMA and other targets in the detection and therapeutic targeting of metastatic prostate cancer.

[68Ga]Ga-iPSMA-Lys3-Bombesin: Biokinetics, dosimetry and first patient PET/CT imaging

BACKGROUND

The prostate-specific membrane antigen (PSMA) and the gastrin-releasing peptide receptor (GRPR) are overexpressed in prostate cancer (PCa). In preclinical studies, the iPSMA-Lys3-Bombesin (iPSMA-BN) heterodimeric ligand has shown a suitable affinity for PSMA and GRPR. This research aimed to assess the biokinetics and radiation dosimetry of [68Ga]Ga-iPSMA-BN in four healthy volunteers based on biodistribution data obtained from whole-body PET/CT studies, as well as to visualize the [68Ga]Ga-iPSMA-BN tumor uptake in a patient with PCa.

METHODS

PET/CT images acquired at 5 min, 0.5, 1, and 2 h after radiotracer administration (124.5 ± 2.1 MBq) were corrected for attenuation, scattering, dead-time, and decay. The activity in the segmented volumes of interest (VOIs) in each source organ at different times was adjusted to mono- and bi-exponential biokinetic models (A(t)VOI), from which the total disintegrations (N) were calculated to assess the internal radiation doses by using the OLINDA V1.1 code.

RESULTS

Images from the patient showed an evident uptake by the metastasis (SUVmax of 4.7) and by the organs expressing GRPR (pancreas) and PSMA (salivary glands). The average effective dose was 2.70 ± 0.05 mSv, which was like those known for most of the 68Ga studies, making [68Ga]Ga-iPSMA-BN a promising dual-target PET imaging radiotracer for PCa.

CONCLUSIONS

[68Ga]Ga-iPSMA-BN, capable of detecting both PSMA and GRPR with suitable biokinetics and dosimetric patterns, could be a potential complementary diagnostic tool for the improvement of prostate cancer PET imaging.

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.

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.

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.

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.

Synthesis and Evaluation of 177Lu-DOTA-DN(PTX)-BN for Selective and Concomitant Radio and Drug-Therapeutic Effect on Breast Cancer Cells

The peptide-receptor radionuclide therapy (PRRT) is a successful approach for selectively delivering radiation within tumor sites through specific recognition of radiolabeled peptides by overexpressed receptors on cancer cell surfaces. The efficacy of PRRT could be improved by using polymeric radio- and drug- therapy nanoparticles for a concomitant therapeutic effect on malignant cells. This research aimed to prepare and evaluate, a novel drug and radiation delivery nanosystem based on the ¹⁷⁷Lu-labeled polyamidoamine (PAMAM) dendrimer (DN) loaded with paclitaxel (PTX) and functionalized on the surface with the Lys¹Lys³(DOTA)-bombesin (BN) peptide for specific targeting to gastrin-releasing peptide receptors (GRPr) overexpressed on breast cancer cells. DN was first conjugated covalently to BN and DOTA (chemical moiety for lutetium-177 complexing) and subsequently loaded with PTX. The characterization by microscopic and spectroscopic techniques, in-vitro drug delivery tests as well as in in-vitro and in-vivo cellular uptake of ¹⁷⁷Lu-DOTA-DN(PTX)-BN by T47D breast cancer cells (GRPr-positive), indicated the formation of an improved delivery nanosystem with target-specific recognition by GRPr. Results of the ¹⁷⁷Lu-DOTA-DN(PTX)-BN effect on T47D cell viability (1.3%, compared with 10.9% of ¹⁷⁷Lu-DOTA-DN-BN and 14.0% of DOTA-DN-(PTX)-BN) demonstrated the concomitant radiotherapeutic and chemotherapeutic properties of the polymeric nanosystem as a potential agent for the treatment of GRPr-positive tumors.

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.

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.