Heterobivalent agents targeting PSMA and integrin-αvβ3

Current Status and Perspectives of Novel Radiopharmaceuticals with Heterologous Dual-targeted Functions: 2013-2023

Radiotracers provide molecular- and cellular-level information in a noninvasive manner and have become important tools for precision medicine. In particular, the successful clinical application of radioligand therapeutic (RLT) has further strengthened the role of nuclear medicine in clinical treatment. The complicated microenvironment of the lesion has rendered traditional single-targeted radiopharmaceuticals incapable of fully meeting the requirements. The design and development of dual-targeted and multitargeted radiopharmaceuticals have rapidly emerged. In recent years, significant progress has been made in the development of heterologous dual-targeted radiopharmaceuticals. This perspective aims to provide a comprehensive overview of the recent progress in these heterologous dual-targeted radiopharmaceuticals, with a special focus on the design of ligand structures, pharmacological properties, and preclinical and clinical evaluation. Furthermore, future directions are discussed from this perspective.

PSMA-targeted radiotheranostics in modern nuclear medicine: then, now, and what of the future?

In 1853, the perception of prostate cancer (PCa) as a rare ailment prevailed, was described by the eminent Londoner surgeon John Adams. Rapidly forward to 2018, the landscape dramatically altered. Currently, men face a one-in-nine lifetime risk of PCa, accentuated by improved diagnostic methods and an ageing population. With more than three million men in the United States alone grappling with this disease, the overall risk of succumbing to stands at one in 39. The intricate clinical and biological diversity of PCa poses serious challenges in terms of imaging, ongoing monitoring, and disease management. In the field of theranostics, diagnostic and therapeutic approaches that harmoniously merge targeted imaging with treatments are integrated. A pivotal player in this arena is radiotheranostics, employing radionuclides for both imaging and therapy, with prostate-specific membrane antigen (PSMA) at the forefront. Clinical milestones have been reached, including FDA- and/or EMA-approved PSMA-targeted radiodiagnostic agents, such as [18F]DCFPyL (PYLARIFY®, Lantheus Holdings), [18F]rhPSMA-7.3 (POSLUMA®, Blue Earth Diagnostics) and [68Ga]Ga-PSMA-11 (Locametz®, Novartis/ ILLUCCIX®, Telix Pharmaceuticals), as well as PSMA-targeted radiotherapeutic agents, such as [177Lu]Lu-PSMA-617 (Pluvicto®, Novartis). Concurrently, ligand-drug and immune therapies designed to target PSMA are being advanced through rigorous preclinical research and clinical trials. This review delves into the annals of PSMA-targeted radiotheranostics, exploring its historical evolution as a signature molecule in PCa management. We scrutinise its clinical ramifications, acknowledge its limitations, and peer into the avenues that need further exploration. In the crucible of scientific inquiry, we aim to illuminate the path toward a future where the enigma of PCa is deciphered and where its menace is met with precise and effective countermeasures. In the following sections, we discuss the intriguing terrain of PCa radiotheranostics through the lens of PSMA, with the fervent hope of advancing our understanding and enhancing clinical practice.

Prostate-specific membrane antigen-targeted surgery in prostate cancer: Accurate identification, real-time diagnosis, and precise resection

Radical prostatectomy (RP) combined with pelvic lymph node dissection (PLND) is the first step in multimodal treatment of prostate cancer (PCa) without distant metastases. For a long time, the surgical resection range has been highly dependent on the surgeon's visualization and experience with preoperative imaging. With the rapid development of prostate-specific membrane antigen positron emission tomography and single-photon emission computed tomography (PSMA-PET and PSMA-SPECT), PSMA-targeted surgery has been introduced for a more accurate pathological diagnosis and complete resection of positive surgical margins (PSMs) and micro-lymph node metastases (LNMs). We reviewed PSMA-targeted surgeries, including PSMA-PET-guided prostatic biopsy (PSMA-TB), PSMA-targeted radio-guided surgery (PSMA-RGS), PSMA-targeted fluorescence-guided surgery (PSMA-FGS), and multi-modality/multi-targeted PSMA-targeted surgery. We also discuss the strengths and challenges of PSMA-targeted surgery, and propose that PSMA-targeted surgery could be a great addition to existing surgery protocols, thereby improving the accuracy and convenience of surgery for primary and recurrent PCa in the near future.

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.

Radiolabeling and in vivo evaluation of lanmodulin with biomedically relevant lanthanide isotopes

Short-lived, radioactive lanthanides comprise an emerging class of radioisotopes attractive for biomedical imaging and therapy applications. To deliver such isotopes to target tissues, they must be appended to entities that target antigens overexpressed on the target cell's surface. However, the thermally sensitive nature of biomolecule-derived targeting vectors requires the incorporation of these isotopes without the use of denaturing temperatures or extreme pH conditions; chelating systems that can capture large radioisotopes under mild conditions are therefore highly desirable. Herein, we demonstrate the successful radiolabeling of the lanthanide-binding protein, lanmodulin (LanM), with medicinally relevant radioisotopes: ¹⁷⁷Lu, ^132/135La and ⁸⁹Zr. Radiolabeling of the endogenous metal-binding sites of LanM, as well exogenous labeling of a protein-appended chelator, was successfully conducted at 25 °C and pH 7 with radiochemical yields ranging from 20-82%. The corresponding radiolabeled constructs possess good formulation stability in pH 7 MOPS buffer over 24 hours (>98%) in the presence of 2 equivalents of ^natLa carrier. In vivo experiments with [¹⁷⁷Lu]-LanM, [^132/135La]-LanM, and a prostate cancer targeting-vector linked conjugate, [^132/135La]-LanM-PSMA, reveal that endogenously labeled constructs produce bone uptake in vivo. Exogenous, chelator-tag mediated radiolabeling to produce [⁸⁹Zr]-DFO-LanM enables further study of the protein's in vivo behavior, demonstrating low bone and liver uptake, and renal clearance of the protein itself. While these results indicate that additional stabilization of LanM is required, this study establishes precedence for the radiochemical labeling of LanM with medically relevant lanthanide radioisotopes.

Synthesis and preclinical evaluation of a heterodimeric radioligand targeting fibroblast activation protein and integrin-αvβ3

Tumor progression is accompanied by intrinsic heterogeneity and different phenotypes, which implies a different expression of cell surface receptors. Fibroblast activation protein (FAP) and integrin α_vβ₃ are highly expressed in the cell surface of cancer-associated cells or cancer cells compared with normal cells. Therefore, a FAP/integrin α_vβ₃ bispecific heterodimer was developed for positron emission tomography (PET) diagnostic imaging and radiotherapy. The heterodimer DOTA-FAPI-RGD was labeled with the diagnostic radionuclide gallium-68 or the therapeutic radionuclide lutetium-177, with yields >80%, and high stability. The competitive displacement binding assay showed an IC₅₀ = 6.8 ± 0.6 nM for DOTA-FAPI-RGD towards FAP and IC₅₀ = 2.1 ± 0.4 nM towards integrin α_vβ₃. Radionuclide labeled DOTA-FAPI-RGD showed high specificity and rapid internalization into U87MG cells (FAP/α_vβ₃-positive) in vitro. Micro-PET and biodistribution studies of [⁶⁸Ga]Ga-DOTA-FAPI-RGD in tumor-bearing mice demonstrated that a high and specific tumor uptake of the tracer and a fast body clearance, resulting in high contrast images. In addition to the imaging applications demonstrated in this study, the labeling of the heterodimeric ligand with the radionuclide lutetium-177 used in cancer treatment might allow the therapeutic application of this ligand.

Hetero-bivalent agents targeting FAP and PSMA

PURPOSE

We developed a theranostic radiopharmaceutical that engages two key cell surface proteases, fibroblast activation protein alpha (FAP) and prostate-specific membrane antigen (PSMA), each frequently overexpressed within the tumor microenvironment (TME). The latter is also expressed in most prostate tumor epithelium. To engage a broader spectrum of cancers for imaging and therapy, we conjugated small-molecule FAP and PSMA-targeting moieties using an optimized linker to provide ⁶⁴Cu-labeled compounds.

METHODS

We synthesized FP-L1 and FP-L2 using two linker constructs attaching the FAP and PSMA-binding pharmacophores. We determined in vitro inhibition constants (K_i) for FAP and PSMA. Cell uptake assays and flow cytometry were conducted in human glioma (U87), melanoma (SK-MEL-24), prostate cancer (PSMA + PC3 PIP and PSMA - PC3 flu), and clear cell renal cell carcinoma lines (PSMA + /PSMA - 786-O). Quantitative positron emission tomography/computed tomography (PET/CT) and tissue biodistribution studies were performed using U87, SK-MEL-24, PSMA + PC3 PIP, and PSMA + 786-O experimental xenograft models and the KPC genetically engineered mouse model of pancreatic cancer.

RESULTS

⁶⁴Cu-FP-L1 and ⁶⁴Cu-FP-L2 were produced in high radiochemical yields (> 98%) and molar activities (> 19 MBq/nmol). K_i values were in the nanomolar range for both FAP and PSMA. PET imaging and biodistribution studies revealed high and specific targeting of ⁶⁴Cu-FP-L1 and ⁶⁴Cu-FP-L2 for FAP and PSMA. ⁶⁴Cu-FP-L1 displayed more favorable pharmacokinetics than ⁶⁴Cu-FP-L2. In the U87 tumor model at 2 h post-injection, tumor uptake of ⁶⁴Cu-FP-L1 (10.83 ± 1.02%ID/g) was comparable to ⁶⁴Cu-FAPI-04 (9.53 ± 2.55%ID/g). ⁶⁴Cu-FP-L1 demonstrated high retention 5.34 ± 0.29%ID/g at 48 h in U87 tumor. Additionally, ⁶⁴Cu-FP-L1 showed high retention in PSMA + PC3 PIP tumor (12.06 ± 0.78%ID/g at 2 h and 10.51 ± 1.82%ID/g at 24 h).

CONCLUSIONS

⁶⁴Cu-FP-L1 demonstrated high and specific tumor targeting of FAP and PSMA. This compound should enable imaging of lesions expressing FAP, PSMA, or both on the tumor cell surface or within the TME. FP-L1 can readily be converted into a theranostic for the management of heterogeneous tumors.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

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.

Visualizing Tumors in Real Time: A Highly Sensitive PSMA Probe for NIR-II Imaging and Intraoperative Tumor Resection

Owing to the complex anatomical structure, precise resection of a tumor while maintaining adjacent tissue is a challenge in radical prostatectomy for prostate cancer (PCa). Optical imaging in near-infrared window II (NIR-II) is a promising technology for intraoperative guidance, whereas there is no available probe for PCa yet. In this article, a novel probe (PSMA-1092) bearing two prostate-specific membrane antigen (PSMA) binding motifs was developed, displaying excellent optical properties (λ_max = 1092 nm) and ultrahigh affinity (K_i = 80 pM) toward PSMA. The tumor was visualized with high resolution (tissue-to-normal tissue ratio = 7.62 ± 1.05) and clear margin by NIR-II imaging using PSMA-1092 in a mouse model. During the tumor resection, residual tumors missed by visible inspection were detected by the real-time imaging. Overall, PSMA-1092 displayed excellent performance in delineating the tumor margin and detecting residual tumors, demonstrating promising potential for precise PCa tumor resection in clinical practice.

Inhibitors of prostate-specific membrane antigen in the diagnosis and therapy of metastatic prostate cancer - a review of patent literature

INTRODUCTION

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II, is a potential target protein for imaging and treatment of patients with prostate cancer because of its overexpression during metastasis. Various PSMA-targeted imaging and therapeutic probes have been designed and synthesized based on the Lys-urea-Glu motif. Structural modifications have been made exclusively in the linker region, while maintaining the Lys-urea-Glu structure that interacts with S1 and S1' pockets.

AREA COVERED

This review includes WIPO-listed patents (from January 2017 to June 2020) reporting PSMA-targeted probes based on the Lys-urea-Glu or Glu-urea-Glu structure.

EXPERT OPINION

: PSMA-targeted imaging agents labeled with radionuclides such as fluorine-18, copper-64, gallium-68, and technetium-99m have been successfully translated into clinical phase for the early diagnosis of metastatic prostate cancer. Recently, PSMA-targeted therapeutic agents labeled with iodine-131, lutetium-177, astatine-211, and lead-212 have also been developed with notable progress. Most PSMA-targeted agents are based on the Lys-urea-Glu or Glu-urea-Glu structure, demonstrate strong PSMA-binding affinity in nanomolar range, and achieve diverse structural modifications in the non-pharmacophore pocket. By exploiting the S1 accessory pocket or the tunnel region of the PSMA active site, the in vivo efficacy and pharmacokinetic profiles of the PMSA-targeted agents can be effectively modulated.

RGD-Binding Integrins Revisited: How Recently Discovered Functions and Novel Synthetic Ligands (Re-)Shape an Ever-Evolving Field

Integrins have been extensively investigated as therapeutic targets over the last decades, which has been inspired by their multiple functions in cancer progression, metastasis, and angiogenesis as well as a continuously expanding number of other diseases, e.g., sepsis, fibrosis, and viral infections, possibly also Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). Although integrin-targeted (cancer) therapy trials did not meet the high expectations yet, integrins are still valid and promising targets due to their elevated expression and surface accessibility on diseased cells. Thus, for the future successful clinical translation of integrin-targeted compounds, revisited and innovative treatment strategies have to be explored based on accumulated knowledge of integrin biology. For this, refined approaches are demanded aiming at alternative and improved preclinical models, optimized selectivity and pharmacological properties of integrin ligands, as well as more sophisticated treatment protocols considering dose fine-tuning of compounds. Moreover, integrin ligands exert high accuracy in disease monitoring as diagnostic molecular imaging tools, enabling patient selection for individualized integrin-targeted therapy. The present review comprehensively analyzes the state-of-the-art knowledge on the roles of RGD-binding integrin subtypes in cancer and non-cancerous diseases and outlines the latest achievements in the design and development of synthetic ligands and their application in biomedical, translational, and molecular imaging approaches. Indeed, substantial progress has already been made, including advanced ligand designs, numerous elaborated pre-clinical and first-in-human studies, while the discovery of novel applications for integrin ligands remains to be explored.

Development of Heterobivalent Theranostic Probes Having High Affinity/Selectivity for the GRPR/PSMA

In this study, we describe the development of heterobivalent [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] radiotracers that display very high selectivity/specificity for gastrin-releasing peptide receptor (GRPR)-/prostate-specific membrane antigen (PSMA)-expressing cells. These studies include metallation, purification, characterization, and in vitro and in vivo evaluation of the new small-molecule-/peptide-based radiopharmaceuticals having utility for imaging and potentially therapy. Competitive displacement binding assays using PC-3 cells and LNCaP cell membranes showed high binding affinity for the GRPR or the PSMA. Biodistribution studies showed favorable excretion pharmacokinetics with high tumor uptake in PC-3 or PC-3 prostatic inhibin peptide (PIP) tumor-bearing mice. For example, tumor accumulation at the 1 h time point ranged from (4.74 ± 0.90) to (7.51 ± 2.61)%ID/g. Micro-single-photon emission computed tomography (microSPECT) molecular imaging investigations showed very high uptake in tumors with minimal accumulation of tracers in the surrounding collateral tissues in xenografted mice at 4 h postintravenous injection. In conclusion, [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] tracers displayed favorable pharmacokinetic and excretion profiles with high uptake and retention in tumors.

Image-Guided Surgery: Are We Getting the Most Out of Small-Molecule Prostate-Specific-Membrane-Antigen-Targeted Tracers?

Expressed on virtually all prostate cancers and their metastases, the transmembrane protein prostate-specific membrane antigen (PSMA) provides a valuable target for the imaging of prostate cancer. Not only does PSMA provide a target for noninvasive diagnostic imaging, e.g., PSMA-positron emission tomography (PSMA-PET), it can also be used to guide surgical resections of PSMA-positive lesions. The latter characteristic has led to the development of a plethora of PSMA-targeted tracers, i.e., radiolabeled, fluorescent, or hybrid. With image-guided surgery applications in mind, this review discusses these compounds based on clinical need. Here, the focus is on the chemical aspects (e.g., imaging label, spacer moiety, and targeting vector) and their impact on in vitro and in vivo tracer characteristics (e.g., affinity, tumor uptake, and clearance pattern).

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 Aptamer Induced Artificial Protein-Pairing: A Strategy for Selective Inhibition of Receptor Function

Cell surface receptors play a critical role in modulating intracellular signal transduction, making them important drug targets. However, it remains challenging to develop a selective and efficient strategy for regulating receptor function. Herein, we develop a strategy, called bispecific aptamer induced artificial protein-pairing, to selectively regulate receptor function. In this strategy, bispecific aptamer probes act as molecular mediators to bind to both a target receptor protein and a paired protein, which brings the two proteins into close proximity on the living cell membrane. Importantly, the paired proteins work not only as a cancer biomarker for enhancing cell selectivity but also as a blocking assistant to inhibit target receptor function via strong steric hindrance effect. Compared with single-aptamer-mediated regulation, the proposed bispecific aptamer probes afford substantial improvement in selective and efficient regulation of receptor function and downstream signaling pathways. This work offers a versatile methodology to design molecular mediators that can modulate receptor function, thereby providing a new way for developing novel therapeutic drugs.

Dual-modal in vivo fluorescence and photoacoustic imaging using a heterodimeric peptide

A heterodimeric peptide labeled with IRDye800 is used to perform dual-modal imaging of human esophageal xenograft tumors in vivo. Fluorescence and photoacoustic images provide complementary visualization of tumor dimensions in planar and sagittal views, respectively, demonstrating promise for targeted cancer diagnosis and staging.

Prostate cancer: updates on current strategies for screening, diagnosis and clinical implications of treatment modalities

Prostate cancer is the most common cancer in men by way of diagnosis and a leading cause of cancer-related deaths. Early detection and intervention remains key to its optimum clinical management. This review provides the most updated information on the recent methods of prostate cancer screening, imaging and treatment modalities. Wherever possible, clinical trial data has been supplemented to provide a comprehensive overview of current prostate cancer research and development. Considering the recent success of immunotherapy in prostate cancer, we discuss cell, DNA and viruses based, as well as combinatorial immunotherapeutic strategies in detail. Furthermore, the potential of nanotechnology is increasingly being realized, especially in prostate cancer research, and we provide an overview of nanotechnology-based strategies, with special emphasis on nanotheranostics and multifunctional nanoconstructs. Understanding these recent developments is critical to the design of future therapeutic strategies to counter prostate cancer.

Clinical translation of immunoliposomes for cancer therapy: recent perspectives

INTRODUCTION

Liposomes have been extensively investigated as drug delivery vehicles. Immunoliposomes (ILs) are antibody-conjugated liposomes designed to selectively target antigen-expressing cells. ILs can be used to deliver drugs to tumor cells for improving efficacy and reducing toxicity. In addition, ILs can be used in immunoassays, immunotherapy, and imaging. Although there has been extensive coverage on ILs in the literature, only a limited number of clinical trials have been reported and no IL drug has been approved by the FDA.

AREAS COVERED

Factors to consider in developing ILs are discussed, including the choice of antibody or antibody fragment, the formulation of liposomes, and the conjugation chemistry. In addition, challenges and opportunities in clinical development of ILs are discussed. The purpose of this review is to provide an overview on the state of the art of ILs and to discuss potential future developments.

EXPERT OPINION

IL research has had a lengthy history and numerous preclinical studies have yielded encouraging results. However, there are a number of obstacles to clinical translation of ILs. Given the unique capabilities of ILs, its potential for clinical application is underexplored. There is great potential for expanded role for ILs in the clinic and further efforts to this end are warranted.

ABBREVIATIONS

Ab: antibody; ADCs: antibody-drug conjugates; API: active pharmaceutical ingredient; ADCC: antibody-dependent cellular cytotoxicity; CR: complete remission; cGMP: current good manufacturing practice; DSPE: distearoyl phosphatidylethanolamine; EGF: epidermal growth factor; EGFR: epidermal growth factor receptor; EPR: enhanced permeability and retention; Fc: fragment crystalline; Tf: transferrin; HACA: human-anti-chimeric antibody; HAHA: human-anti-human antibody; HAMA: human-anti-mouse antibody; HER2: human epidermal growth factor 2; IL: immunoliposome; LNPs: lipid nanoparticles; MRI: magnetic resonance imaging; MTD: maximum tolerated dose; PEG: polyethylene glycol; PET: positron emission tomography; PR: partial response; PSMA: prostate-specific membrane antigen; scFv: single-chain variable fragment; SPECT: single photon emission computed tomography; TTR: transthyretin.

Multiplexed Targeting of Barrett's Neoplasia with a Heterobivalent Ligand: Imaging Study on Mouse Xenograft in Vivo and Human Specimens ex Vivo

Esophageal adenocarcinoma (EAC) is a molecularly heterogeneous disease that is rising rapidly in incidence and has poor prognosis. We developed a heterobivalent peptide to target detection of early Barrett's neoplasia by combining monomer heptapeptides specific for either EGFR or ErbB2 in a heterodimer configuration. The structure of a triethylene glycol linker was optimized to maximize binding interactions to the surface receptors on cells. The Cy5.5-labeled heterodimer QRH*-KSP*-E3-Cy5.5 demonstrated specific binding to each target and showed 3-fold greater fluorescence intensity and 2-fold higher affinity compared with those of either monomer alone. Peak uptake in xenograft tumors was observed at 2 h postinjection with systemic clearance by ∼24 h in vivo. Furthermore, ligand binding was evaluated on human esophageal specimens ex vivo, and 88% sensitivity and 87% specificity were found for the detection of either high-grade dysplasia (HGD) or EAC. This peptide heterodimer shows promise for targeted detection of early Barrett's neoplasia in clinical study.

PSMA-Targeted Theranostic Nanocarrier for Prostate Cancer

Herein, we report the use of a theranostic nanocarrier (Folate-HBPE(CT20p)) to deliver a therapeutic peptide to prostate cancer tumors that express PSMA (folate hydrolase 1). The therapeutic peptide (CT20p) targets and inhibits the chaperonin-containing TCP-1 (CCT) protein-folding complex, is selectively cytotoxic to cancer cells, and is non-toxic to normal tissue. With the delivery of CT20p to prostate cancer cells via PSMA, a dual level of cancer specificity is achieved: (1) selective targeting to PSMA-expressing prostate tumors, and (2) specific cytotoxicity to cancer cells with minimal toxicity to normal cells. The PSMA-targeting theranostic nanocarrier can image PSMA-expressing cells and tumors when a near infrared dye is used as cargo. Meanwhile, it can be used to treat PSMA-expressing tumors when a therapeutic, such as the CT20p peptide, is encapsulated within the nanocarrier. Even when these PSMA-targeting nanocarriers are taken up by macrophages, minimal cell death is observed in these cells, in contrast with doxorubicin-based therapeutics that result in significant macrophage death. Incubation of PSMA-expressing prostate cancer cells with the Folate-HBPE(CT20p) nanocarriers induces considerable changes in cell morphology, reduction in the levels of integrin β1, and lower cell adhesion, eventually resulting in cell death. These results are relevant as integrin β1 plays a key role in prostate cancer invasion and metastatic potential. In addition, the use of the developed PSMA-targeting nanocarrier facilitates the selective in vivo delivery of CT20p to PSMA-positive tumor, inducing significant reduction in tumor size.

The therapeutic and diagnostic potential of the prostate specific membrane antigen/glutamate carboxypeptidase II (PSMA/GCPII) in cancer and neurological disease

Prostate specific membrane antigen (PSMA) otherwise known as glutamate carboxypeptidase II (GCPII) is a membrane bound protein that is highly expressed in prostate cancer and in the neovasculature of a wide variety of tumours including glioblastomas, breast and bladder cancers. This protein is also involved in a variety of neurological diseases including schizophrenia and ALS. In recent years, there has been a surge in the development of both diagnostics and therapeutics that take advantage of the expression and activity of PSMA/GCPII. These include gene therapy, immunotherapy, chemotherapy and radiotherapy. In this review, we discuss the biological roles that PSMA/GCPII plays, both in normal and diseased tissues, and the current therapies exploiting its activity that are at the preclinical stage. We conclude by giving an expert opinion on the future direction of PSMA/GCPII based therapies and diagnostics and hurdles that need to be overcome to make them effective and viable.

Design, synthesis and biological evaluation of PSMA/hepsin-targeted heterobivalent ligands

Cell surface biomarkers such as prostate-specific membrane antigen (PSMA) and hepsin have received considerable attention as targets for imaging prostate cancer (PCa) due to their high cell surface expression in such tumors and easy access for imaging probes. Novel amidine-containing indole analogs (13-21) as hepsin inhibitors were designed and synthesized. These compounds showed in vitro inhibitory activity against hepsin with IC50 values from 5.9 to 70 μM. Based on the SAR of amidine-derived analogs, the novel heterobivalent compound 30, targeting both hepsin and PSMA, was synthesized by linking compound 18 with Lys-urea-Glu, the key scaffold for the specific binding to PSMA, followed by the conjugation of the optical dye SulfoCy7. Compound 30 exhibited inhibitory activities against PSMA and hepsin, with IC50 values of 28 nM and 2.8 μM, respectively. In vitro cell uptake and preliminary in vivo optical imaging studies of 30 showed selective binding and retention in both PSMA and hepsin high-expressing PC3/ML-PSMA-HPN cells as compared with low-expressing PC3/ML cells.

Critical Length of PEG Grafts on lPEI/DNA Nanoparticles for Efficient in Vivo Delivery

Nanoparticle-mediated gene delivery is a promising alternative to viral methods; however, its use in vivo, particularly following systemic injection, has suffered from poor delivery efficiency. Although PEGylation of nanoparticles has been successfully demonstrated as a strategy to enhance colloidal stability, its success in improving delivery efficiency has been limited, largely due to reduced cell binding and uptake, leading to poor transfection efficiency. Here we identified an optimized PEGylation scheme for DNA micellar nanoparticles that delivers balanced colloidal stability and transfection activity. Using linear polyethylenimine (lPEI)-g-PEG as a carrier, we characterized the effect of graft length and density of polyethylene glycol (PEG) on nanoparticle assembly, micelle stability, and gene delivery efficiency. Through variation of PEG grafting degree, lPEI with short PEG grafts (molecular weight, MW 500–700 Da) generated micellar nanoparticles with various shapes including spherical, rodlike, and wormlike nanoparticles. DNA micellar nanoparticles prepared with short PEG grafts showed comparable colloidal stability in salt and serum-containing media to those prepared with longer PEG grafts (MW 2 kDa). Corresponding to this trend, nanoparticles prepared with short PEG grafts displayed significantly higher in vitro transfection efficiency compared to those with longer PEG grafts. More importantly, short PEG grafts permitted marked increase in transfection efficiency following ligand conjugation to the PEG terminal in metastatic prostate cancer-bearing mice. This study identifies that lPEI-g-PEG with short PEG grafts (MW 500–700 Da) is the most effective to ensure shape control and deliver high colloidal stability, transfection activity, and ligand effect for DNA nanoparticles in vitro and in vivo following intravenous administration.

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

A new series of bispecific radioligands (BRLs) targeting prostate-specific membrane antigen (PSMA) and gastrin releasing peptide receptor (GRPr), both expressed on prostate cancer cells, was developed. Their design was based on the bombesin (BN) analogue, H2N-PEG2-[D-Tyr(6),β-Ala(11),Thi(13),Nle(14)]BN(6-14), which binds to GRPr with high affinity and specificity, and the peptidomimetic urea-based pseudoirreversible inhibitor of PSMA, Glu-ureido-Lys. The two pharmacophores were coupled through copper(I)-catalyzed azide-alkyne cycloaddition to the bis(tetrafluorophenyl) ester of the chelating agent HBED-CC via amino acid linkers made of positively charged His (H) and negatively charged Glu (E): -(HE)n- (n = 0-3). The BRLs were labeled with (68)Ga, and their preliminary pharmacological properties were evaluated in vitro (competitive and time kinetic binding assays) on prostate cancer (PC-3, LNCaP) and rat pancreatic (AR42J) cell lines and in vivo by biodistribution and small animal PET imaging studies in both normal and tumor-bearing mice. The IC50/Ki values determined for all BRLs essentially matched those of the respective monomers. The maximal cellular uptake of the BLRs was observed between 20 and 30 min. The BRLs showed a synergistic ability in vivo by targeting both PSMA (LNCaP) and GRPr (PC-3) positive tumors, whereas the charged -(HE)n- (n = 1-3) linkers significantly reduced the kidney and spleen uptake. The bispecific (PSMA and GRPr) targeting ability and optimized pharmacokinetics of the compounds developed in this study could lead to their future application in clinical practice as more sensitive radiotracers for noninvasive imaging of prostate cancer (PCa) by PET/CT and PET/MRI.

Prostate-specific membrane antigen as a target for cancer imaging and therapy

The prostate-specific membrane antigen (PSMA) is a molecular target whose use has resulted in some of the most productive work toward imaging and treating prostate cancer over the past two decades. A wide variety of imaging agents extending from intact antibodies to low-molecular-weight compounds permeate the literature. In parallel there is a rapidly expanding pool of antibody-drug conjugates, radiopharmaceutical therapeutics, small-molecule drug conjugates, theranostics and nanomedicines targeting PSMA. Such productivity is motivated by the abundant expression of PSMA on the surface of prostate cancer cells and within the neovasculature of other solid tumors, with limited expression in most normal tissues. Animating the field is a variety of small-molecule scaffolds upon which the radionuclides, drugs, MR-detectable species and nanoparticles can be placed with relative ease. Among those, the urea-based agents have been most extensively leveraged, with expanding clinical use for detection and more recently for radiopharmaceutical therapy of prostate cancer, with surprisingly little toxicity. PSMA imaging of other cancers is also appearing in the clinical literature, and may overtake FDG for certain indications. Targeting PSMA may provide a viable alternative or first-line approach to managing prostate and other cancers.

Electrophile-integrating Smiles rearrangement provides previously inaccessible C4'-O-alkyl heptamethine cyanine fluorophores

New synthetic methods to rapidly access useful fluorophores are needed to advance modern molecular imaging techniques. A new variant of the classical Smiles rearrangement is reported that enables the efficient synthesis of previously inaccessible C4′-O-alkyl heptamethine cyanines. The key reaction involves N- to O- transposition with selective electrophile incorporation on nitrogen. A representative fluorophore exhibits excellent resistance to thiol nucleophiles, undergoes productive bioconjugation, and can be used in near-IR fluorescence imaging applications.

Molecular-genetic imaging of cancer

Molecular-genetic imaging of cancer using nonviral delivery systems has great potential for clinical application as a safe, efficient, noninvasive tool for visualization of various cellular processes including detection of cancer, and its attendant metastases. In recent years, significant effort has been expended in overcoming technical hurdles to enable clinical adoption of molecular-genetic imaging. This chapter will provide an introduction to the components of molecular-genetic imaging and recent advances on each component leading to safe, efficient clinical applications for detecting cancer. Combination with therapy, namely, generating molecular-genetic theranostic constructs, will provide further impetus for clinical translation of this promising technology.