A low molecular weight PSMA-based fluorescent imaging agent for cancer

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.

Membrane-derived particles shed by PSMA-positive cells function as pro-angiogenic stimuli in tumors

Cell membrane-derived particles (Mp) are rounded membrane-enclosed particles that are shed from tumor cells. Mp are formed from tumor membranes and are capable of tumor targeting and immunotherapeutic agents because they share membrane homology with parental cells; thus, they are under consideration as a drug delivery vehicle. Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein with enzymatic functionality, is highly expressed in Mp and extracellular vesicles (EV) from prostate cancer (PCa) with poor clinical prognosis. Although PSMA expression was previously shown in EV and Mp isolated from cell lines and from the blood of patients with high-grade PCa, no pathophysiological effects have been linked to PCa-derived Mp. Here, we compared Mp from PSMA-expressing (PSMA-Mp) and PSMA-non-expressing (WT-Mp) cells side by side in vitro and in vivo. PSMA-Mp can transfer PSMA and new phenotypic characteristics to the tumor microenvironment. The consequence of PSMA transfer to cells and increased secretion of vascular endothelial growth factor-A (VEGF-A), pro-angiogenic and pro-lymphangiogenic mediators, with increased 4E binding protein 1 (4EBP-1) phosphorylation.

Offsetting Low-Affinity Carbohydrate Binding with Covalency to Engage Sugar-Specific Proteins for Tumor-Immune Proximity Induction

Carbohydrate-binding receptors are often used by the innate immune system to potentiate inflammation, target endocytosis/destruction, and adaptive immunity (e.g., CD206, DC-SIGN, MBL, and anticarbohydrate antibodies). To access this class of receptors for cancer immunotherapy, a growing repertoire of bifunctional proximity-inducing therapeutics use high-avidity multivalent carbohydrate binding domains to offset the intrinsically low affinity associated with monomeric carbohydrate-protein binding interactions (Kd ≈ 10-3-10-6 M). For applications aimed at recruiting anticarbohydrate antibodies to tumor cells, large synthetic scaffolds are used that contain both a tumor-binding domain (TBD) and a multivalent antibody-binding domain (ABD) comprising multiple l-rhamnose monosaccharides. This allows for stable bridging between tumor cells and antibodies, which activates tumoricidal immune function. Problematically, such multivalent macromolecules can face limitations including synthetic and/or structural complexity and the potential for off-target immune engagement. We envisioned that small bifunctional "proximity-inducing" molecules containing a low-affinity monovalent ABD could efficiently engage carbohydrate-binding receptors for tumor-immune proximity by coupling weak binding with covalent engagement. Typical covalent drugs and electrophilic chimeras use high-affinity ligands to promote the fast covalent engagement of target proteins (i.e., large kinact/KI), driven by a favorably small KI for binding. We hypothesized the much less favorable KI associated with carbohydrate-protein binding interactions can be offset by a favorably large kinact for the covalent labeling step. In the current study, we test this hypothesis in the context of a model system that uses rhamnose-specific antibodies to induce tumor-immune proximity and tumoricidal function. We discovered that synthetic chimeric molecules capable of preorganizing an optimal electrophile (i.e., SuFEx vs activated ester) for protein engagement can rapidly covalently engage natural sources of antirhamnose antibody using only a single low-affinity rhamnose monosaccharide ABD. Strikingly, we observe chimeric molecules lacking an electrophile, which can only noncovalently bind the antibody, completely lack tumoricidal function. This is in stark contrast to previous work targeting small molecule hapten and peptide-specific antibodies. Our findings underscore the utility of covalency as a strategy to engage low-affinity carbohydrate-specific proteins for tumor-immune proximity induction.

Fluorescent Conjugates Based on Prostate-Specific Membrane Antigen Ligands as an Effective Visualization Tool for Prostate Cancer

Fluorescent dyes are widely used in histological studies and in intraoperative imaging, including surgical treatment of prostate cancer (PC), which is one of the most common types of cancerous tumors among men today. Targeted delivery of fluorescent conjugates greatly improves diagnostic efficiency and allows for timely correct diagnosis. In the case of PC, the protein marker is a prostate-specific membrane antigen (PSMA). To date, a large number of diagnostic conjugates targeting PSMA have been created based on modified urea. The review focuses on the conjugates selectively binding to PSMA and answers the following questions: What fluorescent dyes are already in use in the field of PC diagnosis? What factors influence the structure-activity ratio of the final molecule? What features should be considered when selecting a fluorescent tag to create new diagnostic conjugates? And what could be suggested to further development in this field at the present time?

A Series of PSMA-Targeted Near-Infrared Fluorescent Imaging Agents

We have synthesized a series of 10 new, PSMA-targeted, near-infrared imaging agents intended for use in vivo for fluorescence-guided surgery (FGS). Compounds were synthesized from the commercially available amine-reactive active NHS ester of DyLight800. We altered the linker between the PSMA-targeting urea moiety and the fluorophore with a view to improve the pharmacokinetics. Chemical yields for the conjugates ranged from 51% to 86%. The K_i values ranged from 0.10 to 2.19 nM. Inclusion of an N-bromobenzyl substituent at the ε-amino group of lysine enhanced PSMA⁺ PIP tumor uptake, as did hydrophilic substituents within the linker. The presence of a polyethylene glycol chain within the linker markedly decreased renal uptake. In particular, DyLight800-10 demonstrated high specific uptake relative to background signal within kidney, confirmed by immunohistochemistry. These compounds may be useful for FGS in prostate, renal or other PSMA-expressing cancers.

Design and Validation of Site-Specifically Labeled Single-Domain Antibody-Based Tracers for in Vivo Fluorescence Imaging and Image-Guided Surgery

Near-infrared fluorescence molecular imaging has become an established preclinical technique to investigate molecular processes in vivo and to study novel therapies. Furthermore, fluorescence molecular imaging is gaining significant interest from clinicians as an intra-operative guidance tool. This technique makes use of targeted fluorescent tracers as contrast agents that recognize specific biomarkers expressed at the site of disease. Single-domain antibodies have shown to possess excellent properties for in vivo imaging in comparison to conventional antibodies. In this chapter, we describe a method for site-specific conjugation of a near-infrared fluorophore to single-domain antibodies by exploiting cysteine-maleimide chemistry. As opposed to random conjugation, site-specific conjugation results in a homogenously labeled fluorescent tracer and avoids inference with antigen binding.

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.

A prostate-specific membrane antigen activated molecular rotor for real-time fluorescence imaging

Surgery is an efficient way to treat localized prostate cancer (PCa), however, it is challenging to demarcate rapidly and accurately the tumor boundary intraoperatively, as existing tumor detection methods are seldom performed in real-time. To overcome those limitations, we develop a fluorescent molecular rotor that specifically targets the prostate-specific membrane antigen (PSMA), an established marker for PCa. The probes have picomolar affinity (IC50 = 63-118 pM) for PSMA and generate virtually instantaneous onset of robust fluorescent signal proportional to the concentration of the PSMA-probe complex. In vitro and ex vivo experiments using PCa cell lines and clinical samples, respectively, indicate the utility of the probe for biomedical applications, including real-time monitoring of endocytosis and tumor staging. Experiments performed in a PCa xenograft model reveal suitability of the probe for imaging applications in vivo.

Modular Synthesis of Peptide-Based Single- and Multimodal Targeted Molecular Imaging Agents

A practical, modular synthesis of targeted molecular imaging agents (TMIAs) containing near-infrared dyes for optical molecular imaging (OMI) or chelated metals for magnetic resonance imaging (MRI) and single-photon emission correlation tomography (SPECT) or positron emission tomography (PET) has been developed. In the method, imaging modules are formed early in the synthesis by attaching imaging agents to the side chain of protected lysines. These modules may be assembled to provide a given set of single- or dual-modal imaging agents, which may be conjugated in the last steps of the synthesis under mild conditions to linkers and targeting groups. A key discovery was the ability of a metal such as gadolinium, useful in MRI, to serve as a protecting group for the chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). It was further discovered that two lanthanide metals, La and Ce, can double as protecting groups and placeholder metals, which may be transmetalated under mild conditions by metals used for PET in the final step. The modular method enabled the synthesis of discrete targeted probes with two of the same or different dyes, two same or different metals, or mixtures of dyes and metals. The approach was exemplified by the synthesis of single- or dual-modal imaging modules for MRI-OMI, PET-OMI, and PET-MRI, followed by conjugation to the integrin-seeking peptide, c(RGDyK). For Gd modules, their efficacy for MRI was verified by measuring the NMR spin-lattice relaxivity. To validate functional imaging of TMIAs, dual-modal agents containing Cy5.5 were shown to target A549 cancer cells by confocal fluorescence microscopy.

Methods and techniques for in vitro subcellular localization of radiopharmaceuticals and radionuclides

In oncology, the holy grail of radiotherapy is specific radiation dose deposition in tumours with minimal healthy tissue toxicity. If used appropriately, injectable, systemic radionuclide therapies could meet these criteria, even for treatment of micrometastases and single circulating tumour cells. The clinical use of α and β- particle-emitting molecular radionuclide therapies is rising, however clinical translation of Auger electron-emitting radionuclides is hampered by uncertainty around their exact subcellular localisation, which in turn affects the accuracy of dosimetry. This review aims to discuss and compare the advantages and disadvantages of various subcellular localisation methods available to localise radiopharmaceuticals and radionuclides for in vitro investigations.

Dual contrast agents for fluorescence and photoacoustic imaging: evaluation in a murine model of prostate cancer

Prostate-specific membrane antigen (PSMA) is a promising diagnostic and therapeutic target for prostate cancer (PC). Poly(amidoamine) [PAMAM] dendrimers serve as versatile scaffolds for imaging agents and drug delivery that can be tailored to different sizes and compositions depending upon the application. We have developed PSMA-targeted PAMAM dendrimers for real-time detection of PC using fluorescence (FL) and photoacoustic (PA) imaging. A generation-4, ethylenediamine core, amine-terminated dendrimer was consecutively conjugated with on average 10 lysine-glutamate-urea PSMA targeting moieties and a different number of sulfo-cyanine7.5 (Cy7.5) near-infrared dyes (2, 4, 6 and 8 denoted as conjugates II, III, IV and V, respectively). The remaining terminal primary amines were capped with butane-1,2-diol functionalities. We also prepared a conjugate composed of Cy7.5-lysine-suberic acid-lysine glutamate-urea (I) and control dendrimer conjugate (VI). Among all conjugates, IV showed superior in vivo target specificity in male NOD-SCID mice bearing isogenic PSMA+ PC3 PIP and PSMA- PC3 flu xenografts and suitable physicochemical properties for FL and PA imaging. Such agents may prove useful in PC cancer detection and subsequent surgical guidance during excision of PSMA-expressing lesions.

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.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

Quicker, deeper and stronger imaging: A review of tumor-targeted, near-infrared fluorescent dyes for fluorescence guided surgery in the preclinical and clinical stages

Cancer is a public health problem and the main cause of human mortality and morbidity worldwide. Complete removal of tumors and metastatic lymph nodes in surgery is significantly beneficial for the prognosis of patients. Tumor-targeted, near-infrared fluorescent (NIRF) imaging is an emerging field of real-time intraoperative cancer imaging based on tumor-targeted NIRF dyes. Targeted NIRF dyes contain NIRF fluorophores and specific binding ligands such as antibodies, peptides and small molecules. The present article reviews recently updated tumor-targeted NIRF dyes for the molecular imaging of malignant tumors in the preclinical stage and clinical trials. The strengths and challenges of NIRF agents with tumor-targeting ability are also summarized. Smaller ligands, near infrared II dyes, dual-modality dyes and activatable dyes may contribute to quicker, deeper, stronger imaging in the nearest future. In this review, we highlighted tumor-targeted NIRF dyes for fluorescence-guided surgery and their potential clinical translation.

Functionalized NIR-II Semiconducting Polymer Nanoparticles for Single-cell to Whole-Organ Imaging of PSMA-Positive Prostate Cancer

Development of molecular probes holds great promise for early diagnosis of aggressive prostate cancer. Here, 2-[3-(1,3-dicarboxypropyl) ureido] pentanedioic acid (DUPA)-conjugated ligand and bis-isoindigo-based polymer (BTII) are synthesized to formulate semiconducting polymer nanoparticles (BTII-DUPA SPN) as a prostate-specific membrane antigen (PSMA)-targeted probe for prostate cancer imaging in the NIR-II window. Insights into the interaction of the imaging probes with the biological targets from single cell to whole organ are obtained by transient absorption (TA) microscopy and photoacoustic (PA) tomography. At single-cell level, TA microscopy reveals the targeting efficiency, kinetics, and specificity of BTII-DUPA SPN to PSMA-positive prostate cancer. At organ level, PA tomographic imaging of BTII-DUPA SPN in the NIR-II window demonstrates superior imaging depth and contrast. By intravenous administration, BTII-DUPA SPN demonstrates selective accumulation and retention in the PSMA-positive tumor, allowing noninvasive PA detection of PSMA overexpressing prostate tumors in vivo. The distribution of nanoparticles inside the tumor tissue is further analyzed through TA microscopy. These results collectively demonstrate BTII-DUPA SPN as a promising probe for prostate cancer diagnosis by PA tomography.

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).

Hybrid Tracers Based on Cyanine Backbones Targeting Prostate-Specific Membrane Antigen: Tuning Pharmacokinetic Properties and Exploring Dye-Protein Interaction

Prostate cancer surgery is currently being revolutionized by the use of prostate-specific membrane antigen (PSMA)-targeted radiotracers, for example, ^99mTc-labeled PSMA tracer analogs for radioguided surgery. The purpose of this study was to develop a second-generation ^99mTc-labeled PSMA-targeted tracer incorporating a fluorescent dye. Methods: Several PSMA-targeted hybrid tracers were synthesized: glutamic acid-urea-lysine (EuK)-Cy5-mas₃, EuK-(SO₃)Cy5-mas₃, EuK-Cy5(SO₃)-mas₃, EuK-(Ar)Cy5-mas₃, and EuK-Cy5(Ar)-mas₃; the Cy5 dye acts as a functional backbone between the EuK targeting vector and the 2-mercaptoacetyl-seryl-seryl-seryl (mas₃) chelate to study the dye's interaction with PSMA's amphipathic entrance funnel. The compounds were evaluated for their photophysical and chemical properties and PSMA affinity. After radiolabeling with ^99mTc, we performed in vivo SPECT imaging, biodistribution, and fluorescence imaging on BALB/c nude mice with orthotopically transplanted PC346C tumors. Results: The dye composition influenced the photophysical properties (brightness range 0.3-1.5 × 10⁴ M^-1 × cm^-1), plasma protein interactions (range 85.0% ± 2.3%-90.7% ± 1.3% bound to serum, range 76% ± 0%-89% ± 6% stability in serum), PSMA affinity (half-maximal inhibitory concentration [IC₅₀] range 19.2 ± 5.8-175.3 ± 61.1 nM) and in vivo characteristics (tumor-to-prostate and tumor-to-muscle ratios range 0.02 ± 0.00-154.73 ± 28.48 and 0.46 ± 0.28-5,157.50 ± 949.17, respectively; renal, splenic, and salivary retention). Even though all tracer analogs allowed tumor identification with SPECT and fluorescence imaging, ^99mTc-EuK-(SO₃)Cy5-mas₃ had the most promising properties (e.g., half-maximal inhibitory concentration, 19.2 ± 5.8, tumor-to-muscle ratio, 5,157.50 ± 949.17). Conclusion: Our findings demonstrate the intrinsic integration of a fluorophore in the pharmacophore in PSMA-targeted small-molecule tracers. In this design, having 1 sulfonate on the indole moiety adjacent to EuK (^99mTc-EuK-(SO₃)Cy5-mas₃) yielded the most promising tracer candidate for imaging of PSMA.

A Fluorescent, [18F]-Positron-Emitting Agent for Imaging Prostate-Specific Membrane Antigen Allows Genetic Reporting in Adoptively Transferred, Genetically Modified Cells

Clinical trials involving genome-edited cells are growing in popularity, where CAR-T immunotherapy and CRISPR/Cas9 editing are more recognized strategies. Genetic reporters are needed to localize the molecular events inside these cells in patients. Specifically, a nonimmunogenic genetic reporter is urgently needed as current reporters are immunogenic due to derivation from nonhuman sources. Prostate-specific membrane antigen (PSMA) is potentially nonimmunogenic due to its natural, low-level expression in select tissues (self-MHC display). PSMA overexpression on human prostate adenocarcinoma is also visible with excellent contrast. We exploit these properties in a transduced, two-component, Human-Derived, Genetic, Positron-emitting, and Fluorescent (HD-GPF) reporter system. Mechanistically analogous to the luciferase and luciferin reporter, PSMA is genetically encoded into non-PSMA expressing 8505C cells and tracked with ACUPA-Cy3-BF3, a single, systemically injected small molecule that delivers positron emitting fluoride (¹⁸F) and a fluorophore (Cy3) to report on cells expressing PSMA. PSMA-lentivirus transduced tissues become visible by Cy3 fluorescence, [¹⁸F]-positron emission tomography (PET), and γ-scintillated biodistribution. HD-GPF fluorescence is visible at subcellular resolution, while a reduced PET background is achieved in vivo, due to rapid ACUPA-Cy3-BF3 renal excretion. Co-transduction with luciferase and GFP show specific advantages over popular genetic reporters in advanced murine models including, a "mosaic" model of solid-tumor intratumoral heterogeneity and a survival model for observing postsurgical recurrence. We report an advanced genetic reporter that tracks genetically modified cells in entire animals and with subcellular resolution with PET and fluorescence, respectively. This reporter system is potentially nonimmunogenic and will therefore be useful in human studies. PSMA is a biomarker of prostate adenocarcinoma and ACUPA-Cy3-BF3 potential in radical prostatectomy is demonstrated.

Fluorescence Detection of Prostate Cancer by an Activatable Fluorescence Probe for PSMA Carboxypeptidase Activity

Prostate cancer (PCa) is a common malignant tumor among adult males, and convenient intraoperative detection of PCa would reduce the risk of leaving positive surgical margins, especially during nerve-sparing procedures. To achieve rapid, fluorescence-based visualization of PCa, we focused on the glutamate carboxypeptidase (CP) activity of prostate-specific membrane antigen (PSMA), a type II transmembrane glycoprotein that is attracting attention as a PCa biomarker. Based on our finding that aryl glutamate conjugates with an azoformyl linker are recognized by PSMA and have a sufficiently low LUMO (lowest unoccupied molecular orbital) energy level to quench the fluorophore through photoinduced electron transfer, we designed and synthesized a first-in-class activatable fluorescence probe for CP activity of PSMA. The developed probe allowed us to visualize the CP activity of PSMA in living cells and in clinical specimens from PCa patients and is expected to be useful for rapid intraoperative detection and diagnosis of PCa.

Emerging Fluorescent Molecular Tracers to Guide Intra-Operative Surgical Decision-Making

Fluorescence imaging is an emerging technology that can provide real-time information about the operating field during cancer surgery. Non-specific fluorescent agents, used for the assessment of blood flow and sentinel lymph node detection, have so far dominated this field. However, over the last decade, several clinical studies have demonstrated the great potential of targeted fluorescent tracers to visualize tumor lesions in a more specific way. This has led to an exponential growth in the development of novel molecular fluorescent contrast agents. In this review, the design of fluorescent molecular tracers will be discussed, with particular attention for agents and approaches that are of interest for clinical translation.

Comparison of prostate-specific membrane antigen ligands in clinical translation research for diagnosis of prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA), overexpressed on prostate cancer (PCa), is a well-characterized cell surface protein to selectively diagnose PCa. PSMA's unique characteristics and its 1000-fold higher expression in PCa compared with other tissues renders it as a suitable biomarker for detection of PCa in its early stage. In this report, we critically analyze and recommend the requirements needed for the development of variety of PSMA-targeted molecular imaging agents based on antibodies, small molecule ligands, peptides, and aptamers. The targeting moieties are either conjugated to radionuclear isotopes or near-infrared agents for efficient diagnosis of PCa.

RECENT FINDINGS

From the analysis, it was found that several small molecule-derived PCa imaging agents are approved for clinical trials in Europe and the United States, and few are already in the clinical use for diagnosis of PCa. Even though ¹¹¹In-labeled capromab pendetide was approved by the Food and Drug Administration (FDA) and other engineered antibodies are available for detection of PCa, but high production cost, low shelf life (less than 1 month at 4°C), possibility of human immuno reactions, and low blood clearance rate necessitated a need for developing new imaging agents, which are serum stable, cost-effective, and possesses longer shelf life (6 months), have fast clearance rate from nontargeted tissues during the diagnosis process. It is found that small molecule ligand-derived imaging agents possesses most of the desired properties expected for an ideal diagnostic agent when compared with other targeting moieties.

CONCLUSION

This report discusses in detail the homing moieties used in the development of targeted diagnostic tools for detection of PCa. The merits and demerits of monoclonal antibodies, small molecule ligands, peptides, and aptamers for imaging of PCa and intraoperative guided surgery are extensively analyzed. Among all, urea-based ligands were found to be most successful in preclinical and clinical trials and show a major promise for future commercialization.

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.

Tuning Pharmacokinetics to Improve Tumor Accumulation of a Prostate-Specific Membrane Antigen-Targeted Phototheranostic Agent

We describe a simple and effective bioconjugation strategy to extend the plasma circulation of a low molecular weight targeted phototheranostic agent, which achieves high tumor accumulation (9.74 ± 2.26%ID/g) and high tumor-to-background ratio (10:1). Long-circulating pyropheophorbide (LC-Pyro) was synthesized with three functional building blocks: (1) a porphyrin photosensitizer for positron-emission tomography (PET)/fluorescence imaging and photodynamic therapy (PDT), (2) a urea-based prostate-specific membrane antigen (PSMA) targeting ligand, and (3) a peptide linker to prolong the plasma circulation time. With porphyrin's copper-64 chelating and optical properties, LC-Pyro demonstrated its dual-modality (fluorescence/PET) imaging potential for selective and quantitative tumor detection in subcutaneous, orthotopic, and metastatic murine models. The peptide linker in LC-Pyro prolonged its plasma circulation time about 8.5 times compared to its truncated analog. High tumor accumulation of LC-Pyro enabled potent PDT, which resulted in significantly delayed tumor growth in a subcutaneous xenograft model. This approach can be applied to improve the pharmacokinetics of existing and future targeted PDT agents for enhanced tumor accumulation and treatment efficacy.

Near-Infrared Dye-Labeled Anti-Prostate Stem Cell Antigen Minibody Enables Real-Time Fluorescence Imaging and Targeted Surgery in Translational Mouse Models

PURPOSE

The inability to intraoperatively distinguish primary tumor, as well as lymphatic spread, increases the probability of positive surgical margins, tumor recurrence, and surgical toxicity. The goal of this study was to develop a tumor-specific optical probe for real-time fluorescence-guided surgery.

EXPERIMENTAL DESIGN

A humanized antibody fragment against PSCA (A11 minibody, A11 Mb) was conjugated with a near-infrared fluorophore, IRDye800CW. The integrity and binding of the probe to PSCA were confirmed by gel electrophoresis, size-exclusion chromatography, and flow cytometry, respectively. The ability of the probe to detect tumor-infiltrated lymph nodes and metastatic lesions was evaluated in 2 xenograft models, as well as in transgenic mice expressing human PSCA (hPSCA). An invasive intramuscular model was utilized to evaluate the efficacy of the A11 Mb-IRDye800CW-guided surgery.

RESULTS

A11 Mb was successfully conjugated with IRDye800CW and retained specific binding to PSCA. In vivo imaging showed maximal signal-to-background ratios at 48 hours. The A11 Mb-IRDye800CW specifically detected PSCA-positive primary tumors, tumor-infiltrated lymph nodes, and distant metastases with high contrast. Fluorescence guidance facilitated more complete tumor resection, reduced tumor recurrence, and improved overall survival, compared with conventional white light surgery. The probe successfully identified primary orthotopic tumors and metastatic lesions in hPSCA transgenic mice.

CONCLUSIONS

Real-time fluorescence image-guided surgery with A11 Mb-IRDye800CW enabled detection of lymph node metastases and positive surgical margins, facilitated more complete tumor removal, and improved survival, compared with white light surgery. These results may be translatable into clinical practice to improve surgical and patient outcomes.

Carbonic Anhydrase IX-Targeted Near-Infrared Dye for Fluorescence Imaging of Hypoxic Tumors

Use of tumor-targeted fluorescence dyes to help surgeons identify otherwise undetected tumor nodules, decrease the incidence of cancer-positive margins, and facilitate localization of malignant lymph nodes has demonstrated considerable promise for improving cancer debulking surgery. Unfortunately, the repertoire of available tumor-targeted fluorescent dyes does not permit identification of all cancer types, raising the need to develop additional tumor-specific fluorescent dyes to ensure localization of all malignant lesions during cancer surgeries. By comparing the mRNA levels of the hypoxia-induced plasma membrane protein carbonic anhydrase IX (CA IX) in 13 major human cancers with the same mRNA levels in corresponding normal tissues, we document that CA IX constitutes a nearly universal marker for the design of tumor-targeted fluorescent dyes. Motivated by this expression profile, we synthesize two new CA IX-targeted near-infrared (NIR) fluorescent imaging agents and characterize their physical and biological properties both in vitro and in vivo. We report that conjugation of either acetazolamide or 6-aminosaccharin (i.e., two CA-IX-specific ligands) to the NIR fluorescent dye, S0456, via an extended phenolic spacer creates a brightly fluorescent dye that binds CA IX with high affinity and allows rapid visualization of hypoxic regions of solid tumors at depths >1 cm beneath a tissue surface. Taken together, these data suggest that a CA IX-targeted NIR dye can constitute a useful addition to a cocktail of tumor-targeted NIR dyes designed to image all human cancers.

Evaluation of Novel Prostate-Specific Membrane Antigen-Targeted Near-Infrared Imaging Agent for Fluorescence-Guided Surgery of Prostate Cancer

PURPOSE

The ability to locate and remove all malignant lesions during radical prostatectomy leads not only to prevent biochemical recurrence (BCR) and possible side effects but also to improve the life expectancy of patients with prostate cancer. Fluorescence-guided surgery (FGS) has emerged as a technique that uses fluorescence to highlight cancerous cells and guide surgeons to resect tumors in real time. Thus, development of tumor-specific near-infrared (NIR) agents that target biomarkers solely expressed on prostate cancer cells will enable to assess negative tumor margins and affected lymph nodes.

EXPERIMENTAL DESIGN

Because PSMA is overexpressed in prostate cancer cells in >90% of the prostate cancer patient population, a prostate-specific membrane antigen (PSMA)-targeted NIR agent (OTL78) was designed and synthesized. Optical properties, in vitro and in vivo specificity, tumor-to-background ratio (TBR), accomplishment of negative surgical tumor margins using FGS, pharmacokinetics (PKs) properties, and preclinical toxicology of OTL78 were then evaluated in requisite models.

RESULTS

OTL78 binds to PSMA-expressing cells with high affinity, concentrates selectively to PSMA-positive cancer tissues, and clears rapidly from healthy tissues with a half-time of 17 minutes. It also exhibits an excellent TBR (5:1) as well as safety profile in animals.

CONCLUSIONS

OTL78 is an excellent tumor-specific NIR agent for use in fluorescence-guided radical prostatectomy and FGS of other cancers.

Photoacoustic imaging of integrin-overexpressing tumors using a novel ICG-based contrast agent in mice

PhotoAcoustic Imaging (PAI) is a biomedical imaging modality currently under evaluation in preclinical and clinical settings. In this work, ICG is coupled to an integrin binding vector (ICG-RGD) to combine the good photoacoustic properties of ICG and the favourable α_vβ₃-binding capabilities of a small RGD cyclic peptidomimetic. ICG-RGD is characterized in terms of physicochemical properties, biodistribution and imaging performance. Tumor uptake was assessed in subcutaneous xenograft mouse models of human glioblastoma (U-87MG, high α_vβ₃ expression) and epidermoid carcinoma (A431, low α_vβ₃ expression). ICG and ICG-RGD showed high PA signal in tumors already after 15 min post-injection. At later time points the signal of ICG rapidly decreased, while ICG-RGD showed sustained uptake in U-87MG but not in A431 tumors, likely due to the integrin-mediated retention of the probe. In conclusion, ICG-RGD is a novel targeted contrast agents for PAI with superior biodistribution, tumor uptake properties and diagnostic value compared to ICG.

Prostate-specific membrane antigen-targeted photoacoustic imaging of prostate cancer in vivo

A sensitive, noninvasive method to detect localized prostate cancer, particularly for early detection and repetitive study in patients undergoing active surveillance, remains an unmet need. Here, we propose a molecular photoacoustic (PA) imaging approach by targeting the prostate-specific membrane antigen (PSMA), which is over-expressed in the vast majority of prostate cancers. We performed spectroscopic PA imaging in an experimental model of prostate cancer, namely, in immunocompromised mice bearing PSMA+ (PC3 PIP) and PSMA- (PC3 flu) tumors through administration of the known PSMA-targeted fluorescence agent, YC-27. Differences in contrast between PSMA+ and isogenic control tumors were observed upon PA imaging, with PSMA+ tumors showing higher contrast in average of 66.07-fold with 5 mice at the 24-hour postinjection time points. These results were corroborated using standard near-infrared fluorescence imaging with YC-27, and the squared correlation between PA and fluorescence intensities was 0.89. Spectroscopic PA imaging is a new molecular imaging modality with sufficient sensitivity for targeting PSMA in vivo, demonstrating the potential applications for other saturable targets relevant to cancer and other disorders.

68Ga-PSMA is a novel PET-CT tracer for imaging of hepatocellular carcinoma: A prospective pilot study

Background:⁶⁸Ga-Prostate Specific Membrane Antigen (⁶⁸Ga-PSMA), a positron emission tomography (PET) tracer that was recently introduce for imaging of prostate cancer, may accumulate in other solid tumors including Hepatocellular Carcinoma (HCC). The aim of the study was to assess the potential role of ⁶⁸Ga-PSMA PET-Computed Tomography (CT) for imaging of HCC. Material and Methods: A prospective pilot study in seven patients with HCC with 41 liver lesions: 37 suspected malignant lesions (tumor lesions) and 4 regenerative nodules. For each liver lesion, uptake of ⁶⁸Ga-PSMA and ¹⁸F-FDG uptake were measured [standard uptake value (SUV) and lesion-to-liver background ratios (TBR-SUV)], and correlated with dynamic characteristics (HU and TBR-HU) obtained on contrast enhanced CT data. Immunohistochemistry staining of PSMA in the tumor tissue was analyzed in samples obtained from 5 patients with HCC and compared to control samples from 3 patients with prostate cancer. Results: Thirty-six of the 37 tumor lesions and none of the regenerative nodules showed increased ⁶⁸Ga-PSMA uptake while only 10 lesions were ¹⁸F-FDG avid. Based on contrast enhancement, tumor lesions were categorized into 27 homogeneously enhancing lesions, nine lesions with "mosaic" enhancement composed of enhancing and non-enhancing regions in the same lesion and a single non-enhancing lesion, the latter being the only non-⁶⁸Ga-PSMA avid lesion. Using the Mann-Whitney test, ⁶⁸Ga-PSMA uptake was found significantly higher in enhancing tumor areas compared to non-enhancing areas and in contrast, ¹⁸F-FDG uptake was higher in non-enhancing areas, P<0.001 for both. ⁶⁸Ga-PSMA uptake (TBR SUV_max) was found to correlate with vascularity (TBR-HU) (Spearman r=0.866, p<0.001). Immunohistochemistry showed intense intra-tumoral microvessel staining for PSMA in HCC, in contrast with cytoplasmic and membranous staining, mainly in the luminal border, in prostate cancer samples. In two of the study patients ⁶⁸Ga-PSMA PET-CT identified unexpected extrahepatic metastases. Four regenerative liver nodules showed no increased uptake of either of the PET tracers. Conclusion:⁶⁸Ga-PSMA PET-CT is superior to ¹⁸F-FDG PET-CT in imaging patients with HCC. HCC lesions are more commonly hypervascular taking up ⁶⁸Ga-PSMA in tumoral micro-vessels. ⁶⁸Ga-PSMA PET-CT is a potential novel modality for imaging patients with HCC.

18F-Positron Emitting/Trimethine Cyanine-Fluorescent Contrast for Image-Guided Prostate Cancer Management

[^18/19F]-4, an anionic GCPII/PSMA inhibitor for image-guided intervention in prostate cancer, is described. [¹⁹F]-4 is radiolabeled with a radiochemical yield that is ≥27% and a molar activity of 190 ± 50 mCi/μmol in a <1 h, one-step, aqueous isotopic exchange reaction. [¹⁹F]-4 allows PSMA expression to be imaged by fluorescence (FL) and [¹⁸F]-PET. PC3-PIP (PSMA-positive, EC₅₀ = 6.74 ± 1.33 nM) cancers are specifically delineated in mice that bear 3 million (18 mg) PC3-PIP and PC3 (control, PSMA-negative) cells. Colocalization of [^18/19F]-4 PET, fluorescence, scintillated biodistribution, and PSMA expression are observed.

Evaluation of a Centyrin-Based Near-Infrared Probe for Fluorescence-Guided Surgery of Epidermal Growth Factor Receptor Positive Tumors

Tumor-targeted near-infrared fluorescent dyes have the potential to improve cancer surgery by enabling surgeons to locate and resect more malignant lesions where good visualization tools are required to ensure complete removal of malignant tissue. Although the tumor-targeted fluorescent dyes used in humans to date have been either small organic molecules or high molecular weight antibodies, low molecular weight protein scaffolds have attracted significant attention because they penetrate solid tumors almost as efficiently as small molecules, but can be infinitely mutated to bind almost any antigen. Here we describe the use of a 10 kDa protein scaffold, a Centyrin, to target a near-infrared fluorescent dye to tumors that overexpress the epidermal growth factor receptor (EGFR) for fluorescence-guided surgery (FGS). We have developed and optimized the dose and time required for imaging small tumor burdens with minimal background fluorescence in real-time fluorescence-guided surgery of EGFR-expressing tumor xenografts in murine models. We demonstrate that the Centyrin-near-infrared dye conjugate (CNDC) binds selectively to human EGFR+ cancer cells with an EC50 of 2 nM, localizes to EGFR+ tumor xenografts in athymic nude mice and that uptake of the dye in xenografts is significantly reduced when EGFR are blocked by preinjection of excess unlabeled Centyrin. Taken together, these data suggest that CNDCs can be used for intraoperative identification and surgical removal of EGFR-expressing lesions and that Centyrins targeted to other tumor-specific antigens should prove similarly useful in fluorescence guided surgery of cancer. In addition, we demonstrate that the CNDC is detected in the NIR region of the spectrum and can be utilized for fluorescence-guided surgery (FGS). In addition, we propose that with its eventual complete clearance from EGFR-negative tissues and its quantitative retention in the tumor mass for >24 h, a Centyrin-targeted NIR dye should provide excellent tumor contrast when injected at least 6-8 h before initiation of cancer surgery in human patients.

MR-Guided Delivery of Hydrophilic Molecular Imaging Agents Across the Blood-Brain Barrier Through Focused Ultrasound

PURPOSE

A wide variety of hydrophilic imaging and therapeutic agents are unable to gain access to the central nervous system (CNS) due to the blood-brain barrier (BBB). In particular, unless a particular transporter exists that may transport the agent across the BBB, most agents that are larger than 500 Da or that are hydrophilic will be excluded by the BBB. Glutamate carboxypeptidase II (GCPII), also known as the prostate-specific membrane antigen (PSMA) in the periphery, has been implicated in various neuropsychiatric conditions. As all agents that target GCPII are hydrophilic and thereby excluded from the CNS, we used GCPII as a platform for demonstrating our MR-guided focused ultrasound (MRgFUS) technique for delivery of GCPII/PSMA-specific imaging agents to the brain.

PROCEDURES

Female rats underwent MRgFUS-mediated opening of the BBB. After opening of the BBB, either a radio- or fluorescently labeled ureido-based ligand for GCPII/PSMA was administered intravenously. Brain uptake was assessed for 2-(3-{1-carboxy-5-[(6-[¹⁸F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid ([¹⁸F]DCFPyL) and YC-27, two compounds known to bind GCPII/PSMA with high affinity, using positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging, respectively. Specificity of ligand binding to GCPII/PSMA in the brain was determined with co-administration of a molar excess of ZJ-43, a compound of the same chemical class but different structure from either [¹⁸F]DCFPyL or YC-27, which competes for GCPII/PSMA binding.

RESULTS

Dynamic PET imaging using [¹⁸F]DCFPyL demonstrated that target uptake reached a plateau by ∼1 h after radiotracer administration, with target/background ratios continuing to increase throughout the course of imaging, from a ratio of ∼4:1 at 45 min to ∼7:1 by 80 min. NIRF imaging likewise demonstrated delivery of YC-27 to the brain, with clear visualization of tracer in the brain at 24 h. Tissue uptake of both ligands was greatly diminished by ZJ-43 co-administration, establishing specificity of binding of each to GCPII/PSMA. On gross and histological examination, animals showed no evidence for hemorrhage or other deleterious consequences of MRgFUS.

CONCLUSIONS

MRgFUS provided safe opening of the BBB to enable specific delivery of two hydrophilic agents to target tissues within the brain. This platform might facilitate imaging and therapy using a variety of agents that have heretofore been excluded from the CNS.

Low-Level Endogenous PSMA Expression in Nonprostatic Tumor Xenografts Is Sufficient for In Vivo Tumor Targeting and Imaging

Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancer and within the neovasculature of other solid tumors. The nonprostatic expression of PSMA has been reported exclusively within the neovasculature endothelial cells of nonprostatic cancers; however, there are few reports on PSMA expression in epithelial cells. Herein, we describe PSMA expression in nonprostatic epithelial cells and characterize the potential of PSMA-binding agents to noninvasively detect that expression. Methods: PSMA expression data were extracted from publicly available genomic databases. Genomic data were experimentally validated for PSMA expression-by quantitative reverse transcription polymerase chain reaction, flow cytometry, and Western blotting-in several nonprostatic cell lines and xenografts of melanoma and small cell lung cancer (SCLC) origin. The feasibility of PSMA detection in those tumor models was further established using PSMA-based nuclear and optical imaging agents and by biodistribution, blocking, and ex vivo molecular characterization studies. Results: We discovered that a small percentage of nonprostatic cancer cell lines and tumors express PSMA. Importantly, PSMA expression was sufficiently high to image established melanoma and SCLC xenografts using PSMA-based nuclear and optical imaging agents. Conclusion: These results indicate that PSMA expression in nonprostatic tumors may not be limited to the endothelium but may also include solid tumor tissue of nonprostatic cancers including melanoma and SCLC. Our observations indicate broader applicability of PSMA-targeted imaging and therapeutics.

Mouse glutamate carboxypeptidase II (GCPII) has a similar enzyme activity and inhibition profile but a different tissue distribution to human GCPII

Glutamate carboxypeptidase II (GCPII), also known as prostate-specific membrane antigen (PSMA) or folate hydrolase, is a metallopeptidase expressed predominantly in the human brain and prostate. GCPII expression is considerably increased in prostate carcinoma, and the enzyme also participates in glutamate excitotoxicity in the brain. Therefore, GCPII represents an important diagnostic marker of prostate cancer progression and a putative target for the treatment of both prostate cancer and neuronal disorders associated with glutamate excitotoxicity. For the development of novel therapeutics, mouse models are widely used. However, although mouse GCPII activity has been characterized, a detailed comparison of the enzymatic activity and tissue distribution of the mouse and human GCPII orthologs remains lacking. In this study, we prepared extracellular mouse GCPII and compared it with human GCPII. We found that mouse GCPII possesses lower catalytic efficiency but similar substrate specificity compared with the human protein. Using a panel of GCPII inhibitors, we discovered that inhibition constants are generally similar for mouse and human GCPII. Furthermore, we observed highest expression of GCPII protein in the mouse kidney, brain, and salivary glands. Importantly, we did not detect GCPII in the mouse prostate. Our data suggest that the differences in enzymatic activity and inhibition profile are rather small; therefore, mouse GCPII can approximate human GCPII in drug development and testing. On the other hand, significant differences in GCPII tissue expression must be taken into account when developing novel GCPII-based anticancer and therapeutic methods, including targeted anticancer drug delivery systems, and when using mice as a model organism.

PSMA ligands in prostate cancer - Probe optimization and theranostic applications

Due to its selective overexpression in prostate cancer (PCa), the prostate-specific membrane antigen (PSMA) has been recognized as a highly promising target for diagnostic and therapeutic applications. So far, various PSMA ligands have been developed for radiolabeling with radioisotopes such as ⁶⁸Ga or ¹⁸F which can be used for specific visualization and diagnosis of PSMA-expressing PCa. In addition, PSMA ligands suitable for radiolabeling with ¹³¹I or ¹⁷⁷Lu have become available to the clinics, allowing PSMA-based radioligand therapies. Here, we provide a comprehensive review of the most frequently used PSMA ligands, their structural modifications, and the impact of those on clinical applications.

Next generation NIR fluorophores for tumor imaging and fluorescence-guided surgery: A review

Cancer is a group of diseases responsible for the major causes of mortality and morbidity among people of all ages. Even though medical sciences have made enormous growth, complete treatment of this deadly disease is still a challenging task. Last few decades witnessed an impressive growth in the design and development of near infrared (NIR) fluorophores with and without recognition moieties for molecular recognitions, imaging and image guided surgeries. The present article reviews recently reported NIR emitting organic/inorganic fluorophores that targets and accumulates in organelle/organs specifically for molecular imaging of cancerous cells. Near infrared (NIR probe) with or without a tumor-targeting warhead have been considered and discussed for their applications in the field of cancer imaging. In addition, challenges persist in this area are also delineated in this review.

In Vivo 3T Magnetic Resonance Imaging Using a Biologically Specific Contrast Agent for Prostate Cancer: A Nude Mouse Model

We characterized in vivo a functional superparamagnetic iron-oxide magnetic resonance contrast agent that shortens the T2 relaxation time in magnetic resonance imaging (MRI) of prostate cancer xenografts. The agent was developed by conjugating Molday ION™ carboxyl-6 (MIC6), with a deimmunized mouse monoclonal antibody (muJ591) targeting prostate-specific membrane antigen (PSMA). This functional contrast agent could be used as a noninvasive method to detect prostate cancer cells that are PSMA positive and more readily differentiate them from surrounding tissues for treatment. The functional contrast agent was injected intravenously into mice and its effect was compared to both MIC6 (without conjugated antibody) and phosphate-buffered saline (PBS) injection controls. MR imaging was performed on a clinical 3T MRI scanner using a multiecho spin echo (MESE) sequence to obtain T2 relaxation time values. Inductively coupled plasma atomic emission spectroscopy was used to confirm an increase in elemental iron in injected mice tumours relative to controls. Histological examination of H&E stained tissues showed normal morphology of the tissues collected.

Fluorescent chemical probes for accurate tumor diagnosis and targeting therapy

This review focuses on small molecular ligand-targeted fluorescent imaging probes and fluorescent theranostics, including their design strategies and applications in clinical tumor treatment.

A PSMA-targeted theranostic agent for photodynamic therapy

Prostate-specific membrane antigen (PSMA) is over-expressed in the epithelium of prostate cancer and in the neovasculature of many non-prostate solid tumors. PSMA has been increasingly used as a target for cancer imaging and therapy. Here we describe a low-molecular-weight theranostic photosensitizer, YC-9, for PSMA-targeted optical imaging and photodynamic therapy (PDT). YC-9 was synthesized by conjugating IRDye700DX N-hydroxysuccinimide (NHS) ester with a PSMA targeting Lys-Glu urea through a lysine-suberate linker in suitable yield. Optical imaging in vivo demonstrated PSMA-specific tumor uptake of YC-9 with rapid clearance from non-target tissues. PSMA-specific cell kill was demonstrated with YC-9in vitro through PDT in PSMA⁺ PC3-PIP and PSMA^- PC3-flu cells. In vivo PDT in mice bearing PSMA⁺ PC3-PIP tumors at 4h post-injection of YC-9 (A total of four PDT sessions were performed, 48h apart) resulted in significant tumor growth delay, while tumors in control groups continued to grow. PDT with YC-9 significantly increased the median survival of the PSMA⁺ PC3-PIP tumor mice (56.5days) compared to control groups [23.5-30.0days, including untreated, light alone, YC-9 alone (without light) and non-targeted IRDye700DX PDT treatment groups], without noticeable toxicity at the doses used. This study proves in principle that YC-9 is a promising therapeutic agent for targeted PDT of PSMA-expressing tissues, such as prostate tumors, and may also be useful against non-prostate tumors by virtue of neovascular PSMA expression.

A Bright Future for Precision Medicine: Advances in Fluorescent Chemical Probe Design and Their Clinical Application

The Precision Medicine Initiative aims to use advances in basic and clinical research to develop therapeutics that selectively target and kill cancer cells. Under the same doctrine of precision medicine, there is an equally important need to visualize these diseased cells to enable diagnosis, facilitate surgical resection, and monitor therapeutic response. Therefore, there is a great opportunity for chemists to develop chemically tractable probes that can image cancer in vivo. This review focuses on recent advances in the development of optical probes, as well as their current and future applications in the clinical management of cancer. The progress in probe development described here suggests that optical imaging is an important and rapidly developing field of study that encourages continued collaboration among chemists, biologists, and clinicians to further refine these tools for interventional surgical imaging, as well as for diagnostic and therapeutic applications.