PSMA-11–Derived Dual-Labeled PSMA Inhibitors for Preoperative PET Imaging and Precise Fluorescence-Guided Surgery of Prostate Cancer

Photosensitizer-based multimodal PSMA-targeting ligands for intraoperative detection of prostate cancer

Incomplete resection of prostate cancer (PCa) occurs in 15%-50% of PCa patients. Disease recurrence negatively impacts oncological outcome. The use of radio-, fluorescent-, or photosensitizer-labeled ligands to target the prostate-specific membrane antigen (PSMA) has become a well-established method for the detection and treatment of PCa. Methods: Here, we developed and characterized multimodal [¹¹¹In]In-DOTA(GA)-IRDye700DX-PSMA ligands, varying in their molecular composition, for use in intraoperative radiodetection, fluorescence imaging and targeted photodynamic therapy of PCa lesions. PSMA-specificity of these ligands was determined in xenograft tumor models and on fresh human PCa biopsies. Results: Ligand structure optimization showed that addition of the photosensitizer (IRDye700DX) and additional negative charges significantly increased ligand uptake in PSMA-expressing tumors. Moreover, an ex vivo incubation study on human tumor biopsies confirmed the PSMA-specificity of these ligands on human samples, bridging the gap to the clinical situation. Conclusion: We developed a novel PSMA-targeting ligand, optimized for multimodal image-guided PCa surgery combined with targeted photodynamic therapy.

Dual-labeled prostate-specific membrane antigen (PSMA)-targeting agent for preoperative molecular imaging and fluorescence-guided surgery for prostate cancer

The objective of this study was to report the synthesis and characteristics of a dual modality imaging agent, Tc-99m GRFLTGGTGRLLRIS-GHEG-ECG-K(-5-carboxy-X-rhodamine)-NH2 (GRFLT-ECG-ROX), and to verify its feasibility as both molecular imaging and intraoperative guidance agent. GRFLT-ECG-ROX was synthesized using Fmoc solid-phase peptide synthesis. Radiolabeling of GRFLT-ECG-ROX with Tc-99m was accomplished using ligand exchange via tartrate. Binding affinity and in vitro cellular uptake studies were performed. Gamma camera imaging, biodistribution, and ex vivo imaging studies were performed using LNCaP and PC-3 tumor-bearing murine models. Surgical removal of tumor nodules in murine models with peritoneal carcinomatosis was performed under a fluorescence imaging system. After radiolabeling procedures with Tc-99m, Tc-99m GRFLT-ECG-ROX complexes were prepared in high yield (>96%). The binding affinity value (Kd ) of Tc-99m GRFLT-ECG-ROX for LNCaP cells was estimated to be 9.5 ± 1.3 nM. In gamma camera imaging, the tumor to normal muscle uptake ratios of Tc-99m GRFLT-ECG-ROX increased with time (3.1 ± 0.2, 4.0 ± 0.4, and 6.3 ± 0.9 at 1, 2, and 3 h, respectively). Under real-time optical imaging, the removal of visible nodules was successfully performed. Thus, Tc-99m GRFLT-ECG-ROX could provide both preoperative molecular imaging and fluorescence imaging guidance for tumor removal.

Off-Peak Near-Infrared-II (NIR-II) Bioimaging of an Immunoconjugate Having Peak Fluorescence Emission in the NIR-I Spectral Region for Improving Tumor Margin Delineation

The primary treatment for malignant tumors remains to be resection. The strongest predictor of recurrence and postoperative prognosis is whether diseased tissue/cells remain(s) at the surgical margin. Cancer surgery entails surgeons having the capability to visually distinguish between subtle shades of color in attempts of differentiating between diseased tissue and healthy tissue under standard white-light illumination, as such tissue states appear identical at the meso-/macroscopic level. Accordingly, enhancing the capability of surgeons to do so such that they can accurately delineate the tumor margin is of paramount importance. Fluorescence-guided surgery facilitates in enhancing such capability by color-coding the surgical field with overlaid contrasting pseudo-colors from real-time intraoperative fluorescence emission via utilizing fluorescent constructs in tandem. Constructs undergoing clinical trials or that are FDA-approved provide peak fluorescence emission in the visible (405 - 700 nm) or near-infrared-I (NIR-I) spectral region (700-900 nm), whereby differentiation between tissue states progressively improves in sync with using constructs that emit longer wavelengths of light. Here, we repurpose the usage of such fluorescent constructs by establishing feasibility of a tumor-targeting immunoconjugate (cetuximab-IRDye800) having peak fluorescence emission at the NIR-I spectral region to provide improved tumor margin delineation by affording higher tumor-to-background ratios (TBRs) when measuring its off-peak fluorescence emission at the near-infrared-II (NIR-II) spectral region (1000-1700 nm) in in vivo applications. We prepared murine tumor models, administered such immunoconjugate, and imaged such models pre-/post-administration via utilizing imaging systems that separately afforded acquisition of fluorescence emission in the NIR-I or NIR-II spectral region. On doing so, we determined in vivo TBRs, ex vivo TBRs with/-out skin, and ex vivo biodistribution, all via measuring the fluorescence emission of the immunoconjugate at tumor site(s) at both spectral regions. Collectively, we established feasibility of using the immunoconjugate to afford improved tumor margin delineation by providing 2-fold higher TBRs via utilizing the NIR-II spectral region to capture off-peak fluorescence emission from a fluorescent construct having NIR-I peak fluorescence emission.

[Robotics and intraoperative navigation]

Urology has always been closely linked to technological progress. In the last few decades, we have witnessed increasing implementation of various technologies and innovations in subdisciplines of urology. While conventional laparoscopy is increasingly being replaced by robot-assisted procedures and the introduction of new robotic systems from various manufactures will continue for years, the field of endourolgy is still not dominated by robotic systems. However, new systems (e.g., autonomous, robot-controlled aquablation of the prostate) are becoming increasingly popular and numerous development projects will also probably change clinical care in coming years. In addition, further advancements in the combination of robotics with intraoperative navigation through the integration of imaging and augmented-reality (AR) and virtual reality (VR) technology can be expected. This combination of navigation and robotic technology is already being used successfully in prostate biopsy.

Functional Hybrid Molecules for the Visualization of Cancer: PESIN-Homodimers Combined with Multimodal Molecular Imaging Probes for Positron Emission Tomography and Optical Imaging: Suited for Tracking of GRPR-Positive Malignant Tissue*

We describe multimodal imaging probes for gastrin-releasing peptide receptor (GRPR)-specific targeting suited for positron emission tomography and optical imaging (PET/OI), consisting of PESIN (PEG3 -BBN7-14 ) dimers connected to multimodal imaging subunits. These multimodal agents comprise a fluorescent dye for OI and the chelator ((1,4,7-triazacyclononane-4,7-diyl)diacetic acid-1-glutaric acid) (NODA-GA) for PET radiometal isotope labelling. Special focus was put on the influence of the used dyes on the properties of the whole bioconjugates. For this, several compounds with different fluorescent dyes and non-dye carrying subunits were synthesized and investigated. As fluorescent dyes, dansyl, NBD, derivatives of fluorescein, coumarin and rhodamine as well as three pyrilium-based dyes were employed. Considerable influence of the charge of the colored unit on hydrophilicity as well as in vitro target receptor binding was observed and classified. High radiochemical yields and purities were found during radiolabeling of the multimodal imaging subunits as well as their GRPR-specific bioconjugates with 68 Ga. Examinations of the photophysical properties of both molecule species displayed no loss or alteration of fluorescence characteristics.

Cyanine Conjugate-Based Biomedical Imaging Probes

Cyanine is a class of fluorescent dye with meritorious fluorescence properties and has motivated numerous researchers to explore its imaging capabilities by miscellaneous structural modification and functionalization strategies. The covalent conjugation with other functional molecules represents a distinctive design strategy and has shown immense potential in both basic and clinical research. This review article summarizes recent achievements in cyanine conjugate-based probes for biomedical imaging. Particular attention is paid to the conjugation with targeting warheads and other contrast agents for targeted fluorescence imaging and multimodal imaging, respectively. Additionally, their clinical potential in cancer diagnostics is highlighted and some concurrent impediments for clinical translation are discussed.

Hybrid Multimodal Imaging Synthons for Chemoselective and Efficient Biomolecule Modification with Chelator and Near-Infrared Fluorescent Cyanine Dye

The development of hybrid multimodal imaging synthons (MIS), carrying in addition to a chelator for radiometal labeling also a near-infrared (NIR) fluorescent cyanine dye was the aim of this work. The MIS should be introducible into biomolecules of choice via an efficient and chemoselective click chemistry reaction. After chemical optimization, a successful synthetic strategy towards such hybrid MIS was developed, based on solid phase-based synthesis techniques and applying different near-infrared fluorescent cyanine dyes. The developed hybrid agents were shown to be easily introducible into a model homobivalent peptidic gastrin-releasing peptide receptor- (GRPR)-specific carrier without forming any side products and the MIS as well as their bioconjugates were radiolabeled with the positron-emitter ⁶⁸Ga³⁺. The hybrid multimodal agents were characterized with regard to their log_Ds, GRPR target affinities and photophysical characteristics. It could be shown that the properties of the bioconjugates were not per se affected by the introduction of the MIS but that the cyanine dye used and specifically the number of comprised negative charges per dye molecule can have a considerable influence on target receptor binding. Thus, the molecular toolbox described here enables the synthesis of tailored hybrid multimodal imaging synthons for biomolecule modification, meeting the specific need and envisioned application of the combined imaging agent.

A Facile and Reproducible Synthesis of Near-Infrared Fluorescent Conjugates with Small Targeting Molecules for Microbial Infection Imaging

Optical imaging of microbial infections, based on the detection of targeted fluorescent probes, offers high sensitivity and resolution with a relatively simple and portable setup. As the absorbance of near-infrared (NIR) light by human tissues is minimal, using respective tracers, such as IRdye800CW, enables imaging deeper target sites in the body. Herein, we present a general strategy for the conjugation of IRdye800CW and IRdye700DX to small molecules (vancomycin and amphotericin B) to provide conjugates targeted toward bacterial and fungal infections for optical imaging and photodynamic therapy. In particular, we present how the use of coupling agents (such as HBTU or HATU) leads to high yields (over 50%) in the reactions of amines and IRDye-NHS esters and how precipitation can be used as a convenient purification strategy to remove excess of the targeting molecule after the reaction. The high selectivity of the synthesized model compound Vanco-800CW has been proven in vitro, and the development of analogous agents opens up new possibilities for diagnostic and theranostic purposes. In times of increasing microbial resistance, this research gives us access to a platform of new fluorescent tracers for the imaging of infections, enabling early diagnosis and respective treatment.

Circulating Tumor Cells Expressing the Prostate Specific Membrane Antigen (PSMA) Indicate Worse Outcome in Primary, Non-Metastatic Triple-Negative Breast Cancer

Background: We analyzed mRNA profiles of prostate cancer related genes in circulating tumor cells (CTCs) of primary, non-metastatic triple-negative breast cancer (TNBC) patients (pts) before and after neoadjuvant chemotherapy to elucidate the potential of prostate cancer targets in this BC subgroup.

Method: Blood from 41 TNBC pts (n = 41 before / 26 after therapy) was analyzed for CTCs applying the AdnaTest EMT-2/Stem Cell Select. Multimarker RT-qPCR allowed the detection of the prostate specific antigen PSA, the prostate specific membrane antigen PSMA, full-length androgen receptor (AR-FL), and AR splice-variant seven (AR-V7).

Results: Before therapy, at least one prostate cancer related gene was detected in 15/41 pts (37%). Notably, in 73% of AR-FL positive cases, AR-V7 was co-expressed. After therapy, CTCs of only one patient harbored prostate cancer related genes. AR-V7+ and PSMA+ CTCs significantly correlated with early relapse (p = 0.041; p = 0.00039) whereas PSMA+ CTCs also associated with a reduced OS (p = 0.0059). This correlation was confirmed for PSMA+ CTCs in univariate (PFS p = 0.002; OS p = 0.015), but not multivariate analysis.

Conclusion: Although CTCs that expressed prostate cancer related genes were eliminated by the given therapy, PSMA+ CTCs significantly identified pts at high risk for relapse. Furthermore, AR inhibition, often discussed for this BC subgroup, might not be successful in the primary setting of the disease since we identified AR-FL+ CTCs together with AR-V7+ CTCs, associated with therapeutic failure.

[Surgery 4.0-are we ready?]

The increasing networking of data systems in medicine is not only leading to modern interdisciplinarity in the sense of cooperation between different medical departments, but also poses new challenges regarding the building and room infrastructure. The surgical operating room of the future expands or augments its reality, away from the pure building characteristics, towards an intelligent and communicative space platform. The building infrastructure (operating theatre) serves as sensor and actuator. Thus, it is possible to inform about missing diagnostics as well as to register them directly in the contextualization of the planned surgical intervention or to integrate them into the processes. Integrated operating theatres represent a comprehensive computer platform based on a corresponding system architecture with software-based protocols. An underlying modular system consisting of various modules for image acquisition and analysis, interaction and visualization supports the integration and merging of heterogeneous data that are generated in a hospital operation. Integral building data (e.g., air conditioning, lighting control, device registration) are merged with patient-related data (age, type of illness, concomitant diseases, existing diagnostic CT and MRI images). New systems coming onto the market, as well as already existing systems will have to be measured by the extent to which they will be able to guarantee this integration of information-similar to the development from mobile phone to smartphone. Cost reduction should not be the only legitimizing argument for the market launch, but the vision of a new quality of surgical perception and action.

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.

Hybrid Imaging Agents for Pretargeting Applications Based on Fusarinine C-Proof of Concept

Hybrid imaging combining the beneficial properties of radioactivity and optical imaging within one imaging probe has gained increasing interest in radiopharmaceutical research. In this study, we modified the macrocyclic gallium-68 chelator fusarinine C (FSC) by conjugating a fluorescent moiety and tetrazine (Tz) moieties. The resulting hybrid imaging agents were used for pretargeting applications utilizing click reactions with a trans-cyclooctene (TCO) tagged targeting vector for a proof of principle both in vitro and in vivo. Starting from FSC, the fluorophores Sulfocyanine-5, Sulfocyanine-7, or IRDye800CW were conjugated, followed by introduction of one or two Tz motifs, resulting in mono and dimeric Tz conjugates. Evaluation included fluorescence microscopy, binding studies, logD, protein binding, in vivo biodistribution, µPET (micro-positron emission tomography), and optical imaging (OI) studies. 68Ga-labeled conjugates showed suitable hydrophilicity, high stability, and specific targeting properties towards Rituximab-TCO pre-treated CD20 expressing Raji cells. Biodistribution studies showed fast clearance and low accumulation in non-targeted organs for both SulfoCy5- and IRDye800CW-conjugates. In an alendronate-TCO based bone targeting model the dimeric IRDye800CW-conjugate resulted in specific targeting using PET and OI, superior to the monomer. This proof of concept study showed that the preparation of FSC-Tz hybrid imaging agents for pretargeting applications is feasible, making such compounds suitable for hybrid imaging applications.

Ex Vivo Assessment of Tumor-Targeting Fluorescent Tracers for Image-Guided Surgery

Image-guided surgery can aid in achieving complete tumor resection. The development and assessment of tumor-targeted imaging probes for near-infrared fluorescence image-guided surgery relies mainly on preclinical models, but the translation to clinical use remains challenging. In the current study, we introduce and evaluate the application of a dual-labelled tumor-targeting antibody for ex vivo incubation of freshly resected human tumor specimens and assessed the tumor-to-adjacent tissue ratio of the detectable signals. Immediately after surgical resection, peritoneal tumors of colorectal origin were placed in cold medium. Subsequently, tumors were incubated with ¹¹¹In-DOTA-hMN-14-IRDye800CW, an anti-carcinoembryonic antigen (CEA) antibody with a fluorescent and radioactive label. Tumors were then washed, fixed, and analyzed for the presence and location of tumor cells, CEA expression, fluorescence, and radioactivity. Twenty-six of 29 tumor samples obtained from 10 patients contained malignant cells. Overall, fluorescence intensity was higher in tumor areas compared to adjacent non-tumor tissue parts (p < 0.001). The average fluorescence tumor-to-background ratio was 11.8 ± 9.1:1. A similar ratio was found in the autoradiographic analyses. Incubation with a non-specific control antibody confirmed that tumor targeting of our tracer was CEA-specific. Our results demonstrate the feasibility of this tracer for multimodal image-guided surgery. Furthermore, this ex vivo incubation method may help to bridge the gap between preclinical research and clinical application of new agents for radioactive, near infrared fluorescence or multimodal imaging studies.

Design, synthesis, radiolabeling and biological evaluation of new urea-based peptides targeting prostate specific membrane antigen

Early diagnosis of Prostate cancer (PCa) plays a vital role in successful treatment increasing the survival rate of patients. Prostate Specific Membrane Antigen (PSMA) is over-expressed in almost all types of PCa. The goal of present study is to introduce new ^99mTc-labeled peptides as a PSMA inhibitor for specific detection of PCa at early stages. Based on published PSMA-targeting compounds, a set of peptides bearing the well-known Glu-Urea-Lys pharmacophore and new non-urea containing pharmacophore were designed and assessed by in silico docking studies. The selected peptides were synthesized and radiolabeled with ^99mTc. The in-vitro tests (log P, stability in normal saline and fresh human plasma, and affinity toward PSMA-positive LNCaP cell line) and in-vivo characterizations of radiopeptides (biodistribution and Single Photon Emission Computed Tomography-Computed Tomography (SPECT-CT) imaging in normal and tumour-bearing mice) were performed. The peptides 1-3 containing Glu-Urea-Lys and Glu-GABA-Asp as pharmacophores were efficiently interacted with crystal structure of PSMA and showed the highest binding energies range from -8 to -11.2 kcal/mol. Regarding the saturation binding test, ^99mTc-labeled peptide 1 had the highest binding affinity (K_d = 13.58 nM) to PSMA-positive cells. SPECT-CT imaging and biodistribution studies showed high kidneys and tumour uptake 1 h post-injection of radiopeptide 1 and 2 (%ID/g tumour = 3.62 ± 0.78 and 1.8 ± 0.32, respectively). ^99mTc-peptide 1 (Glu-urea-Lys-Gly-Ala-Asp-Naphthylalanine-HYNIC-^99mTc) exhibited the highest binding affinity, high radiochemical purity, the most stability and high specific accumulation in prostate tumour lesions. ^99mTc-peptide 1 being of comparable efficacy and pharmacokinetic properties with the well-known PET tracer (⁶⁸Ga-PSMA-11) seems to be applied as a promising SPECT imaging agent to early diagnose of PCa and consequently increase survival rate of patients.

Results from extended lymphadenectomies with [111In]PSMA-617 for intraoperative detection of PSMA-PET/CT-positive nodal metastatic prostate cancer

Purpose

Identification of suspicious PSMA-PET/CT-positive lymph node (LN) metastases (LNM) from prostate cancer (PCa) during lymphadenectomy (LA) is challenging. We evaluated an ¹¹¹In-labelled PSMA ligand (DKFZ-617, referred to as [¹¹¹In]PSMA-617) as a γ-emitting tracer for intraoperative γ-probe application for resected tissue samples in PCa patients. Forty-eight hours prior to LA, [¹¹¹In]PSMA-617 was administered intravenously in 23 patients with suspected LNM on PSMA-PET/CT (n = 21 with biochemical relapse, n = 2 at primary therapy). Resected tissue samples (LN, LNM and fibrofatty tissue) were measured ex situ by a γ-probe expressed as counts per second (CPS_norm). [¹¹¹In]PSMA-617 tissue sample uptake was measured by a germanium detector for verification and calculated as %IA_lbm (percent injected activity per kilogram lean body mass at time of surgery). Based on a clinical requirement for a specificity > 95%, thresholds for both ex situ measurements were chosen accordingly. Correlation of the results from PET/CT, γ-probe and germanium detector with histopathology was done.

Results

Eight hundred sixty-four LNs (197 LNM) were removed from 275 subregions in 23 patients, on average 8.6 ± 14.9 LNM per patient. One hundred four of 275 tissue samples showed cancer. Median γ-probe and germanium detector results were significantly different between tumour-affected (33.5 CPS_norm, 0.71 %IA_lbm) and tumour-free subregions (3.0 CPS_norm, 0.03 %IA_lbm) (each p value < 0.0001). For the chosen γ-probe cut-off (CPS_norm > 23) and germanium detector cut-off (%IA_lbm > 0.27), 64 and 74 true-positive and 158 true-negative samples for both measurements were identified. Thirty-nine and 30 false-negative and 6 and 5 false-positive tissue samples were identified by γ-probe and germanium detector measurements.

Conclusion

[¹¹¹In]PSMA-617 application for LA is feasible in terms of an intraoperative real-time measurement with a γ-probe for detection of tumour-affected tissue samples. γ-probe results can be confirmed by precise germanium detector measurements and were significantly different between tumour-affected and tumour-free samples.

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).

Prostate specific membrane antigen (PSMA) in urothelial cell carcinoma (UCC) is associated with tumor grading and staging

INTRODUCTION

Prostate specific membrane antigen (PSMA) has become a target for radionuclide imaging and therapy. Previous studies have shown that the expression of PSMA is not specific to prostate tissue. In this study we examine the expression of PSMA in urothelial cell carcinoma (UCC).

METHODS

Immunhistochemical PSMA-staining was performed in 89 UCC samples. PSMA expression in tumor tissue, adjacent healthy tissue and blood vessels was examined. We furthermore analyzed PSMA-mRNA expression in nine human UCC cell lines. We correlated our findings with clinical data regarding recurrence and progression of UCC.

RESULTS

UCC tissue showed a significantly higher PSMA expression compared to healthy urothelial tissue (p < 0.001). Non muscle invasive bladder cancer revealed significantly higher PSMA expression compared to muscle invasive bladder cancer (p < 0.05). PSMA expression significantly differed between various T-stages (p < 0.05) and tumor differentiation (p < 0.001). In four human UCC cell lines PSMA-mRNA was detectable. Those patients who suffered recurrence showed a higher rate of PSMA expression but no correlation to recurrence-free survival was evident. Progression of disease correlated significantly with a higher PSMA expression (p = 0.036).

CONCLUSIONS

Both UCC tissue and healthy urothelial tissue express PSMA, with significantly higher levels in UCC. We confirmed these findings in human UCC cell lines. In this small first cohort expression of PSMA correlates significant with progression of disease but not with recurrence and recurrence-free survival. These first results make PSMA a promising target for future diagnosis and therapy of UCC.

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.

NIR Scaffold Bearing Three Handles for Biocompatible Sequential Click Installation of Multiple Functional Arms

Near-infrared (NIR) probes are ideal for fluorescence labeling and imaging of biological targets in living animals. However, the instability of common NIR dyes hampers the construction of NIR probes bearing multiple functional components such as biomolecules for specific targeting and imaging reagents for multimodality imaging. To overcome these limitations, we designed a novel NIR scaffold bearing two terminal alkynes as clickable handles and a chloride on the heptamethine backbone that allows nucleophilic substitution with an azide to generate the third clickable handle. This unique scaffold allows for facile installation of multiple functional arms for the construction of multifunctional NIR probes. Various biomacromolecules or imaging reagents can be introduced to the NIR scaffold by sequential one-pot click reactions under biocompatible conditions. The preclickable handle chloride on the NIR backbone does not interfere with the initial click reactions, and it can be easily transformed into an azide for a following click reaction. On the basis of this unique NIR scaffold, we developed a highly efficient method to construct diverse NIR probes containing multiple functional biomolecules including peptides, antibodies, nucleic acids, and NIR/PET (positron emission tomography) dual-modality imaging probes bearing tumor-targeting groups. NIR imaging or multimodality imaging using these probes was performed on live cells or tumor models on living mice.

Multimodal Nanocarrier Probes Reveal Superior Biodistribution Quantification by Isotopic Analysis over Fluorescence

Absolute measurements of biodistribution are essential for understanding and optimizing the function of nanomaterials for in vivo diagnostic and therapeutic applications. Biodistribution analysis by optical imaging is desirable due to its low cost, wide accessibility, and high-throughput nature, but it is substantially less accurate than isotopic and chemical techniques. In this work, we developed multimodal probes for optical and nuclear imaging to analyze the quantitative limits of optical contrast in the red and near-infrared spectra for polysaccharide nanocarriers targeting macrophage cells. Probes incorporating three zwitterionic fluorophores together with radioactive copper distributed diffusely to optically dissimilar tissues that were either white (visceral adipose tissue) or dark red (liver and spleen) in obese rodents. We used in vivo positron emission tomography/computed tomography (PET/CT) imaging, in vivo hyperspectral tomographic fluorescence imaging, and ex vivo optical and isotopic analyses to determine correlations between optical and nuclear signals. PET imaging strongly correlated with standardized ex vivo methods for all tissue types, whereas no fluorescence signals exhibited substantial accuracy in quantification or localization in vivo. Optical imaging of resected tissues was most accurate in the 700 nm wavelength window, but only in white tissues. This work suggests that fluorescence can be used to measure diffuse probe distribution in white tissues over time or across animals, but not red tissues and not deep in the body. This work also highlights the importance of choosing validated experimental protocols and describes how optical measurements are impacted by fluorophore class and spectral properties, tissue properties, and imaging workflow.

Chelation with a twist: a bifunctional chelator to enable room temperature radiolabeling and targeted PET imaging with scandium-44

Scandium-44 has emerged as an attractive, novel PET radioisotope with ideal emission properties and half-life (t 1/2 = 3.97 h, E mean β+ = 632 keV) well matched to the pharmacokinetics of small molecules, peptides and small biologics. Conjugates of the current gold-standard chelator for 44Sc, 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid (DOTA), require heating to achieve radiochemical complexation, limiting application of this isotope in conjunction with temperature-sensitive biologics. To establish Sc(iii) isotopes as broadly applicable tools for nuclear medicine, development of alternative bifunctional chelators is required. To address this need, we characterized the Sc(iii)-chelation properties of the small-cavity triaza-macrocycle-based, picolinate-functionalized chelator H3mpatcn. Spectroscopic and radiochemical studies establish the [Sc(mpatcn)] complex as kinetically inert and appropriate for biological applications. A proof-of-concept bifunctional conjugate targeting the prostate-specific membrane antigen (PSMA), picaga-DUPA, chelates 44Sc to form 44Sc(picaga)-DUPA at room temperature with an apparent molar activity of 60 MBq μmol-1 and formation of inert RRR-Λ and SSS-Δ-twist isomers. Sc(picaga)-DUPA exhibits a K i of 1.6 nM for PSMA, comparable to the 18F-based imaging probe DCFPyL (K i = 1.1 nM) currently in phase 3 clinical trials for imaging prostate cancer. Finally, we successfully employed 44Sc(picaga)-DUPA to image PSMA-expressing tumors in a preclinical mouse model, establishing the picaga bifunctional chelator as an optimal choice for the 44Sc PET nuclide.

A novel clickable MSAP agent for dual fluorescence/nuclear labeling of biovectors

Simple and efficient synthesis of dual-modality imaging agents for preoperative surgical planning and intraoperative surgical guidance.

Trending: Radioactive and Fluorescent Bimodal/Hybrid Tracers as Multiplexing Solutions for Surgical Guidance

By contributing to noninvasive molecular imaging and radioguided surgery, nuclear medicine has been instrumental in the realization of precision medicine. During the last decade, it has also become apparent that nuclear medicine (e.g., in the form of bimodal/hybrid tracers) can help to empower fluorescence-guided surgery. More specifically, when using hybrid tracers, lesions can be noninvasively identified and localized with a high sensitivity and precision (guided by the radioisotope) and ultimately resected under real-time optical guidance (fluorescent dye). This topical review discusses early clinical successes, preclinical directions, and key aspects that could have an impact on the future of this field.

PSMA-targeting agents for radio- and fluorescence-guided prostate cancer surgery

Despite recent improvements in imaging and therapy, prostate cancer (PCa) still causes substantial morbidity and mortality. In surgical treatment, incomplete resection of PCa and understaging of possible undetected metastases may lead to disease recurrence and consequently poor patient outcome. To increase the chance of accurate staging and subsequently complete removal of all cancerous tissue, prostate specific membrane antigen (PSMA) targeting agents may provide the surgeon an aid for the intraoperative detection and resection of PCa lesions. Two modalities suitable for this purpose are radionuclide detection, which allows sensitive intraoperative localization of tumor lesions with a gamma probe, and fluorescence imaging, allowing tumor visualization and delineation. Next to fluorescence, use of photosensitizers may enable intraoperative targeted photodynamic therapy to eradicate remaining tumor lesions. Since radiodetection and optical imaging techniques each have their own strengths and weaknesses, a combination of both modalities could be of additional value. Here, we provide an overview of recent preclinical and clinical advances in PSMA-targeted radio- and fluorescence-guided surgery of PCa.

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.

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.

A Dual-Modality Linker Enables Site-Specific Conjugation of Antibody Fragments for 18F-Immuno-PET and Fluorescence Imaging

Antibody-based dual-modality (PET/fluorescence) imaging enables both presurgery antigen-specific immuno-PET for noninvasive whole-body evaluation and intraoperative fluorescence for visualization of superficial tissue layers for image-guided surgery. Methods: We developed a universal dual-modality linker (DML) that facilitates site-specific conjugation to a cysteine residue-bearing antibody fragment, introduction of a commercially available fluorescent dye (via an amine-reactive prosthetic group), and rapid and efficient radiolabeling via click chemistry with ¹⁸F-labeled trans-cyclooctene (¹⁸F-TCO). To generate a dual-modality antibody fragment-based imaging agent, the DML was labeled with the far-red dye sulfonate cyanine 5 (sCy5), site-specifically conjugated to the C-terminal cysteine of the anti-prostate stem cell antigen (PSCA) cys-diabody A2, and subsequently radiolabeled by click chemistry with ¹⁸F-TCO. The new imaging probe was evaluated in a human PSCA-positive prostate cancer xenograft model by sequential immuno-PET and optical imaging. Uptake in target tissues was confirmed by ex vivo biodistribution. Results: We successfully synthesized a DML for conjugation of a fluorescent dye and ¹⁸F. The anti-PSCA cys-diabody A2 was site-specifically conjugated with either DML or sCy5 and radiolabeled via click chemistry with ¹⁸F-TCO. Immuno-PET imaging confirmed in vivo antigen-specific targeting of prostate cancer xenografts as early as 1 h after injection. Rapid renal clearance of the 50-kDa antibody fragment enables same-day imaging. Optical imaging showed antigen-specific fluorescent signal in PSCA-positive xenografts and high contrast to surrounding tissue and PSCA-negative xenografts. Conclusion: The DML enables site-specific conjugation away from the antigen-binding site of antibody fragments, with a controlled linker-to-protein ratio, and combines signaling moieties for 2 imaging systems into 1 molecule. Dual-modality imaging could provide both noninvasive whole-body imaging with organ-level biodistribution and fluorescence image-guided identification of tumor margins during surgery.

Neuroblastoma-targeting triangular gadolinium oxide nanoplates for precise excision of cancer

Neuroblastoma accounts for 8-10% of malignancies in infants and children. It is urgent to develop an appropriate theranostic agent for effective diagnosis and therapy of neuroblastoma. Herein, we constructed RVG peptide and IRDye800-conjugated bovine serum albumin-coated triangular gadolinium oxide nanoplates (RVG&IRDye800-Gd₂O₃ TNs) as a targeting MRI agent for the diagnosis of neuroblastoma preoperation and a fluorescence imaging agent for the guidance of the precise excision of the neuroblastoma in surgery. RVG&IRDye800-Gd₂O₃ TNs have uniform edge length. The RVG&IRDye800-Gd₂O₃ TNs show remarkably enhanced affinity to both mouse- and human-derived neuroblastoma cells compared with IRDye800-Gd₂O₃ TNs (3.07-fold and 3.02-fold, respectively). Because of the increased accumulation in tumor cells, RVG&IRDye800-Gd₂O₃ TNs exhibit signals threefold to fivefold higher than the surrounding normal tissues, which is propitious to the diagnosis of neuroblastoma preoperation and provides real-time visual guidance of the precise excision of the neuroblastoma. Most importantly, with the guidance of the fluorescence imaging agent, the survival rate increased from 0% to 80% 42 days after surgery compared with that in conventional surgery. These findings indicated that the RVG peptide combined with IRDye800-Gd₂O₃ TNs has the potential to improve the diagnosis and treatment of patients with neuroblastoma. STATEMENT OF SIGNIFICANCE: In this study, we prepared RVG peptide and IRDye800-conjugated bovine serum albumin-coated triangular gadolinium oxide nanoplates (RVG&IRDye800-Gd₂O₃ TNs) as a targeting MRI agent for the diagnosis of neuroblastoma preoperation and a fluorescence imaging agent for the guidance of the precise excision of the neuroblastoma during surgery. Neuroblastoma was accurately located by MRI imaging, and the tumor margin could be real-time monitored through near-infrared fluorescence imaging. The RVG&IRDye800-Gd₂O₃ TNs exhibit signals threefold to fivefold higher than those in the surrounding normal tissues, which is propitious to the diagnosis of the neuroblastoma preoperation and provides real-time visual guidance of the precise excision of the neuroblastoma. With the guidance of the fluorescence imaging agent in surgery, the survival rate increased from 0% to 80% 42 days after surgery compared with that in conventional surgery.

New Developments in Dual-Labeled Molecular Imaging Agents

Intraoperative detection of tumors has had a profound impact on how cancer surgery is performed and addresses critical unmet needs in surgical oncology. Tumor deposits, margins, and residual cancer can be imaged through the use of fluorescent contrast agents during surgical procedures to complement visual and tactile guidance. The combination of fluorescent and nuclear contrast into a multimodality agent builds on these capabilities by adding quantitative, noninvasive nuclear imaging capabilities to intraoperative imaging. This review focuses on new strategies for the development and evaluation of targeted dual-labeled molecular imaging agents while highlighting the successful first-in-human application of this technique.

Synthesis and Preclinical Characterization of the PSMA-Targeted Hybrid Tracer PSMA-I&F for Nuclear and Fluorescence Imaging of Prostate Cancer

The prostate-specific membrane antigen (PSMA)–targeted radiotracers ⁶⁸Ga/¹⁷⁷Lu-PSMA-I&T and ^99mTc-PSMA-I&S (for imaging and surgery) are currently successfully used for clinical PET imaging, radionuclide therapy, and radioguided surgery of metastatic prostate cancer. To additionally exploit the high sensitivity and spatial resolution of fluorescence imaging for improved surgical guidance, a PSMA-I&T–based hybrid tracer, PSMA-I&F (DOTAGA-k(Sulfo-Cy5)-y-nal-k-Sub-KuE), has been developed and evaluated. Methods: The in vitro PSMA-targeting efficiency of PSMA-I&F, the reference PSMA-I&T, and their corresponding ^natGa-/⁶⁸Ga- and ^natLu/¹⁷⁷Lu counterparts was determined in LNCaP cells via competitive binding assays (IC₅₀) and dual-tracer radioligand and fluorescence internalization studies. Biodistribution and small-animal PET imaging studies were performed in CB17 SCID and LNCaP xenograft–bearing SHO mice, respectively, and complemented by intraoperative far-red fluorescence imaging using a clinical laparoscope. Additionally, fully automated serial cryosectioning and fluorescence imaging of 1 tumor-bearing animal as well as PSMA immunohistochemistry and fluorescence microscopy of organ cryosections (tumor, kidney, spleen) were also performed. Results: Compared with the parent PSMA-I&T analogs, the PSMA affinities of PSMA-I&F and its ^natGa-/^natLu-complexes remained high and unaffected by dye conjugation (7.9 < IC₅₀ < 10.5 nM for all ligands). The same was observed for the internalization of ⁶⁸Ga- and ¹⁷⁷Lu-PSMA-I&F. In vivo, blood clearance of ⁶⁸Ga- and ¹⁷⁷Lu-PSMA-I&F was only slightly delayed by high plasma protein binding (94%–95%), and very low accumulation in nontarget organs was observed already at 1 h after injection. Dynamic PET imaging confirmed PSMA-specific (as demonstrated by coinjection of 2-PMPA) uptake into the LNCaP xenograft (4.5% ± 1.8 percentage injected dose per gram) and the kidneys (106% ± 23 percentage injected dose per gram). Tumor-to-background ratios of 2.1, 5.2, 9.6, and 9.6 for blood, liver, intestines, and muscle, respectively, at 1 h after injection led to excellent imaging contrast in ⁶⁸Ga-PSMA-I&F PET and in intraoperative fluorescence imaging. Furthermore, fluorescence imaging of tissue cryosections allowed high-resolution visualization of intraorgan PSMA-I&F distribution in vivo and its correlation with PSMA expression as determined by immunohistochemistry. Conclusion: Thus, with its high PSMA-targeting efficiency and favorable pharmacokinetic profile, ⁶⁸Ga/¹⁷⁷Lu-PSMA-I&F serves as an excellent proof-of-concept compound for the general feasibility of PSMA-I&T–based hybrid imaging. The PSMA-I&T scaffold represents a versatile PSMA-targeted lead structure, allowing relatively straightforward adaptation to the different structural requirements of dedicated nuclear or hybrid imaging agents.

Nuclear and Optical Bimodal Imaging Probes Using Sequential Assembly: A Perspective

New, targeted imaging tracers enable improved diagnosis, staging, and planning of treatment of disease and represent an important step toward personalized medicine applications. The combination of radioisotopes for nuclear imaging with fluorophores for fluorescence imaging provides the possibility to noninvasively assess disease burden in a patient using positron emission tomography/single-photon emission computed tomography, followed by fluorescence imaging-assisted surgical intervention in close succession. Probes enabling imaging with both modalities pose a design, synthesis, and pharmacokinetics challenge. In this study, the authors strive to summarize recent efforts toward optimized, discrete, bimodal probes as well as a perspective on future directions of this burgeoning subfield of targeted imaging probe development.

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.