Enhanced tumor retention of NTSR1-targeted agents by employing a hydrophilic cysteine cathepsin inhibitor

Synthesis of 64Cu-, 55Co-, and 68Ga-Labeled Radiopharmaceuticals Targeting Neurotensin Receptor-1 for Theranostics: Adjusting In Vivo Distribution Using Multiamine Macrocycles

The development of theranostic radiotracers relies on their binding to specific molecular markers of a particular disease and the use of corresponding radiopharmaceutical pairs thereafter. This study reports the use of multiamine macrocyclic moieties (MAs), as linkers or chelators, in tracers targeting the neurotensin receptor-1 (NTSR-1). The goal is to achieve elevated tumor uptake, minimal background interference, and prolonged tumor retention in NTSR-1-positive tumors. Methods: We synthesized a series of neurotensin antagonists bearing MA linkers and metal chelators. The MA unit is hypothesized to establish a strong interaction with the cell membrane, and the addition of a second chelator may enhance water solubility, consequently reducing liver uptake. Small-animal PET/CT imaging of [64Cu]Cu-DOTA-SR-3MA, [64Cu]Cu-NT-CB-NOTA, [68Ga]Ga-NT-CB-NOTA, [64Cu]Cu-NT-CB-DOTA, and [64Cu]Cu-NT-Sarcage was acquired at 1, 4, 24, and 48 h after injection using H1299 tumor models. [55Co]Co-NT-CB-NOTA was also tested in HT29 (high NTSR-1 expression) and Caco2 (low NTSR-1 expression) colorectal adenocarcinoma tumor models. Saturation binding assay and internalization of [55Co]Co-NT-CB-NOTA were used to test tracer specificity and internalization in HT29 cells. Results: In vivo PET imaging with [64Cu]Cu-NT-CB-NOTA, [68Ga]Ga-NT-CB-NOTA, and [55Co]Co-NT-CB-NOTA revealed high tumor uptake, high tumor-to-background contrast, and sustained tumor retention (≤48 h after injection) in NTSR-1-positive tumors. Tumor uptake of [64Cu]Cu-NT-CB-NOTA remained at 76.9% at 48 h after injection compared with uptake 1 h after injection in H1299 tumor models, and [55Co]Co-NT-CB-NOTA was retained at 60.2% at 24 h compared with uptake 1 h after injection in HT29 tumor models. [64Cu]Cu-NT-Sarcage also showed high tumor uptake with low background and high tumor retention 48 h after injection Conclusion: Tumor uptake and pharmacokinetic properties of NTSR-1-targeting radiopharmaceuticals were greatly improved when attached with different nitrogen-containing macrocyclic moieties. The study results suggest that NT-CB-NOTA labeled with either 64Cu/67Cu, 55Co/58mCo, or 68Ga (effect of 177Lu in tumor to be determined in future studies) and NT-Sarcage labeled with 64Cu/67Cu or 55Co/58mCo may be excellent diagnostic and therapeutic radiopharmaceuticals targeting NTSR-1-positive cancers. Also, the introduction of MA units to other ligands is warranted in future studies to test the generality of this approach.

Targeted delivery of activatable 131I-radiopharmaceutical for sustained radiotherapy with improved pharmacokinetics

Targeted radionuclide therapy (TRT) is an effective treatment for tumors. Self-condensation strategies can enhance the retention of radionuclides in tumors and enhance the anti-tumor effect. Considering legumain is overexpressed in multiple types of human cancers, a 131I-labeled radiopharmaceutical ([131I]MAAN) based on the self-condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine (Cys) was developed by us recently for treating legumain-overexpressed tumors. However, liver enrichment limits its application. In this study, a new radiopharmaceutical [131I]IM(HE)3AAN was designed and synthesized by introducing a hydrophilic peptide sequence His-Glu-His-Glu-His-Glu ((HE)3) into [131I]MAAN to optimize the pharmacokinetics. Upon activation by legumain under a reducing environment, hydrophilic [131I]IM(HE)3AAN could react with its precursor to form heterologous dimer [131I]H-Dimer that is highly hydrophobic. Cerenkov imaging revealed that [131I]IM(HE)3AAN displayed superior tumor selectivity and longer tumor retention time as compared with [131I]MAAN, with a significant reduction in the liver uptake. After an 18-day treatment with [131I]IM(HE)3AAN, the tumor proliferation was obviously inhibited, while no obvious injury was observed in the normal organs. These findings suggest that [131I]IM(HE)3AAN could serve as a promising drug candidate for treating legumain-overexpressed tumors.

Dual-Targeting PET Tracers Enable Enzyme-Mediated Self-Assembly for the PET Imaging of Legumain Activity

Legumain, a lysosomal cysteine protease overexpressed in a variety of tumors, has been considered a promising biomarker for various cancers. Precise detection of legumain activity in the lysosome represents an important strategy for early diagnosis and prognosis of tumors. Small-molecule probes with the property of target-enabled self-assembly hold great potential for molecular imaging. In this study, we reported two dual-targeting radiotracers ([18F]SF-AAN-M and [18F]SF-AAN-HEM) with a property of legumain-mediated self-assembly for positron emission tomography (PET) imaging. Both the radiotracers were synthesized with high labeling yield (>50%) and the radiochemical purity was over 99% via one-step straightforward 18F-labeling. Both tracers were efficiently activated by the reducing agent and legumain to self-assemble into aggregates and showed enhanced retention in legumain-overexpressed MDA-MB-468 cells and tumors, indicating that the introduction of lysosome-targeting morpholine increased the tumor uptake and extended the retention of radiotracers in legumain-overexpressed tumors. In addition, [18F]SF-AAN-HEM with a hydrophilic (histidine-glutamate)3 tag displayed significantly reduced liver uptake with no conspicuous reduction in tumor uptake, affording high signal-to-noise ratios (tumor/liver and tumor/muscle). All of these results suggest that dual-targeting tracer [18F]SF-AAN-HEM could provide a promising tool for in vivo monitoring legumain activity in tumors.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

In Vitro and In Vivo Evaluation of DTPA-HPMA Copolymers as Potential Decorporating Agents for Prophylactic Therapy of Actinide Contamination

The release of actinides into the environment represents a significant potential public health concern. Chelation therapy utilizing diethylenetriamine pentaacetate (DTPA) is a U.S. Food and Drug Administration (FDA)-approved therapy capable of mitigating the deposition of some absorbed actinides in the body. However, the pharmacokinetic profile of DTPA is not ideal for prophylactic applications. In this study, we examine the incorporation of DTPA into a HPMA copolymer (P-DTPA) to investigate if the enhanced blood circulation time can offer superior prophylactic protection and of improving in vivo radiometal decorporation. Utilizing lutetium-177 (177Lu) as an actinide model, the performance of P-DTPA and DTPA (control) were evaluated using selectivity studies in the presence of competing biological metals, chelation and stability assays in human serum and cytotoxicity studies using human umbilical vein endothelial cells (HUVEC). The in vivo decorporation efficiency of P-DTPA relative to DTPA and untreated controls was also evaluated over two weeks in CF-1 mice. In the experimental groups, the mice were prophylactically treated with P-DTPA or DTPA (30 μmol/kg) 6 or 24 h prior to 177LuCl3 administration. The in vitro results reveal that P-DTPA gives efficient complexation yields relative to DTPA with a tolerable cytotoxicity profile and good serum stability. The in vivo decorporation studies demonstrated enhanced total excretion of the 177Lu using P-DTPA compared to DTPA in both the 6 and 24 h prophylactic treatment study arms. This enhanced decorporation effect is certainly attributable to the expected prolonged biological half-life of DTPA when grafted to the HPMA polymer.

Examination of Charge Modifications of an Endolysosomal Trapping Inhibitor in an Antagonistic NTSR1-Targeted Construct for Colon Cancer

Many low-molecular weight targeted radiotherapeutics (TRTs) are capable of rapidly achieving exceptional tumor to non-target ratios shortly after administration. However, the low tumor residence time of many TRTs limits therapeutic dose delivery and has become the Achilles heel to their clinical translation. To combat the tumor efflux of these otherwise promising agents, we have previously presented a strategy of equipping low-molecular weight TRTs with irreversible cysteine cathepsin inhibitors (e.g., E-64 analogues). These inhibitors are capable of forming irreversible adducts with cysteine proteases within the endolysosomal compartments of cells. Using these endolysosomal trapping agents (ETs), the receptor-targeted constructs are able to increase tumor retention and, thus, deliverable therapeutic doses. In this study, we examine this approach in the development of agents targeting the neurotensin receptor subtype 1 (NTSR1), a receptor overexpressed in numerous cancers. Using an antagonistic NTSR1-targeting vector, we explore the impact of charge modification of the ETs on the in vitro and in vivo biological performance of the constructs using HT-29 colon cancer models. Four ETs (based on the epoxysuccinyl peptide E-64) with various charge states were synthesized and incorporated into the structures of the NTSR1-targeted antagonist. These four ¹⁷⁷Lu-labeled, ET-enhanced, NTSR1-targeted agents (¹⁷⁷Lu-NA-ET1-4), along with the structurally analogous ¹⁷⁷Lu-3BP-227, currently in clinical trials, underwent a battery of in vitro assays using HT-29 xenograft colon cancer cells to examine their NTSR1 binding, internalization and efflux, inhibition, and adduct formation properties. The biodistribution profile of these constructs was studied in an HT-29 mouse model. Charge modification of the terminal carboxylic acid and arginine of the ETs had deleterious effects on inhibition kinetics and in vitro adduct formation. Contrastingly, deletion of the arginine resulted in a modest increase in inhibition kinetics. Incorporation of ETs into the NTSR1-targeted agents was well-tolerated with minimal impact on the in vivo NTSR1 targeting but resulted in increased renal uptake. This study demonstrates that the ETs can be successfully incorporated into antagonistic NTSR1-targeted constructs without compromising their adduct formation capabilities. Based on these results, further exploration of the endolysosomal trapping approach is warranted in NTSR1- and other receptor-targeted antagonistic constructs.

Examination of the impact molecular charge has on NTSR1-targeted agents incorporated with cysteine protease inhibitors

Our laboratory has previously reported a strategy of employing cysteine cathepsin (CC) inhibitors as adduct forming, trapping agents to extend the tumor residence time of neurotensin receptor subtype 1 (NTSR1)-targeted radiopharmaceuticals. As a follow-up, we herein report a small library of CC trapping agent (CCTA)-incorporated, NTSR1-targeted conjugates with structural modifications that reduce the number of charged functional groups for both the CCTA and the peptide targeting sequence. These modifications were pursued to reduce the renal uptake and increase the translational potential of the CCTA-incorporated, NTSR1-targeted agents as radiotherapeutics. The biological performance of these constructs was examined using a battery of in vitro and in vivo studies employing the NTSR1-positive HT-29 human colon cancer cell line as our model. In vitro studies confirmed the ability of these constructs to target the NTSR1 and efficiently form intracellular adducts with cysteine proteases. Biodistribution studies using an HT-29 xenograft mouse model revealed that truncation (removal of Lys₆-Pro₇) of the NTSR1-targeted peptide (¹⁷⁷Lu-NE2a) had the greatest (3.7-fold) effect at lowering renal recognition/uptake relative to our previously reported construct. Other charge-reducing modifications to the CCTA resulted in unexpected increases in renal uptake. All of the constructs demonstrated similar levels of in vivo NTSR1-positive tumor targeting with the highest tumor residualization resulting from the construct containing the zwitterionic CCTA (¹⁷⁷Lu-NE2a). In vivo adduct formation of the conjugates was confirmed using autoradiographic SDS-PAGE analysis.

Cysteine cathepsins as therapeutic targets in inflammatory diseases

Introduction: Cysteine cathepsins are involved in the development and progression of numerous inflammation-associated diseases such as cancer, arthritis, bone and immune disorders. Consequently, there is a drive to progress research efforts focused on cathepsin use in diagnostics and as therapeutic targets in disease.Areas covered: This review discusses the potential of cysteine cathepsins as therapeutic targets in inflammation-associated diseases and recent advances in preclinical and clinical research. We describe direct targeting of cathepsins for treatment purposes and their indirect use in diagnostics.Expert opinion: The targeting of cysteine cathepsins has not translated into the clinic; this failure is attributed to off- and on-target side effects and/or the lack of companion biomarkers. This field now embraces developments in diagnostic imaging, the activation of prodrugs and antibody-drug conjugates for targeted drug delivery. The future lies in improved molecular tools and therapeutic concepts that will find a wide spectrum of uses in diagnostic and therapeutic applications.

Development of Improved Tumor-Residualizing, GRPR-Targeted Agents: Preclinical Comparison of an Endolysosomal Trapping Approach in Agonistic and Antagonistic Constructs

Receptor-targeted radiopharmaceuticals based on low-molecular-weight carriers offer many clinically advantageous attributes relative to macromolecules but have generally been hampered by their rapid clearance from tumors, thus diminishing tumor-to-nontarget tissue ratios. Herein, we present a strategy using irreversible inhibitors (E-64 derivative) of cysteine cathepsins (CCs) as trapping agents to increase the tumor retention of receptor-targeted agents. Methods: We incorporated these CC-trapping agents into agonistic and antagonistic pharmacophores targeting the gastrin-releasing peptide receptor (GRPR). The synthesized radioconjugates with either an incorporated CC inhibitor or a matching control were examined using in vitro and in vivo models of the GRPR-positive, PC-3 human prostate cancer cell line. Results: From the in vitro studies, multiple techniques confirmed that the CC-trapping, GRPR-targeted constructs were able to increase cellular retention by forming intracellular macromolecule adducts. In PC-3 tumor-bearing xenograft mice, the CC-trapping, GRPR-targeted agonistic and antagonistic constructs led to an approximately 2-fold increase in tumor retention with a corresponding improvement in most tumor-to-nontarget tissue ratios over 72 h. Conclusion: CC endolysosomal trapping provides a pathway to increase the efficacy and clinical potential of low-molecular-weight, receptor-targeted agents.