PET Diagnostic Molecules Utilizing Multimeric Cyclic RGD Peptide Analogs for Imaging Integrin αvβ3 Receptors

Family-Wide Photoproximity Profiling of Integrin Protein Social Networks in Cancer

Integrin family transmembrane receptors mediate dynamic interactions between cells and their extracellular microenvironment. The heterogeneous interaction partners of integrins directly regulate cell adhesion, motility, proliferation, and intracellular signaling. Despite the recognized importance of protein-protein interactions and the formation of signaling hubs around integrins, the ability to detect and quantify these dynamic binding partners with high spatial and temporal resolution remains challenging. Here, we developed an integrin-family-directed quantitative photoproximity protein interaction (PhotoPPI) profiling method to detect and quantify native integrin-centered protein social networks on live cells and tissues without the need for genetic manipulation, antibodies, or non-physiologic cell culture conditions. We drafted quantitative maps of integrin-centered protein social networks, highlighting conserved and unique binding partners between different cell types and cellular microenvironments. Comparison of integrin social networks in cancer cell lines of diverse tissue of origin and disease state identified specific AND-gate binding partners involved cell migration, microenvironmental interactions and proliferation that serve as markers of tumor cell metastatic state. Finally, we identified unique combinations - or barcodes - of integrin-proximal proteins on the surface of pre- and post-metastatic triple negative breast cancer (TNBC) cells whose expression strongly correlate with both positive and negative disease progression and outcomes in TNBC patients. Taken together, these data provide the first family-wide high-resolution maps of native protein interactors on live cells and identify dynamic integrin-centered social networks as potential AND-gate markers of cell identity, microenvironmental context and disease state.

EGFR- and Integrin αVβ3-Targeting Peptides as Potential Radiometal-Labeled Radiopharmaceuticals for Cancer Theranostics

The burgeoning field of cancer theranostics has witnessed advancements through the development of targeted molecular agents, particularly peptides. These agents exploit the overexpression or mutations of specific receptors, such as the Epidermal Growth Factor receptor (EGFR) and αVβ3 integrin, which are pivotal in tumor growth, angiogenesis, and metastasis. Despite the extensive research into and promising outcomes associated with antibody-based therapies, peptides offer a compelling alternative due to their smaller size, ease of modification, and rapid bioavailability, factors which potentially enhance tumor penetration and reduce systemic toxicity. However, the application of peptides in clinical settings has challenges. Their lower binding affinity and rapid clearance from the bloodstream compared to antibodies often limit their therapeutic efficacy and diagnostic accuracy. This overview sets the stage for a comprehensive review of the current research landscape as it relates to EGFR- and integrin αVβ3-targeting peptides. We aim to delve into their synthesis, radiolabeling techniques, and preclinical and clinical evaluations, highlighting their potential and limitations in cancer theranostics. This review not only synthesizes the extant literature to outline the advancements in peptide-based agents targeting EGFR and integrin αVβ3 but also identifies critical gaps that could inform future research directions. By addressing these gaps, we contribute to the broader discourse on enhancing the diagnostic precision and therapeutic outcomes of cancer treatments.

The importance of tyrosines in multimers of cyclic RGD nonapeptides: towards αvβ6-integrin targeted radiotherapeutics

In a recent paper in this journal (RSC Med. Chem., 2023, 14, 2429), we described an unusually strong impact of regiospecific exchange of phenylalanines by tyrosines in 10 gallium-68-labeled trimers of certain cyclic RGD peptides, c[XRGDLAXp(NMe)K] (X = F or Y), on non-specific organ uptakes. We found that there was, in part, no correlation of liver uptake with established polarity proxies, such as the octanol-water distribution coefficient (log D). Since this observation could not be explained straightforwardly, we suggested that the symmetry of the compounds had resulted in a synergistic interaction of certain components of the macromolecules. In the present work, we investigated whether a comparable effect also occurred for a series of 5 tetramers labeled with lutetium-177. We found that in contrast to the trimers, liver uptake of the tetramers was well correlated to their polarity, indicating that the unusual observations along the trimer series indeed was a unique feature, probably related to their particular symmetry. Since the Lu-177 labeled tetramers are also potential agents for treatment of a variety of αvβ6-integrin expressing cancers, these were evaluated in mice bearing human lung adenocarcinoma xenografts. Due to their tumor-specific uptake and retention in biodistribution and SPECT imaging experiments, these compounds are considered a step forward on the way to αvβ6-integrin-targeted anticancer agents. Furthermore, we noticed that the presence of tyrosines in general had a positive impact on the in vivo performance of our peptide multimers. In view of the fact that a corresponding rule was already proposed in the context of protein engineering, we argue in favor of considering peptide multimers as a special class of small or medium-sized proteins. In summary, we contend that the performance of peptide multimers is less determined by the in vitro characteristics (particularly, affinity and selectivity) of monomers, but rather by the peptides' suitability for the overall macromolecular design concept, and peptides containing tyrosines are preferred.

Tumor-Homing Peptides as Crucial Component of Magnetic-Based Delivery Systems: Recent Developments and Pharmacoeconomical Perspective

According to data from the World Health Organization (WHO), cancer is considered to be one of the leading causes of death worldwide, and new therapeutic approaches, especially improved novel cancer treatment regimens, are in high demand. Considering that many chemotherapeutic drugs tend to have poor pharmacokinetic profiles, including rapid clearance and limited on-site accumulation, a combined approach with tumor-homing peptide (THP)-functionalized magnetic nanoparticles could lead to remarkable improvements. This is confirmed by an increasing number of papers in this field, showing that the on-target peptide functionalization of magnetic nanoparticles improves their penetration properties and ensures tumor-specific binding, which results in an increased clinical response. This review aims to highlight the potential applications of THPs in combination with magnetic carriers across various fields, including a pharmacoeconomic perspective.

A comparison of [18F]AlF- and 68Ga-labeled dual targeting heterodimer FAPI-RGD in malignant tumor: preclinical evaluation and pilot clinical PET/CT imaging

Due to the heterogeneity of tumors, strategies to improve the effectiveness of dual-targeting tracers in tumor diagnostics have been intensively practiced. In this study, the radiolabeled [18F]AlF-NOTA-FAPI-RGD (denoted as [18F]AlF-LNC1007), a dual-targeting heterodimer tracer targeting both fibroblast activation protein (FAP) and integrin αvβ3 to enhance specific tumor uptake and retention, was synthesized and evaluated. The tracer was compared with [68Ga]Ga-LNC1007 in preclinical and clinical settings.

METHODS

The preparation of [18F]AlF- and 68Ga-labeled FAPI-RGD was carried out with an optimized protocol. The stability was tested in PBS and fetal bovine serum (FBS). Cellular uptake and in vivo distribution of the two products were compared and carried out on the U87MG cell line and its xenograft model. The safety and dosimetry of [18F]AlF-LNC1007 PET/CT scan were evaluated in six patients with malignant tumors.

RESULTS

Two radiolabeling protocols of [18F]AlF-/[68Ga]Ga-LNC1007 were developed and optimized to give a high yield of tracers with good stability. In vivo microPET images showed that the two tracers exhibited comparable pharmacokinetic characteristics, with high tumor uptake and prolonged tumor retention. In vivo distribution data showed that the target-to-non-target ratios of [18F]AlF-LNC1007 were similar to[68Ga]Ga-LNC1007. A total of six patients underwent [18F]AlF-LNC1007 PET/CT evaluation while two had head-to-head [18F]FDG PET/CT scans. The total body effective dose was 9.94E-03 mSv/MBq. The biodistribution curve showed optimal normal organ uptake with high tumor uptake and long retention of up to 3h p.i., and notably, the tumor-to-background ratio increased over time.

CONCLUSION

We successfully prepared an [18F]AlF-LNC1007 dual-targeting PET probe with comparable performances as [68Ga]Ga-LNC1007. With prolonged tumor retention and tumor specificity, it produced good imaging quality in preclinical and clinical translational studies, indicating that [18F]AlF-LNC1007 is a promising non-invasive tracer for detecting tumors expressing FAP and/or integrin avβ3, with the prospect of clinical implementation.

Development of 5-Fluorouracil/pH-Responsive Adjuvant-Embedded Extracellular Vesicles for Targeting αvβ3 Integrin Receptors in Tumors

To selectively target and treat murine melanoma B16BL6 tumors expressing αvβ3 integrin receptors, we engineered tumor-specific functional extracellular vesicles (EVs) tailored for the targeted delivery of antitumor drugs. This objective was achieved through the incorporation of a pH-responsive adjuvant, cyclic arginine-glycine-aspartic acid peptide (cRGD, serving as a tumor-targeting ligand), and 5-fluorouracil (5-FU, employed as a model antitumor drug). The pH-responsive adjuvant, essential for modulating drug release, was synthesized by chemically conjugating 3-(diethylamino)propylamine (DEAP) to deoxycholic acid (DOCA, a lipophilic substance capable of integrating into EVs' membranes), denoted as DEAP-DOCA. The DOCA, preactivated using N-(2-aminoethyl)maleimide (AEM), was chemically coupled with the thiol group of the cRGD-DOCA through the thiol-maleimide click reaction, resulting in the formation of cRGD-DOCA. Subsequently, DEAP-DOCA, cRGD-DOCA, and 5-FU were efficiently incorporated into EVs using a sonication method. The resulting tumor-targeting EVs, expressing cRGD ligands, demonstrated enhanced in vitro/in vivo cellular uptake specifically for B16BL6 tumors expressing αvβ3 integrin receptors. The ionization characteristics of the DEAP in DEAP-DOCA induced destabilization of the EVs membrane at pH 6.5 through protonation of the DEAP substance, thereby expediting 5-FU release. Consequently, an improvement in the in vivo antitumor efficacy was observed for B16BL6 tumors. Based on these comprehensive in vitro/in vivo findings, we anticipate that this EV system holds substantial promise as an exceptionally effective platform for antitumor therapeutic delivery.

A comparison study of dynamic [18F]Alfatide II imaging and [11C]MET in orthotopic rat models of glioblastoma

PURPOSE

To investigate and compare the dynamic positron emission tomography (PET) imaging with [18F]Alfatide II Imaging and [11C]Methionine ([11C]MET) in orthotopic rat models of glioblastoma multiforme (GBM), and to assess the utility of [18F]Alfatide II in detecting and evaluating neoangiogenesis in GBM.

METHODS

[18F]Alfatide II and [11C]MET were injected into the orthotopic GBM rat models (n = 20, C6 glioma cells), followed by dynamic PET/MR scans 21 days after surgery of tumor implantation. On the PET image with both radiotracers, the MRI-based volume-of-interest (VOI) was manually delineated encompassing glioblastoma. Time-activity curves were expressed as tumor-to-normal brain ratio (TNR) parameters and PET pharmacokinetic modeling (PKM) performed using 2-tissue-compartment models (2TCM). Immunofluorescent staining (IFS), western blotting and blocking experiment of tumor tissue were performed for the validation.

RESULTS

Compared to 11C-MET, [18F]Alfatide II presented a persistent accumulation in the tumor, albeit with a slightly lower SUVmean of 0.79 ± 0.25, and a reduced uptake in the contralateral normal brain tissue, respectively. This resulted in a markedly higher tumor-to-normal brain ratio (TNR) of 18.22 ± 1.91. The time-activity curve (TACs) showed a significant increase in radioactive uptake in tumor tissue, followed by a plateau phase up to 60 min for [18F]Alfatide II (time to peak:255 s) and 40 min for [11C]MET (time to peak:135 s) post injection. PKM confirmed significantly higher K1 (0.23/0.07) and K3 (0.26/0.09) in the tumor region compared to the normal brain with [18F]Alfatide II. Compared to [11C]MET imaging, PKM confirmed both significantly higher K1/K2 (1.24 ± 0.79/1.05 ± 0.39) and K3/K4 (11.93 ± 4.28/3.89 ± 1.29) in the tumor region with [18F]Alfatide II. IFS confirmed significant expression of integrin and tumor vascularization in tumor region.

CONCLUSION

[18F]Alfatide II demonstrates potential in imaging tumor-associated neovascularization in the context of glioblastoma multiforme (GBM), suggesting its utility as a tool for further exploration in neovascular characterization.

RGD peptide in cancer targeting: Benefits, challenges, solutions, and possible integrin-RGD interactions

RGD peptide can be found in cell adhesion and signaling proteins, such as fibronectin, vitronectin, and fibrinogen. RGD peptides' principal function is to facilitate cell adhesion by interacting with integrin receptors on the cell surface. They have been intensively researched for use in biotechnology and medicine, including incorporation into biomaterials, conjugation to medicinal molecules or nanoparticles, and labeling with imaging agents. RGD peptides can be utilized to specifically target cancer cells and the tumor vasculature by engaging with these integrins, improving drug delivery efficiency and minimizing adverse effects on healthy tissues. RGD-functionalized drug carriers are a viable option for cancer therapy as this focused approach has demonstrated promise in the future. Writing a review on the RGD peptide can significantly influence how drugs are developed in the future by improving our understanding of the peptide, finding knowledge gaps, fostering innovation, and making drug design easier.

Preclinical evaluation of a dual-receptor targeted tracer [68Ga]Ga-HX01 in 10 different subcutaneous and orthotopic tumor models

PURPOSE

The integrin αvβ3 and aminopeptidase N (APN/CD13) play vital roles in the tumor angiogenesis process. They are highly expressed in a variety of tumor cells and proliferating endothelial cells during angiogenesis, which have been considered as highly promising targets for tumor imaging. Arginine-glycine-aspartic (RGD) and asparagine-glycine-arginine (NGR) are two peptides specifically binding to the integrin αvβ3 and CD13, respectively. In this study, we optimized our previously developed probe and preclinically evaluated the new integrin αvβ3 and CD13 dual-targeted probe, NOTA-RGD-NGR (denoted as HX01) radiolabeled with 68Ga, in 10 different subcutaneous and orthotopic tumor models.

METHODS

The specific activity and radiochemical purity of [68Ga]Ga-HX01 were identified. The dual-receptor targeting ability was confirmed by a series of blocking studies and partly muted tracers using BxPC-3 xenograft model. The dynamic imaging study and dose escalation study were explored to determine the optimal imaging time point and dosage in the BxPC-3 xenograft model. Next, we established a variety of subcutaneous and orthotopic tumor models including pancreas (BxPC-3), breast (MCF-7), gallbladder (NOZ), lung (HCC827), ovary (SK-OV-3), colorectal (HCT-8), liver (HuH-7), stomach (NUGC-4), and glioma (U87) cancers. All models underwent [68Ga]Ga-HX01 PET/CT imaging about 2 weeks post-inoculation, with a subset of them undergoing [18F]FDG PET/CT scan performed concurrently, and their results were compared. In addition, ex vivo biodistribution studies were also performed for verifying the semi-quantitative results of the non-invasive PET images.

RESULTS

[68Ga]Ga-HX01 significantly outperformed single target probes in the BxPC-3 xenograft model. All blocking and single target groups exhibited significantly descending tumor uptake. The high tumor uptakes were found in BxPC-3, MCF-7, and NOZ subcutaneous tumors (%ID/g > 1.1), while middle uptakes were observed in HCC827, SK-OV-3, HCT-8, and HuH-7 subcutaneous tumor (%ID/g 0.7-1.0). Due to the low background, the tumor-to-muscle and tumor-to-blood ratios of [68Ga]Ga-HX01 were higher than that of [18F]FDG.

CONCLUSIONS

[68Ga]Ga-HX01, as a dual target imaging agent, exhibited superior in vivo performance in different subcutaneous and orthotopic mice models of human tumors over [18F]FDG and its respectively mono-receptor targeted agents, which warrants the future clinical translation for tumor imaging.

Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

Experimental study of 131I-caerin 1.1 and 131I-c(RGD)2 for internal radiation therapy of esophageal cancer xenografts

OBJECTIVE

The aim of this study was to analyze and compare the therapeutic effects of 131I-caerin 1.1 and 131I-c(RGD)2 on TE-1 esophageal cancer cell xenografts.

METHODS

(1) The in vitro antitumor effects of the polypeptides caerin 1.1 and c(RGD)2 were verified by MTT and clonogenic assays. 131I-caerin 1.1 and 131I-c(RGD)2 were prepared by chloramine-T (Ch-T) direct labeling, and their basic properties were measured. The binding and elution of 131I-caerin 1.1, 131I-c(RGD)2, and Na131I (control group) in esophageal cancer TE-1 cells were studied through cell binding and elution assays. (2) The antiproliferative effect and cytotoxicity of 131I-caerin 1.1, 131I-c(RGD)2, Na131I, caerin 1.1 and c(RGD)2 on TE-1 cells were detected by Cell Counting Kit-8 (CCK-8) assay. (3) A nude mouse esophageal cancer (TE-1) xenograft model was established to study and compare the efficacy of 131I-caerin 1.1 and 131I-c(RGD)2 in internal radiation therapy for esophageal cancer.

RESULTS

(1) Caerin 1.1 inhibited the in vitro proliferation of TE-1 cells in a concentration-dependent manner, with an IC50 of 13.00 µg/mL. The polypeptide c(RGD)2 had no evident inhibitory effect on the in vitro proliferation of TE-1 cells. Therefore, the antiproliferative effects of caerin 1.1 and c(RGD)2 on esophageal cancer cells were significantly different (P < 0.05). The clonogenic assay showed that the clonal proliferation of TE-1 cells decreased as the concentration of caerin 1.1 increased. Compared with the control group (drug concentration of 0 µg/mL), the caerin 1.1 group showed significantly lower clonal proliferation of TE-1 cells (P < 0.05). (2) The CCK-8 assay showed that 131I-caerin 1.1 inhibited the in vitro proliferation of TE-1 cells, while 131I-c(RGD)2 had no evident inhibitory effect on proliferation. The two polypeptides showed significantly different antiproliferative effects on esophageal cancer cells at higher concentrations (P < 0.05). Cell binding and elution assays showed that 131I-caerin 1.1 stably bound to TE-1 cells. The cell binding rate of 131I-caerin 1.1 was 15.8 % ± 1.09 % at 24 h and 6.95 % ± 0.22 % after 24 h of incubation and elution. The cell binding rate of 131I-c(RGD)2 was 0.06 % ± 0.02 % at 24 h and 0.23 % ± 0.11 % after 24 h of incubation and elution. (3) In the in vivo experiment, 3 days after the last treatment, the tumor sizes of the phosphate-buffered saline (PBS) group, caerin 1.1 group, c(RGD)2 group, 131I group, 131I-caerin 1.1 group, and 131I-c(RGD)2 group were 68.29 ± 2.67 mm3, 61.78 ± 3.58 mm3, 56.67 ± 5.65 mm3, 58.88 ± 1.71 mm3, 14.40 ± 1.38 mm3, and 60.14 ± 0.47 mm3, respectively. Compared with the other treatment groups, the 131I-caerin 1.1 group had significantly smaller tumor sizes (P < 0.001). After treatment, the tumors were isolated and weighed. The tumor weights in the PBS group, caerin 1.1 group, c(RGD)2 group, 131I group, 131I-caerin 1.1 group, and 131I-c(RGD)2 group were 39.50 ± 9.54 mg, 38.25 ± 5.38 mg, 38.35 ± 9.53 mg, 28.25 ± 8.50 mg, 9.50 ± 4.43 mg, and 34.75 ± 8.06 mg, respectively. The tumor weights in the 131I-caerin 1.1 group were significantly lighter than those in the other groups (P < 0.01).

CONCLUSION

131I-caerin 1.1 has tumor-targeting properties, is capable of targeted binding to TE-1 esophageal cancer cells, can be stably retained in tumor cells, and has an evident cytotoxic killing effect, while 131I-c(RGD)2 has no evident cytotoxic effect. 131I-caerin 1.1 better suppressed tumor cell proliferation and tumor growth than pure caerin 1.1, 131I-c(RGD)2, and pure c(RGD)2.

Synthesis of Novel Carborane-Containing Derivatives of RGD Peptide

Short peptides containing the Arg-Gly-Asp (RGD) fragment can selectively bind to integrins on the surface of tumor cells and are attractive transport molecules for the targeted delivery of therapeutic and diagnostic agents to tumors (for example, glioblastoma). We have demonstrated the possibility of obtaining the N- and C-protected RGD peptide containing 3-amino-closo-carborane and a glutaric acid residue as a linker fragment. The resulting carboranyl derivatives of the protected RGD peptide are of interest as starting compounds in the synthesis of unprotected or selectively protected peptides, as well as building blocks for preparation of boron-containing derivatives of the RGD peptide of a more complex structure.

Synthesis and Preclinical Evaluation of a Novel FAPI-04 Dimer for Cancer Theranostics

Overexpression of fibroblast activation protein (FAP) in cancer-associated fibroblasts in a wide variety of tumors enables a highly selective targeting strategy using FAP inhibitors (FAPIs). Quinoline-based FAPIs labeled with radionuclides have been widely developed for tumor-targeted nuclear medicine imaging. However, the short retention time of FAPIs at the tumor site limits their application in radionuclide therapy. In this study, a novel FAPI-04 dimer was synthesized and labeled with radionuclides to prolong the retention time in tumors for imaging and therapy. To prepare the FAPI-04 dimer complex, DOTA-Suc-Lys-(FAPI-04)₂, we used Fmoc-Lys(Boc)-OH as the linker to conjugate two FAPI-04 structures by an amide reaction. The resulting product was further modified by DOTA groups to allow for conjugation with radioactive metals. Both [⁶⁸Ga]Ga-(FAPI-04)₂ and [¹⁷⁷Lu]Lu-(FAPI-04)₂ showed a radiochemical purity of >99% and remained stable in vitro. In vivo, micro-PET images of SKOV3, A431, and H1299 xenografts revealed that the tumor uptake of [⁶⁸Ga]Ga-(FAPI-04)₂ was about twice that of [⁶⁸Ga]Ga-FAPI-04 and that the accumulation of [⁶⁸Ga]Ga-(FAPI-04)₂ at the tumor site did not significantly decrease even 3h after injection. The tumor-abdomen ratio of [⁶⁸Ga]Ga-(FAPI-04)₂ images was significantly higher than that of [¹⁸F]F-FDG images. For radionuclide therapy, [¹⁷⁷Lu]Lu-(FAPI-04)₂ effectively retarded tumor growth and displayed good tolerance. In conclusion, the DOTA-Suc-Lys-(FAPI-04)₂ design enhanced its uptake in FAP-expressing tumors, improved its retention time at the tumor site, and produced high-contrast imaging in xenografts after radionuclide labeling. Furthermore, it showed a noticeable antitumor effect. DOTA-Suc-Lys-(FAPI-04)₂ provides a new approach for applying FAPI derivatives in tumor theranostics.

18F-alfatide II internal dosimetry using the ICRP 110 adult reference phantoms and the ICRP 103 tissue weighting factors

PURPOSE

¹⁸F-alfatide II is an arginine-glycine-aspartate (RGD) peptide-based PET tracer with promising imaging properties and pharmacokinetics. This study aims to calculate the absorbed and effective doses of ¹⁸F-alfatide II using the ICRP 110 adult reference phantoms and the ICRP 103 tissue weighting factors.

METHODS

The MIRD method was used in this study to calculate the absorbed dose of organs and tissues. The biokinetic data were taken from a previous study. These data are based on the whole-body PET imaging of mice.

RESULTS

The results show that the effective dose per unit activity administered of ¹⁸F-alfatide II is 1.33E-02 mSv/MBq. The urinary bladder wall receives the highest absorbed dose due to the administration of this radiopharmaceutical. Also, the effective dose of ¹⁸F-alfatide II is lower than that of ¹⁸F-FDG and some other RGD peptide-based tracers.

CONCLUSIONS

Dose calculation using ICRP 110 voxelized adult reference phantoms and ICRP 103 tissue weighting factors leads to more realistic and accurate results for 18F-alfatide II compared to the stylized phantoms. The calculated effective dose of ¹⁸F-alfatide II in the present study is lower than that of previously published data.

Improved Boron Neutron Capture Therapy Using Integrin αvβ3-Targeted Long-Retention-Type Boron Carrier in a F98 Rat Glioma Model

Integrin αvβ3 is more highly expressed in high-grade glioma cells than in normal tissues. In this study, a novel boron-10 carrier containing maleimide-functionalized closo-dodecaborate (MID), serum albumin as a drug delivery system, and cyclic arginine-glycine-aspartate (cRGD) that can target integrin αvβ3 was developed. The efficacy of boron neutron capture therapy (BNCT) targeting integrin αvβ3 in glioma cells in the brain of rats using a cRGD-functionalized MID-albumin conjugate (cRGD-MID-AC) was evaluated. F98 glioma cells exposed to boronophenylalanine (BPA), cRGD-MID-AC, and cRGD + MID were used for cellular uptake and neutron-irradiation experiments. An F98 glioma-bearing rat brain tumor model was used for biodistribution and neutron-irradiation experiments after BPA or cRGD-MID-AC administration. BNCT using cRGD-MID-AC had a sufficient cell-killing effect in vitro, similar to that with BNCT using BPA. In biodistribution experiments, cRGD-MID-AC accumulated in the brain tumor, with the highest boron concentration observed 8 h after administration. Significant differences were observed between the untreated group and BNCT using cRGD-MID-AC groups in the in vivo neutron-irradiation experiments through the log-rank test. Long-term survivors were observed only in BNCT using cRGD-MID-AC groups 8 h after intravenous administration. These findings suggest that BNCT with cRGD-MID-AC is highly selective against gliomas through a mechanism that is different from that of BNCT with BPA.

Prospective Phase II Trial of [68Ga]Ga-NODAGA-E[c(RGDyK)]2 PET/CT Imaging of Integrin αvβ3 for Prognostication in Patients with Neuroendocrine Neoplasms

Integrin α_vβ₃, a subtype of the arginine-glycine-aspartate (RGD)-recognizing cell surface integrins, is upregulated on endothelial cells during angiogenesis and on tumor cells. Because of involvement in tumor growth, invasiveness and metastases, and angiogenesis, integrin α_vβ₃ is an attractive target in cancers. In this study, we applied ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ for imaging of integrin α_vβ₃ in patients with neuroendocrine neoplasms (NENs) and its potential use for prognostication. We hypothesized that ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ PET/CT would show tumor lesion uptake and that higher tumor lesion uptake was associated with a poorer prognosis. Methods: Between December 2017 and November 2020 we prospectively enrolled 113 patients with NEN of all grades (2019 World Health Organization classification) for ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ PET/CT. The scan was acquired 45 min after injection of 200 MBq of ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ Board-certified specialists in nuclear medicine and radiology analyzed the PET/CT measuring SUV_max in tumor lesions. Positive tumor lesions were defined as those with tumor-to-liver background ≥ 2. Maximal tumor SUV_max for each patient was used as a predictor of outcome. Patients were followed for at least 1 y to assess progression-free survival and overall survival. Results: Of 113 patients enrolled in the trial, 99 underwent ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ PET/CT, with 97 patients having evaluable lesions. The patients predominantly had small intestinal (64%) or pancreatic (20%) NEN and most had metastatic disease (93%). Most patients had low-grade tumors (78%), whereas 22% had high-grade tumors. During a median follow-up of 31 mo (interquartile range, 26-38 mo), 62 patients (64%) experienced disease progression and 25 (26%) patients died. In total, 76% of patients had positive tumor lesions, and of the patients with high-grade tumors 91% had positive tumor lesions. High integrin α_vβ₃ expression, defined as an SUV_max of at least 5.25, had a hazard ratio of 2.11 (95% CI, 1.18-3.78) and 6.95 (95% CI, 1.64-29.51) for progression-free survival and overall survival, respectively (P = 0.01 for both). Conclusion: Tumor lesion uptake of ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ was evident in patients with all grades of NEN. High uptake was associated with a poorer prognosis. Further studies are warranted to establish whether ⁶⁸Ga-NODAGA-E[c(RGDyK)]₂ PET/CT may become a prediction tool for identification of patients eligible for treatments targeting integrin α_vβ_3.

Controlling Disassembly of Paramagnetic Prodrug and Photosensitizer Nanoassemblies for On-Demand Orthotopic Glioma Theranostics

Controlling delivery and release of therapeutic agents to accomplish on-demand synergistic therapy of orthotopic gliomas is desired but challenging. Here, we report a glioma targeting and redox activatable theranostic nanoprobe (Co-NP-RGD_1/1) for magnetic resonance (MR) and fluorescence (FL) bimodal imaging-guided on-demand synergistic chemotherapy/photodynamic therapy (Chemo-PDT) of orthotopic gliomas. Co-NP-RGD_1/1 is formed via molecular coassembly of two paramagnetic and fluorogenic small-molecule probes CPT-RGD and PPa-RGD at an optimized molar ratio of 1/1, which shows a high longitudinal relaxivity (r₁ = 17.0 ± 0.6 mM^-1 s^-1, 0.5 T) but weak FL emissions and low Chemo-PDT activity. Upon reduction by endogenous glutathione (GSH), Co-NP-RGD_1/1 disassemble and release small molecules 2-RGD, chemodrug camptothecin (CPT), and near-infrared (NIR) photosensitizer (PS) PPa-SH that further binds to endogenous albumin to form PPa-SH-albumin complex, allowing to turn on FL, chemotherapeutic efficacy, and PDT activity for synergistic Chemo-PDT of orthotopic U87MG or U251 gliomas in living mice. Moreover, Co-NP-RGD_1/1 can also allow noninvasive detection and monitoring of orthotopic brain tumor growth via FL and MR imaging. Findings suggest the potential of cascade coassembly and stimuli-controlled intracellular disassembly strategy for constructing targeted and activatable nanoagents for improving combinational cancer theranostics.

[Study of the efficiency of cellular accumulation of doxorubicin supplied with a targeted delivery system based on phospholipid nanoparticles with integrin-directed peptide]

Chemotherapeutic agents containing targeted systems are a promising pathway to increase the effectiveness of glioblastoma treatment. Specific proteins characterized by increased expression on the surface of tumor cells are considered as possible targets. Integrin αvβ3 is one of such proteins on the cell surface. It effectively binds the cyclic Arg-Gly-Asp (cRGD) peptide. In this study, the cRGD peptide-modified doxorubicin (Dox) phospholipid composition was investigated. The particle size of this composition was 43.76±2.09 nm, the ζ-potential was 4.33±0.54 mV. Dox was almost completely incorporated into the nanoparticles (99.7±0.58%). The drug release increased in an acidic medium (at pH 5.0 of about 35±3.2%). The total accumulation and internalization of Dox used the composition of phospholipid nanoparticles with the targeted vector was 1.4-fold higher as compared to the free form. In the HeLa cell line (not expressing αvβ3 integrin) this effect was not observed. These results suggest the prospects of using the cyclic RGD peptide in the delivery of Dox to glioblastoma cells and the feasibility of further investigation of the mechanism of action of the entire composition as a whole.

Investigational PET tracers in neuro-oncology-What's on the horizon? A report of the PET/RANO group

Many studies in patients with brain tumors evaluating innovative PET tracers have been published in recent years, and the initial results are promising. Here, the Response Assessment in Neuro-Oncology (RANO) PET working group provides an overview of the literature on novel investigational PET tracers for brain tumor patients. Furthermore, newer indications of more established PET tracers for the evaluation of glucose metabolism, amino acid transport, hypoxia, cell proliferation, and others are also discussed. Based on the preliminary findings, these novel investigational PET tracers should be further evaluated considering their promising potential. In particular, novel PET probes for imaging of translocator protein and somatostatin receptor overexpression as well as for immune system reactions appear to be of additional clinical value for tumor delineation and therapy monitoring. Progress in developing these radiotracers may contribute to improving brain tumor diagnostics and advancing clinical translational research.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

Homomultimer strategy for improvement of radiolabeled peptides and antibody fragments in tumor targeting

A homomultimeric radioligand is composed of multiple identical ligands connected to the linker and radionuclide to detect a variety of overexpressed receptors on cancer cells. Multimer strategy holds great potential for introducing new radiotracers based on peptide and monoclonal antibody (mAb) derivatives in molecular imaging and therapy. It offers a reliable procedure for the preparation of biological-based targeting with diverse affinities and pharmacokinetics. In this context, we provide a useful summary and interpretation of the main results by a comprehensive look at multimeric radiopharmaceuticals in nuclear oncology. Therefore, there will be explanations for the strategy mechanisms and the main variables affecting the biodistribution results. The discussion is followed by highlights of recent work in the targeting of various types of receptors. The consequences are expressed based on comparing some parameters between monomer and multimer counterparts in each relevant section.

First-in-Human Study of [68Ga]Ga-NODAGA-E[c(RGDyK)]2 PET for Integrin αvβ3 Imaging in Patients with Breast Cancer and Neuroendocrine Neoplasms: Safety, Dosimetry and Tumor Imaging Ability

Arginine-Glycine-Aspartate (RGD)-recognizing cell surface integrins are involved in tumor growth, invasiveness/metastases, and angiogenesis, and are therefore an attractive treatment target in cancers. The subtype integrin α_vβ₃ is upregulated on endothelial cells during angiogenesis and on tumor cells. In vivo assessment of integrin α_vβ₃ is possible with positron emission tomography (PET). Preclinical data on radiochemical properties, tumor uptake and radiation exposure identified [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ as a promising candidate for clinical translation. In this first-in-human phase I study, we evaluate [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ PET in patients with neuroendocrine neoplasms (NEN) and breast cancer (BC). The aim was to investigate safety, biodistribution and dosimetry as well as tracer uptake in tumor lesions. A total of 10 patients (5 breast cancer, 5 neuroendocrine neoplasm) received a single intravenous dose of approximately 200 MBq [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂. Biodistribution profile and dosimetry were assessed by whole-body PET/CT performed at 10 min, 1 h and 2 h after injection. Safety assessment with vital parameters, electrocardiograms and blood tests were performed before and after injection. In vivo stability of [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ was determined by analysis of blood and urine. PET images were analyzed for tracer uptake in tumors and background organs. No adverse events or pharmacologic effects were observed in the 10 patients. [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ exhibited good in vivo stability and fast clearance, primarily by renal excretion. The effective dose was 0.022 mSv/MBq, equaling a radiation exposure of 4.4 mSv at an injected activity of 200 MBq. The tracer demonstrated stable tumor retention and good image contrast. In conclusion, this first-in-human phase I trial demonstrated safe use of [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ for integrin α_vβ₃ imaging in cancer patients, low radiation exposure and favorable uptake in tumors. Further studies are warranted to establish whether [⁶⁸Ga]Ga-NODAGA-E[c(RGDyK)]₂ may become a tool for early identification of patients eligible for treatments targeting integrin α_vβ₃ and for risk stratification of patients.

Using ELP Repeats as a Scaffold for De Novo Construction of Gadolinium-Binding Domains within Multifunctional Recombinant Proteins for Targeted Delivery of Gadolinium to Tumour Cells

Three artificial proteins that bind the gadolinium ion (Gd3+) with tumour-specific ligands were de novo engineered and tested as candidate drugs for binary radiotherapy (BRT) and contrast agents for magnetic resonance imaging (MRI). Gd3+-binding modules were derived from calmodulin. They were joined with elastin-like polypeptide (ELP) repeats from human elastin to form the four-centre Gd3+-binding domain (4MBS-domain) that further was combined with F3 peptide (a ligand of nucleolin, a tumour marker) to form the F3-W4 block. The F3-W4 block was taken alone (E2-13W4 protein), as two repeats (E1-W8) and as three repeats (E1-W12). Each protein was supplemented with three copies of the RGD motif (a ligand of integrin αvβ3) and green fluorescent protein (GFP). In contrast to Magnevist (a Gd-containing contrast agent), the proteins exhibited three to four times higher accumulation in U87MG glioma and A375 melanoma cell lines than in normal fibroblasts. The proteins remained for >24 h in tumours induced by Ca755 adenocarcinoma in C57BL/6 mice. They exhibited stability towards blood proteases and only accumulated in the liver and kidney. The technological advantages of using the engineered proteins as a basis for developing efficient and non-toxic agents for early diagnosis of tumours by MRI as well as part of BRT were demonstrated.

Fibronectin-Coated Metal-Phenolic Networks for Cooperative Tumor Chemo-/Chemodynamic/Immune Therapy via Enhanced Ferroptosis-Mediated Immunogenic Cell Death

The development of nanomedicine formulations to overcome the disadvantages of traditional chemotherapeutic drugs and integrate cooperative theranostic modes still remains challenging. Herein, we report the facile construction of a multifunctional theranostic nanoplatform based on doxorubicin (DOX)-loaded tannic acid (TA)-iron (Fe) networks (for short, TAF) coated with fibronectin (FN) for combination tumor chemo-/chemodynamic/immune therapy under the guidance of magnetic resonance (MR) imaging. We show that the DOX-TAF@FN nanocomplexes created through in situ coordination of TA and Fe(III) and physical coating with FN have a mean particle size of 45.0 nm, are stable, and can release both DOX and Fe in a pH-dependent manner. Due to the coexistence of the TAF network and DOX, significant immunogenic cell death can be caused through enhanced ferroptosis of cancer cells via cooperative Fe-based chemodynamic therapy and DOX chemotherapy. Through further treatment with programmed cell death ligand 1 antibody for an immune checkpoint blockade, the tumor treatment efficacy and the associated immune response can be further enhanced. Meanwhile, with FN-mediated targeting, the DOX-TAF@FN platform can specifically target tumor cells with high expression of α_vβ₃ integrin. Finally, the TAF network also enables the DOX-TAF@FN to have an r₁ relaxivity of 6.1 mM^-1 s^-1 for T₁-weighted MR imaging of tumors. The developed DOX-TAF@FN nanocomplexes may represent an updated multifunctional nanosystem with simple compositions for cooperative MR imaging-guided targeted chemo-/chemodynamic/immune therapy of tumors.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

There is a world beyond αvβ3-integrin: Multimeric ligands for imaging of the integrin subtypes αvβ6, αvβ8, αvβ3, and α5β1 by positron emission tomography

BACKGROUND

In the context of nuclear medicine and theranostics, integrin-related research and development was, for most of the time, focused predominantly on 'RGD peptides' and the subtype αvβ3-integrin. However, there are no less than 24 known integrins, and peptides without the RGD sequence as well as non-peptidic ligands play an equally important role as selective integrin ligands. On the other hand, multimerization is a well-established method to increase the avidity of binding structures, but multimeric radiopharmaceuticals have not made their way into clinics yet. In this review, we describe how these aspects have been interwoven in the framework of the German Research Foundation's multi-group interdisciplinary funding scheme CRC 824, yielding a series of potent PET imaging agents for selective imaging of various integrin subtypes.

RESULTS

The gallium-68 chelator TRAP was utilized to elaborate symmetrical trimers of various peptidic and non-peptidic integrin ligands. Preclinical data suggested a high potential of the resulting Ga-68-tracers for PET-imaging of the integrins α5β1, αvβ8, αvβ6, and αvβ3. For the first three, we provide some additional immunohistochemistry data in human cancers, which suggest several future clinical applications. Finally, application of αvβ3- and αvβ6-integrin tracers in pancreatic carcinoma patients revealed that unlike αvβ3-targeted PET, αvβ6-integrin PET is not characterized by off-target uptake and thus, enables a substantially improved imaging of this type of cancer.

CONCLUSIONS

Novel radiopharmaceuticals targeting a number of different integrins, above all, αvβ6, have proven their clinical potential and will play an increasingly important role in future theranostics.

Assessment of 177 Lu-labeled carboxyl-terminated polyamidoamine (PAMAM) dendrimer-RGD peptide conjugate

Structurally unique polyamidoamine (PAMAM) dendrimers implanted with targeting biological moiety along with complexed radiometal constitute a favorable nano-system for diagnosis and therapy of targeted tumor sites. In the present study, carboxyl functionalities of PAMAM- generation 4 dendrimer (PAMAM-G4-COOH) were conjugated with ε-amino group of lysine of cRGDfK peptide to impart integrin α_v β₃ targeting capability. Reaction of p-NH₂ -Bn-DOTA with PAMAM was accomplished via acid-amine coupling using EDC/NHS for ¹⁷⁷ Lu-complexation. ¹⁷⁷ Lu-labeled nano-system, ¹⁷⁷ Lu-PAMAM-DOTA-cRGDfK demonstrated receptor-mediated uptake in murine melanoma B16F10 cells during in vitro cell uptake studies. In vivo biodistribution studies demonstrated low tumor uptake and retention of ¹⁷⁷ Lu-PAMAM-DOTA-cRGDfK which may be attributed to rapid blood clearance. However, fast clearance from non-target organs resulted in higher target to background ratio. Tumor uptake of targeted nano-system, ¹⁷⁷ Lu-PAMAM-DOTA-cRGDfK was observed to be significantly (p < 0.05) higher in comparison to ¹⁷⁷ Lu-PAMAM-DOTA without the targeting peptide. Inhibition studies with unlabeled cRGDfK resulted in 60% reduction in tumor uptake of ¹⁷⁷ Lu-PAMAM-DOTA-cRGDfK, indicating specificity of the developed nano-system towards integrin α_v β₃ receptors.