(64)Cu- and (68)Ga-Based PET Imaging of Folate Receptor-Positive Tumors: Development and Evaluation of an Albumin-Binding NODAGA-Folate

Development of radioligands with an albumin-binding moiety of 4-(P-Iodophenyl) butyric acid for theranostic applications

The rapid clearance of imaging probes from blood circulation is beneficial for receptor imaging, as it minimizes non-target tissue exposure and improves tumor-to-background contrast. However, this rapid clearance can hinder radioligand therapy by limiting tumor uptake of radiolabeled compounds. An optimal blood half-life is crucial, as it enhances the uptake of radiolabeled compounds in targets, improving tumor uptake and retention of small molecule drugs, and thus therapeutic outcomes. To address this, strategies to extend blood half-life have been developed, with the addition of an albumin-binding moiety (ABM) being particularly effective. Among these, 4-(p-iodophenyl)butyric acid (IPBA) has emerged as a versatile ABM for radiopharmaceutical design. IPBA conjugation has successfully enhanced tissue distribution profiles across various cancer types. This review highlights recent progress in the design, radiosynthesis, and application of IPBA-based small molecular radioligands, providing insights for future clinical development of IPBA-based radiopharmaceuticals.

Development of a Novel 99mTc-Labeled Folate Derivative Containing Phenyl Isonitrile to Target Folate Receptor with Reduced Renal Uptake

The folate receptor has attracted much attention in the field of radiolabeled imaging agents due to the significant difference in its expression levels between tumor cells and most normal cells. However, the development of folate-based imaging agents has been limited by their high uptake in the kidney. In this study, to reduce the high renal uptake of radiolabeled folate-based tracers, a phenyl-isonitrile folate derivative (CNMBFA) was designed and labeled with technetium-99m. The complex obtained via the one-step kit labeling method had a high labeling yield (>95%) and high in vitro stability and hydrophilicity (log D7.4 = -1.72 ± 0.13). The results of the in vitro cell uptake and blocking studies and competitive binding experiments revealed that the [[99mTc]Tc-(CNMBFA)6]+ complex was specific for the folate receptor. Biodistribution and inhibition studies in KB tumor-bearing mice revealed moderate uptake and significant inhibition of the complex in tumors, whereas the renal uptake of [[99mTc]Tc-(CNMBFA)6]+ was significantly lower than that of previously reported tracers. Micro-SPECT/CT images further supported its ability to target the folate receptor for tumor imaging. Taken together, these results indicate that [[99mTc]Tc-(CNMBFA)6]+ is a potential tumor imaging agent that has good tumor-targeting properties with minimal radiation damage to the kidney.

Chemical Modifications on the αvβ6 Integrin Targeting A20FMDV2 Peptide: A Review

Integrin proteins have received a significant increase in attention in recent scientific endeavors. The current trend uses the pre-established knowledge that the arginyl-glycyl-aspartic acid (RGD) structural motif present in the A20FMDV2 peptide is highly selective for the integrin class αvβ6 which is overexpressed in many cancer types. This review will provide an extensive overview of the existing literature research to date to the best of our knowledge, highlighting significant improvements and drawbacks of structure-activity relationships (SAR) work undertaken, aiding future research to identify established SAR for an informed design of future A20FMDV2 mimetic inhibitors. Herein, the review aims to collate the existing structural chemical modifications present on A20FMDV2 in the literature to highlight key structural analogues that display more potent biological activity.

Effect of Ibuprofen as an Albumin Binder on Melanoma-Targeting Properties of 177Lu-Labeled Ibuprofen-Conjugated Alpha-Melanocyte-Stimulating Hormone Peptides

The purpose of this study was to examine how the introduction of ibuprofen (IBU) affected tumor-targeting and biodistribution properties of 177Lu-labeled IBU-conjugated alpha-melanocyte-stimulating hormone peptides. The IBU was used as an albumin binder and conjugated to the DOTA-Lys moiety without or with a linker to yield DOTA-Lys(IBU)-GG-Nle-CycMSHhex {1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-Lys(IBU)-Gly-Gly-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2}, DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex, DOTA-Lys(Asn-IBU)-GGNle-CycMSHhex, and DOTA-Lys(Dab-IBU)-GGNle-CycMSHhex peptides. Their melanocortin-receptor 1 (MC1R) binding affinities were determined on B16/F10 melanoma cells first. Then the biodistribution of 177Lu-labeled peptides was determined on B16/F10 melanoma-bearing C57 mice at 2 h postinjection to choose the lead peptide for further examination. The full biodistribution and melanoma imaging properties of 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex were further evaluated using B16/F10 melanoma-bearing C57 mice. DOTA-Lys(IBU)-GG-Nle-CycMSHhex, DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex, DOTA-Lys(Asn-IBU)-GGNle-CycMSHhex, and DOTA-Lys(Dab-IBU)-GGNle-CycMSHhex displayed the IC50 values of 1.41 ± 0.37, 1.52 ± 0.08, 0.03 ± 0.01, and 0.58 ± 0.06 nM on B16/F10 melanoma cells, respectively. 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex exhibited the lowest liver and kidney uptake among all four designed 177Lu peptides. Therefore, 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex was further evaluated for its full biodistribution and melanoma imaging properties. The B16/F10 melanoma uptake of 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex was 19.5 ± 3.12, 24.12 ± 3.35, 23.85 ± 2.08, and 10.80 ± 2.89% ID/g at 0.5, 2, 4, and 24 h postinjection, respectively. Moreover, 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex could clearly visualize the B16/F10 melanoma lesions at 2 h postinjection. The conjugation of IBU with or without a linker to GGNle-CycMSHhex affected the MC1R binding affinities of the designed peptides. The charge of the linker played a key role in the liver and kidney uptake of 177Lu-Asp-IBU, 177Lu-Asn-IBU, and 177Lu-Dab-IBU. 177Lu-Asp-IBU exhibited higher tumor/liver and tumor/kidney uptake ratios than those of 177Lu-Asn-IBU and 177Lu-Dab-IBU, underscoring its potential evaluation for melanoma therapy in the future.

Synthesis and preclinical evaluation of a novel probe [18F]AlF-NOTA-IPB-GPC3P for PET imaging of GPC3 positive tumor

Glypican-3 (GPC3) is markedly overexpressed in hepatocellular carcinoma (HCC) and not expressed in normal liver tissues. In this study, a novel peptide PET imaging agent ([18F]AlF-NOTA-IPB-GPC3P) was developed to target GPC3 expressed in tumors. The overall radiochemical yield of [18F]AlF-NOTA-IPB-GPC3P was 10-15 %, and its lipophilicity, expressed as the logD value at a pH of 7.4, was -1.18 ± 0.06 (n = 3). Compared to the previously reported tracer [18F]AlF-GP2633, [18F]AlF-NOTA-IPB-GPC3P exhibited higher cellular uptake (15.13 vs 5.96) and internalized rate (80.63 % vs 35.93 %) in Huh7 cells at 120 min. Micro-PET/CT and biodistribution studies further demonstrated that [18F]AlF-NOTA-IPB-GPC3P exhibited significantly increased tumor uptake and prolonged tumor residence in Huh7 tumors compared to [18F]AlF-GP2633 (4.66 ± 0.22 % ID/g vs 0.72 ± 0.09 % ID/g at 60 min, p < 0.001; 5.05 ± 0.23 % ID/g vs 0.35 ± 0.08 % ID/g at 120 min, p < 0.001, respectively). Furthermore, the tumor-to-organ ratios of [18F]AlF-NOTA-IPB-GPC3P surpassed those of [18F]AlF-GP2633. Our results support the utilization of [18F]AlF-NOTA-IPB-GPC3P as a PET imaging agent targeting the GPC3 receptor for tumor detection.

Customizable Porphyrin Platform Enables Folate Receptor PET Imaging Using Copper-64

Folate receptors including folate receptor α (FRα) are overexpressed in up to 90% of ovarian cancers. Ovarian cancers overexpressing FRα often exhibit high degrees of drug resistance and poor outcomes. A porphyrin chassis has been developed that is readily customizable according to the desired targeting properties. Thus, compound O5 includes a free base porphyrin, two water-solubilizing groups that project above and below the macrocycle plane, and a folate targeting moiety. Compound O5 was synthesized (>95% purity) and exhibited aqueous solubility of at least 0.48 mM (1 mg/mL). Radiolabeling of O5 with 64Cu in HEPES buffer at 37 °C gave a molar activity of 1000 μCi/μg (88 MBq/nmol). [64Cu]Cu-O5 was stable in human serum for 24 h. Cell uptake studies showed 535 ± 12% bound/mg [64Cu]Cu-O5 in FRα-positive IGROV1 cells when incubated at 0.04 nM. Subcellular fractionation showed that most radioactivity was associated with the cytoplasmic (39.4 ± 2.7%) and chromatin-bound nuclear (53.0 ± 4.2%) fractions. In mice bearing IGROV1 xenografts, PET imaging studies showed clear tumor uptake of [64Cu]Cu-O5 from 1 to 24 h post injection with a low degree of liver uptake. The tumor standardized uptake value at 24 h post injection was 0.34 ± 0.16 versus 0.06 ± 0.07 in the blocking group. In summary, [64Cu]Cu-O5 was synthesized at high molar activity, was stable in serum, exhibited high binding to FRα-overexpressing cells with high nuclear translocation, and gave uptake that was clearly visible in mouse tumor xenografts.

Nano-Drug Delivery Systems Based on Natural Products

Natural products have proven to have significant curative effects and are increasingly considered as potential candidates for clinical prevention, diagnosis, and treatment. Compared with synthetic drugs, natural products not only have diverse structures but also exhibit a range of biological activities against different disease states and molecular targets, making them attractive for development in the field of medicine. Despite advancements in the use of natural products for clinical purposes, there remain obstacles that hinder their full potential. These challenges include issues such as limited solubility and stability when administered orally, as well as short durations of effectiveness. To address these concerns, nano-drug delivery systems have emerged as a promising solution to overcome the barriers faced in the clinical application of natural products. These systems offer notable advantages, such as a large specific surface area, enhanced targeting capabilities, and the ability to achieve sustained and controlled release. Extensive in vitro and in vivo studies have provided further evidence supporting the efficacy and safety of nanoparticle-based systems in delivering natural products in preclinical disease models. This review describes the limitations of natural product applications and the current status of natural products combined with nanotechnology. The latest advances in nano-drug delivery systems for delivery of natural products are considered from three aspects: connecting targeting warheads, self-assembly, and co-delivery. Finally, the challenges faced in the clinical translation of nano-drugs are discussed.

Preclinical Evaluation of 68Ga- and 177Lu-Labeled Integrin αvβ6-Targeting Radiotheranostic Peptides

The integrin αvβ6, an epithelium-specific cell surface receptor, is overexpressed on numerous malignancies, including the highly lethal pancreatic ductal adenocarcinomas. Here, we developed and tested a novel αvβ6-targeting peptide, DOTA-5G (1) radiolabeled with 68Ga, for PET/CT imaging and 177Lu for treatment. With the goal to develop a radiotheranostic, further modifications were made for increased circulation time, renal recycling, and tumor uptake, yielding DOTA-albumin-binding moiety-5G (2). Methods: Peptides 1 and 2 were synthesized on solid phase, and their affinity for αvβ6 was assessed by enzyme-linked immunosorbent assay. The peptides were radiolabeled with 68Ga and 177Lu. In vitro cell binding, internalization, and efflux of 68Ga-1 and 177Lu-2 were evaluated in αvβ6-positive BxPC-3 human pancreatic cancer cells. PET/CT imaging of 68Ga-1 and 68Ga-2 was performed on female nu/nu mice bearing subcutaneous BxPC-3 tumors. Biodistribution was performed for 68Ga-1 (1 and 2 h after injection), 68Ga-2 (2 and 4 h after injection), and 177Lu-1 and 177Lu-2 (1, 24, 48, and 72 h after injection). The 177Lu-2 biodistribution data were extrapolated for human dosimetry data estimates using OLINDA/EXM 1.1. Therapeutic efficacy of 177Lu-2 was evaluated in mice bearing BxPC-3 tumors. Results: Peptides 1 and 2 demonstrated high affinity (<55 nM) for αvβ6 by enzyme-linked immunosorbent assay. 68Ga-1, 68Ga-2, 177Lu-1, and 177Lu-2 were synthesized in high radiochemical purity. Rapid in vitro binding and internalization of 68Ga-1 and 177Lu-2 were observed in BxPC-3 cells. PET/CT imaging and biodistribution studies demonstrated uptake in BxPC-3 tumors. Introduction of the albumin-binding moiety in 177Lu-2 resulted in a 5-fold increase in tumor uptake and retention over time. Based on the extended dosimetry data, the dose-limiting organ for 177Lu-2 is the kidney. Treatment with 177Lu-2 prolonged median survival by 1.5- to 2-fold versus controls. Conclusion: 68Ga-1 and 177Lu-2 demonstrated high affinity for the integrin αvβ6 both in vitro and in vivo, were rapidly internalized into BxPC-3 cells, and were stable in mouse and human serum. Both radiotracers showed favorable pharmacokinetics in preclinical studies, with predominantly renal excretion and good tumor-to-normal-tissue ratios. Favorable human dosimetry data suggest the potential of 177Lu-2 as a treatment for pancreatic ductal adenocarcinoma.

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.

The Effect of Albumin-Binding Moiety on Tumor Targeting and Biodistribution Properties of 67Ga-Labeled Albumin Binder-Conjugated Alpha-Melanocyte-Stimulating Hormone Peptides

Background: The purpose of this study was to examine the effect of 4-p-(tolyl)butyric acid as an albumin-binding (ALB) moiety on tumor targeting and biodistribution properties of ⁶⁷Ga-labeled albumin binder-conjugated alpha-melanocyte-stimulating hormone peptides. Materials and Methods: DOTA-Lys(ALB)-G/GG/GGG-Nle-CycMSH_hex {1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-Lys(ALB)-Gly/GlyGly/GlyGlyGly-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂} were synthesized with 4-p-(tolyl)butyric acid serving as an ALB moiety. The melanocortin-1 receptor (MC1R)-binding affinities of the peptides were determined on B16/F10 melanoma cells. The biodistribution of ⁶⁷Ga-DOTA-Lys(ALB)-G/GG/GGG-Nle-CycMSH_hex was examined on B16/F10 melanoma-bearing C57 mice at 2 h postinjection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of ⁶⁷Ga-DOTA-Lys(ALB)-GGNle-CycMSH_hex {⁶⁷Ga-ALB-G2} were determined on B16/F10 melanoma-bearing C57 mice. Results: The IC₅₀ value of DOTA-Lys(ALB)-G/GG/GGG-Nle-CycMSH_hex {ALB-G1, ALB-G2, ALB-G3} was 0.67 ± 0.07, 0.5 ± 0.09 and 0.51 ± 0.03 nM on B16/F10 cells, respectively. ⁶⁷Ga-ALB-G2 was further evaluated as a lead peptide because of its higher tumor uptake (30.25 ± 3.24%ID/g) and lower kidney uptake (7.09 ± 2.22%ID/g) than ⁶⁷Ga-ALB-G1 and ⁶⁷Ga-ALB-G3 at 2 h postinjection. The B16/F10 melanoma uptake of ⁶⁷Ga-ALB-G2 was 15.64 ± 4.55, 30.25 ± 3.24, 26.76 ± 3.23, and 10.71 ± 1.21%ID/g at 0.5, 2, 4, and 24 h postinjection, respectively. The B16/F10 melanoma lesions were clearly visualized by SPECT/CT using ⁶⁷Ga-ALB-G2 as an imaging probe at 2 h postinjection. Conclusions: The introduction of 4-p-(tolyl)butyric acid as an ALB moiety increased the blood retention, and resulted in higher tumor/kidney ratio of ⁶⁷Ga-ALB-G2 as compared with its counterpart without an albumin binder. However, the resulting high uptake of ⁶⁷Ga-ALB-G2 in blood and liver need to be further reduced to facilitate its therapeutic application when replacing ⁶⁷Ga with therapeutic radionuclides.

Active Targeted Nanoformulations via Folate Receptors: State of the Art and Future Perspectives

In normal tissues, the expression of folate receptors is low and limited to cells that are important for embryonic development or for folate reabsorption. However, in several pathological conditions some cells, such as cancer cells and activated macrophages, overexpress folate receptors (FRs). This overexpression makes them a potential therapeutic target in the treatment of cancer and inflammatory diseases to obtain a selective delivery of drugs at altered cells level, and thus to improve the therapeutic efficacy and decrease the systemic toxicity of the pharmacological treatments. Two strategies have been used to achieve this folate receptor targeting: (i) the use of ligands with high affinity to FRs (e.g., folic acid or anti-FRs monoclonal antibodies) linked to the therapeutic agents or (ii) the use of nanocarriers whose surface is decorated with these ligands and in which the drug is encapsulated. This manuscript analyzes the use of FRs as a target to develop new therapeutic tools in the treatment of cancer and inflammatory diseases with an emphasis on the nanoformulations that have been developed for both therapeutic and imaging purposes.

Evaluation of Copper-64-Labeled αvβ6-Targeting Peptides: Addition of an Albumin Binding Moiety to Improve Pharmacokinetics

The incorporation of non-covalent albumin binding moieties (ABMs) into radiotracers results in increased circulation time, leading to a higher uptake in the target tissues such as the tumor, and, in some cases, reduced kidney retention. We previously developed [¹⁸F]AlF NOTA-K(ABM)-α_vβ₆-BP, where α_vβ₆-BP is a peptide with high affinity for the cell surface receptor integrin α_vβ₆ that is overexpressed in several cancers, and the ABM is an iodophenyl-based moiety. [¹⁸F]AlF NOTA-K(ABM)-α_vβ₆-BP demonstrated prolonged blood circulation compared to the non-ABM parent peptide, resulting in high, α_vβ₆-targeted uptake with continuously improving detection of α_vβ₆(+) tumors using PET/CT. To further extend the imaging window beyond that of fluorine-18 (t_1/2 = 110 min) and to investigate the pharmacokinetics at later time points, we radiolabeled the α_vβ₆-BP with copper-64 (t_1/2 = 12.7 h). Two peptides were synthesized without (1) and with (2) the ABM and radiolabeled with copper-64 to yield [⁶⁴Cu]1 and [⁶⁴Cu]2, respectively. The affinity of [^natCu]1 and [^natCu]2 for the integrin α_vβ₆ was assessed by enzyme-linked immunosorbent assay. [⁶⁴Cu]1 and [⁶⁴Cu]2 were evaluated in vitro (cell binding and internalization) using DX3puroβ6 (α_vβ₆(+)), DX3puro (α_vβ₆(-)), and pancreatic BxPC-3 (α_vβ₆(+)) cells, in an albumin binding assay, and for stability in both mouse and human serum. In vivo (PET/CT imaging) and biodistribution studies were done in mouse models bearing either the paired DX3puroβ6/DX3puro or BxPC-3 xenograft tumors. [⁶⁴Cu]1 and [⁶⁴Cu]2 were synthesized in ≥97% radiochemical purity. In vitro, [^natCu]1 and [^natCu]2 maintained low nanomolar affinity for integrin α_vβ₆ (IC₅₀ = 28 ± 3 and 19 ± 5 nM, respectively); [⁶⁴Cu]1 and [⁶⁴Cu]2 showed comparable binding to α_vβ₆(+) cells (DX3puroβ6: ≥70%, ≥42% internalized; BxPC-3: ≥19%, ≥12% internalized) and ≤3% to the α_vβ₆(-) DX3puro cells. Both radiotracers were ≥98% stable in human serum at 24 h, and [⁶⁴Cu]2 showed a 6-fold higher binding to human serum protein than [⁶⁴Cu]1. In vivo, selective uptake in the α_vβ₆(+) tumors was observed with tumor visualization up to 72 h for [⁶⁴Cu]2. A 3-5-fold higher α_vβ₆(+) tumor uptake of [⁶⁴Cu]2 vs [⁶⁴Cu]1 was observed throughout, at least 2.7-fold improved BxPC-3-to-kidney and BxPC-3-to-blood ratios, and 2-fold improved BxPC-3-to-stomach ratios were noted for [⁶⁴Cu]2 at 48 h. Incorporation of an iodophenyl-based ABM into the α_vβ₆-BP ([⁶⁴Cu]2) prolonged circulation time and resulted in improved pharmacokinetics, including increased uptake in α_vβ₆(+) tumors that enabled visualization of α_vβ₆(+) tumors up to 72 h by PET/CT imaging.

The Development of Nanoparticles for the Detection and Imaging of Ovarian Cancers

Ovarian cancer remains as one of the most lethal gynecological cancers to date, with major challenges associated with screening, diagnosis and treatment of the disease and an urgent need for new technologies that can meet these challenges. Nanomaterials provide new opportunities in diagnosis and therapeutic management of many different types of cancers. In this review, we highlight recent promising developments of nanoparticles designed specifically for the detection or imaging of ovarian cancer that have reached the preclinical stage of development. This includes contrast agents, molecular imaging agents and intraoperative aids that have been designed for integration into standard imaging procedures. While numerous nanoparticle systems have been developed for ovarian cancer detection and imaging, specific design criteria governing nanomaterial targeting, biodistribution and clearance from the peritoneal cavity remain key challenges that need to be overcome before these promising tools can accomplish significant breakthroughs into the clinical setting.

Preparation and Evaluation of Novel Folate Isonitrile 99mTc Complexes as Potential Tumor Imaging Agents to Target Folate Receptors

In order to seek novel technetium-99m folate receptor-targeting agents, two folate derivatives (CN5FA and CNPFA) were synthesized and radiolabeled to obtain [^99mTc]Tc-CN5FA and [^99mTc]Tc-CNPFA complexes, which exhibited high radiochemical purity (>95%) without purification, hydrophilicity, and good stability in vitro. The KB cell competitive binding experiments indicated that [^99mTc]Tc-CN5FA and [^99mTc]Tc-CNPFA had specificity to folate receptor. Biodistribution studies in KB tumor-bearing mice illustrated that [^99mTc]Tc-CN5FA and [^99mTc]Tc-CNPFA had specific tumor uptake. Compared with [^99mTc]Tc-CN5FA, the tumor/muscle ratios of [^99mTc]Tc-CNPFA were higher, resulting in a better SPECT/CT imaging background. According to the results, the two ^99mTc complexes have potential as tumor imaging agents to target folate receptors.

Production, Purification, and Applications of a Potential Theranostic Pair: Cobalt-55 and Cobalt-58m

The emerging success of [⁶⁸Ga/¹⁷⁷Lu]Ga/Lu-DOTATATE as a theranostic pair has spurred interest in other isotopes as potential theranostic combinations. Here, we review cobalt-55 and cobalt-58m as a potential theranostic pair. Radionuclidically pure cobalt-55 and cobalt-58m have been produced on small cyclotrons with high molar activity. In vitro, DOTATOC labeled with cobalt has shown greater affinity for SSTR2 than DOTATOC labeled with gallium and yttrium. Similarly, [^58mCo]Co-DOTATATE has shown improved cell-killing capabilities as compared to DOTATATE labeled with either indium-111 or lutetium-177. Finally, PET imaging with an isotope such as cobalt-55 allows for image acquisition at much later timepoints than gallium, allowing for an increased degree of biological clearance of non-bound radiotracer. We discuss the accelerator targetry and radiochemistry used to produce cobalt-55,58m, emphasizing the implications of these techniques to downstream radiotracers being developed for imaging and therapy.

Design of Neuraminidase-Targeted Imaging and Therapeutic Agents for the Diagnosis and Treatment of Influenza Virus Infections

The last step in influenza virus replication involves the assembly of viral components on the infected cell's plasma membrane followed by budding of intact virus from the host cell surface. Because viral neuraminidase and hemagglutinin are both inserted into the host cell's membrane during this process, influenza virus-infected cells are distinguished from uninfected cells by the presence of viral neuraminidase and hemagglutinin on their cell surfaces. In an effort to exploit this difference in cell surface markers for development of diagnostic and therapeutic agents, we have modified an influenza neuraminidase inhibitor, zanamivir, for targeting of attached imaging and therapeutic agents selectively to influenza viruses and virus-infected cells. We have designed here a zanamivir-conjugated rhodamine dye that allows visual monitoring of binding, internalization, and intracellular trafficking of the fluorescence-labeled neuraminidase in virus-infected cells. We also synthesize a zanamivir-^99mTc radioimaging conjugate that permits whole body imaging of the virus's biodistribution and abundance in infected mice. Finally, we create both a zanamivir-targeted cytotoxic drug (i.e., zanamivir-tubulysin B) and a viral neuraminidase-targeted CAR T cell and demonstrate that they are both able to kill viral neuraminidase-expressing cells without damaging healthy cells. Taken together, these data suggest that the influenza virus neuraminidase inhibitor, zanamivir, can be exploited to improve the diagnosis, imaging, and treatment of influenza virus infections.

A Trifunctional Theranostic Ligand Targeting Fibroblast Activation Protein-α (FAPα)

PURPOSE

Fibroblast activation protein-α (FAPα) is uniquely expressed in activated fibroblasts, including cancer-associated fibroblasts that populate tumor stroma and contribute to proliferation and immunosuppression. Radiolabeled FAPα inhibitors enable imaging of multiple human cancers, but time-dependent clearance from tumors currently limits their utility as FAPα-targeted radiotherapeutics. We sought to increase the area under the curve (AUC) by constructing a trifunctional ligand that binds FAPα with high affinity and also binds albumin and theranostic radiometals.

PROCEDURES

RPS-309 comprised a FAPα-targeting moiety, an albumin-binding group, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Inhibition of recombinant human FAPα (rhFAPα) was determined by colorimetric assay. Affinity for human serum albumin (HSA) was determined by high-performance affinity chromatography. The tissue distribution of [⁶⁸Ga]Ga-RPS-309 in SW872 tumor xenograft-bearing mice was imaged by microPET/CT and quantified by biodistribution studies performed from 30 min to 3 h post injection (p.i.). The biodistribution of [¹⁷⁷Lu]Lu-RPS-309 was determined at 4, 24, and 96 h p.i.

RESULTS

RPS-309 inhibits rhFAPα with IC₅₀ = 7.3 ± 1.4 nM. [⁶⁸Ga]Ga-RPS-309 is taken up specifically by FAPα-expressing cells and binds HSA with K_d = 4.6 ± 0.1 μM. Uptake of the radiolabeled ligand in tumors was evident from 30 min p.i. (> 5 %ID/g) and was significantly reduced by co-injection of RPS-309. Specific skeletal uptake was also observed. Activity in tumors was constant through 4 h p.i., but cleared significantly by 24 h. The AUC in this period was 127 (%ID/g) × h.

CONCLUSIONS

RPS-309 is a high-affinity FAPα inhibitor with prolonged plasma residence. Introduction of the albumin-binding group did not compromise FAPα binding. Although initial tumor uptake was high and FAPα-specific, RPS-309 also progressively cleared from tumors. Nevertheless, RPS-309 incorporates multiple sites in which structural diversity can be introduced, and therefore serves as a platform for future structure-activity relationship studies.

Immunotherapy with Immunocytokines and PD-1 Blockade Enhances the Anticancer Activity of Small Molecule-Drug Conjugates Targeting Carbonic Anhydrase IX

Small molecule-drug conjugates (SMDCs) represent an alternative to conventional antitumor chemotherapeutic agents, with the potential to improve the therapeutic window of cytotoxic payloads through active delivery at the site of the disease. In this article, we describe novel combination therapies consisting of anti-carbonic anhydrase IX SMDCs combined with different immunomodulatory products. The therapeutic effect of the SMDCs was potentiated by combination with PD-1 blockade and with tumor-homing antibody-cytokine fusions in mouse models of renal cell carcinoma and colorectal cancer. The combination with L19-IL12, a fusion protein specific to the alternatively spliced EDB domain of fibronectin containing the murine IL12 moiety, was also active against large established tumors. Analysis of the microscopic structures of healthy organs performed 3 months after tumor eradication confirmed absence of pathologic abnormalities in the healthy kidney, liver, lung, stomach, and intestine. Our findings may be of clinical significance as they provide motivation for the development of combinations based on SMDCs and immunotherapy for the treatment of renal cell carcinoma and hypoxic tumors.

Pharmacokinetic Properties of 68Ga-labelled Folic Acid Conjugates: Improvement Using HEHE Tag

The folate receptor (FR) is a promising cell membrane-associated target for molecular imaging and radionuclide therapy of cancer (FR-α) and potentially also inflammatory diseases (FR-β) through use of folic acid-based radioconjugate. FR is often overexpressed by cells of epithelial tumors, including tumors of ovary, cervix, endometrium, lungs, kidneys, etc. In healthy tissues, FR can be found in small numbers by the epithelial cells, mainly in the kidneys. Extremely high undesired accumulation of the folate radioconjugates in the renal tissue is a main drawback of FR-targeting concept. In the course of this work, we aimed to reduce the undesirable accumulation of folate radioconjugates in the kidneys by introducing a histidine/glutamic acid tag into their structure. Two folic acid based compounds were synthesized: NODAGA-1,4-butanediamine-folic acid (FA-I, as control) and NODAGA-[Lys-(HE)₂]-folic acid (FA-II) which contains a (His-Glu)₂ fragment. In vitro studies with FR (+) cells (KB and others) showed that both compounds have specificity for FR. Introduction of (HE)₂-tag does not affect FR binding ability of the conjugates. In vivo biodistribution studies with normal laboratory animals, as well as with KB tumor bearing animals, were carried out. The results showed that introduction of the (HE)₂ tag into the structure of folate radioconjugates can significantly reduce the accumulation of these compounds in non-target tissues and important organs (the accumulation in the kidneys is reduced 2-4 times), leaving the accumulation in tumor at least at the same level, and even increasing it.

Development of Folate Receptor-Targeted PET Radiopharmaceuticals for Tumor Imaging-A Bench-to-Bedside Journey

The folate receptor-α (FR-α) is overexpressed in many epithelial cancers, including ovary, uterus, kidneys, breast, lung, colon and prostate carcinomas, but shows limited expression in normal tissues such as kidneys, salivary glands, choroid plexus and placenta. FR-α has therefore emerged as a promising target for the delivery of therapeutic and imaging agents to FR-positive tumors. A series of folate-based PET (positron emission tomography) radiopharmaceuticals have been developed for the selective targeting of FR-positive malignancies. This review provides an overview on the research progress made so far regarding the design, radiosynthesis and the utility of the folate-derived PET radioconjugates for targeting FR-positive tumors. For the most part, results from folate radioconjugates labeled with fluorine-18 (t_1/2 = 109.8 min) and gallium-68 (t_1/2 = 67.7 min) have been presented but folates labeled with "exotic" and new PET radionuclides such as copper-64 (t_1/2 = 12.7 h), terbium-152 (t_1/2 = 17.5 h), scandium-44 (t_1/2 = 3.97 h), cobalt-55 (t_1/2 = 17.5 h) and zirconium-89 (t_1/2 = 78.4 h) are also discussed. For tumor imaging, none of the reported PET radiolabeled folates reported to date has made the complete bench-to-bedside journey except [¹⁸F]AzaFol, which made it to patients with metastatic ovarian and lung cancers in a multicenter first-in-human trial. In the near future, however, we expect more clinical trials with folate-based PET radiopharmaceuticals given the increasing clinical interest in imaging and the treatment of FR-related malignancies.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Alpha-PET for Prostate Cancer: Preclinical investigation using 149Tb-PSMA-617

In this study, it was aimed to investigate ¹⁴⁹Tb-PSMA-617 for targeted α-therapy (TAT) using a mouse model of prostate-specific membrane antigen (PSMA)-expressing prostate cancer. ¹⁴⁹Tb-PSMA-617 was prepared with >98% radiochemical purity (6 MBq/nmol) for the treatment of mice with PSMA-positive PC-3 PIP tumors. ¹⁴⁹Tb-PSMA-617 was applied at 1 × 6 MBq (Day 0) or 2 × 3 MBq (Day 0 & Day 1 or Day 0 & Day 3) and the mice were monitored over time until they had reached a pre-defined endpoint which required euthanasia. The tumor growth was significantly delayed in mice of the treated groups as compared to untreated controls (p < 0.05). TAT was most effective in mice injected with 2 × 3 MBq (Day 0 & 1) resulting in a median lifetime of 36 days, whereas in untreated mice, the median lifetime was only 20 days. Due to the β⁺-emission of ¹⁴⁹Tb, tumor localization was feasible using PET/CT after injection of ¹⁴⁹Tb-PSMA-617 (5 MBq). The PET images confirmed the selective accumulation of ¹⁴⁹Tb-PSMA-617 in PC-3 PIP tumor xenografts. The unique characteristics of ¹⁴⁹Tb for TAT make this radionuclide of particular interest for future clinical translation, thereby, potentially enabling PET-based imaging to monitor the radioligand’s tissue distribution.

New 55Co-labeled Albumin-Binding Folate Derivatives as Potential PET Agents for Folate Receptor Imaging

Overexpression of folate receptors (FRs) on different tumor types (e.g., ovarian, lung) make FRs attractive in vivo targets for directed diagnostic/therapeutic agents. Currently, no diagnostic agent suitable for positron emission tomography (PET) has been adopted for clinical FR imaging. In this work, two ⁵⁵Co-labeled albumin-binding folate derivatives-[⁵⁵Co]Co-cm10 and [⁵⁵Co]Co-rf42-with characteristics suitable for PET imaging have been developed and evaluated. High radiochemical yields (≥95%) and in vitro stabilities (≥93%) were achieved for both compounds, and cell assays demonstrated FR-mediated uptake. Both ⁵⁵Co-labeled folate conjugates demonstrated high tumor uptake of 17% injected activity per gram of tissue (IA/g) at 4 h in biodistribution studies performed in KB tumor-bearing mice. Renal uptake was similar to other albumin-binding folate derivatives, and liver uptake was lower than that of previously reported [⁶⁴Cu]Cu-rf42. Small animal PET/CT images confirmed the biodistribution results and showed the clear delineation of FR-expressing tumors.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Radioiodinated Portable Albumin Binder as a Versatile Agent for in Vivo Imaging with Single-Photon Emission Computed Tomography

In this study, radioiodinated 4-( p-iodophenyl)butyric acid ([¹³¹I]IBA) was synthesized and evaluated as a portable albumin-binder for potential applications in single photon emission computed tomography imaging of blood pool, tumor, and lymph node with significantly improved pharmacokinetic properties. The [¹³¹I]IBA was prepared under the catalyst of Cu₂O/1,10-phenanthroline. After that, the albumin-binding capability of [¹³¹I]IBA was tested in vitro, ex vivo, and in vivo, respectively. [¹³¹I]IBA was obtained with very high radiolabeling yield (>99%) and good radiochemical purity (>98%) within 10 min. It binds to albumin effectively with high affinity (IC₅₀= 46.5 μM) and has good stability. The results of biodistribution indicated that the [¹³¹I]IBA was mainly accumulated in blood with good retention (10.51 ± 2.58%ID/g at 30 min p.i. and 4.63 ± 0.17%ID/g at 4 h p.i.). In the SPECT imaging of mice models with [¹³¹I]IBA, blood pool, lymph node, and tumors could be imaged clearly with high target-to-background ratio. Overall, the radioiodinated albumin binder of [¹³¹I]IBA with long blood half-life and excellent stability could be used to decorate diversified albumin-binding radioligands and developed as a versatile theranostic agent.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Positron emission tomography imaging in evaluation of MS pathology in vivo

Positron emission tomography (PET) gives an opportunity to quantitate the expression of specific molecular targets in vivo and longitudinally in brain and thus enhances our possibilities to understand and follow up multiple sclerosis (MS)-related pathology. For successful PET imaging, one needs a relevant target molecule within the brain, to which a blood–brain barrier–penetrating specific radioligand will bind. 18-kDa translocator protein (TSPO)-binding radioligands have been used to detect activated microglial cells at different stages of MS, and remyelination has been measured using amyloid PET. Several PET ligands for the detection of other inflammatory targets, besides TSPO, have been developed but not yet been used for imaging MS patients. Finally, synaptic density evaluation has been successfully tested in human subjects and gives opportunities for the evaluation of the development of cortical and deep gray matter pathology in MS. This review will discuss PET imaging modalities relevant for MS today.

Application of Cu-64 NODAGA-PSMA PET in Prostate Cancer

INTRODUCTION

The high diagnostic potential of ⁶⁴Cu-PSMA PET-CT imaging was clinically investigated in prostate cancer patients with recurrent disease and in the primary staging of selected patients with advanced local disease. The aim of our study is to assess the uptake behavior in the clinical setting of 64Copper Prostate-Specific Membrane Antigen (⁶⁴Cu PSMA) Positron Emission Tomography/Computed Tomography (PET/CT) in prostate cancer.

METHODS

A retrospective study was performed in 23 patients with intermediate, high risk and progressive disease at primary staging of prostate cancer. All patients underwent ⁶⁴Cu-PSMA PET. Overall, 250 MBq (4 MBq per kg bodyweight, range 230-290 MBq) of ⁶⁴Cu-NODAGA PSMA was intravenously applied. PET images were performed 30 min (pelvis and abdomen) and 1-2 h post-injection (skull base to mid-thigh). Maximum standardized uptake values (SUVmax) were measured in the organs with high physiological uptake such as liver and kidney, and, additionally, background activity was measured in the gluteal area and in suspected tumor lesions using a HERMES workstation.

RESULTS

PSMA uptake was detected in prostate bed in nine patients, in six patients in distant metastases (bone, lung and liver) and in nine patients in lymph nodes. Of 23 patients, 5 (20.8%) did not show any focal pathological uptake in the whole body. The number of sites (prostate bed, lymph nodes, distant metastases) with positive PSMA uptake was significantly associated with PSA values before imaging (P = 0.0032). The ⁶⁴Cu PSMA uptake increased significantly from 30 min to 1-3 h post-injection (Wilcoxon signed rank test, P = 0.002).

CONCLUSIONS

⁶⁴Cu NODAGA-PSMA PET is a promising imaging tool in the detection of residual disease in patients with recurrent or primary progressive prostate cancer. Furthermore, the increased tracer uptake over time indicates in vivo stability of the diagnostic radiopharmaceutical.

Albumin-Binding PSMA Ligands: Optimization of the Tissue Distribution Profile

The prostate-specific membrane antigen (PSMA) has emerged as an attractive prostate cancer associated target for radiotheragnostic application using PSMA-specific radioligands. The aim of this study was to design new PSMA ligands modified with an albumin-binding moiety in order to optimize their tissue distribution profile. The compounds were prepared by conjugation of a urea-based PSMA-binding entity, a DOTA chelator, and 4-( p-iodophenyl)butyric acid using multistep solid phase synthesis. The three ligands (PSMA-ALB-02, PSMA-ALB-05, and PSMA-ALB-07) were designed with varying linker entities. Radiolabeling with ¹⁷⁷Lu was performed at a specific activity of up to 50 MBq/nmol resulting in radioligands of >98% radiochemical purity and high stability. In vitro investigations revealed high binding of all three PSMA radioligands to mouse (>64%) and human plasma proteins (>94%). Uptake and internalization into PSMA-positive PC-3 PIP tumor cells was equally high for all radioligands. Negligible accumulation was found in PSMA-negative PC-3 flu cells, indicating PSMA-specific binding of all radioligands. Biodistribution and imaging studies performed in PC-3 PIP/flu tumor-bearing mice showed enhanced blood circulation of the new radioligands when compared to the clinically employed ¹⁷⁷Lu-PSMA-617. The PC-3 PIP tumor uptake of all three radioligands was very high (76.4 ± 2.5% IA/g, 79.4 ± 11.1% IA/g, and 84.6 ± 14.2% IA/g, respectively) at 24 h post injection (p.i.) resulting in tumor-to-blood ratios of ∼176, ∼48, and ∼107, respectively, whereas uptake into PC-3 flu tumors was negligible. Kidney uptake at 24 h p.i. was lowest for ¹⁷⁷Lu-PSMA-ALB-02 (10.7 ± 0.92% IA/g), while ¹⁷⁷Lu-PSMA-ALB-05 and ¹⁷⁷Lu-PSMA-ALB-07 showed higher renal retention (23.9 ± 4.02% IA/g and 51.9 ± 6.34% IA/g, respectively). Tumor-to-background ratios calculated from values of the area under the curve (AUC) of time-dependent biodistribution data were in favor of ¹⁷⁷Lu-PSMA-ALB-02 (tumor-to-blood, 46; tumor-to-kidney, 5.9) when compared to ¹⁷⁷Lu-PSMA-ALB-05 (17 and 3.7, respectively) and ¹⁷⁷Lu-PSMA-ALB-07 (39 and 2.1, respectively). The high accumulation of the radioligands in PC-3 PIP tumors was visualized on SPECT/CT images demonstrating increasing tumor-to-kidney ratios over time. Taking all of the characteristics into account, ¹⁷⁷Lu-PSMA-ALB-02 emerged as the most promising candidate. The applied concept may be attractive for future clinical translation potentially enabling more potent and convenient prostate cancer radionuclide therapy.

Advances in targeting the folate receptor in the treatment/imaging of cancers

The folate receptor (FR) is a recognised biomarker for tumour cells due to its overexpression on a large number of tumours. Consequently, the FR has been exploited by many diagnostic and therapeutic tools to allow targeted delivery to, and imaging of, cancer cells. Herein, we describe the many different approaches by which this has been achieved, including the attachment of folate to potent chemotherapeutic drugs to form FR-targeting small molecule-drug conjugates (SMDCs), FR-targeting antibodies (as antibody alone and as an antibody-drug conjugate), and in the form of complementary nanotechnology-folate platforms; as well as imaging variants thereof. The potential of exploiting the FR for targeted therapy/imaging has the potential to revolutionise the way several cancers are treated. These FR-targeted technologies can also pave the way for inspiring further sophisticated drug conjugates, especially as this receptor is being targeted by use of several complementary technologies: small molecule, nanoparticle and protein-based - thus providing broad and distinct knowledge in the area.

Development of a New FR-Targeting Agent 99mTc-HYNFA with Improved Imaging Contrast and Comparison of Multimerization and/or PEGylation Strategies for Radio-Folate Modification

This study aims to develop a new folate receptor (FR)-targeting agent for SPECT imaging with improved contrast and evaluate the modification strategies of multimerization and/or PEGylation in the development of new radio-folates. A series of novel folate derivatives have been synthesized and radiolabeled with ^99mTc using tricine and TPPTS as coligands. To better investigate their pharmacokinetics, cell uptake, biodistribution, and microSPECT/CT imaging were evaluated. Four radioligands displayed high KB cell uptake after incubation for 2 and 4 h. Presaturated with excess folic acid (FA) resulted in a significant blocking effect in KB cells, indicating specificity of these radioligands toward FR. Biodistribution and microSPECT imaging studies in KB tumor-bearing mice showed that the folate conjugate ^99mTc-HYNFA with poly(ethylene glycol) (PEG) and triazole linkage displayed the highest tumor uptake (16.30 ± 2.01 %ID/g at 2 h p.i. and 14.9 ± 0.62 %ID/g at 4 h p.i. in mice biodistribution) and best imaging contrast, indicating promising application prospect. More interestingly, the in vivo performance of this monomeric ^99mTc-HYNFA was much better than that of FA multimers and non-PEGylated monomers, suggesting that multimerization may not be a feasible method for the design of radio-folates. PEG linkage rather than FA multimerization should be taken into consideration in the development of folate-based radiopharmaceuticals in the future.

Radiolabelled Polymeric Materials for Imaging and Treatment of Cancer: Quo Vadis?

Owing to their tunable blood circulation time and suitable plasma stability, polymer-based nanomaterials hold a great potential for designing and utilising multifunctional nanocarriers for efficient imaging and effective treatment of cancer. When tagged with appropriate radionuclides, they may allow for specific detection (diagnosis) as well as the destruction of tumours (therapy) or even customization of materials, aiming to both diagnosis and therapy (theranostic approach). This review provides an overview of recent developments of radiolabelled polymeric nanomaterials (natural and synthetic polymers) for molecular imaging of cancer, specifically, applying nuclear techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Different approaches to radiolabel polymers are evaluated from the methodical radiochemical point of view. This includes new bifunctional chelating agents (BFCAs) for radiometals as well as novel labelling methods. Special emphasis is given to eligible strategies employed to evade the mononuclear phagocytic system (MPS) in view of efficient targeting. The discussion encompasses promising strategies currently employed as well as emerging possibilities in radionuclide-based cancer therapy. Key issues involved in the clinical translation of radiolabelled polymers and future scopes of this intriguing research field are also discussed.

64Cu-PSMA-617 PET/CT Imaging of Prostate Adenocarcinoma: First In-Human Studies

AIM

The prostate-specific membrane antigen (PSMA) is a cell surface protein, which is overexpressed in nearly all cases of prostate cancer (PCa). PET imaging with ⁶⁸Ga-PSMA-HBED-CC has recently found widespread application in the diagnosis of recurrent PCa. In this study, the diagnostic potential of ⁶⁴Cu-labeled PSMA ligand (PSMA-617) PET in patients with PCa has been investigated.

MATERIALS AND METHODS

The study was conducted simultaneously at two nuclear medicine centers, Austria (Vienna, Center 1) and Germany (Bad Berka, Center 2). The patients (n = 29) included in this study were referred for PET (Center 1, 21 patients) or PET/CT (Center 2, 8 patients) imaging with either a high suspicion of recurrent disease or for possible surgical or PSMA radioligand therapy planning. PET images of the whole body were performed at 1 hour p.i. and additional images of the pelvis at 2 hours p.i.

RESULTS

In 23 of 29 patients, at least one focus of pathological tracer uptake suspicious for primary disease in the prostate lobe or recurrent disease was detected. Among healthy organs, the salivary glands, kidneys, and liver showed the highest radiotracer uptake. Lesions suspicious for PCa were detected with excellent contrast as early as 1 hour p.i. with high detection rates even at low prostate-specific antigen (PSA) levels.

CONCLUSION

The preliminary results of this study demonstrate the high potential of ⁶⁴Cu-PSMA ligand PET/CT imaging in patients with recurrent disease and in the primary staging of selected patients with progressive local disease. The acquired PET images showed an excellent resolution of the detected lesions with very high lesion-to- background contrast. Furthermore, the long half-life of ⁶⁴Cu allows distribution of the tracer to clinical PET centers that lack radiochemistry facilities for the preparation of ⁶⁸Ga-PSMA ligand (satellite concept).