Imaging Bradykinin B1 Receptor with 68Ga-Labeled [des-Arg10]Kallidin Derivatives: Effect of the Linker on Biodistribution and Tumor Uptake

Assessment of Radiolabelled Derivatives of R954 for Detection of Bradykinin B1 Receptor in Cancer Cells: Studies on Glioblastoma Xenografts in Mice

Bradykinin B1 receptor (B1R) has garnered attention as a cancer therapeutic and diagnostic target. Several reports on radiolabelled derivatives of B1R antagonists have shown favourable properties as imaging agents in cells highly expressing hB1R following transfection. In the present study, we assessed whether radiolabelled probes can detect B1R endogenously expressed in cancer cells. To this end, we evaluated 111In-labelled derivatives of a B1R antagonist ([111In]In-DOTA-Ahx-R954) using glioblastoma cell lines (U87MG and U251MG) with different B1R expression levels. Cellular uptake studies showed that the specific accumulation of [111In]In-DOTA-Ahx-R954 in U87MG was higher than that in U251MG, which correlated with B1R expression levels. Tissue distribution in U87MG-bearing mice revealed approximately 2-fold higher radioactivity in tumours than in the muscle in the contralateral leg. The specific accumulation of [111In]In-DOTA-Ahx-R954 in the tumour was demonstrated by the reduction in the tumour-to-plasma ratios in nonlabelled R954-treated mice. Moreover, ex vivo autoradiographic images revealed that the intratumoural distribution of [111In]In-DOTA-Ahx-R954 correlated with the localisation of B1R-expressing glioblastoma cells. In conclusion, we demonstrated that [111In]In-DOTA-Ahx-R954 radioactivity correlated with B1R expression in glioblastoma cells, indicating that radiolabelled derivatives of the B1R antagonist could serve as promising tools for elucidating the involvement of B1R in cancer.

Synthesis and Evaluation of Novel 68Ga-Labeled [D-Phe6,Leu13ψThz14]bombesin(6-14) Analogs for Cancer Imaging with Positron Emission Tomography

Gastrin-releasing peptide receptor (GRPR) is overexpressed in various cancers and is a promising target for cancer diagnosis and therapy. However, the high pancreas uptake and/or metabolic instability observed for most reported GRPR-targeted radioligands might limit their clinical applications. Our group recently reported a GRPR-targeted antagonist tracer, [68Ga]Ga-TacsBOMB2 ([68Ga]Ga-DOTA-Pip-D-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Leu13ψThz14-NH2), which showed a minimal pancreas uptake in a preclinical mouse model. In this study, we synthesized four derivatives with unnatural amino acid substitutions (Tle10-derived Ga-LW01158, NMe-His12-derived Ga-LW01160, α-Me-Trp8- and Tle10-derived Ga-LW01186, and Tle10- and N-Me-Gly11-derived Ga-LW02002) and evaluated their potential for detecting GRPR-expressing tumors with positron emission tomography (PET). The binding affinities (Ki(GRPR)) of Ga-LW01158, Ga-LW01160, Ga-LW01186, and Ga-LW02002 were 5.11 ± 0.47, 187 ± 17.8, 6.94 ± 0.95, and 11.0 ± 0.39 nM, respectively. [68Ga]Ga-LW01158, [68Ga]Ga-LW01186, and [68Ga]Ga-LW02002 enabled clear visualization of subcutaneously implanted human prostate cancer PC-3 tumor xenografts in mice in PET images. Ex vivo biodistribution studies showed that [68Ga]Ga-LW01158 had the highest tumor uptake (11.2 ± 0.65 %ID/g) and good tumor-to-background uptake ratios at 1 h post-injection. Comparable in vivo stabilities were observed for [68Ga]Ga-LW01158, [68Ga]Ga-LW01186, and [68Ga]Ga-LW02002 (76.5-80.7% remaining intact in mouse plasma at 15 min post-injection). In summary, the Tle10 substitution, either alone or combined with α-Me-Trp8 or NMe-Gly11 substitution, in Ga-TacsBOMB2 generates derivatives that retained good GRPR binding affinity and in vivo stability. With good tumor uptake and tumor-to-background imaging contrast, [68Ga]Ga-LW01158 is promising for detecting GRPR-expressing lesions with PET.

[212Pb]Pb-eSOMA-01: A Promising Radioligand for Targeted Alpha Therapy of Neuroendocrine Tumors

Peptide receptor radionuclide therapy (PRRT) has been applied to the treatment of neuroendocrine tumors (NETs) for over two decades. However, improvement is still needed, and targeted alpha therapy (TAT) with alpha emitters such as lead-212 (²¹²Pb) represents a promising avenue. A series of ligands based on octreotate was developed. Lead-203 was used as an imaging surrogate for the selection of the best candidate for the studies with lead-212. ^203/212Pb radiolabeling and in vitro assays were carried out, followed by SPECT/CT imaging and ex vivo biodistribution in NCI-H69 tumor-bearing mice. High radiochemical yields (≥99%) and purity (≥96%) were obtained for all ligands. [²⁰³Pb]Pb-eSOMA-01 and [²⁰³Pb]Pb-eSOMA-02 showed high stability in PBS and mouse serum up to 24 h, whereas [²⁰³Pb]Pb-eSOMA-03 was unstable in those conditions. All compounds exhibited a nanomolar affinity (2.5-3.1 nM) for SSTR2. SPECT/CT images revealed high tumor uptake at 1, 4, and 24 h post-injection of [²⁰³Pb]Pb-eSOMA-01/02. Ex vivo biodistribution studies confirmed that the highest uptake in tumors was observed with [²¹²Pb]Pb-eSOMA-01. [²¹²Pb]Pb-eESOMA-01 displayed the highest absorbed dose in the tumor (35.49 Gy/MBq) and the lowest absorbed dose in the kidneys (121.73 Gy/MBq) among the three tested radioligands. [²¹²Pb]Pb-eSOMA-01 is a promising candidate for targeted alpha therapy of NETs. Further investigations are required to confirm its potential.

68Ga-Labeled [Thz14]Bombesin(7-14) Analogs: Promising GRPR-Targeting Agonist PET Tracers with Low Pancreas Uptake

With overexpression in various cancers, the gastrin-releasing peptide receptor (GRPR) is a promising target for cancer imaging and therapy. However, the high pancreas uptake of reported GRPR-targeting radioligands limits their clinical application. Our goal was to develop ⁶⁸Ga-labeled agonist tracers for detecting GRPR-expressing tumors with positron emission tomography (PET), and compare them with the clinically validated agonist PET tracer, [⁶⁸Ga]Ga-AMBA. Ga-TacBOMB2, TacBOMB3, and TacBOMB4, derived from [Thz¹⁴]Bombesin(7-14), were confirmed to be GRPR agonists by a calcium mobilization study, and their binding affinities (K_i(GRPR)) were determined to be 7.62 ± 0.19, 6.02 ± 0.59, and 590 ± 36.5 nM, respectively, via in vitro competition binding assays. [⁶⁸Ga]Ga-TacBOMB2, [⁶⁸Ga]Ga-TacBOMB3, and [⁶⁸Ga]Ga-AMBA clearly visualized PC-3 tumor xenografts in a PET imaging study. [⁶⁸Ga]Ga-TacBOMB2 showed comparable tumor uptake but superior tumor-to-background contrast ratios when compared to [⁶⁸Ga]Ga-AMBA. Moreover, [⁶⁸Ga]Ga-TacBOMB2 and [⁶⁸Ga]Ga-TacBOMB3 showed a much lower rate of uptake in the pancreas (1.30 ± 0.14 and 2.41 ± 0.72%ID/g, respectively) than [⁶⁸Ga]Ga-AMBA (62.4 ± 4.26%ID/g). In conclusion, replacing Met¹⁴ in the GRPR-targeting sequence with Thz¹⁴ retains high GRPR-binding affinity and agonist properties. With good tumor uptake and tumor-to-background uptake ratios, [⁶⁸Ga]Ga-TacBOMB2 is promising for detecting GRPR-expressing tumors. The much lower pancreas uptake of [⁶⁸Ga]Ga-TacBOMB2 and [⁶⁸Ga]Ga-TacBOMB3 suggests that [Thz¹⁴]Bombesin(7-14) is a promising targeting vector for the design of GRPR-targeting radiopharmaceuticals, especially for radioligand therapy application.

68Ga-Labeled [Leu13ψThz14]Bombesin(7-14) Derivatives: Promising GRPR-Targeting PET Tracers with Low Pancreas Uptake

The gastrin-releasing peptide receptor (GRPR) is a G-protein-coupled receptor that is overexpressed in many solid cancers and is a promising target for cancer imaging and therapy. However, high pancreas uptake is a major concern in the application of reported GRPR-targeting radiopharmaceuticals, particularly for targeted radioligand therapy. To lower pancreas uptake, we explored Ga-complexed TacsBOMB2, TacsBOMB3, TacsBOMB4, TacsBOMB5, and TacsBOMB6 derived from a potent GRPR antagonist sequence, [Leu¹³ψThz¹⁴]Bombesin(7-14), and compared their potential for cancer imaging with [⁶⁸Ga]Ga-RM2. The K_i(GRPR) values of Ga-TacsBOMB2, Ga-TacsBOMB3, Ga-TacsBOMB4, Ga-TacsBOMB5, Ga-TacsBOMB6, and Ga-RM2 were 7.08 ± 0.65, 4.29 ± 0.46, 458 ± 38.6, 6.09 ± 0.95, 5.12 ± 0.57, and 1.51 ± 0.24 nM, respectively. [⁶⁸Ga]Ga-TacsBOMB2, [⁶⁸Ga]Ga-TacsBOMB3, [⁶⁸Ga]Ga-TacsBOMB5, [⁶⁸Ga]Ga-TacsBOMB6, and [⁶⁸Ga]Ga-RM2 clearly show PC-3 tumor xenografts in positron emission tomography (PET) images, while [⁶⁸Ga]Ga-TacsBOMB5 shows the highest tumor uptake (15.7 ± 2.17 %ID/g) among them. Most importantly, the pancreas uptake values of [⁶⁸Ga]Ga-TacsBOMB2 (2.81 ± 0.78 %ID/g), [⁶⁸Ga]Ga-TacsBOMB3 (7.26 ± 1.00 %ID/g), [⁶⁸Ga]Ga-TacsBOMB5 (1.98 ± 0.10 %ID/g), and [⁶⁸Ga]Ga-TacsBOMB6 (6.50 ± 0.36 %ID/g) were much lower than the value of [⁶⁸Ga]Ga-RM2 (41.9 ± 10.1 %ID/g). Among the tested [Leu¹³ψThz¹⁴]Bombesin(7-14) derivatives, [⁶⁸Ga]Ga-TacsBOMB5 has the highest tumor uptake and tumor-to-background contrast ratios, which is promising for clinical translation to detect GRPR-expressing tumors. Due to the low pancreas uptake of its derivatives, [Leu¹³ψThz¹⁴]Bombesin(7-14) represents a promising pharmacophore for the design of GRPR-targeting radiopharmaceuticals, especially for targeted radioligand therapy application.

A Novel Radiotracer for Molecular Imaging and Therapy of Gastrin-Releasing Peptide Receptor Positive Prostate Cancer

The gastrin-releasing peptide receptor (GRPR) is overexpressed in many solid malignancies, particularly in prostate and breast cancers, among others. We synthesized ProBOMB2, a novel bombesin derivative radiolabeled with ⁶⁸Ga and ¹⁷⁷Lu, and evaluated its ability to target GRPR in a preclinical model of human prostate cancer. Methods: ProBOMB2 was synthesized on solid phase using Fmoc chemistry. The chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid was coupled to the N-terminus and separated from the GRPR-targeting sequence by a cationic 4-amino-(1-carboxymethyl)-piperidine spacer. Binding affinity for both human and murine GRPR was determined using a cell-based competition assay, while a calcium efflux assay was used to measure the agonist/antagonist properties of the derivatives. ProBOMB2 was radiolabeled with ¹⁷⁷Lu and ⁶⁸Ga. SPECT and PET imaging, and biodistribution studies were conducted using a preclinical prostate cancer model of male immunocompromised mice bearing GRPR-positive PC-3 human prostate cancer xenografts. Results: Ga-ProBOMB2 and Lu-ProBOMB2 bound to PC-3 cells with a Ki of 4.58±0.67 and 7.29±1.73 nM, respectively. ⁶⁸Ga-ProBOMB2 and ¹⁷⁷Lu-ProBOMB2 were radiolabeled with a radiochemical purity greater than 95%. Both radiotracers were primarily excreted via the renal pathway. PET images of PC-3 tumor xenografts were visualized with excellent contrast at 1 h and 2 h post-injection (p.i.) with ⁶⁸Ga-ProBOMB2, and very low off-target organ accumulation. ¹⁷⁷Lu-ProBOMB2 enabled clear visualization of PC-3 tumor xenografts by SPECT imaging at 1 h, 4 h, and 24 h p.i. ¹⁷⁷Lu-ProBOMB2 displayed higher tumor uptake than ⁶⁸Ga-ProBOMB2 at 1 h p.i. ¹⁷⁷Lu-ProBOMB2 tumor uptake at 1 h, 4 h, and 24 h p.i. was 14.9±3.1, 4.8±2.1, and 1.7±0.3 %ID/g, respectively. Conclusion: ⁶⁸Ga-ProBOMB2 and ¹⁷⁷Lu-ProBOMB2 are promising radiotracers with limited pancreas uptake, good tumor uptake, and favorable pharmacokinetics for imaging and therapy of GRPR-expressing tumors.

Design and Evaluation of Rhein-Based MRI Contrast Agents for Visualization of Tumor Necrosis Induced by Combretastatin A-4 Disodium Phosphate

PURPOSE

Visualization of tumor necrosis can determine tumor response to therapy. Our previous study showed that the rhein-based magnetic resonance imaging (MRI) contrast agent with alkane linker (GdL2) could clearly image tumor necrosis. However, its water solubility and cell safety needed to be improved. Herein, three rhein-based MRI agents with ether or lysine linkers were designed.

PROCEDURES

Three rhein-based MRI agents were synthesized with a tetracarbon ether (GdP1), a hexacarbon ether (GdP2), and a lysine (GdP3) linker, respectively. Their octanol-water partition coefficients (log P) and cytotoxicity were determined. Necrosis avidity of the leading agent was explored on HepG2 cells and ischemia reperfusion-induced liver necrosis (IRLN) rats by MRI. The effect of visualization of tumor necrosis was tested on nude mice with W256 tumor treated by combretastatin-A4 phosphate (CA4P). DNA binding assays were applied to evaluate the possible necrosis-avidity mechanism of the leading agent.

RESULTS

The log P of three agents (- 1.66 ± 0.09, - 1.74 ± 0.01, - 1.95 ± 0.01) decreased when compared with GdL2, indicating higher water solubility. GdP1 not only presented lower cytotoxicity and good necrotic affinity in vitro and in vivo, but also can be fast excreted by renal. According to MRI results of tumor, distinct visualization of tumor necrosis can be discernible from 3 to 4.5 h post-injection of GdP1. In DNA-binding assays, the fluorescence quenching constant KSV (1.00 × 104 M-1) and the ultraviolet binding constant Kb (1.11 × 104 M-1) suggested that GdP1 may bind to DNA through intercalation.

CONCLUSION

GdP1 may serve as a potential candidate for early evaluation of tumor response to CA4P treatment.

177Lu-Labeled Albumin-Binder-Conjugated PSMA-Targeting Agents with Extremely High Tumor Uptake and Enhanced Tumor-to-Kidney Absorbed Dose Ratio

The use of an albumin binder has been shown to improve tumor uptake of prostate-specific membrane antigen (PSMA)-targeting radiotherapeutic agents. The aim of this study was to develop improved radiotherapeutic agents that combine an optimized affinity-modifying group and optimized albumin binders to maximize the tumor-to-kidney absorbed dose ratio. Methods:68Ga-labeled DOTA-conjugated lysine-ureido-glutamate-based PSMA-targeting agents bearing various affinity-modifying groups or albumin binders were synthesized and evaluated by PET/CT imaging and biodistribution studies in LNCaP tumor-bearing mice. The optimized affinity-modifying group and albumin binders were combined, and the resulting derivatives were radiolabeled with 177Lu and evaluated by SPECT/CT imaging and biodistribution studies in LNCaP tumor-bearing mice. Radiation dosimetry was calculated using the OLINDA/EXM software. Results: Affinity-modifying group optimization revealed that 68Ga-HTK03041 bearing a tranexamic acid-9-anthrylalanine affinity-modifying group had the highest tumor uptake (23.1 ± 6.11 percentage injected dose [%ID]/g at 1 h after injection). Albumin binder optimization showed that 68Ga-HTK03055 and 68Ga-HTK03086 bearing the N-(4-(p-chlorophenyl)butanoyl)-Gly and N-(4-(p-methoxyphenyl)butanoyl)-Gly motifs, respectively, had relatively faster tumor accumulation (∼30 %ID/g at 3 h after injection) and lower average kidney uptake (<55 %ID/g at both 1 and 3 h after injection). Combining the tranexamic acid-9-anthrylalanine affinity-modifying group with N-(4-(p-chlorophenyl)butanoyl)-Gly and N-(4-(p-methoxyphenyl)butanoyl)-Gly albumin-binding motifs generated HTK03121 and HTK03123, respectively. 177Lu-HTK03121 and 177Lu-HTK03123 had extremely high peak uptake (104 ± 20.3 and 70.8 ± 23.7 %ID/g, respectively) in LNCaP tumor xenografts, and this peak was sustained up to 120 h after injection. Dosimetry calculation showed that compared with 177Lu-PSMA-617, 177Lu-HTK03121 and 177Lu-HTK03123 delivered 18.7- and 12.7-fold higher absorbed dose to tumor but only 6.4- and 6.3-fold higher absorbed dose to kidneys, leading to 2.9- and 2.0-fold improvement in the tumor-to-kidney absorbed dose ratios. Conclusion: With greatly enhanced tumor uptake and tumor-to-kidney absorbed dose ratio, 177Lu-HTK03121 and 177Lu-HTK03123 have the potential to improve treatment efficacy using significantly lower quantities of 177Lu and are promising candidates for clinical translation to treat metastatic castration-resistant prostate cancer.

A Systematic Review of Molecular Imaging Agents Targeting Bradykinin B1 and B2 Receptors

Kinins, bradykinin and kallidin are vasoactive peptides that signal through the bradykinin B1 and B2 receptors (B1R and B2R). B2R is constitutively expressed in healthy tissues and mediates responses such as vasodilation, fluid balance and retention, smooth muscle contraction, and algesia, while B1R is absent in normal tissues and is induced by tissue trauma or inflammation. B2R is activated by kinins, while B1R is activated by kinins that lack the C-terminal arginine residue. Perturbations of the kinin system have been implicated in inflammation, chronic pain, vasculopathy, neuropathy, obesity, diabetes, and cancer. In general, excess activation and signaling of the kinin system lead to a pro-inflammatory state. Depending on the disease context, agonism or antagonism of the bradykinin receptors have been considered as therapeutic options. In this review, we summarize molecular imaging agents targeting these G protein-coupled receptors, including optical and radioactive probes that have been used to interrogate B1R/B2R expression at the cellular and anatomical levels, respectively. Several of these preclinical agents, described herein, have the potential to guide therapeutic interventions for these receptors.

Positron Emission Tomography Imaging of the Gastrin-Releasing Peptide Receptor with a Novel Bombesin Analogue

The gastrin-releasing peptide receptor (GRPR), a G protein-coupled receptor, is overexpressed in solid malignancies and particularly in prostate cancer. We synthesized a novel bombesin derivative, [⁶⁸Ga]Ga-ProBOMB1, evaluated its pharmacokinetics and potential to image GRPR expression with positron emission tomography (PET), and compared it with [⁶⁸Ga]Ga-NeoBOMB1. ProBOMB1 (DOTA-pABzA-DIG-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ(CH₂N)-Pro-NH₂) was synthesized by solid-phase peptide synthesis. The polyaminocarboxylate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to the N-terminal and separated from the GRPR-targeting sequence by a p-aminomethylaniline-diglycolic acid (pABzA-DIG) linker. The binding affinity to GRPR was determined using a cell-based competition assay, whereas the agonist/antagonist property was determined with a calcium efflux assay. ProBOMB1 was radiolabeled with ⁶⁸GaCl₃. PET imaging and biodistribution studies were performed in male immunocompromised mice bearing PC-3 prostate cancer xenografts. Blocking experiments were performed with coinjection of [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14). Dosimetry calculations were performed with OLINDA software. ProBOMB1 and the nonradioactive Ga-ProBOMB were obtained in 1.1 and 67% yield, respectively. The K _i value of Ga-ProBOMB1 for GRPR was 3.97 ± 0.76 nM. Ga-ProBOMB1 behaved as an antagonist for GRPR. [⁶⁸Ga]Ga-ProBOMB1 was obtained in 48.2 ± 10.9% decay-corrected radiochemical yield with 121 ± 46.9 GBq/μmol molar activity and >95% radiochemical purity. Imaging/biodistribution studies showed that the excretion of [⁶⁸Ga]Ga-ProBOMB1 was primarily through the renal pathway. At 1 h postinjection (p.i.), PC-3 tumor xenografts were clearly delineated in PET images with excellent contrast. The tumor uptake for [⁶⁸Ga]Ga-ProBOMB1 was 8.17 ± 2.57 percent injected dose per gram (% ID/g) and 9.83 ± 1.48% ID/g for [⁶⁸Ga]Ga-NeoBOMB1, based on biodistribution studies at 1 h p.i. This corresponded to tumor-to-blood and tumor-to-muscle uptake ratios of 20.6 ± 6.79 and 106 ± 57.7 for [⁶⁸Ga]Ga-ProBOMB1 and 8.38 ± 0.78 and 39.0 ± 12.6 for [⁶⁸Ga]Ga-NeoBOMB1, respectively. Blockade with [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14) significantly reduced the average uptake of [⁶⁸Ga]Ga-ProBOMB1 in tumors by 62%. The total absorbed dose was lower for [⁶⁸Ga]Ga-ProBOMB1 in all organs except for bladder compared with [⁶⁸Ga]Ga-NeoBOMB1. Our data suggest that [⁶⁸Ga]Ga-ProBOMB1 is an excellent radiotracer for imaging GRPR expression with PET. [⁶⁸Ga]Ga-ProBOMB1 achieved a similar uptake as [⁶⁸Ga]Ga-NeoBOMB1 in tumors, with enhanced contrast and lower whole-body absorbed dose.

Design, synthesis and evaluation of 18F-labeled cationic carbonic anhydrase IX inhibitors for PET imaging

Carbonic anhydrase IX (CA-IX) is a marker for tumor hypoxia, and its expression is negatively correlated with patient survival. CA-IX represents a potential target for eliminating hypoxic cancers. We synthesized fluorinated cationic sulfonamide inhibitors 1-3 designed to target CA-IX. The binding affinity for CA-IX ranged from 0.22 to 0.96 μM. We evaluated compound 2 as a diagnostic PET imaging agent. Compound 2 was radiolabeled with ¹⁸F in 10 ± 4% decay-corrected radiochemical yield with 85.1 ± 70.3 GBq/μmol specific activity and >98% radiochemical purity. ¹⁸F-labeled 2 was stable in mouse plasma at 37 °C after 1 h incubation. PET/CT imaging was conducted at 1 h post-injection in a human colorectal cancer xenograft model. ¹⁸F-labeled 2 cleared through hepatobiliary and renal pathways. Tumor uptake was approximately 0.41 ± 0.06% ID/g, with a tumor-to-muscle ratio of 1.99 ± 0.25. Subsequently, tumor xenografts were visualized with moderate contrast. This study demonstrates the use of a cationic motif for conferring isoform selectively for CA-IX imaging agents.

Preclinical Melanoma Imaging with 68Ga-Labeled α-Melanocyte-Stimulating Hormone Derivatives Using PET

It is estimated that melanoma accounted for 76,380 new cases and 10,130 deaths in the United States in 2016. The melanocortin 1 receptor (MC1R) is highly expressed in the vast majority of melanomas, which makes it an attractive target for molecular imaging and radionuclide therapy. Lactam bridge-cyclized α-melanocyte-stimulating hormone (Ac-Nle⁴-cyclo[Asp⁵-His-D-Phe⁷-Arg-Trp-Lys¹⁰]-NH₂, or Nle-CycMSH_hex) analogues have been successfully developed and studied for MC1R-targeted imaging, predominantly with single-photon emission computed tomography (SPECT). The goal of this study was to design and evaluate novel peptides for melanoma imaging with positron emission tomography (PET). We designed and synthesized three peptides, DOTA-PEG₂-Nle-CycMSH_hex (CCZ01047), DOTA-4-amino-(1-carboxymethyl) piperidine (Pip)-Nle-CycMSH_hex (CCZ01048), and DOTA-Pip-Pip-Nle-CycMSH_hex (CCZ01056). All three peptides exhibited high binding affinity to MC1R with sub-nanomolar K_i values, rapid internalization into B16F10 melanoma cells and high in vivo stability with more than 93% remaining intact at 15 min post-injection (p.i.) in blood plasma. All three ⁶⁸Ga-labeled tracers produced high contrast PET images in C57BL/6J mice bearing B16F10 tumors, and their respective tumor uptakes were 8.0 ± 3.0, 12.3 ± 3.3, and 6.5 ± 1.4 %ID/g at 1 h p.i. Minimal normal organ activity was observed at 1 h p.i., except for kidneys (5.1 ± 1.4, 4.7 ± 0.5, and 6.2 ± 2.0 %ID/g, respectively), and thyroid (4.1 ± 0.6 %ID/g for CCZ01047 and 2.4 ± 0.6 %ID/g for CCZ01048). Due to high accumulation at tumor sites and rapid background clearance of ⁶⁸Ga-CCZ01048, we further evaluated it at 2 h p.i., and a tumor uptake of 21.9 ± 4.6 %ID/g was observed, with background activity further decreased. Exceptional image contrast was also achieved, i.e. tumor-to-blood, tumor-to-muscle, tumor-to-bone and tumor-to-kidney ratios were 96.4 ± 13.9, 210.9 ± 20.9, 39.6 ± 11.9 and 4.0 ± 0.9, respectively. A blocking study was also performed by co-injection of excess amount of non-radioactive Ga-coupled of CCZ01048, which confirmed that the tumor uptake was MC1R mediated. In conclusion, the introduction of a cationic Pip linker to Nle-CycMSH_hex, CCZ01048, not only improved tumor uptake, but also generated high tumor-to-normal tissue contrast with PET imaging in a preclinical melanoma model. Therefore, CCZ01048 is a promising candidate for PET imaging of melanoma, and potentially as a theranostic agent for radionuclide therapy of melanoma when labeled with α or β emitters.

Radiolabeled R954 Derivatives for Imaging Bradykinin B1 Receptor Expression with Positron Emission Tomography

Peptide receptors have emerged as promising targets for diagnosis and therapy. The aberrant overexpression of these receptors in different cancer subtypes allows for the adoption of new treatment strategies that complement conventional chemotherapies. Bradykinin B1 receptor (B1R) is a G protein-coupled receptor that is overexpressed in many cancers, with limited expression in healthy tissues. Previously, we developed ⁶⁸Ga- and ¹⁸F-labeled derivatives of B1R antagonist peptides B9858 and B9958, and successfully targeted B1R-expressing tumor xenografts in vivo. R954 (Ac-Orn-Arg-Oic-Pro-Gly-αMePhe-Ser-d-2-Nal-Ile), a potent B1R antagonist, is reportedly more stable than B9858 against peptidase degradation. We evaluated two radiolabeled derivatives of R954 (⁶⁸Ga-HTK01083 and ¹⁸F-HTK01146) for B1R PET imaging. Peptides were synthesized via solid phase strategy. Nonradioactive standards were obtain by reacting GaCl₃ with DOTA-dPEG2-R954 and by clicking N-propargyl-N,N-dimethylammoniomethyl-trifluoroborate with azidoacetyl-dPEG2-R954. Binding affinity for B1R was determined by an in vitro competition binding assay. ⁶⁸Ga-HTK01083 was obtained by incubating DOTA-dPEG2-R954 with ⁶⁸GaCl₃ under acidic conditions, while ¹⁸F-HTK01146 was prepared via an ¹⁸F-¹⁹F isotope exchange reaction. Biodistribution and imaging studies were conducted at 1 h postinjection (p.i.) in mice inoculated with B1R-expressing (B1R+) and B1R-nonexpressing (B1R-) cells. HTK01083 and HTK01146 bound B1R with good affinity (K_i = 30.5 and 24.8 nM, respectively). ⁶⁸Ga/¹⁸F-labeled R954 were obtained on average in ≥10% decay-corrected radiochemical yield with >99% radiochemical purity and ≥52 GBq/μmol specific activity. For both tracers, clearance was predominantly renal with minimal involvement of the hepatobiliary system. For PET images, B1R+ tumors, kidneys, and bladder were visible. At 1 h p.i., uptake in B1R+ tumor was comparable between ⁶⁸Ga-HTK01083 (8.46 ± 1.44%ID/g) and ¹⁸F-HTK01146 (9.25 ± 0.69%ID/g). B1R+ tumor-to-blood and B1R+ tumor-to-muscle ratios were 6.32 ± 1.44 and 20.7 ± 3.58 for ⁶⁸Ga-HTK01083, and 7.24 ± 2.56 and 19.5 ± 4.29 for ¹⁸F-HTK01146. Our results indicate R954 is a good lead sequence for optimization of B1R tracers for cancer imaging.

Synthesis and evaluation of a 68Ga-labeled bradykinin B1 receptor agonist for imaging with positron emission tomography

A novel ⁶⁸Ga-labeled bradykinin B1 receptor (B1R) agonist, ⁶⁸Ga-Z01115, was synthesized and evaluated for imaging with positron emission tomography (PET). Z01115 exhibited good binding affinity (K_i=25.4±5.1nM) to hB1R. ⁶⁸Ga-Z01115 was prepared in 74±5 decay-corrected radiochemical yield with >99% radiochemical purity and 155±89GBq/µmol (4.2±2.4Ci/μmol) specific activity. ⁶⁸Ga-Z01115 was stable in vitro in mouse plasma (93% remaining intact after 60min incubation), and relatively stable in vivo (51±5% remaining intact at 5min post-injection). PET imaging and biodistribution studies in mice showed that ⁶⁸Ga-Z01115 cleared rapidly from nontarget tissues/organs, and generated high target-to-nontarget contrast images. The uptake of ⁶⁸Ga-Z01115 in B1R-positive (B1R+) tumor was 5.65±0.59%ID/g at 1h post-injection. Average contrast ratios of B1R+ tumor-to-B1R- tumor, -to-blood and -to-muscle were 24.3, 24.4 and 82.9, respectively. Uptake of ⁶⁸Ga-Z01115 in B1R+ tumors was reduced by ∼90% with co-injection of cold standard, confirming it was mediated by B1R. Our data suggest that ⁶⁸Ga-Z01115 is a promising tracer for imaging the expression of B1R that is overexpressed in a variety of cancers.

ATP/pH Dual Responsive Nanoparticle with d-[des-Arg10 ]Kallidin Mediated Efficient In Vivo Targeting Drug Delivery

Inflammation has been reported as one significant hallmark of breast cancer in relation to tumor development, metastasis, and invasion. The bradykinin receptor 1 (B1R) is highly expressed on inflammatory breast tumor cells thus providing a promising targeting site for tumor recognition and sufficient receptor mediated endocytosis. In this study, the authors evaluate the targeting efficiency of l-form and d-form [des-Arg¹⁰ ]kallidin both in vitro and in vivo. To further improve the drug delivery efficiency, the authors establish a dandelion like nanoparticle by combining the polymeric drug conjugates and aptamer complex together. The doxorubicin conjugated polymer is complexed with adenosine-5'-triphosphate (ATP) sensitive hybridized aptamer in self-assembly process by intercalating into the double strand scaffolds. The acid labile conjugating bond and ATP sensitive aptamer endow the nanoparticle with dual responsiveness to intracellular milieu, thus triggering a quick drug release in tumor cells. Remarkable therapeutic effects and tuned in vivo pharmacokinetics profiles are shown by the aptamer complexed drug conjugates nanoparticle with B1R active targeting modification. Therefore the strategies of B1R targeting and ATP/pH dual-responsiveness nanoparticle help achieve enhanced drug accumulation within tumor cells and efficient chemotherapy for breast cancer.