Preclinical evaluation of MC1R targeting theranostic pair [Tb]Tb-crown-αMSH and [Tb]Tb-crown-αMSH.

Background: Targeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (Tb, Tb, Tb, Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving Tb- and Tb-labeled crown-αMSH, a small peptide-based bioconjugate suitable for targeting melanoma.

5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II.

With peptides increasingly favored as drugs, natural product motifs, namely the tryptathionine staple, found in amatoxins and phallotoxins, and the 2,2'-bis-indole found in staurosporine represent unexplored staples for unnatural peptide macrocycles. We disclose the efficient condensation of a 5-hydroxypyrroloindoline with either a cysteine-thiol or a tryptophan-indole to form a tryptathionine or 2-2'-bis-indole staple. Judicious use of protecting groups provides for chemoselective stapling using α-MSH, which provides a basis for investigating both chemoselectivity and affinity.

Synthesis and Evaluation of Ga-Labeled (2,4)-4-Fluoropyrrolidine-2-Carbonitrile and (4)-Thiazolidine-4-Carbonitrile Derivatives as Novel Fibroblast Activation Protein-Targeted PET Tracers for Cancer Imaging.

Fibroblast activation protein α (FAP-α) is a cell-surface protein overexpressed on cancer-associated fibroblasts that constitute a substantial component of tumor stroma and drive tumorigenesis. FAP is minimally expressed by most healthy tissues, including normal fibroblasts. This makes it a promising pan-cancer diagnostic and therapeutic target.

Novel Ga-Labeled Pyridine-Based Fibroblast Activation Protein-Targeted Tracers with High Tumor-to-Background Contrast.

Compared to quinoline-based fibroblast activation protein (FAP)-targeted radiotracers, pyridine-based FAP-targeted tracers are expected to have faster pharmacokinetics due to their smaller molecular size and higher hydrophilicity, which we hypothesize would improve the tumor-to-background image contrast. We aim to develop Ga-labeled pyridine-based FAP-targeted tracers for cancer imaging with positron emission tomography (PET), and compare their imaging potential with the clinically validated [Ga]Ga-FAPI-04. Two DOTA-conjugated pyridine-based AV02053 and AV02070 were synthesized through multi-step organic synthesis.

Ga-Labeled [Thz]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 Ga-labeled agonist tracers for detecting GRPR-expressing tumors with positron emission tomography (PET), and compare them with the clinically validated agonist PET tracer, [Ga]Ga-AMBA.

Synthesis and Preclinical Evaluation of Three Novel Ga-Labeled Bispecific PSMA/FAP-Targeting Tracers for Prostate Cancer Imaging.

Tumor heterogeneity limits the efficacy and reliability of monospecific radiopharmaceuticals in prostate cancer diagnosis and therapy. To overcome this limitation and improve lesion detection sensitivity, we developed and evaluated three bispecific radiotracers that can target both prostate-specific membrane antigen (PSMA) and fibroblast activation protein (FAP), which are the two key proteins overexpressed in prostate cancer. Three FAP-targeting ligands with various linker lengths were synthesized through multistep organic synthesis, and then connected to the PSMA-targeting motif.

HTPAN-Triazole-Bn-NH: Tripicolinate Clicked-Bifunctional Chelate for [Ac]/[In] Theranostics.

A new, high-denticity, bifunctional ligand─HTPAN-triazole-Bn-NH─has been synthesized and studied in complexation with [Ac]Ac and [In]In for radiopharmaceutical applications. The bifunctional chelator is readily synthesized, using a high-yielding four-step prep, which is highly adaptable and allows for straightforward incorporation of different covalent linkers using Cu-catalyzed alkyne-azide cycloaddition (click) chemistry. Nuclear magnetic resonance (NMR) studies of HTPAN-triazole-Bn-NH with La and In metal ions show the formation of a single, asymmetric complex with each ion in solution, corroborated by density functional theory (DFT) calculations.

Hpicoopa─Robust Chelate for Ac/In Theranostics.

The nuclear decay characteristics of Ac ( = 5-8 MeV, linear energy transfer (LET) = ∼100 keV/μm, = 9.92 days) are well recognized as advantageous for the treatment of primary and metastatic tumors; however, suitable chelation systems are required, which can accommodate this radiometal. Since Ac does not possess any suitable low-energy, high abundance γ-ray emissions for nuclear imaging, there is a clear need for the development of other companion radionuclides with similar coordination characteristics and comparable half-lives, which can be applied in diagnostics.

Reducing the Kidney Uptake of High Contrast CXCR4 PET Imaging Agents via Linker Modifications.

Purpose: The C-X-C chemokine receptor 4 (CXCR4) is highly expressed in many subtypes of cancers, notably in several kidney-based malignancies. We synthesized, labeled, and assessed a series of radiotracers based on a previous high contrast PET imaging radiopharmaceutical [Ga]Ga-BL02, with modifications to its linker and metal chelator, in order to improve its tumor-to-kidney contrast ratio.

Methods: Based on the design of BL02, a piperidine-based cationic linker (BL06) and several anionic linkers (tri-Aad (BL17); tri-D-Glu (BL20); tri-Asp (BL25); and tri-cysteic acid (BL31)) were substituted for the triglutamate linker.

Ga-Labeled [LeuψThz]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, [LeuψThz]Bombesin(7-14), and compared their potential for cancer imaging with [Ga]Ga-RM2.

[Bi]Bi/[In]In-neunpa-cycMSH: Theranostic Radiopharmaceutical Targeting Melanoma─Structural, Radiochemical, and Biological Evaluation.

With the emergence of [Ac]Ac as a therapeutic radionuclide for targeted α therapy (TAT), access to clinical quantities of the potent, short-lived α-emitter [Bi]Bi ( = 45.6 min) will increase over the next decade. With this in mind, the nonadentate chelator, Hneunpa-NH, has been investigated as a ligand for chelation of [Bi]Bi in combination with [In]In as a suitable radionuclidic pair for TAT and single photon emission computed tomography (SPECT) diagnostics.

Targeting Refractory Mantle Cell Lymphoma for Imaging and Therapy Using C-X-C Chemokine Receptor Type 4 Radioligands.

Purpose: Mantle cell lymphoma (MCL) is associated with poor survival. The purpose of this study was to assess whether the C-X-C chemokine receptor type 4 (CXCR4) is a useful target for imaging and radioligand therapy of MCL, using a novel pair of radioligands, [68Ga]Ga and [177Lu]Lu-BL02.

Experimental Design: We performed a retrospective analysis of 146 patients with MCL to evaluate CXCR4 expression and its correlation with outcomes.

A 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 Ga and Lu, and evaluated its ability to target GRPR in a preclinical model of human prostate cancer. ProBOMB2 was synthesized in solid phase using fluorenylmethoxycarbonyl chemistry.

Synthesis and F-radiolabeling of thymidine AMBF conjugates.

In pursuit of F-labeled nucleosides for positron emission tomography (PET) imaging, we report on the chemical and radiochemical synthesis of two thymidine (dT) analogs, dT-C-AMBF and dT-N-AMBF, that are radiofluorinated by isotope exchange (IEX) and studied as PET imaging agents in mice with tumor xenografts. dT-C-AMBF shows preferential, and tumor-specific, uptake over dT-N-AMBF. This work provides a new synthetic method in order to access new nucleoside tracers for PET imaging.

Synthesis of DOTA-pyridine chelates for Cu coordination and radiolabeling of αMSH peptide.

Background: Cu is one of the few radioisotopes that can be used for both imaging and therapy, enabling theranostics with identical chemical composition. Development of stable chelators is essential to harness the potential of this isotope, challenged by the presence of endogenous copper chelators. Pyridyl type chelators show good coordination ability with copper, prompting the present study of a series of chelates DOTA-xPy (x = 1-4) that sequentially substitute carboxyl moieties with pyridyl moieties on a DOTA backbone.

High-Contrast CXCR4-Targeted F-PET Imaging Using a Potent and Selective Antagonist.

C-X-C chemokine receptor 4 (CXCR4) is highly expressed in cancers, contributing to proliferation, metastasis, and a poor prognosis. The noninvasive imaging of CXCR4 can enable the detection and characterization of aggressive cancers with poor outcomes. Currently, no F-labeled CXCR4 positron emission tomography (PET) radiotracer has demonstrated imaging contrast comparable to [Ga]Ga-Pentixafor, a CXCR4-targeting radioligand.

Selective Cyclized α-Melanocyte-Stimulating Hormone Derivative with Multiple -Methylations for Melanoma Imaging with Positron Emission Tomography.

In this study, we designed and evaluated a novel α-melanocyte-stimulating hormone derivative with four -methylations for melanocortin 1 receptor-targeted melanoma imaging with positron emission tomography (PET). The resulting peptide, DOTA-Pip-Nle-Cyclo[Asp--Me-His-d-Phe--Me-Arg--Me-Trp--Me-Lys]αMSH-NH (CCZ01099), showed high receptor selectivity, greatly improved stability, and rapid internalization. [Ga]Ga-CCZ01099 showed clear tumor visualization and excellent tumor-to-normal tissue contrast with PET imaging in a preclinical melanoma model.

Comparison of biological properties of [ Lu]Lu-ProBOMB1 and [ Lu]Lu-NeoBOMB1 for GRPR targeting.

The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer and other solid malignancies. Following up on our work on [ Ga]Ga-ProBOMB1 that had better imaging characteristics than [ Ga]Ga-NeoBOMB1, we investigated the effects of substituting Ga for Lu to determine if the resulting radiopharmaceuticals could be used with a therapeutic aim. We radiolabeled the bombesin antagonist ProBOMB1 (DOTA-pABzA-DIG-D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ-Pro-NH ) with lutetium-177 and compared it with [ Lu]Lu-NeoBOMB1 (obtained in 54.

[Ga]Ga/[Lu]Lu-BL01, a Novel Theranostic Pair for Targeting C-X-C Chemokine Receptor 4.

C-X-C chemokine receptor type 4 (CXCR4) is overexpressed in hematological and solid malignancies. LY2510924 is a potent peptide antagonist of CXCR4. A derivative of LY2510924, BL01, was evaluated for theranostic applications targeting CXCR4.

F-Labeled Cyclized α-Melanocyte-Stimulating Hormone Derivatives for Imaging Human Melanoma Xenograft with Positron Emission Tomography.

Since metastatic melanoma is deadly, early diagnosis thereof is crucial for managing the disease. We recently developed α-melanocyte-stimulating hormone (αMSH) derivatives, [Ga]Ga-CCZ01048 and [F]CCZ01064, that target the melanocortin 1 receptor (MC1R) for mouse melanoma imaging. In this study, we aim to evaluate [F]CCZ01064 as well as a novel dual-ammoniomethyl-trifluoroborate (AmBF) derivative, [F]CCZ01096, for targeting human melanoma xenograft using μPET imaging.

Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User-friendly Stapling Reaction.

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (K ≈1 nm) are found to be as potent as α-MSH.

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, [Ga]Ga-ProBOMB1, evaluated its pharmacokinetics and potential to image GRPR expression with positron emission tomography (PET), and compared it with [Ga]Ga-NeoBOMB1. ProBOMB1 (DOTA-pABzA-DIG-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ(CHN)-Pro-NH) was synthesized by solid-phase peptide synthesis.

Effects of adding an albumin binder chain on [Lu]Lu-DOTATATE.

Introduction: [Lu]Lu-DOTATATE peptide receptor radionuclide therapy is used for treatment of neuroendocrine tumours. We investigated whether prolonging blood residence time of [Lu]Lu-DOTATATE with albumin binders could increase tumour accumulation and tumour-to-kidney ratios for improved therapeutic efficacy.

Methods: DOTATATE and its derivatives with an albumin-binder motif (GluAB-DOTATATE and AspAB-DOTATATE) were prepared by solid-phase peptide synthesis.

Melanoma Imaging Using F-Labeled α-Melanocyte-Stimulating Hormone Derivatives with Positron Emission Tomography.

Melanocortin 1 receptor (MC1R) is specifically expressed in the majority of melanomas, a leading cause of death related to skin cancers. Accurate staging and early detection is crucial in managing melanoma. Based on the α-melanocyte-stimulating hormone (αMSH) sequence, MC1R-targeted peptides have been studied for melanoma imaging, predominately for use with single-photon emission computed tomography, with few attempts made for positron emission tomography (PET).

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.