Solid-phase synthesis and fluorine-18 radiolabeling of cycloRGDyK

Exploring Aqueous Coordination Chemistry of Highly Lewis Acidic Metals with Emerging Isotopes for Nuclear Medicine

Nuclear medicine harnesses radioisotopes for the diagnosis and treatment of disease. While the isotopes 99mTc and 111In have enabled the clinical diagnosis of millions of patients over the past 3 decades, more recent clinical translation of numerous 68Ga/177Lu-based radiopharmaceuticals for diagnostic imaging and therapy underscores the clinical utility of metal-based radiopharmaceuticals in mainstream cancer treatment. In addition to such established radionuclides, advancements in radioisotope production have enabled the production of radionuclides with a broad range of half-lives and emission properties of interest for nuclear medicine. Chemical means to form kinetically inert, in vivo-compatible species that can be modified with disease-targeting vectors is imperative. This presents a challenge for radiosiotopes of elements where the aqueous chemistry is still underdeveloped and poorly understood. Here, we discuss our efforts to date in exploring the aqueous, radioactive coordination chemistry of highly Lewis acidic metal ions and how our discoveries apply to the diagnosis and treatment of cancer in preclinical models of disease. The scope of this Account includes approaches to aqueous coordination of to-date understudied highly Lewis acidic metal ions with radioisotopes of emerging interest and the modulation of well-understood coordination environments of radio-coordination complexes to induce metal-catalyzed reactivity for separation and pro-drug applications.First, we discuss the development of seven-coordinate, small-cavity macrocyclic chelator platform mpatcn/picaga as an exemplary case study, which forms robust complexes with 44Sc/47Sc isotopes. Due to the high chemical hardness and pronounced Lewis acidity of the Sc3+ ion, the displacement of ternary ligand H2O by 18/natF- can be achieved to form an inert Sc-18/natF bond. Corresponding coordination complex natSc-18F is in vivo compatible and forms a theranostic tetrad with corresponding 44Sc/47Sc, 177Lu complexes all exhibiting homologous biodistribution profiles. Another exceptionally hard, highly Lewis acidic ion with underdeveloped aqueous chemistry and emerging interest in nuclear medicine is 45Ti4+. To develop de novo approaches to the mononuclear chelation of this ion under aqueous conditions, we employed a fragment-based bidentate ligand screening approach which identified two leads. The screen successfully predicted the formation of [45Ti][Ti(TREN-CAM)], a Ti-triscatechol complex that exhibits remarkable in vivo stability. Furthermore, the fragment-based screen also identified approaches that enabled solid-phase separation of Ti4+ and Sc3+ of interest in streamlining the isotope production of 45Ti and accessing new ways to separate 44Ti/44Sc for the development of a long-lived generator system. In addition to establishing the inert chelation of Ti4+ and Sc3+, we introduce controlled, metal-induced reactivity of corresponding coordination complexes on macroscopic and radiotracer scales. Metal-mediated autolytic amide bond cleavage (MMAAC) enables the temperature-dependent release of high-molar-activity, ready-to-inject radiopharmaceuticals; cleavage is selectively triggered by coordinated trivalent Lewis acid nat/68Ga3+ or Sc3+. Following the scope of reactivity and mechanistic studies, we validated MMAAC for the synthesis of high-molar-activity radiopharmaceuticals to image molecular targets with low expression and metal-mediated prodrug hydrolysis in vivo.This Account summarizes how developing the aqueous coordination chemistry and tuning the chemical reactivity of metal ions with high Lewis acidity at the macroscopic and tracer scales directly apply to the radiopharmaceutical synthesis with clinical potential.

Synthesis and Anti-Angiogenic Activity of Novel c(RGDyK) Peptide-Based JH-VII-139-1 Conjugates

Peptide-drug conjugates are delivery systems for selective delivery of cytotoxic agents to target cancer cells. In this work, the optimized synthesis of JH-VII-139-1 and its c(RGDyK) peptide conjugates is presented. The low nanomolar SRPK1 inhibitor, JH-VII-139-1, which is an analogue of Alectinib, was linked to the α_νβ₃ targeting oligopeptide c(RGDyK) through amide, carbamate and urea linkers. The chemostability, cytotoxic and antiangiogenic properties of the synthesized hybrids were thoroughly studied. All conjugates retained mid nanomolar-level inhibitory activity against SRPK1 kinase and two out of four conjugates, geo75 and geo77 exhibited antiproliferative effects with low micromolar IC₅₀ values against HeLa, K562, MDA-MB231 and MCF7 cancer cells. The activities were strongly related to the stability of the linkers and the release of JH-VII-139-1. In vivo zebrafish screening assays demonstrated the ability of the synthesized conjugates to inhibit the length or width of intersegmental vessels (ISVs). Flow cytometry experiments were used to test the cellular uptake of a fluorescein tagged hybrid in MCF7 and MDA-MB231 cells that revealed a receptor-mediated endocytosis process. In conclusion, most conjugates retained the inhibitory potency against SRPK1 as JH-VII-139-1 and demonstrated antiproliferative and antiangiogenic activities. Further animal model experiments are needed to uncover the full potential of such peptide conjugates in cancer therapy and angiogenesis-related diseases.

Solid phase radiosynthesis of an olaparib derivative using 4-[18F] fluorobenzoic acid and in vivo evaluation in breast and prostate cancer xenograft models for PARP-1 expression

INTRODUCTION

Solid-phase synthesis and conjugation reactions of acids and amines using coupling reagents are common in organic synthesis, but rare in 18F radiochemistry. 4-[18F]Fluorobenzoic acid (FBA) is a useful building block, but is seldom used directly with coupling reagents for the preparation of 18F radiopharmaceuticals. To overcome the inconveniences associated with using [18F]FBA in conjugation reactions, we have developed a non-covalent solid-phase synthesis (SPS) strategy for the radiosynthesis of [18F]PARPi, a derivative of olaparib as a Poly (ADP-ribose) polymerase-1 (PARP-1) radioligand.

METHODS

Fluoro-, bromo- and iodo-benzoic derivatives of olaparib were synthesized, and their PARP-1 affinities were measured using a recently developed cell culture-based competitive assay. To produce [18F]PARPi, [18F]FBA was radiosynthesized and purified using a cation-exchange cartridge, and then trapped by an anion-exchange resin cartridge, on which the solid-phase radiosynthesis was carried out to produce the desired product. [18F]PARPi was evaluated in vivo in breast and prostate xenograft tumor models by microPET imaging, biodistribution and autoradiography.

RESULTS

The best derivatives of olaparib were identified as compound 4, 7 and 8. [18F]4 ([18F]PARPi) was radiosynthesized in high radiochemical yield, high molar activity and high radiochemical purity using this SPS strategy. The in vivo evaluation of [18F]PARPi demonstrates the PARP-1 specific uptake of [18F]PARPi in the animal models.

CONCLUSIONS

This method is simple and efficient, having great potential for the synthesis of radiopharmaceuticals starting from [18F]FBA or other radiolabeled aromatic acids. Using [18F]PARPi prepared by this method, we demonstrated the promise of [18F]PARPi in the nuclear imaging of PARP-1 expression.

Fully automated peptide radiolabeling from [18F]fluoride

The biological properties of receptor-targeted peptides have made them popular diagnostic imaging and therapeutic agents. Typically, the synthesis of fluorine-18 radiolabeled receptor-targeted peptides for positron emission tomography (PET) imaging is a time consuming, complex, multi-step synthetic process that is highly variable based on the peptide. The complexity associated with the radiolabeling route and lack of robust automated protocols can hinder translation into the clinic. A fully automated batch production to radiolabel three peptides (YGGFL, cRGDyK, and Pyr-QKLGNQWAVGHLM) from fluorine-18 using the ELIXYS FLEX/CHEM® radiosynthesizer in a two-step process is described. First, the prosthetic group, 6-[¹⁸F]fluoronicotinyl-2,3,5,6-tetrafluorophenyl ester ([¹⁸F]FPy-TFP) was synthesized and subsequently attached to the peptide. The [¹⁸F]FPy-peptides were synthesized in 13-26% decay corrected yields from fluorine-18 with high molar activity 1-5 Ci μmol^-1 and radiochemical purity of >99% in an overall synthesis time of 97 ± 3 minutes.

Preclinical Development and First-in-Human Imaging of the Integrin αvβ6 with [18F]αvβ6-Binding Peptide in Metastatic Carcinoma

PURPOSE

The study was undertaken to develop and evaluate the potential of an integrin αvβ6-binding peptide (αvβ6-BP) for noninvasive imaging of a diverse range of malignancies with PET.

EXPERIMENTAL DESIGN

The peptide αvβ6-BP was prepared on solid phase and radiolabeled with 4-[18F]fluorobenzoic acid. In vitro testing included ELISA, serum stability, and cell binding studies using paired αvβ6-expressing and αvβ6-null cell lines. In vivo evaluation (PET/CT, biodistribution, and autoradiography) was performed in a mouse model bearing the same paired αvβ6-expressing and αvβ6-null cell xenografts. A first-in-human PET/CT imaging study was performed in patients with metastatic lung, colon, breast, or pancreatic cancer.

RESULTS

[18F]αvβ6-BP displayed excellent affinity and selectivity for the integrin αvβ6 in vitro [IC50(αvβ6) = 1.2 nmol/L vs IC50(αvβ3) >10 μmol/L] in addition to rapid target-specific cell binding and internalization (72.5% ± 0.9% binding and 52.5% ± 1.8%, respectively). Favorable tumor affinity and selectivity were retained in the mouse model and excretion of unbound [18F]αvβ6-BP was rapid, primarily via the kidneys. In patients, [18F]αvβ6-BP was well tolerated without noticeable adverse side effects. PET images showed significant uptake of [18F]αvβ6-BP in both the primary lesion and metastases, including metastasis to brain, bone, liver, and lung.

CONCLUSIONS

The clinical impact of [18F]αvβ6-BP PET imaging demonstrated in this first-in-human study is immediate for a broad spectrum of malignancies.

Improved synthesis on solid phase of dithiocarbamic cRGD-derivative and 99m Tc-radiolabelling

In the field of angiogenesis, small cyclic pentapeptides containing the RGD motif are playing a relevant role for their high affinity and specificity for integrin receptors and for the possibility to act at both therapeutic and diagnostic level by inhibiting pathological angiogenesis and by serving as shuttles to deliver imaging-probe including SPECT/PET radionuclides to specific tissues. In the last decade, several new protocols were reported in literature for the direct synthesis of cyclic RDG either in solution or by SPPS. Here, we have elaborated and tested some alternative approaches using different resins and different protective groups. The introduction of the dithiocarbamate function, useful to complex radio-metals suitable for nuclear medicine applications, has also been considered and achieved.

Identification, Characterization, and Optimization of Integrin αvβ₆-Targeting Peptides from a One-Bead One-Compound (OBOC) Library: Towards the Development of Positron Emission Tomography (PET) Imaging Agents

The current translation of peptides identified through the one-bead one-compound (OBOC) technology into positron emission tomography (PET) imaging agents is a slow process, with a major delay between ligand identification and subsequent lead optimization. This work aims to streamline the development process of ¹⁸F-peptide based PET imaging agents to target the integrin α_vβ₆. By directly identify α_vβ₆–targeting peptides from a 9-mer 4-fluorobenzoyl peptide library using the on-bead two-color (OBTC) cell-screening assay, a total of 185 peptide beads were identified and 5 beads sequenced for further evaluation. The lead peptide 1 (VGDLTYLKK(FB), IC₅₀ = 0.45 ± 0.06 μM, 25% stable in serum at 1 h) was further modified at the N-, C-, and bi-termini. C-terminal PEGylation increased the metabolic stability (>95% stable), but decreased binding affinity (IC₅₀ = 3.7 ± 1 μM) was noted. C-terminal extension (1i, VGDLTYLKK(FB)KVART) significantly increased binding affinity for integrin α_vβ₆ (IC₅₀ = 0.021 ± 0.002 μM), binding selectivity for α_vβ₆-expressing cells (3.1 ± 0.8:1), and the serum stability (>99% stable). Our results demonstrate the challenges in optimizing OBOC-derived peptides, indicate both termini of 1 are sensitive to modifications, and show that further modification of 1 is necessary to demonstrate utility as an ¹⁸F-peptide imaging agent.