N-Succinimidyl 4-[(18)F]-fluoromethylbenzoate-labeled dimeric RGD peptide for imaging tumor integrin expression

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

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

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Design and Investigation of a [18F]-Labeled Benzamide Derivative as a High Affinity Dual Sigma Receptor Subtype Radioligand for Prostate Tumor Imaging

High overexpression of sigma (σ) receptors (σ₁ and σ₂ subtypes) in a variety of human solid tumors has prompted the development of σ receptor-targeting radioligands, as imaging agents for tumor detection. A majority of these radioligands to date target the σ₂ receptor, a potential marker of tumor proliferative status. The identification of approximately equal proportions of both σ receptor subtypes in prostate tumors suggests that a high affinity, dual σ receptor-targeting radioligand could potentially provide enhanced tumor targeting efficacy in prostate cancer. To accomplish this goal, we designed a series of ligands which bind to both σ receptor subtypes with high affinity. Ligand 3a in this series, displaying optimal dual σ receptor subtype affinity (σ₁, 6.3 nM; σ₂, 10.2 nM) was radiolabeled with fluorine-18 (¹⁸F) to give [¹⁸F]3a and evaluated as a σ receptor-targeting radioligand in the mouse PC-3 prostate tumor model. Cellular assays with PC-3 cells demonstrated that a major proportion of [¹⁸F]3a was localized to cell surface σ receptors, while ∼10% of [¹⁸F]3a was internalized within cells after incubation for 3.5 h. Serial PET imaging in mice bearing PC-3 tumors revealed that uptake of [¹⁸F]3a was 1.6 ± 0.8, 4.4 ± 0.3, and 3.6 ± 0.6% ID/g (% injection dose per gram) in σ receptor-positive prostate tumors at 15 min, 1.5 h, and 3.5 h postinjection, respectively (n = 3) resulting in clear tumor visualization. Blocking studies conducted with haloperidol (a nonselective inhibitor for both σ receptor subtypes) confirmed that the uptake of [¹⁸F]3a was σ receptor-mediated. Histology analysis confirmed similar expression of σ₁ and σ₂ in PC-3 tumors which was significantly greater than its expression in normal organs/tissues such as liver, kidney, and muscle. Metabolite studies revealed that >50% of radioactivity in PC-3 tumors at 30 min postinjection represented intact [¹⁸F]3a. Prominent σ receptor-specific uptake of [¹⁸F]3a in prostate tumors and its subsequent clear visualization with PET imaging indicate potential utility for the diagnosis of prostate carcinoma.

Evaluation of Tc-99 m Labeled Dimeric GX1 Peptides for Imaging of Colorectal Cancer Vasculature

PURPOSE

This study aimed to evaluate the potential of PEGylated dimeric GX1 peptide as a radiotracer for imaging of colorectal cancer vasculature in a LoVo tumor xenografted mouse model.

PROCEDURES

The [(99m)Tc]PEG-(GX1)2 peptide was synthesized and identified. Confocal immunofluorescence analysis, receptor binding assay, and competitive inhibition assay were performed to evaluate the binding specificity and the receptor binding affinity of PEG-(GX1)2 to Co-human umbilical vein endothelial cells (HUVECs). Single photon emission computed tomography imaging and biodistribution were performed to evaluate the targeting ability of PEG-(GX1)2 to colorectal cancer.

RESULTS

The studies in vitro suggested that PEG-(GX1)2 co-localized with Factor VIII in the perinuclear cytoplasm of Co-HUVECs and bound specifically to Co-HUVECs with a high affinity. The studies in vivo demonstrated that the targeting efficacy of PEG-(GX1)2 was superior to GX1.

CONCLUSIONS

PEGylation improved the affinity and the targeting ability of the GX1 peptide. PEG-(GX1)2 is a more promising probe for imaging of colorectal vasculature than GX1.

Development and Preliminary Evaluation of TFIB, a New Bimodal Prosthetic Group for Bioactive Molecule Labeling

The new readily available prosthetic group, tetrafluorophenyl 4-fluoro-3-iodobenzoate (TFIB), designed for both molecular imaging and targeted radionuclide therapy purposes was radiolabeled either with fluorine or iodine radionuclides with excellent radiochemical yields and purities. These radiolabeled tags were conjugated to N,N-diethylethylenediamine to give melanin-targeting radiotracers [ (125) I]9 and [ (18) F]9, which were successfully evaluated by PET and gamma scintigraphic imaging in B16F0 pigmented melanoma-bearing C57BL/6J mice. Then, radiolabeled [ (125) I]/[ (18) F]TFIB was used to tag tumor-targeting peptides (i.e., PEG3[c(RGDyK)]2 and NDP-MSH targeting αvβ3 integrin and MC1R receptors, respectively) in mild conditions and with good radiochemical yields (47-83% d.c.) and purities (>99%). The resulting radiolabeled peptides were assessed both in vitro and by PET imaging in animal models.

Synthesis and biological evaluation (in vitro and in vivo) of cyclic arginine-glycine-aspartate (RGD) peptidomimetic-paclitaxel conjugates targeting integrin αVβ3

A small library of integrin ligand-paclitaxel conjugates 10-13 was synthesized with the aim of using the tumor-homing cyclo[DKP-RGD] peptidomimetics for site-directed delivery of the cytotoxic drug. All the paclitaxel-RGD constructs 10-13 inhibited biotinylated vitronectin binding to the purified αVβ3 integrin receptor at low nanomolar concentration and showed in vitro cytotoxic activity against a panel of human tumor cell lines similar to that of paclitaxel. Among the cell lines, the cisplatin-resistant IGROV-1/Pt1 cells expressed high levels of integrin αVβ3, making them attractive to be tested in in vivo models. cyclo[DKP-f3-RGD]-PTX 11 displayed sufficient stability in physiological solution and in both human and murine plasma to be a good candidate for in vivo testing. In tumor-targeting experiments against the IGROV-1/Pt1 human ovarian carcinoma xenotransplanted in nude mice, compound 11 exhibited a superior activity compared with paclitaxel, despite the lower (about half) molar dosage used.