Development of Candidates for Positron Emission Tomography (PET) Imaging of Ghrelin Receptor in Disease: Design, Synthesis, and Evaluation of Fluorine-Bearing Quinazolinone Derivatives

Molecular imaging with positron emission tomography (PET) is an attractive platform for noninvasive detection and assessment of disease. The development of a PET imaging agent targeting the ghrelin receptor (growth hormone secretagogue receptor type 1a or GHS-R1a) has the potential to lead to the detection and assessment of the higher than normal expression of GHS-R1a in diseases such as prostate, breast, and ovarian cancer. To enable the development of ¹⁸F radiopharmaceuticals, we have designed and synthesized three series of quinazolinone derivatives, resulting in the identification of two compound (<b>5i</b>, <b>17</b>) with subnanomolar binding affinity and one fluorine-bearing compound (<b>10b</b>) with picomolar binding affinity (20 pM), representing the highest binding affinity for GHS-R1a reported to date. Two lead compounds (<b>5b</b>, IC₅₀ = 20.6 nM; <b>5e</b>, IC₅₀ = 9.3 nM) were successfully ¹⁸F-radiolabeled with radiochemical purity of greater than 99%. Molecular modeling studies were performed to shed light on ligand–receptor interactions

Discovery of Novel Integrin Ligands from Combinatorial Libraries Using a Multiplex “Beads on a Bead” Approach

The development of screening approaches to identify novel affinity ligands has paved the way for a new generation of molecular targeted nanomedicines. Conventional methods typically bias the display of the target protein to ligands during the screening process. We have developed an unbiased multiplex “beads on a bead” strategy to isolate, characterize, and validate high affinity ligands from OBOC libraries. Novel non-RGD peptides that target α_vβ₃ integrin were discovered that do not affect cancer or endothelial cell biology. The peptides identified here represent novel integrin-targeted agents that can be used to develop targeted nanomedicines without the risk of increased tumor invasion and metastasis

Structure–Activity Study of Ghrelin(1–8) Resulting in High Affinity Fluorine-Bearing Ligands for the Ghrelin Receptor

The ghrelin receptor, also known as the growth hormone secretagogue receptor 1a (GHS-R1a), is a G-protein-coupled receptor that is differentially expressed in healthy tissue and several cancers, including prostate, testicular, and ovarian. Selectively targeting the ghrelin receptor using fluorine-18 tagged entities would allow localization and visualization of ghrelin receptor expressing carcinomas using PET imaging. The endogenous ligand ghrelin, a 28 amino acid peptide with 3.1 nM affinity, has poor in vivo stability. Here we report on ghrelin(1–8) analogues bearing modifications at residues 1, 3, 4, and 8. The lead analogue, [Inp¹,Dpr³(6-fluoro-2-naphthoate),1-Nal⁴,Thr⁸]­ghrelin­(1–8), possessed an IC₅₀ value of 0.11 nM that is a 28-fold improvement compared to the natural ligand. A novel 6-fluoro-2-pentafluorophenyl naphthoate (PFPN) prosthetic group was synthesized to incorporate fluorine-18 for PET imaging. This is not only the highest affinity ghrelin analogue reported but also the shortest ghrelin analogue capable of binding GHS-R1a with better affinity than ghrelin(1–28)

Dual-Modal Magnetic Resonance and Fluorescence Imaging of Atherosclerotic Plaques in Vivo Using VCAM‑1 Targeted Tobacco Mosaic Virus

The underlying cause of major cardiovascular events, such as myocardial infarctions and strokes, is atherosclerosis. For accurate diagnosis of this inflammatory disease, molecular imaging is required. Toward this goal, we sought to develop a nanoparticle-based, high aspect ratio, molecularly targeted magnetic resonance (MR) imaging contrast agent. Specifically, we engineered the plant viral nanoparticle platform tobacco mosaic virus (TMV) to target vascular cell adhesion molecule (VCAM)-1, which is highly expressed on activated endothelial cells at atherosclerotic plaques. To achieve dual optical and MR imaging in an atherosclerotic ApoE^–/– mouse model, TMV was modified to carry near-infrared dyes and chelated Gd ions. Our results indicate molecular targeting of atherosclerotic plaques. On the basis of the multivalency and multifunctionality, the targeted TMV-based MR probe increased the detection limit significantly; the injected dose of Gd ions could be further reduced 400x compared to the suggested clinical use, demonstrating the utility of targeted nanoparticle cargo delivery

Design of a Microfluidic Chip for Magnetic-Activated Sorting of One-Bead-One-Compound Libraries

Molecular targeting using ligands specific to disease markers has shown great promise for early detection and directed therapy. Bead-based combinatorial libraries have served as powerful tools for the discovery of novel targeting agents. Screening platforms employing magnetic capture have been used to achieve rapid and efficient identification of high-affinity ligands from one-bead-one-compound (OBOC) libraries. Traditional manual methodologies to isolate magnetized “hit” beads are tedious and lack accuracy, and existing instruments to expedite bead sorting tend to be costly and complex. Here, we describe the design and construction of a simple and inexpensive microfluidic magnetic sorting device using standard photolithography and soft lithography approaches to facilitate high-throughput isolation of magnetized positive hit beads from combinatorial libraries. We have demonstrated that the device is able to sort magnetized beads with superior accuracy compared to conventional manual sorting approaches. This chip offers a very convenient yet inexpensive alternative for screening OBOC libraries