Ring Expansion of Thiolactams via Imide Intermediates: An Amino Acid Insertion Strategy.

The AgI -promoted reaction of thiolactams with N-Boc amino acids yields an N-(α-aminoacyl) lactam that can rearrange through an acyl transfer process. Boc-deprotection results in convergence to the ring-expanded adduct, thereby facilitating an overall insertion of an amino acid into the thioamide bond to generate medium-sized heterocycles. Application to the site-specific insertion of amino acids into cyclic peptides is demonstrated.

A dual modality 99mTc/Re(i)-labelled T140 analogue for imaging of CXCR4 expression.

The C-X-C chemokine receptor 4 (CXCR4) has been shown to be overexpressed in at least 23 types of cancer, including prostate cancer which has been shown to have a significant distinction of expression rates between cancerous compared to healthy or benign tissue. In an attempt to exploit the difference in expression, we have synthesized a derivative of T140, a peptide antagonist for CXCR4, containing a fluorescent 4-amino-1,8-naphthalimide appended with a di-(2-picolyl)amine binding unit to chelate rhenium or technetium-99m for fluorescence or SPECT imaging. The rhenium-coordinated variant was shown to have similar binding affinity for the receptor as T140 and showed specific uptake by fluorescence microscopy in CXCR4 expressing cells. The peptide was radiolabelled with technetium-99m in decay corrected radiochemical yields ranging from 60-85%, radiochemical purities >95%, and molar activities of 36-44 GBq µmol-1. The technetium-99m labelled peptide showed two-fold higher uptake in U87 cells expressing CXCR4 compared to non-transfected cells. Ex vivo biodistribution studies were performed using the technetium-99m labelled peptide in NOD/SCID mice bearing tumors derived from U87 cells with CXCR4. Tumor uptake of 0.51 ± 0.09% ID g-1 was observed two-hours post-injection. Our novel T140 derivative is suitable for imaging of CXCR4 expression by confocal microscopy. Further structural modifications to the peptide or metal complex may result in improved biodistribution for use in SPECT imaging of CXCR4 expressing tumors.

Target Validation and Identification of Novel Boronate Inhibitors of the Plasmodium falciparum Proteasome.

The Plasmodium proteasome represents a potential antimalarial drug target for compounds with activity against multiple life cycle stages. We screened a library of human proteasome inhibitors (peptidyl boronic acids) and compared activities against purified P. falciparum and human 20S proteasomes. We chose four hits that potently inhibit parasite growth and show a range of selectivities for inhibition of the growth of P. falciparum compared with human cell lines. P. falciparum was selected for resistance in vitro to the clinically used proteasome inhibitor, bortezomib, and whole genome sequencing was applied to identify mutations in the proteasome ß5 subunit. Active site profiling revealed inhibitor features that enable retention of potent activity against the bortezomib-resistant line. Substrate profiling reveals P. falciparum 20S proteasome active site preferences that will inform attempts to design more selective inhibitors. This work provides a starting point for the identification of antimalarial drug leads that selectively target the P. falciparum proteasome.

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).

Bridging computational modeling with amino acid replacements to investigate GHS-R1a-peptidomimetic recognition.

The ghrelin receptor, also referred to as the growth hormone secretagogue receptor 1a (GHS-R1a), is a G protein-coupled receptor (GPCR) primarily expressed in the brain and pituitary. The wide spectrum of biological functions of GHS-R1a has rendered it a target for therapeutic drugs and for molecular imaging agents, for a variety of diseases. An improved understanding of the binding mechanism of a ligand to GHS-R1a would provide guidance on ligand design. This study investigates the binding of G-7039, a peptidomimetic agonist, to the GHS-R1a. A series of computational studies including homology modeling, molecular docking, molecular dynamics (MD) simulations, and binding free energy calculations were carried out in conjunction with amino acid replacements on G-7039. The results suggest that the first three residues on the N-terminal segment of the peptidomimetic are bound to three hydrophobic sub-pockets in the receptor binding site, with the driving force for binding mainly from hydrophobic interactions. It has been reported that a charge-charge interaction between the positively charged terminal amine of the agonist and Glu124 on the receptor serves as an anchor point for binding. However, our studies suggest that this interaction is not strong enough to anchor a ligand to the ghrelin receptor in the absence of hydrophobic interactions. The resulting computational model provides insight into structure activity relationship analysis for the ghrelin receptor and will assist in future ligand design.