Optimal His-Tag Design for Efficient [99mTc(CO)3]+ and [188Re(CO)3]+ Labeling of Proteins for Molecular Imaging and Radionuclide Therapy by Analysis of Peptide Arrays

Hexahistidine tags (His-tags), incorporated into recombinant proteins to facilitate purification using metal-affinity chromatography, are useful binding sites for radiolabeling with [^99mTc(CO)₃]⁺ and [¹⁸⁸Re(CO)₃]⁺ for molecular imaging and radionuclide therapy. Labeling efficiencies vary unpredictably, and the method is therefore not universally useful. To overcome this, we have made quantitative comparisons of radiolabeling of a bespoke Celluspots array library of 382 His-tag-containing peptide sequences with [^99mTc(CO)₃]⁺ and [¹⁸⁸Re(CO)₃]⁺ to identify key features that enhance labeling. A selected sequence with 10-fold enhanced labeling efficiency compared to the most effective literature-reported sequences was incorporated into an exemplar protein and compared biologically with non-optimized analogues, in vitro and in vivo. Optimal labeling with either [^99mTc(CO)₃]⁺ or [¹⁸⁸Re(CO)₃]⁺ required six consecutive His residues in the protein sequence, surrounded by several positively charged residues (Arg or Lys), and the presence of phosphate in the buffer. Cys or Met residues in the sequence were beneficial, to a lesser extent. Negatively charged residues were deleterious to labeling. His-tags with adjacent positively charged residues could be labeled as much as 40 times more efficiently than those with adjacent negatively charged residues. ³¹P NMR of [Re(CO)₃(H₂O)₃]⁺ and electrophoresis of solutions of [^99mTc(CO)₃(H₂O)₃]⁺ suggest that phosphate bridges form between cationic residues and the cationic metal synthon during labeling. The trial optimized protein, a scFv targeted to the PSMA antigen expressed in prostate cancer, was readily labeled in >95% radiochemical yield, without the need for subsequent purification. Labeling occurred more quickly and to higher specific activity than comparable non-optimized proteins, while retaining specific binding to PSMA and prostate cancer in vivo. Thus, optimized His-tags greatly simplify radiolabeling of recombinant proteins making them potentially more widely and economically available for imaging and treating patients.

Incorporation of Fluorine into an OBOC Peptide Library by Copper-Free Click Chemistry toward the Discovery of PET Imaging Agents

A one-bead one-compound (OBOC) library of peptide-based imaging agents was developed where a ¹⁹F-containing moiety was added onto the N-terminus of octamer peptides through copper-free click chemistry prior to screening of the library. This created a library of complete imaging agents that was screened against CXCR4, a receptor of interest for cancer imaging. The screen directly resulted in the discovery of a peptide-based imaging agent with an IC₅₀ of 138 μM. This proof-of-concept study describes a new type of OBOC peptide library design, where hits discovered from screening can be easily translated into their fluorine-18 counterpart for PET imaging without loss of affinity.

Bead-based screening in chemical biology and drug discovery

High-throughput screening is an important component of the drug discovery process. The screening of libraries containing hundreds of thousands of compounds requires assays amenable to miniaturisation and automization. Combinatorial chemistry holds a unique promise to deliver structurally diverse libraries for early drug discovery. Among the various library forms, the one-bead-one-compound (OBOC) library, where each bead carries many copies of a single compound, holds the greatest potential for the rapid identification of novel hits against emerging drug targets. However, this potential has not yet been fully realized due to a number of technical obstacles. In this feature article, we review the progress that has been made in bead-based library screening and its application to the discovery of bioactive compounds. We identify the key challenges of this approach and highlight key steps needed for making a greater impact in the field.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.