Fast and versatile microwave-assisted intramolecular Heck reaction in peptide macrocyclization using microwave energy

Heck Macrocyclization in Forging Non-Natural Large Rings including Macrocyclic Drugs

The intramolecular Heck reaction is a well-established strategy for natural product total synthesis. When constructing large rings, this reaction is also referred to as Heck macrocyclization, which has proved a viable avenue to access diverse naturally occurring macrocycles. Less noticed but likewise valuable, it has created novel macrocycles of non-natural origin that neither serve as nor derive from natural products. This review presents a systematic account of the title reaction in forging this non-natural subset of large rings, thereby addressing a topic rarely covered in the literature. Walking through two complementary sections, namely (1) drug discovery research and (2) synthetic methodology development, it demonstrates that beyond the well-known domain of natural product synthesis, Heck macrocyclization also plays a remarkable role in forming synthetic macrocycles, in particular macrocyclic drugs.

Peptide Carbocycles: From -SS- to -CC- via a Late-Stage "Snip-and-Stitch"

One way to improve the therapeutic potential of peptides is through cyclization. This is commonly done using a disulfide bond between two cysteine residues in the peptide. However, disulfide bonds are susceptible to reductive cleavage, and this can deactivate the peptide and endanger endogenous proteins through covalent modification. Substituting disulfide bonds with more chemically robust carbon-based linkers has proven to be an effective strategy to better develop cyclic peptides as drugs, but finding the optimal carbon replacement is synthetically laborious. We report a new late-stage platform wherein a single disulfide bond in a cyclic peptide can serve as the progenitor for any number of new carbon-rich groups, derived from organodiiodides, using a Zn:Cu couple and a hydrosilane. We show that this platform can furnish entirely new carbocyclic scaffolds with enhanced permeability and structural integrity and that the stereochemistry of the new cycles can be biased by a judicious choice in silane.

Synthesis and Evaluation of a Dimeric RGD Peptide as a Preliminary Study for Radiotheranostics with Radiohalogens

We recently developed ¹²⁵I- and ²¹¹At-labeled monomer RGD peptides using a novel radiolabeling method. Both labeled peptides showed high accumulation in the tumor and exhibited similar biodistribution, demonstrating their usefulness for radiotheranostics. This study applied the labeling method to a dimer RGD peptide with the aim of gaining higher accumulation in tumor tissues based on improved affinity with α_vβ₃ integrin. We synthesized an iodine-introduced dimer RGD peptide, E[c(RGDfK)] (6), and an ^125/131I-labeled dimer RGD peptide, E[c(RGDfK)]{[^125/131I]c[RGDf(4-I)K]} ([^125/131I]6), and evaluated them as a preliminary step to the synthesis of an ²¹¹At-labeled dimer RGD peptide. The affinity of 6 for α_vβ₃ integrin was higher than that of a monomer RGD peptide. In the biodistribution experiment at 4 h postinjection, the accumulation of [¹²⁵I]6 (4.12 ± 0.42% ID/g) in the tumor was significantly increased compared with that of ¹²⁵I-labeled monomer RGD peptide (2.93 ± 0.08% ID/g). Moreover, the accumulation of [¹²⁵I]6 in the tumor was greatly inhibited by co-injection of an excess RGD peptide. However, a single injection of [¹³¹I]6 (11.1 MBq) did not inhibit tumor growth in tumor-bearing mice. We expect that the labeling method for targeted alpha therapy with ²¹¹At using a dimer RGD peptide could prove useful in future clinical applications.

68Ga- and 211At-Labeled RGD Peptides for Radiotheranostics with Multiradionuclides

Probes for radiotheranostics could be produced by introducing radionuclides with similar chemical characteristics into the same precursors. We recently developed an ²¹¹At-labeled RGD peptide and a corresponding radioiodine-labeled RGD peptide. Both labeled peptides accumulated in large quantities in the tumor with similar biodistribution, demonstrating their usefulness for radiotheranostics. In this study, we hypothesized that probes for radiotheranostics combined with multiradionuclides, such as ⁶⁸Ga and ²¹¹At, have useful clinical applications. New radiolabeled RGD peptide probes were synthesized via a molecular design approach, with two labeling sites for metal and halogen. These probes were evaluated in biodistribution experiments using tumor-bearing mice. [⁶⁷Ga]Ga-DOTA-c[RGDf(4-I)K] ([⁶⁷Ga]4), Ga-DOTA-[¹²⁵I]c[RGDf(4-I)K] ([¹²⁵I]4), and Ga-DOTA-[²¹¹At]c[RGDf(4-At)K] ([²¹¹At]7) showed similar biodistribution, with high and equivalent accumulation in tumors. These results indicate the usefulness of these probes in radiotheranostics with multiradionuclides, such as a radiometal and a radiohalogen, and they could contribute to a personalized medicine regimen.