Gold(I)-Mediated Rapid Cyclization of Propargylated Peptides via Imine Formation

Synthesis of Fluorescent Cyclic Peptides via Gold(I)-Catalyzed Macrocyclization

Rapid and efficient cyclization methods that form structurally novel peptidic macrocycles are of high importance for medicinal chemistry. Herein, we report the first gold(I)-catalyzed macrocyclization of peptide-EBXs (ethynylbenziodoxolones) via C2-Trp C-H activation. This reaction was carried out in the presence of protecting group free peptide sequences and is enabled by a simple commercial gold catalyst (AuCl·Me2S). The method displayed a rapid reaction rate (within 10 min), wide functional group tolerance (27 unprotected peptides were cyclized), and up to 86% isolated yield. The obtained highly conjugated cyclic peptide linker, formed through C-H alkynylation, can be directly applied to live-cell imaging as a fluorescent probe without further attachment of fluorophores.

Metal-Mediated, Autolytic Amide Bond Cleavage: A Strategy for the Selective, Metal Complexation-Catalyzed, Controlled Release of Metallodrugs

Activation of metalloprodrugs or prodrug activation using transition metal catalysts represents emerging strategies for drug development; however, they are frequently hampered by poor spatiotemporal control and limited catalytic turnover. Here, we demonstrate that metal complex-mediated, autolytic release of active metallodrugs can be successfully employed to prepare clinical grade (radio-)pharmaceuticals. Optimization of the Lewis-acidic metal ion, chelate, amino acid linker, and biological targeting vector provides means to release peptide-based (radio-)metallopharmaceuticals in solution and from the solid phase using metal-mediated, autolytic amide bond cleavage (MMAAC). Our findings indicate that coordinative polarization of an amide bond by strong, trivalent Lewis acids such as Ga3+ and Sc3+ adjacent to serine results in the N, O acyl shift and hydrolysis of the corresponding ester without dissociation of the corresponding metal complex. Compound [68Ga]Ga-10, incorporating a cleavable and noncleavable functionalization, was used to demonstrate that only the amide bond-adjacent serine effectively triggered hydrolysis in solution and from the solid phase. The corresponding solid-phase released compound [68Ga]Ga-8 demonstrated superior in vivo performance in a mouse tumor model compared to [68Ga]Ga-8 produced using conventional, solution-phase radiolabeling. A second proof-of-concept system, [67Ga]Ga-17A (serine-linked) and [67Ga]Ga-17B (glycine-linked) binding to serum albumin via the incorporated ibuprofen moiety, was also synthesized. These constructs demonstrated that complete hydrolysis of the corresponding [68Ga]Ga-NOTA complex from [67Ga]Ga-17A can be achieved in naïve mice within 12 h, as traceable in urine and blood metabolites. The glycine-linked control [68Ga]Ga-17B remained intact. Conclusively, MMAAC provides an attractive tool for selective, thermal, and metal ion-mediated control of metallodrug activation compatible with biological conditions.

Cyclic Peptides in Pipeline: What Future for These Great Molecules?

Cyclic peptides are molecules that are already used as drugs in therapies approved for various pharmacological activities, for example, as antibiotics, antifungals, anticancer, and immunosuppressants. Interest in these molecules has been growing due to the improved pharmacokinetic and pharmacodynamic properties of the cyclic structure over linear peptides and by the evolution of chemical synthesis, computational, and in vitro methods. To date, 53 cyclic peptides have been approved by different regulatory authorities, and many others are in clinical trials for a wide diversity of conditions. In this review, the potential of cyclic peptides is presented, and general aspects of their synthesis and development are discussed. Furthermore, an overview of already approved cyclic peptides is also given, and the cyclic peptides in clinical trials are summarized.

Gold-Mediated Multiple Cysteine Arylation for the Construction of Highly Constrained Bicycle Peptides

Bicycles are constrained bicyclic peptides formed through reaction of three cysteine residues within a linear sequence with a trivalent, symmetrical small molecule scaffold. Bicycles with high binding affinities to therapeutically important targets can be discovered using screening technologies such as phage display. Increasing the chemical diversity of Bicycles should improve the probability of finding hits to new targets and can be achieved by expanding the toolbox of Bicycle forming chemistries. Gold(III) S-arylation has recently been described as a method for the efficient bioconjugation of cysteine residues under conditions compatible with phage display. Herein, we explore the scope and generality of this methodology for Bicycle construction through the synthesis and evaluation of four novel tris-Gold complexes. These new scaffolds were systematically reacted with a variety of peptide sequences, varying in amino acid loop lengths. All four scaffolds proved to be capable and selective reactive partners for each peptide sequence and afforded the desired Bicycle products in 13-48% isolated yield. This work exemplifies Gold-mediated arylation as a general approach for construction of novel, highly constrained Bicycles.