Design, Synthesis, and Biochemical Evaluation of Alpha-Amanitin Derivatives Containing Analogs of the trans-Hydroxyproline Residue for Potential Use in Antibody-Drug Conjugates

Progress in the Stereoselective Synthesis Methods of Pyrrolidine-Containing Drugs and Their Precursors

The presented review systematizes and summarizes the data on the synthesis of pyrrolidine derivatives, which are precursors for obtaining drugs. Based on the analysis of published data, the most promising directions in the synthesis of biologically active compounds containing a pyrrolidine ring are identified. Stereoselective synthesis methods are classified based on the source of the pyrrolidine ring. The first group includes methods that use a pyrrolidine ring as the starting compound. The second group combines stereoselective methods of cyclization of acyclic starting compounds, which lead to optically pure pyrrolidine derivatives.

Antibody-drug conjugates for targeted cancer therapy: Recent advances in potential payloads

Antibody-drug conjugates (ADCs) represent a promising cancer therapy modality which specifically delivers highly toxic payloads to cancer cells through antigen-specific monoclonal antibodies (mAbs). To date, 15 ADCs have been approved and more than 100 ADC candidates have advanced to clinical trials for the treatment of various cancers. Among these ADCs, microtubule-targeting and DNA-damaging agents are at the forefront of payload development. However, several challenges including toxicity and drug resistance limit the potential of this modality. To tackle these issues, multiple innovative payloads such as immunomodulators and proteolysis targeting chimeras (PROTACs) are incorporated into ADCs to enable multimodal cancer therapy. In this review, we describe the mechanism of ADCs, highlight the importance of ADC payloads and summarize recent progresses of conventional and unconventional ADC payloads, trying to provide an insight into payload diversification as a key step in future ADC development.

Synthesis of (2S,3R,4R)-Dihydroxyisoleucine for Use in Amatoxin Synthesis

We report a streamlined synthesis of (2S,3R,4R)-4,5-dihydroxy isoleucine (DHIle), an amino acid found in α-amanitin, which appears to be critical for toxicity. This synthetic route is transition metal-free and enables the production of significant quantities of DHIle with suitable protection for use in peptide synthesis. Its incorporation into a cytotoxic amatoxin analog is reported.

Antibody Drug Conjugates for Cancer Therapy: From Metallodrugs to Nature-Inspired Payloads

This review highlights significant advancements in antibody-drug conjugates (ADCs) equipped with metal-based and nature-inspired payloads, focusing on synthetic strategies for antibody conjugation. Traditional methods such us maleimide and succinimide conjugation and classical condensation reactions are prevalent for metallodrugs and natural compounds. However, emerging non-conventional strategies such as photoconjugation are gaining traction due to their milder conditions and, in an aspect which minimizes side reactions, selective formation of ADC. The review also summarizes the therapeutic and diagnostic properties of these ADCs, highlighting their enhanced selectivity and reduced side effects in cancer treatment compared to non-conjugated payloads. ADCs combine the specificity of monoclonal antibodies with the cytotoxicity of chemotherapy drugs, offering a targeted approach to the elimination of cancer cells while sparing healthy tissues. This targeted mechanism has demonstrated impressive clinical efficacy in various malignancies. Key future advancements include improved linker technology for enhanced stability and controlled release of cytotoxic agents, incorporation of novel, more potent, cytotoxic agents, and the identification of new cancer-specific antigens through genomic and proteomic technologies. ADCs are also expected to play a crucial role in combination therapies with immune checkpoint inhibitors, CAR-T cells, and small molecule inhibitors, leading to more durable and potentially curative outcomes. Ongoing research and clinical trials are expanding their capabilities, paving the way for more effective, safer, and personalized treatments, positioning ADCs as a cornerstone of modern medicine and offering new hope to patients.

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

N-Terminal Proline Editing for the Synthesis of Peptides with Mercaptoproline and Selenoproline: Mechanistic Insights Lead to Greater Efficiency in Proline Native Chemical Ligation

Native chemical ligation (NCL) at proline has been limited by cost and synthetic access. In addition, prior examples of NCL using mercaptoproline have exhibited stalling of the reaction after thioester exchange, due to inefficient S → N acyl transfer. Herein, we develop methods, using inexpensive Boc-4R-hydroxyproline, for the solid-phase synthesis of peptides containing N-terminal 4R-mercaptoproline and 4R-selenoproline. The synthesis proceeds via proline editing on the N-terminus of fully synthesized peptides on the solid phase, converting an N-terminal Boc-4R-hydroxyproline to the 4S-bromoproline, followed by an SN2 reaction with potassium thioacetate or selenobenzoic acid. After cleavage from the resin and deprotection, peptides with functionalized N-terminal proline amino acids were obtained. NCL reactions with mercaptoproline proceeded slowly under standard NCL conditions, with the S-acyl transthioesterification intermediate observed as a major species. Computational investigations indicated that the bicyclic intermediates and transition states for S → N acyl transfer are sufficiently low in energy (10-15 kcal mol-1 above starting material) that ring strain cannot explain the slow S → N acyl transfer. Instead, the bicyclic zwitterionic tetrahedral intermediate has a low barrier for reversion to the S-acyl intermediate, causing reversion to the thioester (reverse reaction) to occur preferentially over elimination to generate the amide (forward reaction). We hypothesized that a buffer capable of general acid and/or general base catalysis could promote S → N acyl transfer and thus achieve greater efficiency in proline NCL. In the presence of 2 M imidazole at pH 6.8, NCL with mercaptoproline proceeded efficiently to generate the peptide with a native amide bond. NCL with selenoproline also proceeded efficiently to generate the desired products when a thiophenol thioester was employed as a ligation partner. After desulfurization or deselenization, the products obtained were identical to those synthesized directly, confirming that the solid-phase proline editing reactions proceeded stereospecifically and without epimerization.

Convenient access to β-substituted cysteines and β- and γ-mercapto prolines

Herein we describe a convenient method for the synthesis of the β-substituted Cys analogs 1a, b and trans-β-mercapto proline 1c, from their corresponding α,β-unsaturated-α-amino esters using p-methoxybenzyl mercaptan as the sulfur source. The intermediate β-thioethers are stable toward chromatographic purification, and after global deprotection the β-mercapto amino acids 1a–c are obtained in good to high yields (64%–99%).

Rationally Designed Amanitins Achieve Enhanced Cytotoxicity

For 70 years, α-amanitin, the most cytotoxic peptide in its class, has been without a synthetic rival; through synthesis, we address the structure-activity relationships to inform the design of new amatoxins and disclose analogues that are more cytotoxic than the natural product when evaluated on CHO, HEK293, and HeLa cells, whereas on liver-derived HepG2 cells, the same toxins show diminished cytotoxicity.

Synthesis and evaluation of "Ama-Flash", a photocaged amatoxin prodrug for light-activated RNA Pol II inhibition and cell death

Amanitin is used extensively as a research tool to inhibit RNA Pol II thereby implicating its role in mRNA transcription. Recently, amanitin has gained traction as a toxic payload for targeted therapy. Here we report the first-ever photocaged amanitin analog, that is non-toxic and can be pre-loaded into cells. Light provides a means to inhibit RNA Pol II and provoke cell death on-demand.