The antibody-drug conjugate (ADC), a humanized or human monoclonal antibody conjugated with highly cytotoxic small molecules (payloads) through chemical linkers, is a novel therapeutic format and has great potential to make a paradigm shift in cancer chemotherapy. This new antibody-based molecular platform enables selective delivery of a potent cytotoxic payload to target cancer cells, resulting in improved efficacy, reduced systemic toxicity, and preferable pharmacokinetics (PK)/pharmacodynamics (PD) and biodistribution compared to traditional chemotherapy. Boosted by the successes of FDA-approved Adcetris® and Kadcyla®, this drug class has been rapidly growing along with about 60 ADCs currently in clinical trials. In this article, we briefly review molecular aspects of each component (the antibody, payload, and linker) of ADCs, and then mainly discuss traditional and new technologies of the conjugation and linker chemistries for successful construction of clinically effective ADCs. Current efforts in the conjugation and linker chemistries will provide greater insights into molecular design and strategies for clinically effective ADCs from medicinal chemistry and pharmacology standpoints. The development of site-specific conjugation methodologies for constructing homogeneous ADCs is an especially promising path to improving ADC design, which will open the way for novel cancer therapeutics.

Valine–citrulline linkers are commonly used as enzymatically cleavable linkers for antibody–drug conjugates. While stable in human plasma, these linkers are unstable in mouse plasma due to susceptibility to an extracellular carboxylesterase. This instability often triggers premature release of drugs in mouse circulation, presenting a molecular design challenge. Here, we report that an antibody–drug conjugate with glutamic acid–valine–citrulline linkers is responsive to enzymatic drug release but undergoes almost no premature cleavage in mice. We demonstrate that this construct exhibits greater treatment efficacy in mouse tumor models than does a valine–citrulline-based variant. Notably, our antibody–drug conjugate contains long spacers facilitating the protease access to the linker moiety, indicating that our linker assures high in vivo stability despite a high degree of exposure. This technology could add flexibility to antibody–drug conjugate design and help minimize failure rates in pre-clinical studies caused by linker instability.

Breast tumors generally consist of a diverse population of cells with varying gene expression profiles. Breast tumor heterogeneity is a major factor contributing to drug resistance, recurrence, and metastasis after chemotherapy. Antibody-drug conjugates (ADCs) are emerging chemotherapeutic agents with striking clinical success, including T-DM1 for HER2-positive breast cancer. However, these ADCs often suffer from issues associated with intratumor heterogeneity. Here, we show that homogeneous ADCs containing two distinct payloads are a promising drug class for addressing this clinical challenge. Our conjugates show HER2-specific cell killing potency, desirable pharmacokinetic profiles, minimal inflammatory response, and marginal toxicity at therapeutic doses. Notably, a dual-drug ADC exerts greater treatment effect and survival benefit than does co-administration of two single-drug variants in xenograft mouse models representing intratumor HER2 heterogeneity and elevated drug resistance. Our findings highlight the therapeutic potential of the dual-drug ADC format for treating refractory breast cancer and perhaps other cancers.

Antibody-drug conjugates (ADCs) are emerging therapeutic agents in the treatment of cancer, and various conjugation strategies and chemical linkers have been developed to efficiently construct ADCs. Despite previous extensive efforts for improving conjugation efficiency and ADC homogeneity, most ADC linkers developed to date load only single payloads. Branched linkers that can load multiple payload molecules have yet to be fully explored. It is logical to envisage that a multi-loading strategy allows for increase in drug-to-antibody ratio (DAR) with less chemical or enzymatic modification to the antibody structure compared to traditional linear linkers, leading to efficient ADC construction, minimal destabilization of the antibody structure, and enhanced ADC efficacy. Herein, we report that the branched linkers we designed can be quantitatively installed on an anti-HER2 monoclonal antibody by microbial transglutaminase (MTGase)-mediated conjugation without impairing its antigen binding affinity, enabling modular installation of payload molecules and construction of homogeneous ADCs with increased DARs (up to 8). An anti-HER2 antibody-monomethyl auristatin F conjugate constructed using our branched linkers showed greater in vitro cytotoxicity against HER2-expressing breast cancer cell lines than that consisting of linear linkers, demonstrating the effectiveness of the branched linker-based payload delivery. Our finding demonstrates that enzymatic ADC construction using branched linkers is a promising strategy, which may lead to innovative cancer therapeutics.

Therapeutic strategies are urgently needed for patients with acute myeloid leukemia (AML). Leukocyte immunoglobulinlike receptor B4 (LILRB4), which suppresses T-cell activation and supports tissue infiltration of AML cells, represents an attractive drug target for anti-AML therapeutics. Here, we report the identification and development of an LILRB4specific humanized mAb that blocks LILRB4 activation. This mAb, h128-3, showed potent activity in blocking the development of monocytic AML in various models including patient-derived xenograft mice and syngeneic immunocom-petent AML mice. MAb h128-3 enhanced the anti-AML efficacy of chemotherapy treatment by stimulating mobilization of leukemia cells. Mechanistic studies revealed four concordant modes of action for the anti-AML activity of h128-3: (i) reversal of T-cell suppression, (ii) inhibition of monocytic AML cell tissue infiltration, (iii) antibodydependent cellular cytotoxicity, and (iv) antibodydependent cellular phagocytosis. Therefore, targeting LILRB4 with antibody represents an effective therapeutic strategy for treating monocytic AML.

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.

No abstract

Bacteria have developed cell-to-cell communication mechanisms, termed quorum sensing (QS), which regulate bacterial gene expression in a cell population-dependent manner. Autoinducer-2 (AI-2), a class of QS signaling molecules derived from (4S)-4,5-dihydroxy-2,3-pentanedione (DPD), has been identified in both Gram-negative and Gram-positive bacteria. Despite considerable interest in the AI-2 QS system, the biomolecular communication used by distinct bacterial species still remains shrouded. Herein we report the synthesis and evaluation of a new class of DPD analogs, C4-alkoxy-5-hydroxy-2,3-pentanediones, termed C4-alkoxy-HPDs. Remarkably, two of the analogs were more potent QS agonists than the natural ligand, DPD, in Vibrio harveyi. The findings presented extend insights into ligand-receptor recognition/signaling in the AI-2 mediated QS system.