Synergistic potentiation of (-)-lomaiviticin A cytotoxicity by the ATR inhibitor VE-821

Synthesis and Biological Evaluation of (2S,2'S)-Lomaiviticin A

(-)-Lomaiviticin A (1) is a genotoxic C2-symmetric metabolite that arises from the formal dimerization of two bis(glycosylated) diazotetrahydrobenzo[b]fluorenes. Here we present a synthesis of the monomer 17 and its coupling to form (2S,2'S)-lomaiviticin A (4), an unnatural diastereomer of 1. (2S,2'S)-Lomaiviticin A (4) is significantly less genotoxic, a result we attribute to changes in the orientation of the diazofluorene and carbohydrate residues, relative to 1. These data bring the importance of the configuration of the conjoining bond to light and place the total synthesis of 1 itself within reach.

Discovery of ATR kinase inhibitor berzosertib (VX-970, M6620): Clinical candidate for cancer therapy

Chemoresistance, radioresistance, and the challenge of achieving complete resection are major driving forces in the search for more robust and targeted anticancer therapies. Targeting the DNA damage response has recently attracted research interest, as these processes are enhanced in tumour cells. The major replication stress responder is ATM and Rad3-related (ATR) kinase, which is attracting attention worldwide with four drug candidates currently in phase I/II clinical trials. This review addresses a potent and selective small-molecule ATR inhibitor, which is known as VX-970 (also known as berzosertib or M6620), and summarizes the existing preclinical data to provide deep insight regarding its real potential. We also outline the transition from preclinical to clinical studies, as well as its relationships with other clinical candidates (AZD6738, VX-803 [M4344], and BAY1895344). The results suggest that VX-970 is indeed a promising anticancer drug that can be used both as monotherapy and in combination with either chemotherapy or radiotherapy strategies. Based on patient anamnesis and biomarker identification, VX-970 could become a valuable tool for oncologists in the fight against cancer.

The Mechanism of Action of (-)-Lomaiviticin A

(-)-Lomaiviticin A (4) is a complex C2-symmetric bacterial metabolite that contains two diazofluorene functional groups. The diazofluorene consists of naphthoquinone, cyclopentadiene, and diazo substituents fused through a σ- and π-bonding network. Additionally, (-)-lomaiviticin A (4) is a potent cytotoxin, with half-maximal inhibitory potency (IC50) values in the low nanomolar range against many cancer cell lines. Because of limitations in supply, its mechanism of action had remained a "black box" since its isolation in the early 2000s. In this Account, I describe how studies directed toward the total synthesis of (-)-lomaiviticin A (4) provided a platform to elucidate the emergent properties of this metabolite and thereby connect chemical reactivity with cellular phenotype. We first developed a convergent strategy to prepare the diazofluorene (9 + 10 → 13). We then adapted this chemistry to the synthesis of lomaiviticin aglycon (21/22) and the natural monomeric diazofluorene (-)-kinamycin F (3). The key step in the lomaiviticin aglycon (21/22) synthesis involved the stereoselective oxidative coupling of two monomeric diazofluorenes (2 × 18 → 20) to establish the cojoining carbon-carbon bond of the target. As the absolute stereochemistry of the aglycon and carbohydrate residues of (-)-lomaiviticin A (4) were unknown, we developed a semisynthetic route to the metabolite that proceeds in one step and 42% yield by diazo transfer to the more abundant isolate (-)-lomaiviticin C (6). This allowed us to complete the stereochemical assignment of (-)-lomaiviticin A (4) and provided a renewable source of material. Using this material, we established that the remarkable cytotoxic effects of (-)-lomaiviticin A (4) derive from the induction of highly toxic double-strand breaks (DSBs) in DNA. At the molecular level, 1,7-nucleophilic additions to each electrophilic diazofluorene trigger homolytic decomposition pathways that produce sp2 radicals at the carbon atoms of each diazo group. These radicals abstract hydrogen atoms from the deoxyribose of DNA, a process known to initiate strand cleavage. NMR spectroscopy and molecular mechanics simulations were used to elucidate the mode of DNA binding. These studies showed that both diazofluorenes of (-)-lomaiviticin A (4) penetrate into the duplex. This mode of non-covalent binding places each diazo carbon atom in close proximity to each DNA strand. Throughout these studies, isolates containing one diazofluorene, such as (-)-lomaiviticin C (6) and (-)-kinamycin C (2), were used as controls. Consistent with our mechanistic model, these compounds do not induce DSBs in DNA and are several orders of magnitude less potent. Reactivity studies suggest that (-)-lomaiviticin A (4) is more electrophilic than simple monomeric diazofluorenes. We attribute this to through-space delocalization of the developing negative charge in the transition state for 1,7-addition. Consistent with this mechanism of action, (-)-lomaiviticin A (4) displays selective low-picomolar potencies toward DNA DSB repair-deficient cell types. The emergent properties of (-)-lomaiviticin A (4) derive from the specific arrangement of diazo, naphthoquinone, cyclopentadiene, and ketone functional groups. These functional groups work together to yield, essentially, a masked vinyl radical that can be exposed under biological conditions. Furthermore, the rotational symmetry of the metabolite, deriving from dimerization, allows it to interact with the antiparallel symmetry of DNA and affect cleavage of the duplex.

Mechanism of Nucleophilic Activation of (-)-Lomaiviticin A

(-)-Lomaiviticin A (1) is a C₂-symmetric cytotoxin that contains two diazofluorene functional groups and which induces double-strand breaks (DSBs) in DNA. Evidence suggests DNA cleavage is initiated by hydrogen atom abstraction from the deoxyribose backbone. Here we demonstrate the formation of the vinyl radicals 1· and 2· from 1 by 1,7-addition of thiols to the diazofluorenes. These radicals can affect hydrogen atom abstraction from methanol and acetone. The first addition of thiol to 1 proceeds at a much greater rate than the second. The diazosulfide 5 formed en route to 1· has been detected at -50 °C and undergoes decomposition to 1· with a half-life of 110 min at -20 °C under air. These data, which constitute the first direct evidence for the generation of 1· and 2· from 1, provide insights into the mechanism of DNA cleavage by 1.