Gold(III) Macrocycles: Nucleotide-Specific Unconventional Catalytic Inhibitors of Human Topoisomerase I

Abstract

Topoisomerase IB (Top1) is a key eukaryotic nuclear enzyme that regulates the topology of DNA during replication and gene transcription. Anticancer drugs that block Top1 are either well-characterized interfacial poisons or lesser-known catalytic inhibitor compounds. Here we describe a new class of cytotoxic redox-stable cationic Au3+ macrocycles which, through hierarchical cluster analysis of cytotoxicity data for the lead compound, 3, were identified as either poisons or inhibitors of Top1. Two pivotal enzyme inhibition assays prove that the compounds are true catalytic inhibitors of Top1. Inhibition of human topoisomerase IIα (Top2α) by 3 was 2 orders of magnitude weaker than its inhibition of Top1, confirming that 3 is a type I-specific catalytic inhibitor. Importantly, Au3+ is essential for both DNA intercalation and enzyme inhibition. Macromolecular simulations show that 3 intercalates directly at the 5′-TA-3′ dinucleotide sequence targeted by Top1 via crucial electrostatic interactions, which include π–π stacking and an Au···O contact involving a thymine carbonyl group, resolving the ambiguity of conventional (drug binds protein) vs unconventional (drug binds substrate) catalytic inhibition of the enzyme. Surface plasmon resonance studies confirm the molecular mechanism of action elucidated by the simulations.

The Benzyl Moiety in a Quinoxaline‐Based Scaffold Acts as a DNA Intercalation Switch

“…[1][2][3] In addition to studies on synthetic derivatives, [4,5] potent anticancer agents have been designed using the depsipeptide-biquinoxaline scaffold. [6] In contrast, known DNA-binding monoquinoxaline scaffolds are either metal chelates [7] or contain af used quinoxaline moeity. [8] TheD NA binding affinity of the only reported simple synthetic monoquinoxaline DNA-binding scaffold, however, was only moderate.…”

Mechanism and resistance for antimycobacterial activity of a fluoroquinophenoxazine compound

“…This might be due to FP-11g acting as an unconventional catalytic inhibitor. These types of inhibitors are described as compounds that bind to the DNA as well as to the topoisomerase enzyme to inhibit the topoisomerase activity [39]. Mutations that affect the essential activity of topoisomerase I could compromise cell growth to a significant extent and would less likely be selected for resistance than the mutations detected here that can limit compound transport into the cell.…”

Transition Metal Intercalators as Anticancer Agents—Recent Advances

“…For example, gold complexes have been relatively successful in medicinal chemistry [69,70,71], although most active anticancer complexes do not target DNA [72,73,74]. Recently reported macrocyclic gold(III) complexes that incorporated a quinoxaline moiety to promote DNA intercalation demonstrated cytotoxic activity [75]. The lead compound, [Au(12,13,14,15-tetrahydro-6,9:18,21-diepimino[1,6]diazacycloctadecino[12,13- b ]quinoxaline)] + ( Au1 , Figure 11) exhibited low micromolar activity in a panel of human cell lines, particularly in leukaemia and central nervous system cancers, and was well-tolerated by nude tumour-less mice at high doses.…”