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

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.