TriplatinNC
is a highly positively charged, substitution-inert
derivative of the phase II clinical anticancer drug, BBR3464. Such
substitution-inert complexes form a distinct subset of polynuclear
platinum complexes (PPCs) interacting with DNA and other biomolecules
through noncovalent interactions. Rapid cellular entry is facilitated
via interaction with cell surface glycosoaminoglycans and is a mechanism
unique to PPCs. Nanoscale secondary ion mass spectrometry (nanoSIMS)
showed rapid distribution within cytoplasmic and nucleolar compartments,
but not the nucleus. In this article, the downstream effects of nucleolar
localization are described. In human colon carcinoma cells, HCT116,
the production rate of 47S rRNA precursor transcripts was dramatically
reduced as an early event after drug treatment. Transcriptional inhibition
of rRNA was followed by a robust G1 arrest, and activation
of apoptotic proteins caspase-8, -9, and -3 and PARP-1 in a p53-independent
manner. Using cell synchronization and flow cytometry, it was determined
that cells treated while in G1 arrest immediately, but
cells treated in S or G2 successfully complete mitosis.
Twenty-four hours after treatment, the majority of cells finally arrest
in G1, but nearly one-third contained highly compacted
DNA; a distinct biological feature that cannot be associated with
mitosis, senescence, or apoptosis. This unique effect mirrored the
efficient condensation of tRNA and DNA in cell-free systems. The combination
of DNA compaction and apoptosis by TriplatinNC treatment conferred
striking activity in platinum-resistant and/or p53 mutant or null
cell lines. Taken together, our results support that the biological
activity of TriplatinNC reflects reduced metabolic deactivation (substitution-inert
compound not reactive to sulfur nucleophiles), high cellular accumulation,
and novel consequences of high-affinity noncovalent DNA binding, producing
a new profile and a further shift in the structure–activity
paradigms for antitumor complexes.