Time-Dependent Studies of Oxaliplatin and Other Nucleolar Stress-Inducing Pt(II) Derivatives.

The properties of small molecule Pt(II) compounds that drive specific cellular responses are of interest due to their broad clinical use as chemotherapeutics as well as to provide a better mechanistic understanding of bioinorganic processes. The chemotherapeutic compound cisplatin causes cell death through DNA damage, while oxaliplatin may induce cell death through inhibition of ribosome biogenesis, also referred to as nucleolar stress induction. Previous work has found a subset of oxaliplatin derivatives that cause nucleolar stress at 24 h drug treatment. Here we report that these different Pt(II) derivatives exhibit a range of rates and degrees of global nucleolar stress induction as well as inhibition of rRNA transcription. Potential explanations for these variations include both the ring size and stereochemistry of the non-aquation-labile ligand. We observe that Pt(II) compounds containing a 6-membered ring show faster onset and a higher overall degree of nucleolar stress than those containing a 5-membered ring, and that compounds having the 1R,2R-stereoisomeric conformation show faster onset and a higher overall degree of stress than those having the 1S,2S-conformation. Pt(II) cellular accumulation and cellular Pt(II)-DNA adduct formation did not correlate with nucleolar stress induction, indicating that the effect is not due to global interactions. Together these results suggest that Pt(II) compounds induce nucleolar stress through a mechanism that likely involves one or a few key intermolecular interactions.

Influence of Ring Modifications on Nucleolar Stress Caused by Oxaliplatin-Like Compounds.

Oxaliplatin, a platinum compound in broad clinical use, can induce cell death through a nucleolar stress pathway rather than the canonical DNA damage response studied for other Pt(II) compounds. Previous work has found that the oxaliplatin 1,2-diaminocyclohexane (DACH) ring but not the oxalate leaving group is important to the ability to induce nucleolar stress. Here we study the influence of DACH ring substituents at the 4-position on the ability of DACH-Pt(II) compounds to cause nucleolar stress. We determine that DACH-Pt(II) compounds with 4-position methyl, ethyl, or propyl substituents induce nucleolar stress, but DACH-Pt(II) compounds with 4-isopropyl substituents do not induce nucleolar stress. This effect is independent of whether the substituent is in the axial or equatorial position relative to the trans diamines of the ligand. These results suggest that spatially sensitive interactions could be involved in the ability of platinum compounds to cause nucleolar stress.

Nucleolar Stress Induction by Oxaliplatin and Derivatives.

Platinum(II) compounds are a critical class of chemotherapeutic agents. Recent studies have highlighted the ability of a subset of Pt(II) compounds, including oxaliplatin but not cisplatin, to induce cytotoxicity via nucleolar stress rather than a canonical DNA damage response. In this study, influential properties of Pt(II) compounds were investigated using redistribution of nucleophosmin (NPM1) as a marker of nucleolar stress. NPM1 assays were coupled to calculated and measured properties such as compound size and hydrophobicity. The oxalate leaving group of oxaliplatin is not required for NPM1 redistribution. Interestingly, although changes in diaminocyclohexane (DACH) ligand ring size and aromaticity can be tolerated, ring orientation appears important for stress induction. The specificity of ligand requirements provides insight into the striking ability of only certain Pt(II) compounds to activate nucleolar processes.

Monofunctional platinum(II) compounds and nucleolar stress: is phenanthriplatin unique?

Platinum anticancer therapeutics are widely used in a variety of chemotherapy regimens. Recent work has revealed that the cytotoxicity of oxaliplatin and phenanthriplatin is through induction of ribosome biogenesis stress pathways, differentiating them from cisplatin and other compounds that mainly work through DNA damage response mechanisms. To probe the structure-activity relationships in phenanthriplatin's ability to cause nucleolar stress, a series of monofunctional platinum(II) compounds differing in ring number, size and orientation was tested by nucleophosmin (NPM1) relocalization assays using A549 cells. Phenanthriplatin was found to be unique among these compounds in inducing NPM1 relocalization. To decipher underlying reasons, computational predictions of steric bulk, platinum(II) compound surface length and hydrophobicity were performed for all compounds. Of the monofunctional platinum(II) compounds tested, phenanthriplatin has the highest calculated hydrophobicity and volume but does not exhibit the largest distance from platinum(II) to the surface. Thus, spatial orientation and/or hydrophobicity caused by the presence of a third aromatic ring may be significant factors in the ability of phenanthriplatin to cause nucleolar stress.