Olaparib Metabolism: Quantum Chemistry, Docking, and Dynamics-Based Insights into Mechanisms and Reactivity

Olaparib, an anticancer drug, has been recently associated with major side effects (hepatotoxicity and hematotoxicity). Human CYP450 3A4/5 metabolizes olaparib and forms dehydrogenated (<b>M11</b>) and hydroxylated (<b>M6</b>, <b>M15)</b> metabolites. The major (dehydrogenated: <b>M11</b>) metabolite is unreactive due to the stability of its amide bonds. Thus, the recently reported toxicities (hepato- and hemato) remain mysterious. Here, we investigate olaparib's metabolic pathways using <b>Cpd I</b> model systems to gain insights into metabolic preferences, reactive metabolite formation, and associated toxicities. Potential energy surface (PES) analysis using activation (Δ<i>G</i>^‡), reaction (Δ<i>G</i>°) free energies, and molecular docking, dynamics-based accessibility (distance of site of metabolism: SOM from heme-Fe) is utilized to explain metabolic preferences. Quantum chemical calculations showed that the formation of dehydrogenated (<b>M11)</b> and hydroxylated (<b>M6) metabolites</b> is favored relative to aromatic hydroxylated (<b>M15)</b> metabolites (reaction free energies: <i>k</i>_BT = 18.5 kcal/mol as cutoff). The detailed analysis of the metabolic pathway for the major metabolite (<b>M11)</b> formation showed that hydroxylation follows the E1 mechanism, leading to dehydration and the formation of a tetrahydropyrazine derivative. The olaparib piperazine ring C approaches the heme-Fe within activating distance (6 ± 2 Å) in most docked poses and during 200 ns MD simulations. The C10 leading to hydroxylated metabolite (<b>M6)</b> remains at >10 Å, making the reactive <b>M12</b> formation less likely. Furthermore, the MM-GBSA-based per-residue calculations showed that 13 active-site residues, including Arg105, contribute significantly to the binding energy (avg: -1.24 kcal/mol). DFT-based global and local reactivity (electrophilicity: ω) analysis showed that the 4-acetylphthalazin-1(2H)-one group in the <b>M12</b> metabolite (formed from <b>M6</b>) is highly electrophilic and might explain the idiosyncratic toxicities. These findings may offer valuable insights into the mechanisms of toxicity and for the design of novel and less toxic olaparib analogs.