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 (M11) and hydroxylated (M6, M15) metabolites. The major (dehydrogenated: M11) 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 Cpd I model systems to gain insights into metabolic preferences, reactive metabolite formation, and associated toxicities. Potential energy surface (PES) analysis using activation (ΔG^‡), reaction (ΔG°) 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 (M11) and hydroxylated (M6) metabolites is favored relative to aromatic hydroxylated (M15) metabolites (reaction free energies: k_BT = 18.5 kcal/mol as cutoff). The detailed analysis of the metabolic pathway for the major metabolite (M11) 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 (M6) remains at >10 Å, making the reactive M12 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 M12 metabolite (formed from M6) 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.