Quantum Chemical Modelling and Experimental Validation of Reactive Metabolite Formation from Selected Pyrrolizidine Alkaloids in Hepatotoxic Herbal Medicines Commonly Used in North East Nigeria

Pyrrolizidine alkaloids present in several herbal plant species used in North East Nigerian traditional medicine are metabolically bioactivated by CYP3A4 and CYP2B6 to reactive pyrrolic esters that form DNA and protein adducts, causing veno-occlusive liver disease. The quantitative relationship between PA chemical structure, metabolic activation barrier, and reactive metabolite hepatotoxic potency has not been systematically characterised using quantum chemical approaches integrated with experimental liver metabolism data. This study applied density functional theory quantum chemical modelling, coupled with experimental microsomal metabolism studies and reactive metabolite trapping, to characterise structure-toxicity relationships for eight pyrrolizidine alkaloids identified in herbal medicines frequently used in Borno, Adamawa, and Gombe States. DFT calculations at the B3LYP/6-311+G** level of theory quantified activation energy barriers and frontier molecular orbital energies for CYP-mediated oxidation reactions of each PA. In vitro metabolic activation was conducted in human liver microsomes, and reactive pyrrolic metabolites were trapped by glutathione and potassium cyanide to form stable adducts quantified by LC-MS/MS. Correlation between calculated activation energies and experimentally measured reactive metabolite formation rates was evaluated. Strong negative correlations (r2 above 0.88) were found between DFT-calculated activation barriers and experimental reactive metabolite yields. Retronecine-type PAs with unsaturated 1,2-positions showed the lowest activation barriers and highest reactive metabolite formation. Quantum chemical descriptors for PA hepatotoxic potential were developed and validated against existing in vivo hepatotoxicity data. This study provides an original integrated quantum chemical framework for predictive hepatotoxicity assessment of PA-containing herbal medicines with direct regulatory applicability. Keywords: pyrrolizidine alkaloids, quantum chemical modelling, reactive metabolites, hepatotoxicity, North East Nigeria.