A weight of evidence approach to assess the human genotoxicity risk of Chinese goldthread botanical extract and its primary constituent berberine

The genotoxicity potential of Chinese Goldthread Extract (CGPE) was initially explored using the Ames test and in vitro micronucleus assay. CGPE induced gene mutations in the Ames test and micronuclei in TK6 human lymphoblastoid cells, and berberine, present in CGPE above 20 %, was identified as the causative phytoconstituent. We confirmed berberine's ability to bind to, and interfere with, human topoisomerase II and bacterial DNA gyrase, enzymes that play a key role in maintaining DNA integrity and conclude that this likely explains the genotoxic effects we observed in the above-mentioned in vitro genotoxicity assays and in an HPRT assay using L5178Y cells. No micronuclei were induced in primary human blood lymphocytes, and in vitro effects were negated when external metabolic activation was included. These factors support a lack of relevance of the positive in vitro findings for human risk assessment. This assumption is further strengthened by comparing human pharmacokinetic Cmax blood levels with in vitro concentrations of berberine that trigger impairment of topo II, a known secondary genotoxicity pathway, revealing a large Margin of Exposure of ≥11900-fold. The overall Weight of Evidence supports the absence of a genotoxicity concern at typical use levels in commercial CGPE and berberine supplements.