Characterisation of anti-prostate cancer metabolites from a prefractionated South African medicinal plant library reveals a bioactive metabolite conversion.

Natural products remain important sources of structurally diverse metabolites with anticancer potential, but biological screening of crude plant extracts can obscure active constituents. In this study, a prefractionated high-throughput screening-ready medicinal plant library from South Africa was evaluated against the prostate cancer cell lines DU145, 22Rv1 and PC-3. Standardised prefractionation improved the resolution of cytotoxic activity relative to crude extracts and enabled the prioritisation of an active hit fraction for further chemical investigation. Bioassay-guided isolation from Zanthoxylum capense yielded six alkaloids and one fatty amide, where chelerythrine (3) showed the strongest activity with IC50 values of 1.3-2.5 μM across the tested cell lines. During isolation, chelerythrine (3) underwent solvent-dependent disproportionation conversion to a less active derivative dihydrochelerythrine (7), as confirmed by UPLC-HRMS and NMR. This transformation was associated with solvent removal from acidic acetonitrile-containing fractions and was not observed under equivalent methanol-containing conditions. These findings highlight the importance of monitoring solvent history and compound stability during natural product isolation, particularly for benzophenanthridine alkaloids. This work demonstrates the value of prefractionated medicinal plant libraries for identifying cytotoxic metabolites while also revealing a processing-induced transformation that may affect compound identity and biological attribution.