Semisynthesis of SY‑1 for Investigation of Breast Cancer Stem Cell Selectivity of C‑Ring-Modified Salinomycin Analogues

Salinomycin, a naturally occurring polyether ionophore was recently found to selectively reduce the proportion of CD44⁺/CD24^– cells, a phenotype associated with breast cancer stem cells. Subsequent studies from our group showed that chemical modification of the allylic C20 hydroxyl of salinomycin, located at the C-ring, can enhance the activity of derivatives against breast cancer cells over 5-fold compared to the native structure. Access to C-ring-modified salinomycin analogues is thus of interest from both a mechanistic and a synthetic perspective. Here, we report efficient strategies for gram scale synthesis of the natural product SY-1 (20-deoxy salinomycin), and a saturated analogue, 18,19-dihydro SY-1, for a comparative in vitro investigation of the biological profiles of these compounds with that of salinomycin. Across several assays, the deoxygenated structures required higher concentrations to elicit similar cellular responses to that of salinomycin. Similarly to salinomycin, SY-1 or 18,19-dihydro SY-1 treatment was found to reduce the proportion of CD44⁺/CD24^– cells with essentially complete selectivity up to ∼IC₂₅. Importantly, the proportion of CD44⁺/CD24^– cells showed a pronounced U-shaped dose response curve for salinomycin and its derivatives, but not for paclitaxel. The concentration for maximum response in this assay followed differences in IC₅₀ for salinomycin and its analogues, which emphasizes the importance of taking concentration dependence into account when comparing effects on the CD44⁺/CD24^– phenotype. Small differences in the global conformation within the triad of compounds investigated together with differences in activity across assays emphasize the importance of substitution at C20 for the activity of salinomycin and its derivatives.