The first total synthesis of the marine bromotyrosine purpurealidin I (1) using trifluoroacetoxy protection group and its dimethylated analog (29) is reported along with 16 simplified bromotyrosine derivatives lacking the tyramine moiety.

Their cytotoxicity was evaluated against the human malignant melanoma cell line (A-375) and normal skin fibroblast cells (Hs27) together with 33 purpurealidin-inspired simplified amides, and the structure-activity relationships were investigated. The synthesized simplified analogs without the tyramine part retained the cytotoxic activity.

Purpurealidin I (1) showed no selectivity but its simplified pyridin-2-yl derivative (36) had the best improvement in selectivity (Selectivity index 4.1). This shows that the marine bromotyrosines are promising scaffolds for developing cytotoxic agents and the full understanding of the elements of their SAR and improving the selectivity requires further optimization of simplified bromotyrosine derivatives.

Several syntheses of bromotyrosines have been reported but the synthesis of bromotyrosines with monomethylated tyramine part has not been reported before. The selective removal of the protective groups from the tyramine fragment before the coupling reaction is a challenging step in the total synthesis of purpurealidin I (1).

The researchers succeeded at this by using trifluoroacetyl protection. This route can be utilized further for the synthesis of additional bromotyrosine derivatives possessing the monomethylated tyramine fragment. The synthesized simplified analogs without the tyramine fragment retained the cytotoxic activity.

The selectivity towards melanoma cell line was generally low. The highest selectivity (SI 4.1) was demonstrated in the case of pyridin-2-yl compound (36).

This showed that the marine cytotoxic bromotyrosines are promising scaffolds for developing cytotoxic agents and the full understanding of the elements of their SAR is still in a very early stage. Further optimization of simplified bromotyrosine derivatives is needed to attain high selectivity.

Analysis of Selectivity to Cancer Cells

The cells were seeded to white frame and clear bottom 96-well plates (Perkin Elmer) at the density of 10,000 cells/well for A-375 cell line and 7500 cells/well for Hs27 cell line. The cells were grown at 37 °C, 5% CO2 until they reached 70–80% confluence (approximately 24 h).

Stock solutions of test compounds and a positive control (camptothecin, Sigma-Aldrich, Saint Louis, MO, USA) were prepared in DMSO and diluted into assay medium (growth medium with 5% FBS) to the final concentration.

Final DMSO concentration was 0.5% in all samples. The culture medium was removed from the plate and compounds added, 200 µL/well. After 48-h incubation, the amount of ATP, which is directly proportional to the number of cells present in the culture, was quantified using CellTiter-Glo® Luminescent Cell Viability kit (Promega, Madison, WI, USA), per manufacturer’s instructions.