IC50 values ≤ 0.6 μM. In a COMPARE analysis, diazirine-functionalized photopsammaplin exhibited a very similar cytotoxicity profile to that of the natural product 1. Furthermore, photopsammaplin 8 was found to be a potent HDAC inhibitor (IC50 35 nM) and can be considered as a suitable candidate for photoaffinity labeling, which may assist in the identification of new targets of psammaplin A. Irradiation