Synthesis of OSW-1 Photoaffinity Probe and Its Biological Evaluation

<p>OSW-1 (1) is a steroidal saponin, isolated from the bulbs of Ornithogalumn Saundersiae, which displays potent and selective anti-proliferative activity toward several types of cancer cell lines and its mechanism has been attracted. However, the mechanism of action at molecular level is still unclear. Identification of cellular targets of OSW-1 is important for elucidating its mechanism of action.</p><p> To identify OSW-1 binding proteins, we designed and synthesized a photoaffinity probe (2) based on OSW-1, which bears diazirine as a photoreactive group and an alkyne tag as a detection group. We previously showed by using NMR and X-ray structural analysis that the sterol side chain, the acyl groups on the sugar moiety formed a hydrophobic cluster. Based on our hypothesis that the hydrophobic cluster is a protein binding domain, we chose the C4’’-OH of 1, which is distant from the cluster moiety as the site of modification. We also developed the site-selective acylation method of 1 by using Me₂SnCl₂previously.</p><p> To develop the synthetic method, we first examined the Me₂SnCl₂-mediated acylation conditions to introduce the glutaryl linker which could be coupled to the lysine moiety bearing diazirine and alkyne (4) by using a model xylose derivative 3. Based on the optimized acylation condition, we synthesized 2 in 58% yield. The photoaffinity probe 2 showed a potent anti-proliferative activity toward HeLa cells (IC₅₀= 2.7 nM), which is similar to that of 1. The photoaffinity labeling studies using 2were also presented.</p>