Design, solid-phase synthesis and biological evaluation of phenyl-piperazine-triazine a-helix mimetics

Abstract

This thesis describes design, and solid-phase synthesis of phenyl-piperazine-triazines as an a-helix mimicking small molecule, and their biological evaluations in a-helix mediated protein-protein interactions (PPIs). In a-Helix mediated PPIs, short helical segments with 2-3 turns function as recognition motif by organizing three important residues whose positions are at i, i+3 or 4, and i+7. The side chains of these residues contribute the majority of interaction by making key contacts with binding partner proteins. Based on the feature, there have been a number of trials to develop small molecules able to reproduce the role of these three side chains in natural a-helices. Especially, it was reported in 2001 that terphenyl structure with appropriate functional groups can work as a-helix mimetics. However, low water solubility and synthetic difficulty prevent this scaffold from further biological studies. Since this seminal work, it has been of great importance to develop molecules with improved water solubility and great synthetic accessibility. Inspired by terphenyl scaffold, phenyl-piperazine-triazines were designed. From the energy-minimization study, it was envisioned that phenyl-piperazine-triazines with appropriate functional groups can mimic the three dimensional orientation of three key residues of a-helix involved in PPIs, as in the case of terphenyl scaffold. To determine water solubility partition coefficient, ClogP, was calculated and compared with terphenyl structure. When methyl groups were substituted for the position of three side chains, the values were 0.96 and 6.02 for phenyl-piperazine-triazine and terphenyl, respectively, suggesting the great improvement in terms of water solubility. Next, a convenient and divergent solid-phase synthetic route for phenyl-piperazine-triazines was developed. Low synthetic accessibility such as long synthesis route not only limits the size of chemical library but also make it difficult to modify structures of lead compounds for improved potency. However, the solid-phase synthesis of phenyl-piperazine-triazines in this work was possible throughout less than 10 steps, and simple to introduce various functional groups, thereby facilitating the preparation of a huge size of chemical library in a short period of time. The average purity, which was monitored from HPLC spectra, was above 85 percent illustrating the rigidity of the proposed synthesis. To evaluate the ability as a-helix mimetics, the inhibitory activity on Mcl-1/BH3 interactions was checked. Due to its anti-apoptotic function, Mcl-1 is regarded as a therapeutic target in cancer. As a result of screening using 36-membered focused library, PPT-31 compound was shown to act as the most potent inhibitor against Mcl-1 (Ki = 7.3 uM). However, this compound did not disrupt the interaction between Bcl-xL, another member of Bcl-2 family, and BH3 protein, suggesting that it can be a selective inhibitor. Together it was shown that phenyl-piperazine-triazines can serve as a-helix mimetics with improved physicochemical properties. It is expected that along with a simple and divergent solid-phase synthesis, this scaffold have a great potential in exploring a-helix mediated PPIs as powerful tools.