Rsc_cc_c4cc01199f 3..6

The development of agents for selective modulation of protein– protein interactions (PPIs) constitutes a prominent goal in drug discovery and chemical biology. Since PPIs are often mediated by well defined secondary structural elements, a promising strategy in this area has involved the stabilization or mimicry of these motifs via compact molecular scaffolds. Reflecting their abundance in protein structures, a-helices are often encountered at the interface of protein–protein complexes. Accordingly, a number of strategies have been developed for stabilization of a-helical peptides, which include the use of hydrogen bond surrogates as well as of a variety of inter-side-chain linkages such as disulfide, lactam, thioether or triazole bridges, ‘hydrocarbon staples’, and cysteine cross-linking moieties. We recently reported strategies for the synthesis of macrocyclic organo-peptide hybrids (MOrPHs) via the chemoand regioselective ligation of bifunctional synthetic precursors to genetically encoded precursor polypeptides (e.g. Fig. 1A). A key feature of this new class of peptide-based macrocycles is their modular architecture, as given by the diverse non-peptidic and peptidic moieties amenable to incorporation into these scaffolds. As part of ongoing studies directed at evaluatingMOrPHs as disruptors of biomedically relevant PPIs, we were interested in assessing the potential of these macrocyclic scaffolds to accommodate, and possibly, stabilize a functional a-helical motif. In this work, we describe the successful implementation of this idea through the design and development of a-helical MOrPHs that can effectively disrupt the interaction between the tumor suppressor p53 and the oncoproteins HDM2 and HDMX. HDM2/X are implicated in the negative regulation of p53 activity and overexpression of these proteins has been linked to several malignancies. While dual inhibition of HDM2/X has emerged as a most promising strategy for anticancer therapy, small-molecule inhibitors of HDM2 typically fail to potently interfere with p53:HDMX interaction due to subtle differences in the p53 binding clefts of these protein homologs. These limitations make the development of dual HDM2/X inhibitors a topic of current interest. HDM2 and HMDX bind to the N-terminal transactivation domain of p53 (p5315–29), which upon complex formation adopts a well defined a-helix. Thus, in addition to its biomedical relevance, these structural features have made the p53:HDM2 interaction an ideal test bed to probe strategies for a-helix stabilization and mimicry. Fig. 1 (A) MOrPH macrocyclization strategy and chemical structure of synthetic precursors (box) investigated in this study. (B) Crystal structure of HDM2:PMI complex (pdb 3EQS) and model of representative example of designer a-helical MOrPH (i/i + 10 peptide cyclization with SP8).