A technique called hydrocarbon stapling has been shown to improve the pharmacological properties of peptides used to manipulate interactions between proteins. Writing in Science, Loren Walensky and colleagues describe how using these stapled peptides to modulate the BCL2 pathway in apoptosis provides both an attractive anticancer strategy and a tool for modulating protein interactions in many biological contexts.

Despite the attraction of modulating protein interactions, achieving this has proved difficult. Protein-interaction surfaces that are shallow, hydrophobic and extensive can present a challenge for targeting by small molecules. An alternative approach is to use peptides, but their use in vivo is hindered by poor permeability and sensitivity to proteases.

Focusing on the BCL2 pathway, Walensky et al. generated a panel of hydrocarbon-stapled peptides that mimic the so-called death domain (BH3) of the pro-apoptotic BCL2 family member BH3-interacting death domain agonist (BID). The BH3 domain is an essential feature of all BCL2 members and comprises an amphipathic α-helical segment that mediates many protein interactions. However, when taken out of their biological context, these helices lose their secondary structure, and in vivo function is compromised.

Most approaches to stabilize peptides leave them susceptible to degradation or unable to penetrate cells, and so the authors developed an alternative method. The reaction (a ruthenium-catalysed olefin metathesis) essentially stabilizes the helical conformation of the BH3 mimics, referred to as stabilized α-helix of BCL2 domains (SAHBs). Insertion of the staple also has another benefit: it shields the amide backbone, making the peptide less sensitive to proteolysis. Indeed, the SAHBs displayed improved serum stability in vitro and in vivo in a comparison with unmodified peptide.

The authors then studied the effects of SAHBs in an assay of cytochrome c release from mouse liver mitochondria (an early apoptotic event) and found that they caused a dose-dependent increase in cytochrome c, whereas the unmodified peptide caused a negligible effect in the low-dose range used. Furthermore, in Bak-null mitochondria (which do not release cytochrome c in response to a pro-apoptotic signal) SAHBA also failed to induce cytochrome c release, thereby proving that the peptide exerts its effects through the expected pathway. SAHBs also inhibited the growth of a wide variety of leukaemic cell lines and leukaemic xenografts in mice, and therefore offer not only a new method for manipulating biological pathways, but potentially a new class of anticancer drugs.