Steven Fletcher

Targeting protein–protein interactions by rational design: mimicry of protein surfaces

Protein–protein interactions play key roles in a range of biological processes, and are therefore important targets for the design of novel therapeutics. Unlike in the design of enzyme active site inhibitors, the disruption of protein–protein interactions is far more challenging, due to such factors as the large interfacial areas involved and the relatively flat and featureless topologies of these surfaces. Nevertheless, in spite of such challenges, there has been considerable progress in recent years. In this review, we discuss this progress in the context of mimicry of protein surfaces: targeting protein–protein interactions by rational design.

Protein-Protein Interaction Inhibitors: Small Molecules from Screening Techniques

Protein-protein interactions play crucial roles in a number of biological processes, and, as such, their disruption is becoming an area of intense research. Despite the many challenges associated with the development of protein-protein interaction inhibitors, such as the large and relatively featureless interfacial areas involved, there has been considerable success in recent years. Importantly, through the existence of protein hot spots, some of this success takes the form of small molecule inhibitors that have been identified from a variety of screening techniques.

Small Molecule STAT5-SH2 Domain Inhibitors Exhibit Potent Antileukemia Activity

A growing body of evidence shows that Signal Transducer and Activator of Transcription 5 (STAT5) protein, a key member of the STAT family of signaling proteins, plays a pivotal role in the progression of many human cancers, including acute myeloid leukemia and prostate cancer. Unlike STAT3, where significant medicinal effort has been expended to identify potent direct inhibitors, Stat5 has been poorly investigated as a molecular therapeutic target. Thus, in an effort to identify direct inhibitors of STAT5 protein, we conducted an in vitro screen of a focused library of SH2 domain binding salicylic acid-containing inhibitors (∼150) against STAT5, as well as against STAT3 and STAT1 proteins for SH2 domain selectivity. We herein report the identification of several potent (K(i) < 5 μM) and STAT5 selective (>3-fold specificity for STAT5 cf. STAT1 and STAT3) inhibitors, BP-1-107, BP-1-108, SF-1-087, and SF-1-088. Lead agents, evaluated in K562 and MV-4-11 human leukemia cells, showed potent induction of apoptosis (IC(50)s ∼ 20 μM) which correlated with potent and selective suppression of STAT5 phosphorylation, as well as inhibition of STAT5 target genes cyclin D1, cyclin D2, C-MYC, and MCL-1. Moreover, lead agent BP-1-108 showed negligible cytotoxic effects in normal bone marrow cells not expressing activated STAT5 protein. Inhibitors identified in this study represent some of the most potent direct small molecule, nonphosphorylated inhibitors of STAT5 to date.

Amphipathic α-helix mimetics based on a 1,2-diphenylacetylene scaffold.

In order to mimic amphipathic α-helices, a novel scaffold based on a 1,2-diphenylacetylene was designed. NMR and computational modeling confirmed that an intramolecular hydrogen bond favors conformations of the 1,2-diphenylacetylene that allow for accurate mimicry of the i, i + 7 and i + 2, i + 5 side chains found on opposing faces of an α-helix.

Structure-Based Design and Synthesis of Potent, Ethylenediamine-Based, Mammalian Farnesyltransferase Inhibitors as Anticancer Agents

A potent class of anticancer, human farnesyltransferase (hFTase) inhibitors has been identified by piggy-backing on potent, antimalarial inhibitors of Plasmodium falciparum farnesyltransferase (PfFTase). On the basis of a 4-fold substituted ethylenediamine scaffold, the inhibitors are structurally simple and readily derivatized, facilitating the extensive structure-activity relationship (SAR) study reported herein. Our most potent inhibitor is compound 1f, which exhibited an in vitro hFTase IC(50) value of 25 nM and a whole cell H-Ras processing IC(50) value of 90 nM. Moreover, it is noteworthy that several of our inhibitors proved highly selective for hFTase (up to 333-fold) over the related prenyltransferase enzyme geranylgeranyltransferase-I (GGTase-I). A crystal structure of inhibitor 1a co-crystallized with farnesyl pyrophosphate (FPP) in the active site of rat FTase illustrates that the para-benzonitrile moiety of 1a is stabilized by a π-π stacking interaction with the Y361β residue, suggesting a structural explanation for the observed importance of this component of our inhibitors.

Recognition of solvent exposed protein surfaces using anthracene derived receptors

A new class of receptor is described that can selectively bind to the solvent exposed surface of proteins such as cytochrome c and lysozyme with low micromolar affinity over cytochrome c551, alpha-lactalbumin, myoglobin and RNase A, under physiologically relevant conditions (5 mM phosphate, pH 7.4). The use of anthracene as a hydrophobic scaffold allows the receptor to act as a selective chemosensor via fluorescence quenching or FRET. The study reveals that co-operative electrostatic interactions over a large surface area dominate binding. Further investigations reveal that the receptor binds to the solvent exposed heme edge of cytochrome c inhibiting its reaction with small reducing agents and validating the strategy for the disruption of protein function.

BRD4 Structure–Activity Relationships of Dual PLK1 Kinase/BRD4 Bromodomain Inhibitor BI-2536

A focused library of analogues of the dual PLK1 kinase/BRD4 bromodomain inhibitor BI-2536 was prepared and then analyzed for BRD4 and PLK1 inhibitory activities. Particularly, replacement of the cyclopentyl group with a 3-bromobenzyl moiety afforded the most potent BRD4 inhibitor of the series (39j) with a K i = 8.7 nM, which was equipotent against PLK1. The superior affinity of 39j over the parental compound to BRD4 possibly derives from improved interactions with the WPF shelf. Meanwhile, substitution of the pyrimidine NH with an oxygen atom reversed the PLK1/BRD4 selectivity to convert BI-2536 into a BRD4-selective inhibitor, likely owing to the loss of a critical hydrogen bond in PLK1. We believe further fine-tuning will furnish a BRD4 magic bullet or an even more potent PLK1/BRD4 dual inhibitor toward the expansion and improved efficacy of the chemotherapy arsenal.

Structural basis for binding and selectivity of antimalarial and anticancer ethylenediamine inhibitors to protein farnesyltransferase.

Protein farnesyltransferase (FTase) catalyzes an essential posttranslational lipid modification of more than 60 proteins involved in intracellular signal transduction networks. FTase inhibitors have emerged as a significant target for development of anticancer therapeutics and, more recently, for the treatment of parasitic diseases caused by protozoan pathogens, including malaria (Plasmodium falciparum). We present the X-ray crystallographic structures of complexes of mammalian FTase with five inhibitors based on an ethylenediamine scaffold, two of which exhibit over 1000-fold selective inhibition of P. falciparum FTase. These structures reveal the dominant determinants in both the inhibitor and enzyme that control binding and selectivity. Comparison to a homology model constructed for the P. falciparum FTase suggests opportunities for further improving selectivity of a new generation of antimalarial inhibitors.

Perturbation of the c-Myc–Max Protein–Protein Interaction via Synthetic α-Helix Mimetics

The rational design of inhibitors of the bHLH-ZIP oncoprotein c-Myc is hampered by a lack of structure in its monomeric state. We describe herein the design of novel, low-molecular-weight, synthetic α-helix mimetics that recognize helical c-Myc in its transcriptionally active coiled-coil structure in association with its obligate bHLH-ZIP partner Max. These compounds perturb the heterodimers binding to its canonical E-box DNA sequence without causing protein-protein dissociation, heralding a new mechanistic class of direct c-Myc inhibitors. In addition to electrophoretic mobility shift assays, this model was corroborated by further biophysical methods, including NMR spectroscopy and surface plasmon resonance. Several compounds demonstrated a 2-fold or greater selectivity for c-Myc-Max heterodimers over Max-Max homodimers with IC50 values as low as 5.6 μM. Finally, these compounds inhibited the proliferation of c-Myc-expressing cell lines in a concentration-dependent manner that correlated with the loss of expression of a c-Myc-dependent reporter plasmid despite the fact that c-Myc-Max heterodimers remained intact.

Potent, Plasmodium-selective farnesyltransferase inhibitors that arrest the growth of malaria parasites: structure-activity relationships of ethylenediamine-analogue scaffolds and homology model validation.

New chemotherapeutics are urgently needed to combat malaria. We previously reported on a novel series of antimalarial, ethylenediamine-based inhibitors of protein farnesyltransferase (PFT). In the current study, we designed and synthesized a series of second generation inhibitors, wherein the core ethylenediamine scaffold was varied in order to examine both the homology model of Plasmodium falciparum PFT (PfPFT) and our predicted inhibitor binding mode. We identified several PfPFT inhibitors (PfPFTIs) that are selective for PfPFT versus the mammalian isoform of the enzyme (up to 136-fold selectivity), that inhibit the malarial enzyme with IC50 values down to 1 nM, and that block the growth of P. falciparum in infected whole cells (erythrocytes) with ED50 values down to 55 nM. The structure-activity data for these second generation, ethylenediamine-inspired PFT inhibitors were rationalized by consideration of the X-ray crystal structure of mammalian PFT and the homology model of the malarial enzyme.