Development of potent small-molecule inhibitors to drug the undruggable steroid receptor coactivator-3

Abstract

Protein-protein interactions (PPIs) play a central role in most biological processes, and therefore represent an important class of targets for therapeutic development. However, disrupting PPIs using small-molecule inhibitors (SMIs) is challenging and often deemed as "undruggable." We developed a cell-based functional assay for highthroughput screening to identify SMIs for steroid receptor coactivator-3 (SRC-3 or AIB1), a large and mostly unstructured nuclear protein.Without any SRC-3 structural information, we identified SI-2 as a highly promising SMI for SRC-3. SI-2 meets all of the criteria of Lipinski's rule [Lipinski et al. (2001) Adv Drug Deliv Rev 46(1-3):3-26] for a drug-like molecule and has a half-life of 1 h in a pharmacokinetics study and a reasonable oral availability in mice. As a SRC-3 SMI, SI-2 can selectively reduce the transcriptional activities and the protein concentrations of SRC-3 in cells through direct physical interactions with SRC-3, and selectively induce breast cancer cell death with IC 50 values in the low nanomolar range (3-20 nM), but not affect normal cell viability. Furthermore, SI-2 can significantly inhibit primary tumor growth and reduce SRC-3 protein levels in a breast cancer mouse model. In a toxicology study, SI-2 caused minimal acute cardiotoxicity based on a hERG channel blocking assay and an unappreciable chronic toxicity to major organs based on histological analyses. We believe that this work could significantly improve breast cancer treatment through the development of "first-in-class" drugs that target oncogenic coactivators.steroid receptor coactivator | small-molecule inhibitor | breast cancer | drug development | protein-protein interactions P rotein-protein interactions (PPIs) play a central role in most biological processes, and therefore represent an important class of targets for therapeutic development (1). Biologics-based therapeutics, such as antibodies, exemplify success in PPI regulation (2). However, antibodies usually can only be applied to protein targets on cell surfaces because of their impermeability to plasma membranes (2). Although small-molecule drugs can readily cross membranes, applying small-molecule inhibitors (SMIs) to disrupt PPIs is a challenging task because ∼750-1,500 Å 2 of protein surface area is involved at the interface of PPIs (3), which is too large for SMIs to cover. In addition, these interacting protein surfaces do not have pocket-like small-molecule binding sites (2). Therefore, these PPI sites are deemed as "undruggable" targets for SMIs. The Holy Grail of drug development is to render small molecules the power of biologics to regulate PPIs.The current strategies for designing small-molecule PPI inhibitors primarily rely on the structural information of the protein targets (4). Clackson and Wells discovered that only a small set of residues at the PPI interface are critical for their interactions, known as "hot spots" (5). Therefore, current drug design for PPIs is mainly focused on small hot spots that can be covered by a dru...

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“…However, transcription factors are frequently found in multi-protein complexes, and designing small molecules that will potently change complex activity can be difficult 144 . Various confounding factors include the large molecular area involved in protein-protein interactions, which may not contain ideal pockets for small molecules to bind; the large and flexible structure of coactivators which can be accompanied by an unknown crystal structure; a lack of ligand-binding sites; and limited small molecule libraries with properties ideal for targeting transcription 145,146 . However, small molecules may allosterically affect protein-protein interactions to ultimately affect multi-protein complex activity either by disrupting the integrity of the complex or by changing post-translational modifications (PTMs) of specific subunits 147 .…”

“…However, small molecules may allosterically affect protein-protein interactions to ultimately affect multi-protein complex activity either by disrupting the integrity of the complex or by changing post-translational modifications (PTMs) of specific subunits 147 . Indeed, successful approaches have been employed to target transcriptional proteins, including the identification of a direct inhibitor of steroid receptor coactivator 3 (Src-3) through high-throughput screening of small molecules that alter Src-3-dependent transcription 145 . Moreover, modulating transcription factor activity – by intervening with the interaction of transcription factors with their cofactors - has been a major focus in drug development for the treatment of cancer 148,149 .…”

“…Additionally, a direct inhibitor of Src-3 has been identified 145 . However, potent inhibition of these coactivators would be undesirable due to the development of fasting hypoglycemia, and the potential toxicity to non-cancer cell types is unknown.…”

Dynamic assembly and activation of estrogen receptor α enhancers through coregulator switching

“…Given the role of SRCs in ERα enhancer maintenance described above, amplification of SRCs may promote sustained enhancer activation, resulting in prolonged mitogenic responses. Such an effect would justify the ongoing efforts to develop drugs that inhibit the interactions between ERα and SRCs (Rodriguez et al 2004;Song et al 2016), which can be a potent way of inhibiting SRC-, p300-, and Mediator-dependent transcription by ERα (Kim et al 2001;Acevedo and Kraus 2003).…”

Molecular Pathways: Targeting Steroid Receptor Coactivators in Cancer

“…A water soluble analog, 3-phospho bufalin, was developed to overcome the solubility problem of bufalin, and it was proved to be equally efficient in inhibiting tumor growth in an orthotopic breast cancer model (55). Importantly, additional investigations of compounds identified from this high throughput screen led to the identification and medicinal chemistry development of additional compounds with stronger drug-like properties such as SI-2 (56). SI-2 is able to selectively kill cancer cells at low nanomolar ranges (3 to 20 nM) and can inhibit primary tumor growth in a triple-negative breast cancer xenograft tumor model.…”