Approach for targeting Ras with small molecules that activate SOS-mediated nucleotide exchange.

Aberrant activation of the small GTPase Ras by oncogenic mutation or constitutively active upstream receptor tyrosine kinases results in the deregulation of cellular signals governing growth and survival in ∼30% of all human cancers. However, the discovery of potent inhibitors of Ras has been difficult to achieve. Here, we report the identification of small molecules that bind to a unique pocket on the Ras:Son of Sevenless (SOS):Ras complex, increase the rate of SOS-catalyzed nucleotide exchange in vitro, and modulate Ras signaling pathways in cells. X-ray crystallography of Ras:SOS:Ras in complex with these molecules reveals that the compounds bind in a hydrophobic pocket in the CDC25 domain of SOS adjacent to the Switch II region of Ras. The structure-activity relationships exhibited by these compounds can be rationalized on the basis of multiple X-ray cocrystal structures. Mutational analyses confirmed the functional relevance of this binding site and showed it to be essential for compound activity. These molecules increase Ras-GTP levels and disrupt MAPK and PI3K signaling in cells at low micromolar concentrations. These small molecules represent tools to study the acute activation of Ras and highlight a pocket on SOS that may be exploited to modulate Ras signaling.

Surface reengineering of RPA70N enables cocrystallization with an inhibitor of the replication protein A interaction motif of ATR interacting protein.

Replication protein A (RPA) is the primary single-stranded DNA (ssDNA) binding protein in eukaryotes. The N-terminal domain of the RPA70 subunit (RPA70N) interacts via a basic cleft with a wide range of DNA processing proteins, including several that regulate DNA damage response and repair. Small molecule inhibitors that disrupt these protein-protein interactions are therefore of interest as chemical probes of these critical DNA processing pathways and as inhibitors to counter the upregulation of DNA damage response and repair associated with treatment of cancer patients with radiation or DNA-damaging agents. Determination of three-dimensional structures of protein-ligand complexes is an important step for elaboration of small molecule inhibitors. However, although crystal structures of free RPA70N and an RPA70N-peptide fusion construct have been reported, RPA70N-inhibitor complexes have been recalcitrant to crystallization. Analysis of the P61 lattice of RPA70N crystals led us to hypothesize that the ligand-binding surface was occluded. Surface reengineering to alter key crystal lattice contacts led to the design of RPA70N E7R, E100R, and E7R/E100R mutants. These mutants crystallized in a P212121 lattice that clearly had significant solvent channels open to the critical basic cleft. Analysis of X-ray crystal structures, target peptide binding affinities, and (15)N-(1)H heteronuclear single-quantum coherence nuclear magnetic resonance spectra showed that the mutations do not result in perturbations of the RPA70N ligand-binding surface. The success of the design was demonstrated by determining the structure of RPA70N E7R soaked with a ligand discovered in a previously reported molecular fragment screen. A fluorescence anisotropy competition binding assay revealed this compound can inhibit the interaction of RPA70N with the peptide binding motif from the DNA damage response protein ATRIP. The implications of the results are discussed in the context of ongoing efforts to design RPA70N inhibitors.

Insulin reveals Akt signaling as a novel regulator of norepinephrine transporter trafficking and norepinephrine homeostasis.

Noradrenergic signaling in the CNS plays an essential role in circuits involving attention, mood, memory, and stress as well as providing pivotal support for autonomic function in the peripheral nervous system. The high-affinity norepinephrine (NE) transporter (NET) is the primary mechanism by which noradrenergic synaptic transmission is terminated. Data indicate that NET function is regulated by insulin, a hormone critical for the regulation of metabolism. Given the high comorbidity of metabolic disorders such as diabetes and obesity with mental disorders such as depression and schizophrenia, we sought to determine how insulin signaling regulates NET function and thus noradrenergic homeostasis. Here, we show that acute insulin treatment, through the downstream kinase protein kinase B (Akt), significantly decreases NET surface expression in mouse hippocampal slices and superior cervical ganglion neuron boutons (sites of synaptic NE release). In vivo manipulation of insulin/Akt signaling, with streptozotocin, a drug that induces a type 1-like diabetic state in mice, also results in aberrant NET function and NE homeostasis. Notably, we also demonstrate that Akt inhibition or stimulation, independent of insulin, is capable of altering NET surface availability. These data suggest that aberrant states of Akt signaling such as in diabetes and obesity have the potential to alter NET function and noradrenergic tone in the brain. Furthermore, they provide one potential molecular mechanism by which Akt, a candidate gene for mood disorders such as schizophrenia and depression, can impact brain monoamine homeostasis.

A selective allosteric potentiator of the M1 muscarinic acetylcholine receptor increases activity of medial prefrontal cortical neurons and restores impairments in reversal learning.

M(1) muscarinic acetylcholine receptors (mAChRs) may represent a viable target for treatment of disorders involving impaired cognitive function. However, a major limitation to testing this hypothesis has been a lack of highly selective ligands for individual mAChR subtypes. We now report the rigorous molecular characterization of a novel compound, benzylquinolone carboxylic acid (BQCA), which acts as a potent, highly selective positive allosteric modulator (PAM) of the rat M(1) receptor. This compound does not directly activate the receptor, but acts at an allosteric site to increase functional responses to orthosteric agonists. Radioligand binding studies revealed that BQCA increases M(1) receptor affinity for acetylcholine. We found that activation of the M(1) receptor by BQCA induces a robust inward current and increases spontaneous EPSCs in medial prefrontal cortex (mPFC) pyramidal cells, effects which are absent in acute slices from M(1) receptor knock-out mice. Furthermore, to determine the effect of BQCA on intact and functioning brain circuits, multiple single-unit recordings were obtained from the mPFC of rats that showed BQCA increases firing of mPFC pyramidal cells in vivo. BQCA also restored discrimination reversal learning in a transgenic mouse model of Alzheimer's disease and was found to regulate non-amyloidogenic APP processing in vitro, suggesting that M(1) receptor PAMs have the potential to provide both symptomatic and disease modifying effects in Alzheimer's disease patients. Together, these studies provide compelling evidence that M(1) receptor activation induces a dramatic excitation of PFC neurons and suggest that selectively activating the M(1) mAChR subtype may ameliorate impairments in cognitive function.

Heterobiaryl and heterobiaryl ether derived M5 positive allosteric modulators.

This Letter describes a chemical lead optimization campaign directed at VU0238429, the first M(5)-preferring positive allosteric modulator (PAM), discovered through analog work around VU0119498, a pan G(q) mAChR M(1), M(3), M(5) PAM. An iterative parallel synthesis approach was employed to incorporate basic heterocycles to improve physiochemical properties.

Chemical lead optimization of a pan G(q) mAChR M(1), M(3), M(5) positive allosteric modulator (PAM) lead. Part I: Development of the first highly selective M(5) PAM.

This Letter describes a chemical lead optimization campaign directed at VU0238429, the first M(5)-preferring positive allosteric modulator (PAM), discovered through analog work around VU0119498, a pan G(q) mAChR M(1), M(3), M(5) PAM. An iterative library synthesis approach delivered the first selective M(5) PAM (no activity at M(1)-M(4) @ 30microM), and an important tool compound to study the role of M(5) in the CNS.

Novel selective allosteric activator of the M1 muscarinic acetylcholine receptor regulates amyloid processing and produces antipsychotic-like activity in rats.

Recent studies suggest that subtype-selective activators of M(1)/M(4) muscarinic acetylcholine receptors (mAChRs) may offer a novel approach for the treatment of psychotic symptoms associated with schizophrenia and Alzheimer's disease. Previously developed muscarinic agonists have provided clinical data in support of this hypothesis, but failed in clinical development because of a lack of true subtype specificity and adverse effects associated with activation of other mAChR subtypes. We now report characterization of a novel highly selective agonist for the M(1) receptor with no agonist activity at any of the other mAChR subtypes, termed TBPB [1-(1'-2-methylbenzyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one]. Mutagenesis and molecular pharmacology studies revealed that TBPB activates M(1) through an allosteric site rather than the orthosteric acetylcholine binding site, which is likely critical for its unprecedented selectivity. Whole-cell patch-clamp recordings demonstrated that activation of M(1) by TBPB potentiates NMDA receptor currents in hippocampal pyramidal cells but does not alter excitatory or inhibitory synaptic transmission, responses thought to be mediated by M(2) and M(4). TBPB was efficacious in models predictive of antipsychotic-like activity in rats at doses that did not produce catalepsy or peripheral adverse effects of other mAChR agonists. Finally, TBPB had effects on the processing of the amyloid precursor protein toward the non-amyloidogenic pathway and decreased Abeta production in vitro. Together, these data suggest that selective activation of M(1) may provide a novel approach for the treatment of symptoms associated with schizophrenia and Alzheimer's disease.

A novel class of H3 antagonists derived from the natural product guided synthesis of unnatural analogs of the marine bromopyrrole alkaloid dispyrin.

This Letter describes the natural product guided synthesis of unnatural analogs of the marine bromopyrrole alkaloid dispyrin, and the resulting SAR of H(3) antagonism. Multiple rounds of iterative parallel synthesis improved human H(3) IC(50) approximately 33-fold, and afforded a new class of H(3) antagonists based on the novel bromotyramine core of dispyrin.

Centrally active allosteric potentiators of the M4 muscarinic acetylcholine receptor reverse amphetamine-induced hyperlocomotor activity in rats.

Previous clinical and animal studies suggest that selective activators of M(1) and/or M(4) muscarinic acetylcholine receptors (mAChRs) have potential as novel therapeutic agents for treatment of schizophrenia and Alzheimer's disease. However, highly selective centrally penetrant activators of either M(1) or M(4) have not been available, making it impossible to determine the in vivo effects of selective activation of these receptors. We previously identified VU10010 [3-amino-N-(4-chlorobenzyl)-4, 6-dimethylthieno[2,3-b]pyridine-2-carboxamide] as a potent and selective allosteric potentiator of M(4) mAChRs. However, unfavorable physiochemical properties prevented use of this compound for in vivo studies. We now report that chemical optimization of VU10010 has afforded two centrally penetrant analogs, VU0152099 [3-amino-N-(benzo[d][1,3]dioxol-5-ylmethyl)-4,6-dimethylthieno[2,3-b]pyridine carboxamide] and VU0152100 [3-amino-N-(4-methoxybenzyl)-4,6-dimethylthieno[2,3-b]pyridine carboxamide], that are potent and selective positive allosteric modulators of M(4). VU0152099 and VU0152100 had no agonist activity but potentiated responses of M(4) to acetylcholine. Both compounds were devoid of activity at other mAChR subtypes or at a panel of other GPCRs. The improved physiochemical properties of VU0152099 and VU0152100 allowed in vivo dosing and evaluation of behavioral effects in rats. Interestingly, these selective allosteric potentiators of M(4) reverse amphetamine-induced hyperlocomotion in rats, a model that is sensitive to known antipsychotic agents and to nonselective mAChR agonists. This is consistent with the hypothesis that M(4) plays an important role in regulating midbrain dopaminergic activity and raises the possibility that positive allosteric modulation of M(4) may mimic some of the antipsychotic-like effects of less selective mAChR agonists.

Synthesis and SAR of selective muscarinic acetylcholine receptor subtype 1 (M1 mAChR) antagonists.

This Letter describes the synthesis and SAR, developed through an iterative analogue library approach, of a novel series of selective M1 mAChR antagonists for the potential treatment of Parkinson's disease, dystonia and other movement disorders. Compounds in this series possess M1 antagonist IC(50)s in the 441nM-19microM range with 8- to >340-fold functional selectivity versus rM2-rM5.

Synthesis and SAR of analogues of the M1 allosteric agonist TBPB. Part I: Exploration of alternative benzyl and privileged structure moieties.

This Letter describes the first account of the synthesis and SAR, developed through an iterative analogue library approach, of analogues of the highly selective M1 allosteric agonist TBPB. With slight structural changes, mAChR selectivity was maintained, but the degree of partial M1 agonism varied considerably.

Total synthesis and biological evaluation of the marine bromopyrrole alkaloid dispyrin: elucidation of discrete molecular targets with therapeutic potential.

The first total synthesis of dispyrin, a recently reported bromopyrrole alkaloid from Agelas dispar with an unprecedented bromopyrrole tyramine motif, was achieved in three steps on a gram scale (68.4% overall). No biological activity was reported for dispyrin, so we evaluated synthetic dispyrin against>200 discrete molecular targets in radioligand binding and functional assays. Unlike most marine natural products, dispyrin (1) possesses no antibacterial or anticancer activity, but was found to be a potent ligand and antagonist of several therapeutically relevant GPCRs, the alpha1D and alpha2A adrenergic receptors and the H2 and H3 histamine receptors.

Discovery of Quinazolines That Activate SOS1-Mediated Nucleotide Exchange on RAS.

Proteins in the RAS family are important regulators of cellular signaling and, when mutated, can drive cancer pathogenesis. Despite considerable effort over the last 30 years, RAS proteins have proven to be recalcitrant therapeutic targets. One approach for modulating RAS signaling is to target proteins that interact with RAS, such as the guanine nucleotide exchange factor (GEF) son of sevenless homologue 1 (SOS1). Here, we report hit-to-lead studies on quinazoline-containing compounds that bind to SOS1 and activate nucleotide exchange on RAS. Using structure-based design, we refined the substituents attached to the quinazoline nucleus and built in additional interactions not present in the initial HTS hit. Optimized compounds activate nucleotide exchange at single-digit micromolar concentrations in vitro. In HeLa cells, these quinazolines increase the levels of RAS-GTP and cause signaling changes in the mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway.

Discovery of Aminopiperidine Indoles That Activate the Guanine Nucleotide Exchange Factor SOS1 and Modulate RAS Signaling.

Deregulated RAS activity, often the result of mutation, is implicated in approximately 30% of all human cancers. Despite this statistic, no clinically successful treatment for RAS-driven tumors has yet been developed. One approach for modulating RAS activity is to target and affect the activity of proteins that interact with RAS, such as the guanine nucleotide exchange factor (GEF) son of sevenless homologue 1 (SOS1). Here, we report on structure-activity relationships (SAR) in an indole series of compounds. Using structure-based design, we systematically explored substitution patterns on the indole nucleus, the pendant amino acid moiety, and the linker unit that connects these two fragments. Best-in-class compounds activate the nucleotide exchange process at submicromolar concentrations in vitro, increase levels of active RAS-GTP in HeLa cells, and elicit signaling changes in the mitogen-activated protein kinase-extracellular regulated kinase (MAPK-ERK) pathway, resulting in a decrease in pERK1/2T202/Y204 protein levels at higher compound concentrations.

Diphenylpyrazoles as replication protein a inhibitors.

Replication Protein A is the primary eukaryotic ssDNA binding protein that has a central role in initiating the cellular response to DNA damage. RPA recruits multiple proteins to sites of DNA damage via the N-terminal domain of the 70 kDa subunit (RPA70N). Here we describe the optimization of a diphenylpyrazole carboxylic acid series of inhibitors of these RPA-protein interactions. We evaluated substituents on the aromatic rings as well as the type and geometry of the linkers used to combine fragments, ultimately leading to submicromolar inhibitors of RPA70N protein-protein interactions.

Discovery of Protein-Protein Interaction Inhibitors of Replication Protein A.

Replication Protein A (RPA) is a ssDNA binding protein that is essential for DNA replication and repair. The initiation of the DNA damage response by RPA is mediated by protein-protein interactions involving the N-terminal domain of the 70 kDa subunit with partner proteins. Inhibition of these interactions increases sensitivity towards DNA damage and replication stress and may therefore be a potential strategy for cancer drug discovery. Towards this end, we have discovered two lead series of compounds, derived from hits obtained from a fragment-based screen, that bind to RPA70N with low micromolar affinity and inhibit the binding of an ATRIP-derived peptide to RPA. These compounds may offer a promising starting point for the discovery of clinically useful RPA inhibitors.

Discovery of a potent inhibitor of replication protein a protein-protein interactions using a fragment-linking approach.

Replication protein A (RPA), the major eukaryotic single-stranded DNA (ssDNA)-binding protein, is involved in nearly all cellular DNA transactions. The RPA N-terminal domain (RPA70N) is a recruitment site for proteins involved in DNA-damage response and repair. Selective inhibition of these protein-protein interactions has the potential to inhibit the DNA-damage response and to sensitize cancer cells to DNA-damaging agents without affecting other functions of RPA. To discover a potent, selective inhibitor of the RPA70N protein-protein interactions to test this hypothesis, we used NMR spectroscopy to identify fragment hits that bind to two adjacent sites in the basic cleft of RPA70N. High-resolution X-ray crystal structures of RPA70N-ligand complexes revealed how these fragments bind to RPA and guided the design of linked compounds that simultaneously occupy both sites. We have synthesized linked molecules that bind to RPA70N with submicromolar affinity and minimal disruption of RPA's interaction with ssDNA.

Akt-dependent and isoform-specific regulation of dopamine transporter cell surface expression.

Dopamine (DA) is a neurotransmitter implicated in multiple functions, including movement, cognition, motivation, and reward. The DA transporter (DAT) is responsible for clearing extracellular DA, thereby terminating DA neurotransmission. Previously, it has been shown that insulin signaling through protein kinase B/Akt regulates DAT function by fine-tuning DAT cell surface expression. Importantly, specific Akt isoforms (e.g., Akt1, Akt2) serve distinct physiological functions. Here, we demonstrate using isoform-specific Akt inhibitors that basal activity of Akt2, rather than Akt1, regulates DAT cell surface expression. Since Akt2 activation is mediated by insulin, these data further implicate insulin signaling as an important modulator of DAT function and dopaminergic tone.

Discovery and characterization of novel subtype-selective allosteric agonists for the investigation of M(1) receptor function in the central nervous system.

Cholinergic transmission in the forebrain is mediated primarily by five subtypes of muscarinic acetylcholine receptors (mAChRs), termed M(1)-M(5). Of the mAChR subtypes, M(1) is among the most heavily expressed in regions that are critical for learning and memory, and has been viewed as the most critical mAChR subtype for memory and attention mechanisms. Unfortunately, it has been difficult to develop selective activators of M(1) and other individual mAChR subtypes, which has prevented detailed studies of the functional roles of selective activation of M(1). Using a functional HTS screen and subsequent diversity-oriented synthesis approach we have discovered a novel series of highly selective M(1) allosteric agonists. These compounds activate M(1) with EC(50) values in the 150 nM to 500 nM range and have unprecedented, clean ancillary pharmacology (no substantial activity at 10µM across a large panel of targets). Targeted mutagenesis revealed a potentially novel allosteric binding site in the third extracellular loop of the M(1) receptor for these allosteric agonists. Optimized compounds, such as VU0357017, provide excellent brain exposure after systemic dosing and have robust in vivo efficacy in reversing scopolamine-induced deficits in a rodent model of contextual fear conditioning. This series of selective M(1) allosteric agonists provides critical research tools to allow dissection of M(1)-mediated effects in the CNS and potential leads for novel treatments for Alzheimer's disease and schizophrenia.