Trivalent PROTACs enhance protein degradation via combined avidity and cooperativity.

Bivalent proteolysis-targeting chimeras (PROTACs) drive protein degradation by simultaneously binding a target protein and an E3 ligase and forming a productive ternary complex. We hypothesized that increasing binding valency within a PROTAC could enhance degradation. Here, we designed trivalent PROTACs consisting of a bivalent bromo and extra terminal (BET) inhibitor and an E3 ligand tethered via a branched linker. We identified von Hippel-Lindau (VHL)-based SIM1 as a low picomolar BET degrader with preference for bromodomain containing 2 (BRD2). Compared to bivalent PROTACs, SIM1 showed more sustained and higher degradation efficacy, which led to more potent anticancer activity. Mechanistically, SIM1 simultaneously engages with high avidity both BET bromodomains in a cis intramolecular fashion and forms a 1:1:1 ternary complex with VHL, exhibiting positive cooperativity and high cellular stability with prolonged residence time. Collectively, our data along with favorable in vivo pharmacokinetics demonstrate that augmenting the binding valency of proximity-induced modalities can be an enabling strategy for advancing functional outcomes.

Transformation of naltrexone into mesembrane and investigation of the binding properties of its intermediate derivatives to opioid receptors.

We transformed naltrexone (5) with the morphinan skeleton into mesembrane (4) belonging to the Sceletium alkaloids via key intermediate 6, characterized by a cis-fused hydroindole skeleton with a suspended phenyl ring fixed by an epoxy bridge. We then investigated the binding affinities of 4 and the key intermediate 6 derivatives to the opioid receptors. Among the tested compounds, 15', with a cis-fused hydroindole core, bound to the three opioid receptor types with strong to moderate affinities. The observed differences of binding affinities among the tested compounds were reasonably explained by the conformational analyses of the compounds. The structure-activity relationship (SAR) of the tested compounds like 15' with the hydroindole structure was completely different from the reported SAR of morphinan derivatives with the hydroisoquinoline skeleton. Compound 15' with a structure that differs from the morphinans represents a useful fundamental skeleton with a novel chemotype that may contribute to the development of new opioid ligands.

Essential structure of opioid κ receptor agonist nalfurafine for binding to the κ receptor 2: synthesis of decahydro(iminoethano)phenanthrene derivatives and their pharmacologies.

To clarify the essential structures of an opioid κ receptor selective agonist, nalfurafine, for binding to the κ receptor, we designed and synthesized some nalfurafine derivatives and the decahydro(iminoethano)phenanthrene derivatives with a cyclohexene moiety as a surrogate for the phenol ring. In addition to the 6-amide side chain and the 17-nitrogen substituted by a cyclopropylmethyl group, the 4,5-epoxy ring, phenolic hydroxy group, and angular hydroxy group played important roles in eliciting the binding properties of nalfurafine but these three moieties were not indispensable for binding to the κ receptor. Moreover, the phenol ring was also not essential for the binding to the κ receptor, and the cyclohexene moiety would play an important role in fixing the conformation of decahydro(iminoethano)phenanthrene derivatives to effectively raise the amide side chain, rendering a conformation that resembled the active one of nalfurafine.

Essential structure of opioid κ receptor agonist nalfurafine for binding to the κ receptor 3: synthesis of decahydro(iminoethano)phenanthrene derivatives with an oxygen functionality at the 3-position and their pharmacologies.

To clarify the essential structures of an opioid κ receptor selective agonist, nalfurafine, for binding to the κ receptor, we designed and synthesized the decahydro(iminoethano)phenanthrene derivatives with an oxygen functionality at the 3-position. The introduction of a hydroxy group to the derivatives increased the affinity and selectivity to the κ receptor regardless of the configuration at the 3-position. However, their affinities were lower than those of nalfurafine with the phenolic hydroxy group. The results suggested that the acidity of the hydroxy group would play an important role in the interaction with the opioid receptor. The low affinities of the 3-keto derivatives indicated that the 3-hydroxy group may participate in the hydrogen bonding with the receptor site not as a hydrogen acceptor but as a hydrogen donor. This is the first experimental evidence for a role as a hydrogen donor for the 3-hydroxy group in morphinans. Furthermore, the κ selectivities in these derivatives with the 6α-amide side chain were affected by the the 3-hydroxy group. The obtained structure-activity relationship information is expected to be useful for the design of more selective ligands for the κ receptor.

Essential structure of opioid κ receptor agonist nalfurafine for binding to κ receptor 1: synthesis of decahydroisoquinoline derivatives and their pharmacologies.

On the basis of the three-dimensional pharmacophore model of opioid κ agonists, we simplified the structure of nalfurafine (selective κ agonist) to find the essential structural moieties for binding the opioid receptors, especially κ receptor type. As a result, we found that the trans-fused decahydroisoquinoline derivatives without a phenol ring bound the opioid receptor in micromolar order and that both the amide side chain and the nitrogen substituted by the cyclopropylmethyl group were indispensable moieties for eliciting the κ selectivity. The simple decahydroisoquinoline without amide side chain also bound the opioid receptor without receptor type selectivity, suggesting that the message-address concept would be applicable to even these simple derivatives. These findings that the simple decahydroisoquinoline derivatives showed the affinities for the opioid receptors, especially some of the compounds showed κ selectivity, are the first example in the opioid field.

Discovery of Benzo[d]imidazole-6-sulfonamides as Bromodomain and Extra-Terminal Domain (BET) Inhibitors with Selectivity for the First Bromodomain.

The bromodomain and extra-terminal (BET) family of proteins includes BRD2, BRD3, BRD4, and the testis-specific protein, BRDT, each containing two N-terminal tandem bromodomain (BRD) modules. Potent and selective inhibitors targeting the two bromodomains are required to elucidate their biological role(s), with potential clinical applications. In this study, we designed and synthesized a series of benzimidazole-6-sulfonamides starting from the azobenzene compounds MS436 (7 a) and MS611 (7 b) that exhibited preference for the first (BD1) over the second (BD2) BRD of BET family members. The most-promising compound (9 a) showed good binding potency and improved metabolic stability and selectivity towards BD1 with respect to the parent compounds.

Synthesis of (-)-homogalanthamine from naltrexone.

Acetylcholinesterase inhibitor (-)-homogalanthamine 3 was synthesized from µ opioid antagonist naltrexone (2) in 16% total yield. The synthesis features Grob fragmentation as a key reaction, which was especially accelerated in the presence of 15-crown-5.

A double decarboxylation reaction of an oxazolidinone and carboxylic Acid: its application to the synthesis of a new opioid lead compound.

Treatment of oxazolidinone carboxylic acid 6 with potassium carbonate gave olefin 7 by a double decarboxylation reaction. The reaction was proposed to proceed via decarboxylation followed by E1cB-like mechanism. 15,16-Nornaltrexone derivative 17 prepared from double decarboxylation product 7 showed strong affinity for the mu opioid receptor, indicating it to be a new opioid lead compound.

Investigation of Beckett-Casy model 1: synthesis of novel 16,17-seco-naltrexone derivatives and their pharmacology.

Novel 16,17-seco-naltrexone derivatives 3 were synthesized using a 16-17 bond cleavage reaction of naltrexone as the key reaction to examine the Beckett-Casy model. All the prepared 16,17-seco-naltrexone derivatives 3 showed lower affinities for opioid receptors than naltrexone. Although the results of binding assay seem to support the existence of a cavity in the model, further investigation using 15,16-nornaltrexone derivatives 26 will be needed to confirm the model.

Investigation of Beckett-Casy model 2: synthesis of novel 15-16 nornaltrexone derivatives and their pharmacology.

We synthesized novel 15-16 nornaltrexone derivatives 9, 11 and 22 to examine the importance of the cavity in the Beckett-Casy model, which was proposed to interact with the 15-16 ethylene moiety in the morphine structure. All the synthesized compounds showed lower affinities for the opioid receptor than did the naltrexone (10). The binding affinities of 14-OH derivatives 11, in which the rotation of the 9-17 bond would be restricted by an intramolecular hydrogen bond, was improved compared to the corresponding 14-H derivatives 9. Compound 22 whose 9-17 bond was strictly fixed by the ethylene bridge hardly bound to the opioid receptor. Compound 26 also showed very weak binding affinity in spite of the existence of the 15-16 ethylene unit. We proposed an important role for the orientation of the lone electron pair on the 17-nitrogen rather than the significance of the cavity in the Beckett-Casy model.

Investigation of Beckett-Casy model 3: synthesis of novel naltrexone derivatives with contracted and expanded D-rings and their pharmacology.

Novel naltrexone derivatives 7 and 8 with contracted and expanded D-rings were synthesized to investigate the importance of orientation of lone electron pair on the nitrogen for binding abilities to the opioid receptor. Compound 7 showed almost no binding affinity, whereas compound 8 was comparable to naltrexone (6) in binding affinity. Conformational analyses and NOE experiments in D(2)O of compounds 6-8 suggested that the lone electron pairs of compounds 6 and 8 with respective six- and seven-membered D-rings would project in the pseudo-axial orientation, whereas compound 7 with five-membered D-ring would have the lone electron pair directing in pseudo-equatorial position. These results strongly supported the proposal that the axial orientation of the lone electron pair on nitrogen would provide sufficient binding abilities to the opioid receptor and that the 15-16 ethylene moiety in the morphine structure would play a role in fixation of the lone electron pair in the axial direction rather than interaction with the putative cavity in the Beckett-Casy model.

Discovery and SAR Studies of Orally Active Somatostatin Receptor Subtype-2 (SSTR2) Agonists for the Treatment of Acromegaly.

Acromegaly is a disease caused by the oversecretion of growth hormone. It is currently treated by intravenous injection with cyclic peptide drugs that activate somatostatin receptor subtype 2 (SSTR2). Here, novel nonpeptidic, small-molecule, and orally active SSTR2 agonists were identified from a hit compound (13). Pharmacophore studies enabled scaffold hopping to obtain a unique 3,4,5-trisubstituted pyridine motif. Further optimization conferred potent SSTR2 agonistic activity and metabolic stability. Several compounds were evaluated and these showed good oral pharmacokinetic profiles in rats, and one representative compound (25) showed highly potent inhibition of growth hormone secretion induced by growth hormone-releasing hormone in rats. Based on these results, 25 was identified as a promising lead for further optimization. A structure-activity relationship (SAR) study and the metabolic stability data for this compound are also described.