Juan R. Del Valle

Inhibition of ER stress–associated IRE-1/XBP-1 pathway reduces leukemic cell survival

Activation of the ER stress response is associated with malignant progression of B cell chronic lymphocytic leukemia (CLL). We developed a murine CLL model that lacks the ER stress-associated transcription factor XBP-1 in B cells and found that XBP-1 deficiency decelerates malignant progression of CLL-associated disease. XBP-1 deficiency resulted in acquisition of phenotypes that are disadvantageous for leukemic cell survival, including compromised BCR signaling capability and increased surface expression of sphingosine-1-phosphate receptor 1 (S1P1). Because XBP-1 expression requires the RNase activity of the ER transmembrane receptor IRE-1, we developed a potent IRE-1 RNase inhibitor through chemical synthesis and modified the structure to facilitate entry into cells to target the IRE-1/XBP-1 pathway. Treatment of CLL cells with this inhibitor (B-I09) mimicked XBP-1 deficiency, including upregulation of IRE-1 expression and compromised BCR signaling. Moreover, B-I09 treatment did not affect the transport of secretory and integral membrane-bound proteins. Administration of B-I09 to CLL tumor-bearing mice suppressed leukemic progression by inducing apoptosis and did not cause systemic toxicity. Additionally, B-I09 and ibrutinib, an FDA-approved BTK inhibitor, synergized to induce apoptosis in B cell leukemia, lymphoma, and multiple myeloma. These data indicate that targeting XBP-1 has potential as a treatment strategy, not only for multiple myeloma, but also for mature B cell leukemia and lymphoma.

Total Synthesis of the Putative Structure of Lucentamycin A

A rapid and stereoselective enolate-Claisen rearrangement provides access to the 4-ethylidene-3-methylproline (Emp) subunit of lucentamycin A. Synthesis of the putative structure of the cytotoxic natural product suggests the need for structural revision.

Synthesis of novel tricyclic chromenone-based inhibitors of IRE-1 RNase activity.

Inositol-requiring enzyme 1 (IRE-1) is a kinase/RNase ER stress sensor that is activated in response to excessive accumulation of unfolded proteins, hypoxic conditions, calcium imbalance, and other stress stimuli. Activation of IRE-1 RNase function exerts a cytoprotective effect and has been implicated in the progression of cancer via increased expression of the transcription factor XBP-1s. Here, we describe the synthesis and biological evaluation of novel chromenone-based covalent inhibitors of IRE-1. Preparation of a family of 8-formyltetrahydrochromeno[3,4-c]pyridines was achieved via a Duff formylation that is attended by an unusual cyclization reaction. Biological evaluation in vitro and in whole cells led to the identification of 30 as a potent inhibitor of IRE-1 RNase activity and XBP-1s expression in wild type B cells and human mantle cell lymphoma cell lines.

Substituted imidazo[1,2-a]pyridines as β-strand peptidomimetics.

New conformationally extended dipeptide surrogates based on an imidazo[1,2-a]pyridine scaffold are described. Efficient synthesis and incorporation into host peptides affords structures with native side-chain functionality and hydrogen bonding elements on one face of the backbone. Structural analysis by NMR suggests that model peptidomimetics adopt a β-strand-like conformation in solution.

Solid-Phase Synthesis of Tetrahydropyridazinedione-ConstrainedPeptides

The design and solid-phase synthesis of tetrahydropyridazine-3,6-dione (Tpd) peptidomimetics derived from backbone-aminated peptides is reported. The described protocol features the synthesis of chiral α-hydrazino acids suitable for chemoselective incorporation into growing peptide chains. Acid-catalyzed cyclization to form the Tpd ring during cleavage affords the target peptidomimetics in good yield and purity. The scope of Tpd incorporation is demonstrated through the synthesis of constrained peptides featuring nucleophilic/electrophilic side chains and sterically encumbered α-substituted hydrazino acid residues.

Total synthesis and structural revision of lucentamycin A.

Lucentamycin A is a marine-derived peptide natural product harboring a unique 4-ethylidene-3-methylproline (Emp) subunit. The proposed structure of lucentamycin A and the core Emp residue have recently been called into question through synthesis. Here, we report the first total synthesis of lucentamycin A, which confirms that the ethylidene substituent in Emp bears an E geometry, in contrast to the originally assigned Z configuration. Synthesis of the desired (E)-Emp subunit required the implementation of a novel strategy starting from Garners aldehyde.

An Ester Enolate–Claisen Rearrangement Route to Substituted 4-Alkylideneprolines. Studies toward a Definitive Structural Revision of Lucentamycin A

Substituted 4-alkylideneprolines represent a rare class of naturally occurring amino acids with promising biological activities. Lucentamycin A is a cytotoxic, marine-derived tripeptide that harbors a 4-ethylidine-3-methylproline (Emp) residue unique among known peptide natural products. In this paper, we examine the synthesis of Emp and related 4-alkylideneprolines employing a versatile ester enolate-Claisen rearrangement. The scope and selectivity of the key rearrangement reaction are described with a number of diversely substituted glycine ester substrates. Treatment of the allyl esters with excess NaHMDS at ambient temperature gives rise to highly substituted α-allylglycine products with good to excellent diastereoselectivities. Resolution of dipeptide diastereomers and cyclization to form the pyrrolidine rings provide rapid access to stereopure prolyl dipeptides. We have applied this strategy to the synthesis of four Emp-containing isomers of lucentamycin A in pursuit of a definitive stereochemical revision of the natural product. Our studies indicate that the Emp stereogenic centers are not the source of structural misassignment. The current strategy should find broad utility in the synthesis of additional natural product analogues and related 3-alkyl-4-alkylidene prolines.

IRE1α RNase–dependent lipid homeostasis promotes survival in Myc-transformed cancers

Myc activation is a primary oncogenic event in many human cancers; however, these transcription factors are difficult to inhibit pharmacologically, suggesting that Myc-dependent downstream effectors may be more tractable therapeutic targets. Here, we show that Myc overexpression induces endoplasmic reticulum (ER) stress and engages the inositol-requiring enzyme 1&agr; (IRE1&agr;)/X-box binding protein 1 (XBP1) pathway through multiple molecular mechanisms in a variety of c-Myc– and N-Myc–dependent cancers. In particular, Myc-overexpressing cells require IRE1&agr;/XBP1 signaling for sustained growth and survival in vitro and in vivo, dependent on elevated stearoyl-CoA-desaturase 1 (SCD1) activity. Pharmacological and genetic XBP1 inhibition induces Myc-dependent apoptosis, which is alleviated by exogenous unsaturated fatty acids. Of note, SCD1 inhibition phenocopies IRE1&agr; RNase activity suppression in vivo. Furthermore, IRE1&agr; inhibition enhances the cytotoxic effects of standard chemotherapy drugs used to treat c-Myc–overexpressing Burkitt’s lymphoma, suggesting that inhibiting the IRE1&agr;/XBP1 pathway is a useful general strategy for treatment of Myc-driven cancers.

Imidazo[1,2-a]pyridine-based peptidomimetics as inhibitors of Akt

We report the design, synthesis, and biological evaluation of imidazopyridine-based peptidomimetics based on the substrate consensus sequence of Akt, an AGC family serine/threonine kinase hyperactivated in over 50% of human tumors. Our ligand-based approach led to the identification of novel substrate mimetic inhibitors of Akt1 featuring an unnatural extended dipeptide surrogate. Compound 11 inhibits Akt isoforms in the sub-micromolar range and exhibits improved proteolytic stability relative to a parent pentapeptide.

Synthesis of GSK3β mimetic inhibitors of Akt featuring a novel extended dipeptide surrogate.

Akt is a cardinal nodal point in PI3K signaling pathway and confers resistance to apoptosis through inactivation of regulatory substrates such as GSK3β. Efforts to inhibit the kinase activity of Akt have largely focused on targeting the ATP-binding domain of Akt. Here, we present the design and synthesis of conformationally constrained GSK3β mimics featuring a novel extended dipeptide surrogate core. This effort resulted in the identification of a novel substrate mimetic Akt inhibitor (11) with low micromolar activity in vitro (Akt1 IC(50)=3.1 μM).