A novel class of ion displacement ligands as antagonists of the αIIbß3 receptor that limit conformational reorganization of the receptor.

A collection of αIIbß3 integrin receptor antagonists possessing a unique MIDAS metal ion displacement mechanism of action is presented. Insight into these agents' structure-activity relationships, binding modality, and pharmacokinetic and pharmacodynamic profiles highlight the potential of these small molecule ion displacement ligands as attractive candidates for clinical development.

Structure-based virtual screening of small-molecule antagonists of platelet integrin αIIbß3 that do not prime the receptor to bind ligand.

Integrin αIIbß3 has emerged as an important therapeutic target for thrombotic vascular diseases owing to its pivotal role in mediating platelet aggregation through interaction with adhesive ligands. In the search for effective anti-thrombotic agents that can be administered orally without inducing the high-affinity ligand binding state, we recently discovered via high-throughput screening of 33,264 compounds a novel, αIIbß3-selective inhibitor (RUC-1) of adenosine-5'-diphosphate (ADP) -induced platelet aggregation that exhibits a different chemical scaffold and mode of binding with respect to classical Arg-Gly-Asp (RGD)-mimicking αIIbß3 antagonists. Most importantly, RUC-1 and its higher-affinity derivative, RUC-2, do not induce major conformational changes in the protein ß3 subunit or prime the receptor to bind ligand. To identify additional αIIbß3-selective chemotypes that inhibit platelet aggregation through similar mechanisms, we screened in silico over 2.5 million commercially available, 'lead-like' small molecules based on complementarity to the predicted binding mode of RUC-2 into the RUC-1-αIIbß3 crystal structure. This first reported structure-based virtual screening application to the αIIbß3 integrin led to the identification of 2 αIIbß3-selective antagonists out of 4 tested, which compares favorably with the 0.003 % "hit rate" of our previous high-throughput chemical screening study. The newly identified compounds, like RUC-1 and RUC-2, showed specificity for αIIbß3 compared to αVß3 and did not prime the receptor to bind ligand. They thus may hold promise as αIIbß3 antagonist therapeutic scaffolds.

Structure-guided design of a high-affinity platelet integrin αIIbß3 receptor antagonist that disrupts Mg²âº binding to the MIDAS.

An integrin found on platelets, α(IIb)ß(3) mediates platelet aggregation, and α(IIb)ß(3) antagonists are effective antithrombotic agents in the clinic. Ligands bind to integrins in part by coordinating a magnesium ion (Mg(2+)) located in the ß subunit metal ion-dependent adhesion site (MIDAS). Drugs patterned on the integrin ligand sequence Arg-Gly-Asp have a basic moiety that binds the α(IIb) subunit and a carboxyl group that coordinates the MIDAS Mg(2+) in the ß(3) subunits. They induce conformational changes in the ß(3) subunit that may have negative consequences such as exposing previously hidden epitopes and inducing the active conformation of the receptor. We recently reported an inhibitor of α(IIb)ß(3) (RUC-1) that binds exclusively to the α(IIb) subunit; here, we report the structure-based design and synthesis of RUC-2, a RUC-1 derivative with a ~100-fold higher affinity. RUC-2 does not induce major conformational changes in ß(3) as judged by monoclonal antibody binding, light scattering, gel chromatography, electron microscopy, and a receptor priming assay. X-ray crystallography of the RUC-2-α(IIb)ß(3) headpiece complex in 1 mM calcium ion (Ca(2+))/5 mM Mg(2+) at 2.6 Å revealed that RUC-2 binds to α(IIb) the way RUC-1 does, but in addition, it binds to the ß(3) MIDAS residue glutamic acid 220, thus displacing Mg(2+) from the MIDAS. When the Mg(2+) concentration was increased to 20 mM, however, Mg(2+) was identified in the MIDAS and RUC-2 was absent. RUC-2's ability to inhibit ligand binding and platelet aggregation was diminished by increasing the Mg(2+) concentration. Thus, RUC-2 inhibits ligand binding by a mechanism different from that of all other α(IIb)ß(3) antagonists and may offer advantages as a therapeutic agent.

Inhibition of the Notch-Hey1 axis blocks embryonal rhabdomyosarcoma tumorigenesis.

PURPOSE: Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and remains refractory to combined-modality therapy in patients with high risk disease. In skeletal myogenesis, Notch signaling prevents muscle differentiation and promotes proliferation of satellite cell progeny. Given its physiologic role in myogenesis and oncogenic role in other human cancers, we hypothesized that aberrant Notch signaling may contribute to RMS tumorigenesis and present novel therapeutic opportunities. EXPERIMENTAL DESIGN: Human RMS cell lines and tumors were evaluated by immunoblot, IHC, and RT-PCR to measure Notch ligand, receptor, and target gene expression. Manipulation of Notch signaling was accomplished using genetic and pharmacologic approaches. In vitro cell growth, proliferation, and differentiation were assessed using colorimetric MTT and BrdU assays, and biochemical/morphologic changes after incubation in differentiation-promoting media, respectively. In vivo tumorigenesis was assessed using xenograft formation in SCID/beige mice. RESULTS: Notch signaling is upregulated in human RMS cell lines and tumors compared with primary skeletal muscle, especially in the embryonal (eRMS) subtype. Inhibition of Notch signaling using Notch1 RNAi or γ-secretase inhibitors reduced eRMS cell proliferation in vitro. Hey1 RNAi phenocopied Notch1 loss and permitted modest myogenic differentiation, while overexpression of an activated Notch moiety, ICN1, promoted eRMS cell proliferation and rescued pharmacologic inhibition. Finally, Notch inhibition using RNAi or γ-secretase inhibitors blocked tumorigenesis in vivo. CONCLUSIONS: Aberrant Notch-Hey1 signaling contributes to eRMS by impeding differentiation and promoting proliferation. The efficacy of Notch pathway inhibition in vivo supports the development of Notch-Hey1 axis inhibitors in the treatment of eRMS.

Defining the cooperative genetic changes that temporally drive alveolar rhabdomyosarcoma.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence. Despite advances in therapy, patients with a histologic variant of RMS known as alveolar (aRMS) have a 5-year survival rate of <30%. aRMS tissues exhibit a number of genetic changes, including loss-of-function of the p53 and Rb tumor suppressor pathways, amplification of MYCN, stabilization of telomeres, and most characteristically, reciprocal translocation of loci involving the PAX and FKHR genes, generating the PAX7-FKHR or PAX3-FKHR fusion proteins. We previously showed that PAX3-FKHR expression in primary human myoblasts, cells that can give rise to RMS, cooperated with loss of p16INK4A to promote extended proliferation. To better understand the genetic events required for aRMS formation, we then stepwise converted these cells to their transformed counterpart. PAX3-FKHR, the catalytic unit of telomerase hTERT, and MycN, in cooperation with down-regulation of p16INK4A/p14ARF expression, were necessary and sufficient to convert normal human myoblasts into tumorigenic cells that gave rise to aRMS tumors. However, the order of expression of these transgenes was critical, as only those cells expressing PAX3-FKHR early could form tumors. We therefore suggest that the translocation of PAX3 to FKHR drives proliferation of myoblasts, and a selection for loss of p16INK4A/p14ARF. These early steps, coupled with MycN amplification and telomere stabilization, then drive the cells to a fully tumorigenic state.

The PAX3-FKHR fusion gene of rhabdomyosarcoma cooperates with loss of p16INK4A to promote bypass of cellular senescence.

Rhabdomyosarcoma is the most common soft tissue sarcoma of childhood and adolescence. Despite advances in therapy, patients with a histologic variant of rhabdomyosarcoma known as alveolar rhabdomyosarcoma (ARMS) have a 5-year survival of <30%. ARMS is characterized by a chromosomal translocation generating the PAX3-FKHR fusion gene. However, ectopic expression of PAX3-FKHR often induces inhibition of cell proliferation, or cell death, when expressed in nonmuscle cells. This prompted us to explore the effect of expressing PAX3-FKHR in more relevant cells, specifically primary human skeletal muscle cells because these cells can be converted to a tumorigenic state that mimics rhabdomyosarcoma. PAX3-FKHR expression promoted both fetal and postnatal primary human skeletal muscle cell precursors to bypass the senescence growth arrest checkpoint. This bypass was accompanied by epigenetic DNA methylation of the p16(INK4A) promoter and correspondingly a loss of expression of this tumor suppressor. Knockdown of p16(INK4A) cooperated with PAX3-FKHR to drive proliferation past senescence, whereas reintroduction of wild-type p16(INK4A) in post-senescent cells caused growth arrest. Thus, PAX3-FKHR acts in concert with loss of p16(INK4A) to promote inappropriate proliferation of skeletal muscle cells. This association between PAX3-FKHR expression and p16(INK4A) loss was seen in human ARMS tumor tissue, as both human rhabdomyosarcoma cell lines and tissue microarrays showed a trend toward down-regulation of p16(INK4A) protein in alveolar subsets. We surmise that the generation of the PAX3-FKHR fusion protein may require loss of p16(INK4A) to promote malignant proliferation of skeletal muscle cells as an early step in ARMS tumorigenesis.