Jon Rosen

Preclinical Activity of Eltrombopag (SB-497115), an Oral, Nonpeptide Thrombopoietin Receptor Agonist†‡§

Eltrombopag is a first‐in‐class, orally bioavailable, small‐molecule, nonpeptide agonist of the thrombopoietin receptor (TpoR), which is being developed as a treatment for thrombocytopenia of various etiologies. In vitro studies have demonstrated that the activity of eltrombopag is dependent on expression of TpoR, which activates the signaling transducers and activators of transcription (STAT) and mitogen‐activated protein kinase signal transduction pathways. The objective of this preclinical study is to determine if eltrombopag interacts selectively with the TpoR to facilitate megakaryocyte differentiation in platelets. Functional thrombopoietic activity was demonstrated by the proliferation and differentiation of primary human CD34+ bone marrow cells into CD41+ megakaryocytes. Measurements in platelets in several species indicated that eltrombopag specifically activates only the human and chimpanzee STAT pathways. The in vivo activity of eltrombopag was demonstrated by an increase of up to 100% in platelet numbers when administered orally (10 mg/kg per day for 5 days) to chimpanzees. In conclusion, eltrombopag interacts selectively with the TpoR without competing with Tpo, leading to the increased proliferation and differentiation of human bone marrow progenitor cells into megakaryocytes and increased platelet production. These results suggest that eltrombopag and Tpo may be able to act additively to increase platelet production. STEM CELLS 2008;27:424–430

Antiinflammatory glucocorticoid receptor ligand with reduced side effects exhibits an altered protein–protein interaction profile

Glucocorticoids are commonly used antiinflammatory agents whose use is limited by side effects. We have developed a series of glucocorticoid receptor (GR) ligands that retain the strong antiinflammatory activity of conventional glucocorticoids with reduced side effects. We present a compound, LGD5552, that binds the receptor efficiently and strongly represses inflammatory gene expression. LGD5552 bound to GR activates gene expression somewhat differently than glucocorticoids. It activates some genes with an efficacy similar to that of the glucocorticoids. However, other glucocorticoid-activated genes are not regulated by LGD5552. These differences may be because of the more efficient binding of corepressor in the presence of LGD5552, compared with glucocorticoid agonists. This class of nonsteroidal, GR-dependent antiinflammatory drugs may offer a safer alternative to steroidal glucocorticoids in the treatment of inflammatory disease.

LGD-5552, an Antiinflammatory Glucocorticoid Receptor Ligand with Reduced Side Effects, in Vivo

Treatment of inflammation is often accomplished through the use of glucocorticoids. However, their use is limited by side effects. We have examined the activity of a novel glucocorticoid receptor ligand that binds the receptor efficiently and strongly represses inflammatory gene expression. This compound has potent antiinflammatory activity in vivo and represses the transcription of the inflammatory cytokine monocyte chemoattractant protein-1 and induces the antiinflammatory cytokine IL-10. The compound demonstrates differential gene regulation, compared with commonly prescribed glucocorticoids, effectively inducing some genes and repressing others in a manner different from the glucocorticoid prednisolone. The separation between the antiinflammatory effects of LGD-5552 and the side effects commonly associated with glucocorticoid treatment suggest that this molecule differs significantly from prednisolone and other steroids and may provide a safer therapeutic window for inflammatory conditions now commonly treated with steroidal glucocorticoids.

Discovery and characterization of an inhibitor of glucosylceramide synthase.

Targeting glycosphingolipid synthesis has emerged as a novel approach for treating metabolic diseases. 32 (EXEL-0346) represents a new class of glucosylceramide synthase (GCS) inhibitors. This report details the elaboration of hit 8 with the goal of achieving and maintaining maximum GCS inhibition in vivo. 32 inhibited GCS with an IC(50) of 2 nM and achieved maximum hepatic GCS inhibition after four or five daily doses in rodents. Robust improvements in glucose tolerance in DIO mice and ZDF rats were observed after 2 weeks of q.d. dosing. Four weeks of dosing resulted in decreased plasma triglycerides and reduced hepatic fat deposition. Thus, 32 provides insight into the amount of metabolic regulation that can be restored following achievement of maximal target knockdown.

Chapter 27. The Role of JAKs and STATs in Transcriptional Regulation by Cytokines

Publisher Summary Cytokines exert their effects on the target cells by binding to specific cell surface receptors, triggering various intracellular events, including rapid changes in gene expression. The binding of a cytokine to its receptor activate the particular members of a family of tyrosine kinases that are known as just another kinases (JAKs). JAKs associate noncovalently with the cytoplasmic domain of the receptor. Activated JAKs then phosphorylate several substrates, including the receptor itself and members of a family of latent cytoplasmic transcription factors known as signal transducers and activators of transcriptions (STATs). Upon phosphorylation of a specific tyrosyl residue, STAT proteins become activated, and form homo- or heterodimeric complexes that translocate to the nucleus and bind to specific DNA sequences termed STAT binding elements (SBEs). The members of the JAK family interact noncovalently with cytoplasmic domains of cytokine receptor subunits. Like the JAKs, STAT proteins also form a complex with the receptor, but only after binding of the cytokine. The STAT1 and STAT2 proteins have been first identified as constituents of a heterotrimeric protein complex termed ISGF3 that bound to specific promoter sequences called IFN α/β-stimulated response elements (ISREs). Recent studies show that STAT1 is phosphorylated on a serine located near the C-terminus of the protein, within a mitogen-activated protein (MAP) kinase consensus sequence that is also present in a similar location in several other STAT proteins. Many cytokines that signal via the JAK/STAT pathway, including IFN α, IFN β IFN γ, IL-2, growth hormone, and the hematopoietic growth factors, Epo, G-CSF, and GMCSF, have proven clinical utility and other cytokines have high potential. The JAK–STAT signal transduction cascade is also amenble to the development of biochemical assays. Thus, the ordered series of steps mediated by JAKs and STATs has presented excellent opportunities for the discovery of novel drugs that modulate cytokine action.