GS-9973

GS-9973 is a selective inhibitor of Syk with IC50 value of 7.7 nM [1].

Spleen tyrosine kinase (Syk) is a cytoplasmic tyrosine kinase that is expressed mainly in hematopoietic cells and acts as a mediator of immunoreceptor signaling. Syk can be used for therapy of autoimmune, oncology indications and allergy [1].

GS-9973 is an orally bioavailable and selective Syk inhibitor. GS-9973 showed good absorption potential and high bioavailability. In monocytes, GS-9973 inhibited immune-complex-stimulated cytokine production and BCR-mediated proliferation and activation of B-cells [1]. In chronic lymphocytic leukemia (CLL) cells, GS-9973 significantly reduced pAkt and ribosomal S6 phosphorylation. These results suggested that GS-9973 inhibited B-cell receptor (BCR) mediated signaling pathways. In CLL samples, the combination of GS-9973 and idelalisib synergistically reduced CLL cell viability [2].

In chronic lymphocytic leukemia patients, GS-9973 inhibited adenopathy and reduced lymph node [3].

References:
[1].  Currie KS, Kropf JE, Lee T, et al. Discovery of GS-9973, a selective and orally efficacious inhibitor of spleen tyrosine kinase. J Med Chem, 2014, 57(9): 3856-3873.
[2].  Burke RT1, Meadows S, Loriaux MM, et al. A potential therapeutic strategy for chronic lymphocytic leukemia by combining Idelalisib and GS-9973, a novel spleen tyrosine kinase (Syk) inhibitor. Oncotarget, 2014, 5(4): 908-915.
[3].  Sharman J, Hawkins M, Kolibaba K, et al. An open-label phase 2 trial of entospletinib (GS-9973), a selective spleen tyrosine kinase inhibitor, in chronic lymphocytic leukemia. Blood, 2015, 125(15): 2336-2343.

An open-label phase 2 trial of entospletinib (GS-9973), a selective spleen tyrosine kinase inhibitor, in chronic lymphocytic leukemia.[Pubmed:25696919]

Blood. 2015 Apr 9;125(15):2336-43.

Small-molecule inhibitors of kinases involved in B-cell receptor signaling are an important advance in managing lymphoid malignancies. Entospletinib (GS-9973) is an oral, selective inhibitor of spleen tyrosine kinase. This multicenter, phase 2 study enrolled subjects with relapsed or refractory chronic lymphocytic leukemia (CLL; n = 41) or non-Hodgkin lymphoma (n = 145). Participants received 800 mg entospletinib twice daily. We report efficacy outcomes in the CLL cohort (n = 41) and safety outcomes in all cohorts (N = 186). The primary end point was a progression-free survival (PFS) rate at 24 weeks in subjects with CLL. The PFS rate at 24 weeks was 70.1% (95% confidence interval [CI], 51.3%-82.7%); median PFS was 13.8 months (95% CI, 7.7 months to not reached). The objective response rate was 61.0% (95% CI, 44.5%-75.8%), including 3 subjects (7.3%) who achieved nodal response with persistent lymphocytosis. Fifty-four subjects (29.0%) had serious adverse events (SAEs). The most common treatment-emergent SAEs included dyspnea, pneumonia, febrile neutropenia, dehydration, and pyrexia. Common grade 3/4 laboratory abnormalities included neutropenia (14.5%) and reversible alanine aminotransferase/aspartate aminotransferase elevations (13.4%). Entospletinib demonstrates clinical activity in subjects with relapsed or refractory CLL with acceptable toxicity. This trial was registered at www.clinicaltrials.gov as #NCT01799889.

A potential therapeutic strategy for chronic lymphocytic leukemia by combining Idelalisib and GS-9973, a novel spleen tyrosine kinase (Syk) inhibitor.[Pubmed:24659719]

Oncotarget. 2014 Feb 28;5(4):908-15.

Agents that target B-cell receptor (BCR) signaling in lymphoid malignancies including idelalisib (GS-1101) and fostamatinib which inhibit the delta isoform of PI3 kinase (PI3Kd) and spleen tyrosine kinase (Syk) respectively have shown significant clinical activity. By disrupting B-cell signaling pathways, idelalisib treatment has been associated with a dramatic lymph node response, but eradication of disease and relapse in high risk disease remain challenges. Targeting the BCR signaling pathway with simultaneous inhibition of PI3Kd and Syk has not yet been reported. We evaluated the pre-clinical activity of idelalisib combined with the novel and selective Syk inhibitor GS-9973 in primary peripheral blood and bone marrow Chronic Lymphocytic Leukemia (CLL) samples. Both PI3Kd and Syk inhibition reduced CLL survival and in combination induced synergistic growth inhibition and further disrupted chemokine signaling at nanomolar concentrations including in bone marrow derived and poor risk samples. Simultaneous targeting of these kinases may significantly increase clinical activity.

Discovery of GS-9973, a selective and orally efficacious inhibitor of spleen tyrosine kinase.[Pubmed:24779514]

J Med Chem. 2014 May 8;57(9):3856-73.

Spleen tyrosine kinase (Syk) is an attractive drug target in autoimmune, inflammatory, and oncology disease indications. The most advanced Syk inhibitor, R406, 1 (or its prodrug form fostamatinib, 2), has shown efficacy in multiple therapeutic indications, but its clinical progress has been hampered by dose-limiting adverse effects that have been attributed, at least in part, to the off-target activities of 1. It is expected that a more selective Syk inhibitor would provide a greater therapeutic window. Herein we report the discovery and optimization of a novel series of imidazo[1,2-a]pyrazine Syk inhibitors. This work culminated in the identification of GS-9973, 68, a highly selective and orally efficacious Syk inhibitor which is currently undergoing clinical evaluation for autoimmune and oncology indications.

The kinase inhibitors R406 and GS-9973 impair T cell functions and macrophage-mediated anti-tumor activity of rituximab in chronic lymphocytic leukemia patients.[Pubmed:28011996]

Cancer Immunol Immunother. 2017 Apr;66(4):461-473.

Small molecules targeting kinases involved in B cell receptor signaling are showing encouraging clinical activity in chronic lymphocytic leukemia (CLL) patients. Fostamatinib (R406) and entospletinib (GS-9973) are ATP-competitive inhibitors designed to target spleen tyrosine kinase (Syk) that have shown clinical activity with acceptable toxicity in trials with CLL patients. Preclinical studies with these inhibitors in CLL have focused on their effect in patient-derived leukemic B cells. In this work we show that clinically relevant doses of R406 and GS-9973 impaired the activation and proliferation of T cells from CLL patients. This effect could not be ascribed to Syk-inhibition given that we show that T cells from CLL patients do not express Syk protein. Interestingly, zeta-chain-associated protein kinase (ZAP)-70 phosphorylation was diminished by both inhibitors upon TCR stimulation on T cells. In addition, we found that both agents reduced macrophage-mediated phagocytosis of rituximab-coated CLL cells. Overall, these results suggest that in CLL patients treated with R406 or GS-9973 T cell functions, as well as macrophage-mediated anti-tumor activity of rituximab, might be impaired. The potential consequences for CLL-treated patients are discussed.