Drugs in Clinical Pipeline: Entospletinib

Entospletinib [6-(1H-indazol-6-yl)-N-(4-morpholinophenyl)imidazo[1,2-a]pyrazin-8-amine] is an orally available, ATP competitive and highly selective Spleen tyrosine kinase (Syk) inhibitor which is being developed for treating autoimmune and oncology conditions. Broad kinase panel screening revealed a greater selectivity of entospletinib vs R406. Dissociation constant (K_d) determinations of the strongest hits from the broad panel KINOMEscan showed that aside from Syk itself, only 1 kinase, TNK1, had a K_d less than 100 nM for Entospletinib, whereas 79 kinases had K_d less than 100 nM for R406. In healthy volunteers, Entospletinib at BID doses higher than 200 mg demonstrated inhibition of Syk activity, as measured by CD63 expression and phospho-Syk. Due to its excellent selectivity profile, Entospletinib was hypothesized to provide high levels of Syk inhibition with potentially fewer off-target adverse effects (AEs) than observed with Fostamatinib [1, 2].

The activity of Entospletinib is as follows:

IC₅₀ (Syk enzyme assay) = 7.7 nM
IC₅₀ (pBLNK) = 26 nM

Common Name: Entospletinib

Synonyms:  GS-9973;  GS 9973; GS9973

IUPAC Name: 6-(1H-indazol-6-yl)-N-(4-morpholinophenyl)imidazo[1,2-a]pyrazin-8-amine

CAS Number: 1229208-44-9

Mechanism of Action: Kinase Inhibitor; Syk Inhibitor; Spleen Tyrosine Kinase Inhibitor

Indication: Various Cancers; Autoimmune diseases

Development Stage: Phase I/II

Company: Gilead Sciences

Spleen tyrosine kinase (Syk) is a 72 kDa multiple-domain intracellular cytoplasmic tyrosine kinase that is expressed primarily in hematopoietic cells (e.g., B-cells, monocytes, macrophages, mast cells, and neutrophils), where it is recognized as an important mediator of immunoreceptor signaling and has been identified as a potential therapeutic target in allergic, autoimmune, and oncology indications. Immunoreceptor engagement triggers phosphorylation of a pair of tyrosine residues in the cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs) by Src family members, and Syk is recruited to the phosphorylated ITAMs via its SH2 domain.4-6 Syk critically regulates immune cell function by propagating signaling cascades through phosphorylation of direct targets (such as BLNK/SLP65), leading to activation of downstream pathways, including PI3K, MAPK, Btk, and PLCγ. The activation of these pathways in immune cells leads to proliferation, differentiation, cytoskeletal remodeling, and cytokine release. Importantly, since Syk is not expressed in mature T-cells, the immunosuppression that is associated with therapeutics that inhibit T-cell signaling should be avoided.

Entospletinib shows better kinase selectivity and oral availability than the most advanced Syk inhibitor, R406, (or its prodrug form Fostamatinib). R406 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 R406. TNK1 is the only kinase that has less than 10-fold selectivity versus Syk for Entospletinib, whereas R406 bound 36 kinases within 10-fold of the potency of Syk and 14 kinases more potently than Syk [1]. 

References:

1. Currie, K. S.; 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. Sharman, J.; 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

3. ClinicalTrials.gov A Phase 1 Study of Novel GS-9973 Tablet Formulations to Evaluate the Effect of Acid Reducing Agents, Relative Bioavailability, and Food Effect on GS-9973 Pharmacokinetics. NCT01841489 (retrieved 25-04-2015)

4. ClinicalTrials.gov A Phase 2 of Entospletinib in Subjects With Relapsed or Refractory Hematologic Malignancies. NCT01799889 (retrieved 25-04-2015)

5. ClinicalTrials.gov  A Phase 2 of GS-9973 in Combination With Idelalisib in Subjects With Relapsed or Refractory Hematologic Malignancies. NCT01796470 (retrieved 25-04-2015)