In vitro activity: PRN1008 is a highly potent, reversible covalent inhibitor of BTK (Bruton’s Tyrosine Kinase) with an IC50 of 1.3 nM. PRN1008 was found to be very potent against BTK and highly selective when tested against a panel of 251 other kinases. Cysteine targeting of BTK by PRN1008 results in a slow off-rate demonstrated by retention of 79 ± 2% of binding to BTK in PBMC 18 hours after washing away the compound in vitro. PRN1008 was safe and well-tolerated following oral administration, and achieved high, sustained levels of BTK occupancy in peripheral blood mononuclear cells. PRN1008 is currently under Phase I development as a therapeutic agent for rheumatoid arthritis.

Kinase Assay: PRN1008 is very potent against BTK (IC50=1.3±0.5 nM) and highly selective against a panel of 251 other kinases. Cysteine targeting of BTK by PRN1008 results in a slow off-rate demonstrated by retention of 79±2% of binding to BTK in PBMC 18 hours after washing away the compound in vitro. The covalent cysteine binding is completely reversible after denaturation of the target. Anti-IgM induces human B cell proliferation (10% serum) and B cell CD69 expression are inhibited by PRN1008 with IC50 of 5±2.4 nM and 123±38 nM, respectively

Cell Assay: Anti-IgM induced human B cell proliferation (10% serum) and B cell CD69 expression (whole blood) were inhibited by PRN1008 with IC50 of 5 ± 2.4 nM and 123 ± 38 nM, respectively. PRN1008 did not block EGFR signaling in epithelial cells or TCR and calcium flux stimulated T cell activation. PRN1008 also did not block IL-4 stimulation of B cells and did not exhibit cytotoxicity in an epithelial cell line HCT-116. In addition, PRN1008 did not block antibody dependent cell-mediated cytotoxicity in combination with anti-CD20
antibodies allowing for potential combination therapies. In vivo PRN1008 demonstrated enduring pharmacodynamic effects after the compound had cleared from circulation, consistent with extended target residence time. PRN1008 also reversed and completely suppressed collagen-induced arthritis in rats in a dose dependent manner which allowed correlation of target occupancy and disease modification.