B-cell receptor (BCR) signals promote survival of chronic lymphocytic leukemia (CLL) cells, and it is believed that overexpressed and constitutively active Lyn mediates this signaling. Here, we show that CLL cells express lymphocyte-specific protein tyrosine kinase (LCK) and that inhibition of this Src family tyrosine kinase with the specific inhibitor [4-amino-5-(4-phenoxyphenyl)-7H-pyrrolo[3,2-d]pyrimidin-7-yl-cyclopentane (Lck-i)], or reduction of its expression with siRNA, blocks the induction of CD79a, Syk, inhibitor of IkB kinase (IKK), Akt, and extracellular signal-regulated kinase (ERK) phosphorylation by BCR cross-linking in these cells. Furthermore, we show that CLL cells with high levels of LCK expression have higher levels of BCR-mediated IKK, Akt, and ERK phosphorylation as well as cell survival than CLL cells with low levels of LCK expression. We also show that treatment of CLL cells with Lck-i inhibits BCR cross-linking-induced cell survival. Taken together, these data show a major role for LCK in proximal and distal BCR-mediated signaling in CLL cells and suggest that LCK expression is important in the pathogenesis of this disease. On a clinical level, these studies advocate the use of specific LCK inhibitors in the treatment of progressive CLL. Mol Cancer Res; 11(5); 541-54. Ó2013 AACR.

IntroductionChronic lymphocytic leukemia (CLL) is a malignant disease characterized by the gradual accumulation of mature, nonproliferative B cells expressing a distinctive group of surface markers, including CD19, CD23, and CD5. 1,2 It is a heterogeneous disease that can follow an indolent or aggressive course, and understanding the nature of this heterogeneity is an area of intense research interest. 2 CLL cells are resistant to apoptosis, and this is thought to be regulated by survival signals from the microenvironment coupled with intrinsic defects in apoptotic machinery. [3][4][5][6][7] Therefore, understanding in greater detail the molecular basis of such apoptotic resistance may help lead to the discovery of agents effective in CLL treatment.The more aggressive, poor prognostic course of CLL positively correlates with a series of cellular markers, such as ZAP70, CD38, and unmutated IgV H . [8][9][10] Recently, it has been reported that high expression levels of the antiapoptotic protein Mcl-1 in CLL cells correlate with both poor disease prognosis 11 and in vitro and in vivo chemoresistance. [11][12][13][14] Moreover, down-regulation of Mcl-1 using antisense oligonucleotides results in cell death during in vitro culture, 15 and elevated Mcl-1 protein expression has been shown to prolong the survival of CLL cells exposed to a variety of apoptosis-inducing stimuli. 16 Taken together, this evidence establishes Mcl-1 as a critical mediator of CLL cell survival.Another signaling protein overexpressed in CLL cells that plays a role in apoptotic resistance is the nonreceptor tyrosine kinase c-Abl. 17 c-Abl appears to be important for normal B-cell development because targeted disruption of the gene encoding this protein in mice results in development of lymphopenia. 18 c-Abl functions in B cells to phosphorylate CD19 after antigen engagement of the B-cell receptor (BCR), highlighting c-Abl as an important regulator of B-cell signaling. 19 In CLL, the overexpression of c-Abl stimulates activity of the nuclear factor-B (NF-B) pathway, leading to increased resistance of the malignant cells to spontaneous apoptosis. 17 Thus, a pathophysiologic role for this kinase in CLL is in the regulation of prosurvival signaling.Active NF-B is known to induce the autocrine expression of cytokines, such as interleukin-6 (IL-6). 20 IL-6 in CLL is associated with adverse disease features and reduced patient survival. 4 It is thought that elevated levels of IL-6 can promote disease progression either indirectly through effects on the microenvironment or directly through effects on cell survival and proliferation. An example of the direct effect of IL-6 is its capability of activating the transcription factor STAT3 via the classic IL-6 receptor (IL-6R) pathway. 21 Such stimulation of STAT3 then promotes up-regulation of antiapoptotic genes, such as Mcl-1. [22][23][24] A role for c-Abl in the activation of STAT3 has been described, 25 as well as a connection, in chronic myelogeneous leukemia (CML) cells, between c-Abl and Mcl-1. 21 Th...

B-lymphocytes are dependent on B-cell receptor (BCR) signaling for the constant maintenance of their physiological function, and in many B-cell malignancies this signaling pathway is prone to aberrant activation. This understanding has led to an ever-increasing interest in the signaling networks activated following ligation of the BCR in both normal and malignant cells, and has been critical in establishing an array of small molecule inhibitors targeting BCR-induced signaling. By dissecting how different malignancies signal through BCR, researchers are contributing to the design of more customized therapeutics which have greater efficacy and lower toxicity than previous therapies. This allows clinicians access to an array of approaches to best treat patients whose malignancies have BCR signaling as a driver of pathogenesis.

Pathogenesis of chronic lymphocytic leukaemia (CLL) is contingent upon antigen receptor (BCR) expressed by malignant cells of this disease. Studies on somatic hypermutation of the antigen binding region, receptor expression levels and signal capacity have all linked BCR on CLL cells to disease prognosis. Our previous work showed that the src-family kinase Lck is a targetable mediator of BCR signalling in CLL cells, and that variance in Lck expression associated with ability of BCR to induce signal upon engagement. This latter finding makes Lck similar to ZAP70, another T-cell kinase whose aberrant expression in CLL cells also associates with BCR signalling capacity, but also different because ZAP70 is not easily pharmacologically targetable. Here we describe a robust method of measuring Lck expression in CLL cells using flow cytometry. However, unlike ZAP70 whose expression in CLL cells predicts prognosis, we find Lck expression and disease outcome in CLL are unrelated despite observations that its inhibition produces effects that biologically resemble the egress phenotype taken on by CLL cells treated with idelalisib. Taken together, our findings provide insight into the pathobiology of CLL to suggest a more complex relationship between expression of molecules within the BCR signalling pathway and disease outcome.

REDX05358 has been identified as a novel, highly selective and potent next generation pan RAF inhibitor with improved therapeutic potential and predicted safety profile. Aberrant signalling via the MAPK pathway is commonly seen in cancer. RAF inhibitors such as Dabrafenib and Vemurafenib have been approved for the treatment of unresectable and metastatic BRAF mutant melanoma, but these agents lack efficacy in BRAF mutant colorectal cancer (CRC), partly because of EGFR-mediated feedback reactivation of the MAPK pathway via CRAF. Furthermore, RAF inhibitor treatment of RAS-mutant, BRAF wildtype melanomas has been associated with other skin cancers, such as cutaneous squamous cell carcinoma due to MAPK pathway paradoxical activation mediated by CRAF. There is therefore a clinical need for novel agents targeting the MAPK pathway that do not have these undesirable properties. Here we present, REDX05358 that demonstrates subnanomolar binding affinity for BRAF and CRAF with high selectivity profile against a panel of 468 kinases that exhibits negligible paradoxical activation due to inhibition of both RAF monomers and dimers. As a result, REDX05358 not only inhibits MAPK signalling in BRAF V600E mutant tumor cells, but also in those harbouring NRAS and KRAS mutations. Furthermore, REDX05358 does not induce feedback reactivation of the pathway through its ability to sustain inhibition of MAPK signalling in CRC cell lines. Correspondingly, profiling of REDX05358 in a panel of CRC, melanoma and NSCLC cell lines shows it has potent anti-proliferative activity in cell lines harbouring BRAF or RAS mutations. REDX05358 is an orally bioavailable, well tolerated small molecule that has demonstrated in vivo efficacy in BRAF V600E CRC xenograft model. In contrast, first generation inhibitors such as Vemarafenib and Dabrafanib have been reported to be ineffective in this genetic background in CRC cell lines and patients. Thus, we have developed a pan RAF inhibitor with unique pharmacological properties enabling it to have utility in treating BRAF and RAS mutant cancers Citation Format: Helen Mason, Simon Scrace, Richard Testar, Julie Rainard, Fatima Talab, Ryaka Poonawala, Philippa Smith, Harriet Brooke, Sarah Frith, Jonathan Ahmet, Jonathan Hall, David Sorrell, Jon Moore, Caroline Phillips, Dennis France, Matilda Bingham, Richard Armer. Development of REDX05358, a novel highly selective and potent pan RAF inhibitor and a potential therapeutic for BRAF and RAS mutant tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5160. doi:10.1158/1538-7445.AM2017-5160

Here we report the preclinical profile of REDX08608 our novel, potent and selective, reversible BTK inhibitor that is equipotent against wild-type and mutant C481S BTK. Bruton's tyrosine kinase (BTK) is a member of the src-related Tec family of cytoplasmic tyrosine kinases and plays a key role in the BCR signaling pathway, which is required for the development, activation and survival of B-cells. BTK is a clinically validated target to treat B-cell malignancies that are dependent on BCR signaling i.e.CLL and NHL with ibrutinib approved for the treatment of CLL, MCL and WM. Irreversible and covalent reversible BTK inhibitors such as ibrutinib, acalabrutinib and GS-4059 specifically target a cysteine residue C481 within BTK and mutations at this site clearly interfere with covalent drug binding. C481S, C481Y, C481R, C481F mutations have been reported and linked to cases of resistance that have emerged in patients with CLL progression following treatment with ibrutinib (Byrd2016, Inhye2016, Maddocks2015, Woaych2014). Redx reversible BTK inhibitor, REDX08608, aims to overcome this resistance mechanism by targeting both wild type and C481-mutated BTK. Redx have recently presented REDX06961 our BTK probe (Guisot2016, AACR#4795) and, following lead optimization, we are now disclosing REDX08608, our lead compound, a potent, reversible and selective BTK inhibitor, which displays an improved profile including superior pharmacokinetics. REDX08608, reversibly, inhibits WT and C481S BTK and displays nanomolar binding affinity and potency in biochemical and cellular-based assays. REDX08608 inhibits BTK signaling and growth in cell lines dependent on the BTK pathway such the OCI-LY10 ABC-DLBCL cell line. Importantly, REDX08608 also inhibits BTK signaling in primary CLL cells. In human whole blood and isolated human PBMCs, REDX08608 inhibits activation of B-cells at nanomolar concentrations measured by inhibition of immunoglobulin-induced CD69 in CD19+cells. REDX08608 is highly selective when tested against a panel of 468 kinases and exhibits improved target specificity with >100-fold selectivity against other Tec and Src kinase family members (ITK, TXK, BMX, TEC, BLK, CSK, FYN, HCK, LCK, SRC) and >400-fold selectivity against EGFR. REDX08608 was fully profiled through DMPK in vitro assays including metabolic stability, plasma stability, cytochrome P450 inhibition, PXR activation/cytochrome P450 activity, time dependent inhibition and cytochrome P450 reaction phenotyping. REDX08608 was shown to have an acceptable metabolic and plasma stability profile across species (mouse, rat, dog, monkey and human). REDX08608 displayed no evidence of PXR activation or time dependent inhibition. IC50s were determined for human cytochrome P450s (1A2, 2D6, 2C9, 2C19 and 3A4) and were all >10 µM. Good exposure, oral bioavailability and half-life were demonstrated for REDX08608 in mouse, rat and dog, with dose linearity assessment performed in mouse (F = 73-100%, CL= 11% liver blood flow in mice; F = 55-84%, CL = 28% liver blood flow in rat; F = 85%, CL= 10% liver blood flow in dog). In vivo efficacy studies in preclinical models will also be disclosed. In conclusion, REDX08608 is a potent and selective, reversible BTK inhibitor with efficacy in lymphoma cell lines that offers the potential to target both wild-type BTK and an important emerging resistance mechanism in patients with CLL progression following ibrutinib-treatment. Disclosures Guisot: Redx Oncology Ltd - Redx Pharma Plc: Employment. Best:Redx Oncology Ltd - Redx Pharma Plc: Employment. Wright:Redx Oncology Ltd - Redx Pharma Plc: Employment. Thomason:Redx Oncology Ltd - Redx Pharma Plc: Employment. Woyach:Acerta: Research Funding; Karyopharm: Research Funding; Morphosys: Research Funding. Abet:Redx Oncology Ltd - Redx Pharma Plc: Employment. Castagna:Redx Oncology Ltd - Redx Pharma Plc: Employment. Cousin:Redx Oncology Ltd - Redx Pharma Plc: Employment. Emmerich:Redx Oncology Ltd - Redx Pharma Plc: Employment. Ho:Redx Oncology Ltd - Redx Pharma Plc: Employment. Kelly:Redx Oncology Ltd - Redx Pharma Plc: Employment. King-Tours:Redx Oncology Ltd - Redx Pharma Plc: Employment. Lyons:Redx Oncology Ltd - Redx Pharma Plc: Employment. Muller:Redx Oncology Ltd - Redx Pharma Plc: Employment. Refuerzo:Redx Oncology Ltd - Redx Pharma Plc: Employment. Sargent:Redx Oncology Ltd - Redx Pharma Plc: Employment. Talab:Redx Oncology Ltd - Redx Pharma Plc: Employment. Bingham:Redx Oncology Ltd - Redx Pharma Plc: Employment. Phillips:Redx Oncology Ltd - Redx Pharma Plc: Employment. Armer:Redx Oncology Ltd - Redx Pharma Plc: Employment.

375 B cell receptor (BCR) signaling promotes survival of the malignant clone in chronic lymphocytic leukaemia (CLL) through its ability to stimulate NFkB pathway signaling. In lymphoid cells, antigen receptor stimulation of this pathway is achieved by engaging the Carma-1 – Bcl10 – MALT1 (CBM) complex for eventual activation of I-kB kinases (IKKs). In B cells, protein kinase C beta (PKCbeta) is an important mediator of CBM complex activation. However, in CLL cells we found that PKCs do not appear to have a role in BCR-mediated NFkB pathway signaling, despite high expression levels of PKCbeta, because the presence of specific inhibitors of this kinase (LY379196 and bisindolylmaleimide-I) has no effect on the induction of IKK phosphorylation during BCR crosslinking. Examination of CBM complex expression suggests an explanation for this phenomenon; the expression levels of Carma-1 and MALT-1 are largely similar in CLL and normal B cells, but the expression of Bcl10 is much reduced in CLL cells. These findings, taken together with the established role of Bcl10 in the pathway of BCR-induced NFkB activation, suggest that CLL cells may employ a different mechanism to activate this pathway during BCR stimulation. Tyrosine kinases are known to play a role in BCR-induced IKK activation in CLL cells because compounds like dasatinib and PP2 inhibit NFkB pathway activation by BCR. One possible tyrosine kinase is c-Abl because we have shown this protein to be overexpressed in CLL cells, where it plays a role in activation of the NFkB pathway. To investigate the role of c-Abl in BCR-induced IKK activation, we used the inhibitor imatinib and found that the presence of this compound partially inhibited IKK phosphorylation in BCR-stimulated CLL cells. However, imatinib can also inhibit Lck, a T cell-specific src-family tyrosine kinase that is expressed by CLL cells. To differentiate between Lck- and c-Abl-mediated BCR signals we used the specific inhibitor 4-amino-5-(4-phenoxyphenyl)-7H-pyrrolo[3,2d] pyrimidin-7-yl-cyclopentane (Lck-i). We found that the presence of this compound in CLL cell cultures undergoing BCR stimulation almost completely inhibited the induction of IKK activation. Investigation of Lck-i specificity revealed this compound did not inhibit either c-Abl or Lyn at the concentration used to inhibit Lck in CLL cell cultures. Further investigation of the effects of Lck-i showed that this compound was also effective in inhibiting BCR-induced activation of the Akt and ERK signaling pathways. Taken together, these data suggest a major role for Lck in BCR-mediated signaling in CLL cells, and question the existing paradigm on the importance of Lyn. Disclosures: No relevant conflicts of interest to declare.

The B-cell receptor (BCR) signaling pathway is required for the survival, activation, proliferation and differentiation of B-cells. Bruton's Tyrosine Kinase (BTK) is a member of the Tec protein tyrosine kinase family that has emerged as an attractive target for the treatment of B-cell malignancies due to the critical role it plays in BCR signaling. Redx Oncology has developed novel differentiated small molecule inhibitors of BTK, combining current best-in-class potency with distinct selectivity profiles, which are suitable for oral once daily dosing. Here we present REDX05194, the result of a successful lead optimization of our proprietary BTK inhibitor series. REDX05194 is a highly selective, covalent BTK inhibitor displaying subnanomolar binding affinity for BTK (0.39 nM) and nanomolar potency towards BTK in a biochemical assay (3.67 nM). In cell proliferation assays, REDX05194 showed significant in vitro potency against ABC-DLBCL cell lines inhibiting the growth of both TMD-8 (0.89 nM) and OCI-Ly10 (1.36 nM) cells. Analysis of BCR signaling in several lymphoma cell lines, including cell lines of ABC-DLBCL, MCL and FL origin, revealed that treatment with REDX05194 inhibits BTK autophosphorylation and downstream activation of PLCγ2. In human PBMCs, REDX05194 inhibited anti-IgM stimulated upregulation of the CD69 activation marker in CD19 positive B-cells. In addition, using a fluorescent probe that binds to BTK, occupancy of BTK in PBMCs has been demonstrated in response to increasing concentrations of REDX05194. To assess selectivity, 456 kinases were screened at 1 μM, confirming that REDX05194 does not significantly inhibit other kinases involved in BCR signaling (e.g. Syk, Lyn). Furthermore, REDX05194 was shown to have high selectivity versus structurally related cysteine-containing kinases such as ITK in binding assays, and EGFR as demonstrated in both binding and cellular assays. REDX05194 also has a favorable in vitro safety profile and drug-like properties, displaying an improved CYP profile and solubility compared to competitor compounds. REDX05194 demonstrated in vivo efficacy in a mouse collagen-induced arthritis (CIA) model. At 10 mg/kg and 30 mg/kg QD, REDX05194 significantly improved all clinical readouts, including disease severity, compared to the vehicle group. Histological data showed that approximately 1/3 of the mice had no or minimal pannus infiltration and no bone resorption, or had bone resorption restricted to small areas. These findings demonstrate potential clinical efficacy and a dose response. In conclusion, REDX05194 is a highly selective and potent BTK inhibitor with proven efficacy in several lymphoma cell lines and human PBMCs and in vivo efficacy demonstrated in a mouse CIA model. Disclosures No relevant conflicts of interest to declare.