Abstract LB-303: Small molecule inhibitor of the BTK pathway disrupts BCR signaling and demonstrates antitumor efficacy in a xenograft model.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Despite the recent advances made in the treatment and management of B cell malignancies, these diseases are not curable and overall survival is limited. Bruton's Tyrosine Kinase (BTK) is a member of the Tec family of intracellular kinases first identified for its signaling via the B-cell Receptor (BCR) and its role in the immune system. More recently, BTK was found to play an important role in B cell malignancies and select solid tumors. Preclinical and clinical results with selective irreversible BTK inhibitors provide validation for BTK as a therapeutic target in B cell malignancies. Aiming to leverage the contribution of the BTK signaling pathway to tumor growth, and its role in progression and drug resistance, we have developed a series of relatively selective, reversible, small molecule BTK inhibitors and evaluated their activity in enzyme, cell-based, and in vivo studies. Data obtained with the orally available MKC4659 compound illustrates our findings. In biochemical assays, compound MKC4659 demonstrated a relatively select targeting profile focused on a narrow group of PTKs including significant activity against BTK with IC50 less than 25 nM. In cellular assays the compound demonstrated significant in vitro potency against B cell lymphoma cell lines, inhibiting the growth of several B cell tumor cell lines including ones unresponsive to currently known BTK inhibitors. Importantly, MKC4659 showed a differential effect on B cell lymphoma, with no significant activity detected in control cells lacking detectable BTK expression. In vitro mode of action studies demonstrated that MKC4659 induces apoptosis and PARP cleavage in B cell lymphoma but not in control cells. Assays evaluating the in vitro on-target effect of compounds showed significant inhibition of the B cell receptor-mediated activation of the BTK pathway. In addition to inhibiting the phosphorylation of BTK, MKC4659 inhibited the phosphorylation of PLCγ in several B cell lymphoma cell lines. With in vitro potency demonstrated, and PK and ADMET profiles amenable to in vivo dosing, MKC4659 was evaluated for in vivo efficacy in a xenograft model of B cell lymphoma. In vivo dosing of MKC4659 inhibited growth of DOHH2 xenograft tumors in a dose dependent manner. In summary, our team has identified BTK pathway inhibitors with demonstrated on-target and anti-tumor activity in cellular assays, and efficacy in a preclinical model of B cell malignancy. This effort provides a platform for compound development and evaluation for the treatment of hematologic malignancies. Optimization efforts on the MKC4659 series are ongoing and have yielded potent and drug-like preclinical candidates that are now moving into advanced animal studies. Citation Format: Mary Faris, Uriel M. Malyankar, Victor Tam, Colleen Schweitzer, Diljeet Joea, Alexis Mollard, Bret Stephens, Steven L. Warner, David J. Bearss, Qingping Zeng. Small molecule inhibitor of the BTK pathway disrupts BCR signaling and demonstrates antitumor efficacy in a xenograft model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-303. doi:10.1158/1538-7445.AM2013-LB-303