Loxo-305, a Highly Selective and Non-Covalent Next Generation BTK Inhibitor, Inhibits Diverse BTK C481 Substitution Mutations

Introduction: Bruton's Tyrosine Kinase (BTK) is an essential component of normal and malignant B-cell receptor signaling. Covalent BTK inhibitors have transformed the treatment of B-cell malignancies but are limited by off-target toxicity and acquired resistance, leading to eventual treatment discontinuation and disease progression. Emerging evidence suggests that acquired resistance is mediated predominantly by BTK C481 substitution mutations at the covalent BTK inhibitors' binding site. There is significant unmet clinical need for new treatment approaches that overcome acquired resistance and minimize toxicity.

LOXO-305 is a highly selective, non-covalent, next generation BTK inhibitor. We previously showed that LOXO-305 potently inhibited both wild-type (WT) BTK and BTK C481S -mediated kinase activity in enzyme and cell-based assays with nanomolar potency, caused regression of BTK-dependent lymphoma mouse xenograft models, and was more than 300-fold selective for BTK over 98% of 370 other kinases tested and showed no significant inhibition of non-kinase off targets at 1 mM (Brandhuber et al. SOHO 2018). In addition, ADME and pharmacokinetic experiments in two preclinical species predicted that LOXO-305 will have high human exposure and sustained BTK C481S target coverage in patients at clinically achievable doses.

Here we describe the activity of LOXO-305 against additional BTK C481 substitution mutations, including mutations identified in patients with acquired resistance to covalent BTK inhibitors. We further determine equilibrium-binding affinities for LOXO-305 for diverse mutant BTK enzymes in comparison to other clinically available BTK inhibitors.

Methods: To assess cellular BTK inhibitor potency, HEK293T cell lines transiently expressing wild-type BTK and BTK C481 substitution mutations were serum starved and incubated with LOXO-305 overnight. Cells were next incubated with serum and orthovanadate for 5 min and the phosphorylated Y223 BTK was analyzed by immunoblot. Bands were quantified and the IC₅₀ values calculated with GraphPad Prism. The equilibrium-binding affinities for targeted BTK inhibitors to BTK enzyme variants were determined by surface plasmon resonance (SPR) using the Biacore T200. Biotinylated BTK variants were immobilized on a docked streptavidin coated sensor chip. Five concentrations of each inhibitor plus blank controls were analyzed. Association/dissociation rate constants were calculated by global fitting of the data to a 1:1 binding interaction model.

Results: While BTK C481S possessed similar levels of basal Y223 autophosphorylation as wild-type BTK in cells, BTK C481T autophosphorylation was reduced by ~50%, C481R by ~90%, and mutants C481F, and C481Y were inactive in HEK293T cells. LOXO-305 inhibited Y223 phosphorylation of all active mutants with similar nanomolar potency. In contrast, autophosphorylation of all BTK C481 mutants were resistant to both Ibrutinib and acalabrutinib. Equilibrium-binding affinities of LOXO-305 for select BTK C481 substitution mutations confirmed LOXO-305's superior potency versus commercially available BTK inhibitors (ibrutinib and acalabrutinib).

Conclusions: The next generation, non-covalent, highly selective BTK inhibitor LOXO-305 potently inhibited the cellular activity of BTK C481S, T and R mutations and displayed strong equilibrium binding to WT BTK and several BTK C481 substitution mutations. Together with high selectivity and significant BTK target coverage in vivo, these results indicate that LOXO-305 may overcome acquired resistance to covalent BTK inhibitors in patients without significant off-target toxicity. A phase 1 clinical trial of LOXO-305 is currently underway.