PF-114, a novel selective inhibitor of BCR-ABL tyrosine kinase, is a potent inducer of apoptosis in chronic myelogenous leukemia cells

A t(9;22) chromosomal translocation which forms the chimeric tyrosine kinase breakpoint cluster region (BCR)-Abelson murine leukemia viral oncogene homolog 1 (ABL) is a key mechanism underlying the pathogenesis of chronic myelogenous leukemia (CML). Pharmacological inhibition of BCR-ABL with imatinib (Gleevec) has been reported as an effective targeted therapy; however, mutations (including the kinase domain of ABL) suppress the efficacy of inhibitors. PF-114, a derivative of the third generation BCR-ABL inhibitor ponatinib, demonstrated a high inhibitory activity against wild-type and mutant BCR-ABL forms, such as the clinically important T315I. Furthermore, PF-114 exhibited preferential kinase selectivity, safety, notable pharmacokinetic properties and therapeutic efficacy in a murine model. Investigation into the mechanisms of CML cell death revealed an exceptional potency of PF-114 (at low nanomolar concentrations) for the CML-derived K562 cell line, whereas leukemia cell lines that lack the chimeric tyrosine kinase were markedly more refractory. The molecular ordering of events mechanistically associated with K562 cell death included the dephosphorylation of CrkL adaptor protein followed by inhibition of ERK1/2 and Akt, G1 arrest, a decrease of phosphorylated Bcl-2-associated death promoter, Bcl-2-like protein 11, BH3 interacting-domain death agonist, Bcl-extra large and Bcl-2 family apoptosis regulator, and reduced mitochondrial transmembrane potential. Increased Annexin V reactivity, activation of caspases and poly(ADP-ribose) polymerase cleavage were proposed to lead to internucleosomal DNA fragmentation. Thus, PF-114 may be a potent inducer of apoptosis in CML cells. Nevertheless, activation of STAT3 phosphorylation in response to PF-114 may permit cell rescue; thus, a combination of BCR-ABL and STAT3 inhibitors should be considered for improved therapeutic outcome. Collectively, the targeted killing of BCR-ABL-positive cells, along with other beneficial properties, such as in vivo characteristics, suggests PF-114 as a potential candidate for analysis in clinical trials with CML patients.