Molecular mechanisms of relapse after therapy discontinuation in chronic myeloid leukemia

Clinical outcomes in chronic myeloid leukemia (CML) were revolutionized by development of Bcr-abl-directed tyrosine kinase inhibitors (TKIs), and many patients now survive for years in with major molecular remission (MMR). Unfortunately, when TKI discontinuation was attempted in CML patients with sustained MMR, ~60% of such patients relapsed. This suggested some leukemia stem cells (LSCs) persist even with an MMR. We used a murine bone marrow transplant model to investigate the characteristics associated with successful TKI discontinuation. In this model, mice transplanted with Bcr-abl transduced bone marrow were treated with TKIs. Bone marrow from mice in sustained MMR was transplanted into recipients, who were followed without additional treatment. 67% of these mice relapsed, but the incidence varied between recipients from different donors (~80% vs ~20%), suggesting underlying characteristics of persistent LSCs in some mice predispose to relapse. Fas-resistance is hypothesized to contribute to LSC persistence during TKI treatment. We identified increased expression of Fap1 (a Fas antagonist) as one mechanism for Fas resistance in CML-LSCs. A tripeptide (SLV) representing the Fas C-terminus blocks Fap1 protein-protein interactions and inhibits its activity. We found the addition of SLV peptide to TKI treatment prevented relapse after treatment discontinuation in mice. Bcr-abl transcript abundance was significantly less in the bone marrow of mice treated with TKI + SLV peptide vs TKI (or SLV) alone, with SLV peptide converting TKI treated mice from MMR to complete molecular response (CMR). The goal of this project is to identify characteristics of CML-LSCs that predict relapse after TKI discontinuation. We hypothesize targeting pathways or intermediates conferring increased risk will permit safe TKI discontinuation, without relapse, in more CML subjects. We will investigate this hypothesis through 3 Aims; Aim 1: Define characteristics predisposing to relapse after TKI discontinuation in CML. Identify molecular mechanisms associated with relapse after TKI discontinuation in a pre-clinical murine model. Results will be compared to CD34+ cells from human CML subjects at presentation, remission and relapse. Aim 2: Determine the contribution of physiologic stress to relapse upon TKI discontinuation. Activation of the innate immune response will be studied as a cell extrinsic mechanism potentially contributing to relapse after TKI discontinuation using the murine model described above. Aim 3: Identify additional potential therapeutic targets to decrease relapse after TKI discontinuation. Events involved in LSC persistence during TKI treatment will be targeted in the pre-clinical murine model to evaluate an impact on successful TKI discontinuation. Focus will be on targets with translational potential. Our studies suggest the pre-clinical murine model employed in this proposal can be used to identify and therapeutically target pathways associated with relapse after TKI discontinuation. Results of these studies will be used to propose a prospective clinical trial to predict relapse and track relevant events in human CML.