P317: TARGETING HYPERACTIVE PDGFR-B IN T-ALL AND T-LBL

Abstract

Background: T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy that accounts for 10-15% of pediatric and 25% of adult ALL cases. General prognosis of T-ALL has been improving over time. However, the outcome of T-ALL patients with primary resistant or relapsed leukemia remains dismal. To this end, we have screened a T-ALL/T-LBL patient cohort for copy number variations and identified a novel MYH9-PDGFR-β fusion in a T-LBL case, driving aberrant activation of platelet derived growth factor β (PDGFR-β). Moreover, RNA sequencing data of an independent T-ALL patient cohort showed overexpression of PDGFR-β in the TLX subgroup and in immature T-ALL patients. PDGFR-β is a transmembrane glycoprotein dimer molecule that functions as a receptor tyrosine kinase. Ligand binding results in receptor dimerization and subsequent activation via autophosphorylation, causing downstream activation of various signaling pathways such as PI3K/AKT, MAPK/ERK, JAK/STAT and Notch. As constitutively active PDGFR-β is observed in a variety of malignancies, multi-target kinase inhibitors are already used in the clinic. However, they are not specific and often result in adverse side-effects. CP-673451 is a novel and selective PDGFR inhibitor. Mechanistically, CP-673451 inhibits autophosphorylation of dimeric PDGFR-β and in this way prevents phosphorylation of downstream PI3K/AKT and GSK-3α/β. Previous studies showed growth inhibition of multiple tumor xenografts (lung and colon carcinomas) mainly due to a direct antitumor effect. Aims: We aim to assess whether specific PDGFR-β inhibitors can be used as a novel and targeted therapy in T-ALL/T-LBL treatment or if they can be of added value to the current chemotherapy regimen for T-ALL/T-LBL patients. In the future, we would like to investigate the transforming capacity of the MYH9-PDFR-β fusion in an in vitro pro-T cell culture system. Methods: T-ALL cell lines were screened for PDGFR-β levels and cell lines were selected with both high (CTV-1, SEM) and low (MOLT-16, Jurkat, Loucy) PDGFR-β levels. CP-673451 was tested on the selected cell lines. Cell viability was measured using an ATP assay (CellTiter Glo) and downstream signaling pathways were quantified with flow cytometry and western blot. Additionally, spleen cells from PDGFR-βhigh and PDGFR-βlow T-ALL PDX models were treated ex vivo using CP-673451. Results: Both PDGFR-βhigh cell lines and PDGFR-βhigh PDX models showed sensitivity to CP-673451 treatment in nanomolar range, whereas PDGFR-βlow models did not (Fig. 1A-B). Further characterization of CP-673451 treatment showed inhibition of PDGFR-β autophosphorylation both on western blot and flow cytometry. Downstream, phosphorylation levels of STAT3, STAT5 and GSK-3β were decreased upon PDGFR-β inhibition (Fig. 1C). Dephosphorylation of GSK-3β results in activation of this kinase, leading to phosphorylation of MCL1, promoting degradation of this antiapoptotic protein. Interestingly, high MCL1 levels are associated with lowered sensitivity to glucocorticoids, core components of T-ALL/T-LBL treatment. To this end, combination therapy of PDGFR-β inhibition with glucocorticoids can result in synergistic effects. Next, we will conduct in vivo studies to verify if specific PDGFR-β inhibitors could be advantageous in the treatment of PDGFR-βhigh T-ALL/T-LBL cases. Image:Summary/Conclusion: In this study, we have identified MYH9-PDGFRB as a novel rare fusion activating PDGFR-β in a T-LBL patient sample. Moreover, in vitro evaluation in T-ALL cell lines and ex vivo treatment of T-ALL PDX models showed therapeutic benefits from PDGFR-β inhibition by CP-673451.