Proteolysis-Targeting Chimeras (PROTAC): an emerging technology for the development of therapeutic strategies in hematological malignancies

More than two decades ago, a new class of heterobifunctional protein degraders was developed, namely, Proteolysis-Targeting Chimeras (PROTACs), in order to improve the drawbacks associated with small-molecule inhibitors in cancer treatment. PROTACs are able to selectively target proteins involved in a specific disease and eliminate them, through the activation of the ubiquitin-proteasome system. PROTACs emerged as a promising approach in hematological cancer. This thesis focuses on PROTAC applications in the development of three specific pre-clinical therapeutic strategies for Acute Myeloid Leukemia (AML) and Chronic Myeloid Leukemia (CML). CML is characterized by the expression of the BCR::ABL1 fusion gene, deriving by genetic translocation between chromosome 9 and chromosome 22. In physiological conditions, ABL1 is a proto-oncogene that encodes a tyrosine kinase protein involved in several cellular processes in human. Its translocation leads to the generation of an abnormal tyrosine kinase that activates several downstream signaling pathways causing an uncontrolled proliferation of myeloid cells in the bone marrow, contributing to the expansion of leukemic cells. The first strategy examined in this thesis consists in a single amino acid PROTAC based on the N-end rule degradation pathway, resulting in an effective BCR-ABL protein degradation and a successful growth inhibition in vitro in K562 cells expressing BCR-ABL and in vivo in K562 xenograft tumor models. The second approach investigated a combination therapy with GMB-475 PROTAC and specific Tyrosine Kinase Inhibitors (TKI) with the aim of overcoming drug resistance, usually associated to TKIs targeting ABL1. Results showed synergic properties of the two drugs, in particular with Dasatinib in terms of apoptosis promotion, growth inhibition and blocking of the cell cycle in BCR::ABL1 Ba/F3 mutant cells, providing a novel strategy for the treatment of CML patients with BCR::ABL1 mutants. The overexpression of anti-apoptotic BCL-2 family proteins, in particular BCL-XL, is an oncogenic feature of AML associated with resistance to chemotherapy and disease progression. The last strategy analyzed describes the pre-clinical single-agent activity of 753B, a new dual BCL-XL/BCL-2 PROTAC. The study demonstrated the properties of PROTAC 753B therapy, highlighting its senolytic activity, enhancing the chemotherapy efficacy, and anti-tumor activity in AML primary samples, in vivo in PDX AML models and in hematopoietic cell lines. With further optimization of formulation and dosing, 753B has the potential to become the first-in-class dual BCL-XL/BCL-2 targeting antitumor agent in AML therapy. Overall, these data although requiring further validation, offer an encouraging and a stimulating perspective in hematological cancers research and treatment.