Novel strategies targeting mutant calreticulin in essential thrombocythemia and myelofibrosis

The discovery of calreticulin (CALR) mutations in patients with myeloproliferative neoplasms (MPNs) has paved the way for the elucidation of a unique disease mechanism that is particularly well suited to targeting by biologics. All MPN-associated pathogenic CALR mutations are characterized by a frameshift, resulting in translation of the same neoantigen peptide. This neoantigen directly activates the thrombopoietin receptor, leading to uncontrolled neoplastic cell proliferation. Current therapeutic approaches for MPNs are focused primarily on blood count control. Furthermore, current approaches are neither disease modifying nor clonally selective. However, because the mutant CALR neoantigen peptide is functional and not expressed in normal physiology, it is an ideal drug target. Here, we review the structure and function of mutant CALR, including the subtle yet clinically and therapeutically relevant differences between the 2 most commonly occurring types of mutation. We also review the current therapeutic landscape for CALR-mutated MPNs, highlighting the areas in which current approaches are inadequate. Finally, we review ongoing clinical and preclinical experimental approaches for targeting mutant CALR in MPNs in a clonally selective manner using monoclonal antibodies, bispecific antibodies, cancer vaccination, chimeric antigen receptor T cells, and antibody-drug conjugates. Taken together, we expect that ongoing developments in mutant CALR-targeted therapeutics will lead to promising novel strategies for long-term disease control.