Discovery of INCA033989, a Monoclonal Antibody That Selectively Antagonizes Mutant Calreticulin Oncogenic Function in Myeloproliferative Neoplasms (MPNs)

Calreticulin (CALR) mutations are responsible for disease development in 20-30% of patients with MPNs. Mutations can be either insertions or deletions in exon 9 of CALR, both of which result in a frameshift that leads to the expression of a novel, positively charged C-terminus as well as the loss of the KDEL endoplasmic reticulum (ER)–retention signal. The mutant CALR (mutCALR) protein has a novel function that involves stable interaction with the thrombopoietin receptor (TPO-R). This interaction originates in the ER and is followed by the transit of the mutCALR/TPO-R complex to the cell surface, resulting in constitutive activation of JAK2/STAT signaling. Current therapeutic options in MPNs provide effective symptom management but have high rates of discontinuation owing to the emergence of resistance and inadequate drug tolerability. The development of novel therapeutics selective for cells expressing mutCALR would allow targeting neoplastic cells without compromising normal hematopoiesis.

We report herein the discovery of INCA033989, a monoclonal antibody selectively targeting mutCALR-driven oncogenesis. INCA033989 was derived from phage and yeast display selections followed by additional sequence engineering. INCA033989 is a high affinity, fully human IgG1 selective for mutCALR binding and functions as an antagonist to suppress TPO-R signaling induced by mutCALR. Additionally, INCA033989 binds to mutCALR expressed on the surface of Ba/F3 cells and disrupts TPO-R oncogenic signaling by inhibiting mutCALR-dependent TPO-R dimerization and subsequent pathogenic signal transduction and cell proliferation. In contrast, INCA033989 exhibited no functional effect on Ba/F3 cells not expressing mutCALR. Potent synergism between INCA033989 and the JAK1/2 inhibitor ruxolitinib was observed when Ba/F3-TPO-R/mutCALR cells were treated with a combination of suboptimal concentrations of both agents. Such treatment resulted in inhibition of cell proliferation, indicating that INCA033989 can enhance the efficacy of ruxolitinib even when suboptimal doses of the latter are used.

The effect of INCA033989 on CD34⁺ cells isolated from patients with MPNs was also evaluated. INCA033989 binds CD34⁺ cells expressing mutCALR and inhibits mutCALR-driven activation of the JAK2/STAT pathway in a dose-dependent manner. Such effects were unique for the mutCALR cells, as no antibody binding or JAK2/STAT inhibition was observed in CD34⁺ cells from either healthy donors or MPN patients carrying the JAK2^V617F mutation. Notably, liquid culture of CD34⁺ cells supported the therapeutic effect of INCA033989 since treatment of CD34⁺ cells with the antibody resulted in decreased proliferation of mutCALR hematopoietic stem and progenitor cells (HSPCs) and megakaryocytes in a dose-dependent manner. This demonstrates that INCA033989 selectively targets mutCALR HSPCs while preserving the proliferation/differentiation of wild-type (WT) counterparts.

To assess the role of INCA033989 in an in vivo MPN model, we used a competitive engraftment mouse model consisting of 30% SCL-Cre-ER^T/ mutCALR and 70% WT CALR UBC-GFP competitor bone marrow cells into lethally irradiated recipient mice. We showed that a 10-week treatment with the antibody prevented the development of thrombocytosis by selectively decreasing mutCALR-positive platelets. At sacrifice, while the antibody therapy did not impact the overall bone marrow cellularity, it induced a dramatic decrease in the percentage of all precursor and progenitor cells from mutCALR origin including megakaryocytes and long-term HSCs. Moreover, the antibody suppressed the mutCALR-induced accumulation of megakaryocytes in the bone marrow, a hallmark of essential thrombocythemia. The targeting of mutCALR disease–initiating stem cells was confirmed by the lack of disease development upon secondary transplantations.

In summary, we have developed a highly potent monoclonal antibody that binds to mutCALR and inhibits oncogenesis in cells expressing mutCALR. Our data demonstrate that INCA033989 selectively targets mutCALR and normalizes TPO-R signaling in patient-derived HSPCs and in a MPN in vivo model. INCA033989 is currently being advanced for investigation in patients with MPNs.