The CD70/CD27 Axis in Tyrosine Kinase Inhibitor-Treated Chronic Phase Chronic Myeloid Leukemia Cells Is Not an Achilles Heel.

Background/Objectives: Revolutionary small molecule, targeted, tyrosine kinase inhibitors (TKIs) in the clinic have engendered the perception that optimal treatment has already been established for diseases like chronic myeloid leukemia (CML). Addressing BCR::ABL1 oncokinase domain mutations in CML is unlikely to redress disease recrudescence owing to persistent leukemia stem cells (LSCs). Therefore, it is necessary to consider an alternative strategic approach: disrupting LSC interactions with the protective microenvironment and/or immune system. The interaction of the TNF-α superfamily member, CD27, with its upregulated ligand, CD70, initiates survival signaling specifically in CML cells when BCR::ABL1 is inhibited by TKIs and thus represents an attractive target. Previous studies modulating the expression of CD27 on LSCs in a mouse model of advanced phase CML were encouraging. In our study, we explored the CD70/CD27 axis as a therapeutic target in early-phase disease. Methods: Primitive CD34+ cells from treatment-naïve chronic phase (CP) CML patients were drug-exposed in an in vitro co-culture system; combination drug treatments of the therapeutic anti-CD70 antibody and TKIs, nilotinib, were assessed in our CP CML murine model. Results: The blockade of the CD70/CD27 axis in combination with TKIs did not result in greater LSC elimination than with nilotinib as a single agent in our CP CML models. Nonetheless, there was an observed reduction in CD70+ cells with combination treatment. Conclusion: Further preclinical study of the antibody as an adjunct in CD70-expressing hematological malignancy is perhaps warranted.