Inhibition of Oxidative Phosphorylation Reverses Bone Marrow Hypoxia Visualized in Imageable Syngeneic B-ALL Mouse Model

Hypoxia, or the abnormally low levels of interstitial oxygen, is a defining characteristic of the tumor microenvironment and is known to contribute to cancer’s resistance to chemotherapy. In solid tumors, hypoxia primarily arises from insufficient oxygenated blood supply due to the sparse or abnormal tumor vasculature, despite low oxygen usage typical of glycolytic metabolism in tumors. However, in acute leukemias, intracellular hypoxia markers, such as increased pimonidazole adduct staining and HIF-1α stabilization, are observed in advanced leukemic bone marrow (BM) despite enhanced vasculogenesis in the BM.

In our study, we employed intravital fast scanning two-photon phosphorescence lifetime imaging microscopy (FaST-PLIM) in a BCR-ABL B-ALL mouse model to measure extracellular oxygen concentrations (pO2) in leukemic BM and examined the relationship between these oxygen levels, intracellular hypoxia, vascular markers, and local leukemia burden. Initially, we detected a temporary increase in BM pO2 during the early stages of the disease, when leukemia burden in the BM was moderate, which corresponded with the expansion of the blood vessel network within the BM. Despite this, we also noted increased formation of intracellular pimonidazole adducts in leukemic BM at the same stage.

As the disease progressed and leukemia cells increasingly populated the BM, we observed a significant drop in extracellular pO2 and a further rise in intracellular hypoxia. Remarkably, treating leukemic mice with IACS-010759, a pharmacological inhibitor of mitochondrial Complex I, led to a substantial increase in BM pO2 in advanced B-ALL and reduced intracellular hypoxia, as indicated by pimonidazole staining. High oxygen consumption rates by B-ALL cells were confirmed through Seahorse assays, including those conducted on ex vivo cells.

Our findings indicate that the expansion of B-ALL in the BM is linked to heightened oxidative phosphorylation (OxPhos), which contributes to metabolic BM hypoxia despite increased vascularization. Targeting mitochondrial respiration may offer a new strategy to counteract BM hypoxia in B-ALL and potentially address tumor hypoxia in other OxPhos-dependent cancers.