Abstract
Abstract 868
Interactions between leukemia cells and the bone marrow (BM) microenvironment are known to promote leukemia cell survival and confer resistance to drugs commonly used in the management of this disease. We investigated the protective role of hypoxia in the BM microenvironment. We observed a marked expansion of hypoxic niches in the bone marrow of immunodeficient mice engrafted with the acute lymphoblastic leukemia (ALL) cell line Nalm6 and with primary ALL cells, as detected by the reductive 2-nitroimidazole compound pimonidazole (PIM), which forms stable adducts in hypoxic regions. We further demonstrated induction of the chemokine receptor CXCR4, and of the enzyme carbonic anhydrase 9 (CAIX), both targets of Hypoxia-Inducible Factor 1α (HIF-1a), in hypoxic areas of BM from mice harboring the Nalm6 xenografts. Furthermore, we used a CML blast crisis model to evaluate the time course of hypoxia expansion in the BM. In C57Bl6/J mice engrafted with murine HSCs co-expressing BCR/ABL and Nup98 we observed a time-dependent increase in PIM positive areas which coincided with the presence of GFP positive cells. In line with these findings, HIF-1α was highly expressed in BM biopsies from newly diagnosed ALL patients (n=15) but was significantly reduced when the patients achieved complete remission (CR). Culture under hypoxic conditions (1% O2) conferred resistance of pre-B ALL cells REH, Nalm-6, and of AML OCI-AML3 leukemic cells against several chemotherapeutic agents including vincristine, methotrexate and idarubicin (% of Annexin V(+) cells at 21% vs 1%O2; REH plus 1ng/ml vincristine:88.5+/−2.3 vs 18.1+/−10.3; REH plus 0.25uM etoposide: 86.4 +/−7 vs 14.4+/−8.6; Nalm6 plus 10ng/ml methotrexate: 33.4+/−2.7 vs 4.6+/−1; OCI-AML3 plus 50ng/ml idarubicin: 28.45+/−5 vs 12.76+/−1) . Taken together, these results provide rationale for examining the potential of hypoxia-activated pro-drugs to eliminate leukemia progenitor cells within hypoxic niches. To this end, we tested the hypoxia-activated prodrug PR104, a dinitrobenzamide nitrogen mustard that is reduced to its active metabolites under hypoxic conditions (Patterson et al., Clin Can Res 2007). In vitro, PR-104 induced cell death in three different leukemia cell lines (Nalm6, REH and the AML line Molm13) selectively under hypoxic (pO2 1%) conditions. The anti-leukemic efficacy of PR-104 as a single agent was next examined in several in vivo leukemia models. Administration of PR-104 prolonged survival and decreased leukemia burden of 1) NOD/Scid/IL2Rg-KO (NOG) mice injected with cells from primary refractory FLT3-mutated AML; 2) NOG mice injected with leukemic cells from an infant with MLL-rearranged B-lineage ALL; and 3) NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice injected with Nalm6-luciferase ALL cells. Next, we evaluated anti-tumor effects of PR-104 at clinically relevant doses (200, 100 and 50 mg/kg) and at maximal tolerated dose (550 mg/kg) in two ALL xenograft models (a T-lineage ALL and a B-cell precursor ALL) (Figure 1, studies supported by NCI NO1CM42216 and by PPTP contract NO1-CM91001-03). Compared to vehicle control, PR-104 significantly delayed progression of the T-ALL xenografts at all doses tested and at three of the four doses (550, 200 and 100 mg/kg) of B-cell pre-ALL, which resulted in significantly increased event-free survival (EFS) of mice in the treatment groups.
Altogether, these findings strongly suggest that targeting hypoxia is feasible. If successful, this approach may significantly impact leukemia therapy and ultimately improve patient survival. This concept is currently being tested in an ongoing Phase I clinical trial of PR-104 in relapsed/refractory AML patients.
Kaplan-Meier EFS plots of PR-104 against T-ALL (A) and B-ALL (B) in vivo.
Wilson:PROACTA: Equity Ownership. Konopleva:PROACTA: PI on clin trial funded by Proacta.
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