Inhibition of Class I PI3K Isoforms Restores the Sensitivity of Acute Myelogenous Leukemia Cells to Multi-Tyrosine Kinase Inhibitors in the Bone Marrow Microenvironment.

Abstract 1734

Poster Board I-760

The bone marrow microenvironment can protect leukemic cells from cytotoxic agents, an effect that has been attributed to both soluble- and contact-mediated factors. Recently, we showed that 3 AML cell lines (MV4-11, THP1 and U937) became resistant to 3 promising TKIs (sorafenib, sunitinib and midostaurin) in a coculture model with mesenchymal cells (MSCs) (Shimada A. et al. Proceedings ASH 2008). In the current investigation, we show that 9 primary AML blast samples (including 7 with wild-type FLT3 and 2 with FLT3-interal tandem duplications) were less sensitive to 10 μM sorafenib in coculture with MSCs vs. suspension culture [mean percentage of total annexin V positive cells, 25% (range, 17-32%) vs. 36% (range, 31-47%)]. We explored potential mechanisms of resistance in AML cell lines. No alterations in drug intracellular accumulation, FLT3 receptor expression/phosphorylation, or acquired mutations in the FLT3, NRAS or KRAS genes were observed in AML cells in coculture with MSCs. However, we found that contact with AML cells enhanced the secretion of IL-6, IL-8, and VCAM-1 by MSCs. In parallel, multiple signaling pathways were activated in AML cells by MSCs including PI3K/AKT, STAT, ERK, and NFKB. Sorafenib-induced inhibition of phosphoproteins in the PI3K/AKT and ERK pathways was decreased in coculture vs. suspension culture. We evaluated PI103, a selective and potent inhibitor of class I PI3K isoforms, as a potential strategy to overcome TKI resistance in the microenvironment. In 3 AML cell lines, the antileukemic activity of PI103 in combination with each of the 3 TKIs was enhanced to a greater extent in coculture than in suspension culture, which was associated with more effective inhibition of both phospho-AKT and phospho-p70 S6 kinase. In primary AML blast samples, the combination of 1.0 μM PI103 + 10.0 μM sorafenib induced more apoptosis [mean percentage of total annexin V positive cells, 38% (range, 32-47%)] than either agent alone [sorafenib, 25% (16-32%); PI103, 24% (20-36%)]. In conclusion, inhibition of PI3K/AKT signaling represents a potential treatment strategy to restore TKI sensitivity in the microenvironment.