Targeting IRAK1/IRAK4 Signaling in Waldenstrom's Macroglobulinemia

Background: MYD88 L265P somatic mutations are highly prevalent in Waldenström's macroglobulinemia (WM) (NEJM 367(9):826-33). MYD88 L265P activates multiple downstream signaling pathways including BTK and IRAK1/IRAK4 that support malignant cell growth and survival (Nature 470(7332):115-9; Blood 122(7):1222-32). Ibrutinib targets BTK, and shows high overall and major clinical response rates, though no complete responses are observed indicating alternative survival signaling.

Methodology: Phospho-flow analysis of IRAK1, IRAK4, and BTK was performed in primary WM cells taken from untreated WM patients, and those on ibrutinib therapy. IRAK1 or IRAK4 knockdown experiments were performed using lentiviral shRNA transduction. Immunoprecipitation, western blot and phospho-flow studies were used to detect protein expression and phosphorylation in WM cells. Cell survival following IRAK4 or IRAK1 knockdown, ibrutinib and/or IRAK4/IRAK1 inhibitor (EMD Millipore) treatment was assessed by Annexin V staining, AlamarBlue® Cell Viability Assay or CellTiter-Glo® Luminescent Cell Viability Assays. 

Results: Phospho-flow analysis of bone marrow lymphoplasmacytic cells taken from WM patients following > 6 months of continued ibrutinib treatment demonstrated highly active IRAK1 and IRAK4, but not BTK. These findings prompted us to dissect the relative impact of IRAK1 and IRAK4 in supporting WM cell survival. Using lentiviral transduction, we identified shRNAs that produced similar levels of protein reduction by western blot analysis for both IRAK1 and IRAK4. Compared to scrambled control vector, knockdown of IRAK1 or IRAK4 both produced decreased tumor cell survival in MYD88 mutated BCWM.1 and MWCL-1 cells. More pronounced apoptosis, as well as sustained reduction in tumor cell growth occurred following knockdown of IRAK1 versus IRAK4. Treatment of primary WM cells taken from untreated patients, patients on ibrutinib therapy, as well as MYD88 mutated WM cells lines with ibrutinib and a toolbox IRAK4/IRAK1 inhibitor resulted in more robust reductions in NFkB signaling, and at least additive tumor cell killing versus either agent alone.

Conclusions: MYD88 L265P mutated WM cells show greater dependence on IRAK1 versus IRAK4 directed signaling. IRAK1/IRAK4 signaling may contribute to persistent WM cell survival following ibrutinib treatment. Combined BTK and IRAK inhibition leads to augmented blockade of NFKB signaling and enhanced WM cell killing. The studies provide a framework for the development and investigation of IRAK inhibitors, alone and in combination with ibrutinib in WM patients.