The Small Molecule IODVA1 Inhibits the Rac Guanine Nucleotide Exchange Factor Vav3 and Overcomes TKI-Resistance in Ph⁺(BCR-ABL1) B-ALL

Aberrant activation of Rho guanine nucleotide exchange factors (RhoGEFs) is a chief mechanism driving abnormal activation of their RhoGTPase targets in cancer. Thus, small molecule inhibitor of RhoGEF activities can be used as a drug lead to treat leukemia and other malignancies. We have identified an active small molecule, IODVA1, in several xenograft mouse models of cancer including Ras-driven cancers. Here, we used cellular and mouse models of Ph⁺(BCR-ABL1) B-ALL to identify Vav3, the multi-domain tyrosine phosphorylation-dependent RacGEF as the target of the small molecule IODVA1.

IODVA1 specifically reduces the proliferation and survival of p190-BCR-ABL but not of empty vector expressing human CD34⁺ blood cells. IODVA1 binds tightly to recombinant Vav3 (K_d= 400 nM) but not to Rac1 or to the RhoGEF LARG. In Ba/F3 cells expressing p190-BCR-ABL, IODVA1 inhibits Rac activation and signaling within minutes of exposure. The decrease in Rac activity is not due to the activation of p50GAP or RhoGDI1 and is accompanied by a decrease in the activity of the pro-survival effectors PAK, JNK, and 4EBP and an increase in pro-apoptotic BAD activity. IODVA1prevents leukemia-related death and eliminates the leukemia burden in a BCR-ABL-induced murine model with no apparent toxicity. It eradicates leukemic propagating activity assessed by serial transplantation. Most importantly, IODVA1 increases the survival of a mouse model of TKI-resistant p210-BCR-ABL1(T315I) B-ALL better than ABL1-TKI imatinib and eliminates leukemic burden. p210-T315 leukemic mice survive 45 days after treatment has ended. Vav3- and Rac1/Rac2-deficient leukemic cells do not respond to IODVA1 in colony formation assay consistent with Vav3 being IODVA1's target. A mouse model of BCR-ABL leukemia deficient in Vav3 also does not respond to IODVA1. Cells from PDX models representing pediatric ALL patients including Ph+ (BCR-ABL(T315I) were found to be highly sensitive to IODVA1 ex vivo. Based on the described mechanism of action, we propose that IODVA1 is an allosteric inhibitor of Vav3 that holds promise as an anti-leukemic agent.