Treatment of Patients with Refractory AML with SU5416 Downregulates VEGF-A, STAT5 and Akt Expression in Leukemic Blasts.

Autocrine or paracrine activation of receptor tyrosine kinases are important pathogenetic factors in AML. Signal transduction via c-kit and FLT3 increase proliferation and apoptosis resistance of leukemic blasts. Additionally bone marrow angiogenesis plays a significant role in disease progression.

This provided the rationale for a multicenter clinical trial in patients with refractory AML with SU5416, a small molecule kinase inhibitor which blocks phosphorylation of c-kit, FLT3, VEGFR-1, VEGFR-2 (KDR) and VEGFR-3.

In the study presented here, we analysed expression levels of VEGF and the downstream signal transduction intermediates STAT5 and Akt in patients before and during treatment with SU5416.

The levels of VEGF mRNA expression were investigated in peripheral blood leukemic blasts by means of quantitative Real-Time PCR. 74% (14 of 19) of analyzed patients had rapid and persistent down regulation of VEGF during therapy. Patients initially expressing high VEGF-levels had a stronger down regulation and a higher clinical response rate (mean 865-fold, n=9) than patients initially expressing low VEGF-levels (mean 4-fold, n=8). These results suggest that downregulation of high pretherapeutic VEGF expression by SU5416 treatment may serve as an early surrogate marker of therapeutic response to anti-angiogenetic therapy.

Additionally, protein expression of STAT5 and Akt was assessed by Western blotting. Incubation of the leukemic cell line M-07e in-vitro with SU5416 using concentrations which are achievable in patients was used as a model system. In the AML patient samples parallel downregulation of both STAT5 and Akt was observed in several cases (STAT5 in 4 of 15, Akt in 3 of 6 examined patients). Decreased intracellular levels of STAT5 and Akt were also found in the line M-07e after incubation with SU5416 in vitro.

In summary, our data provide evidence for suppression of expression of VEGF and of the key signal transduction intermediates STAT5 and Akt in AML blasts in-vivo during treatment with SU5416.