e18535 Background: To individualize therapy for relapsed/refractory AML patients, optimal in-vitro culture conditions to support primary leukemic cells are essential for drug sensitivity testing. Our lab has validated a high throughput chemosensitivity assay with primary AML cells maintained by growth factors (cytokines); however, growth factors have not been shown to support long-term assays of primary AML cells. Stromal cells of the tumor microenvironment are crucial to maintain normal hematopoiesis and leukemic cells and have been shown to support long term in-vitro expansion of primary AML cells. However, there is little information characterizing these growth conditions. The aim of this study was to compare long-term proliferation and phenotypes of primary AML cells with growth factors or stromal support to best determine their utility as a platform for drug sensitivity testing in functional assays. Methods: Patient-derived AML cells were cultured in 96-well plates in: 1) cell culture medium only 2) Human HS5 or HS27 stromal cells 3) HS5 or HS27 stroma-conditioned media or 4) cytokine cocktail. Viability readout by Guava ViaCount and leukemic cell surface phenotypes by fluorescently-conjugated antibodies were performed weekly over 3 weeks. Results: Primary AML cells cultured with only cytokines maintained proliferation at 3 weeks. In comparison, AML cells cultured in HS5 stroma-conditioned medium also maintained proliferation at a similar rate at 3 weeks, while co-culture with HS5 stromal cells demonstrated significantly higher proliferation. Leukemic immunophenotypes were maintained for all growth conditions over 3 weeks. Conclusions: Contrary to known data, primary AML cells with cytokines continued to expand at 3 weeks, at a similar rate to HS5 stroma-conditioned medium, a finding that has not been reported. Consistent with previous studies, we confirmed that stromal cells such as HS5 can provide long-term in-vitro expansion of primary AML cells, which cannot be substituted by stroma-conditioned medium. The ability to maintain long-term expansion of primary AML cells by both cytokines and stromal cells sets up a platform for a direct comparison of high throughput drug sensitivity testing under these growth conditions.