GCS-100 reduces viability and number of primary myeloma cells and overcomes stromal cell protection. Primary myeloma cells (1 × 10⁵ cells/mL) were incubated for 48 hours with GCS-100 (0-700 μg/mL) for 48 hours with or without human stromal cells (HS-5)/IL-6 (10 ng/mL). Viability (A) and cell number (B) were measured by PCA-96 ViaCount Assay. Bar graphs show the mean (± SEM) results from 4 different patient primary cell experiments. *Significant difference between cells incubated with or without HS-5 cells at the same concentration of GCS-100. ● represents a significant difference compared with non–GCS-100–exposed cells.