Pristimerin is selectively cytotoxic to primary myeloma cells in mixed-lineage bone marrow cultures from myeloma patients. Flow cytometric analyses of primary bone marrow samples, obtained from multiple myeloma patients, for viability or apoptosis (annexin V binding, horizontal axis) of CD138⁺ (myeloma) and CD138⁻ (hematopoietic progenitor) fractions (vertical axis), after in vitro drug exposure and culture. (A) Examples of 5 myeloma patient bone marrow samples, each treated separately with either vehicle (left column), pristimerin 100 nM (middle), or pristimerin 200 nM (right column) for 72 hours, showing preferential killing of CD138⁺ tumor cells by pristimerin. Pristimerin induces preferential loss of viable primary tumor cells (top left quadrants, CD138⁺, annexin V⁻) and accumulation of CD138⁻ annexin V⁺ cells (bottom right quadrants). Following induction of apoptosis, myeloma cells shed surface CD138 to become annexin V⁺, CD138⁻,³⁵  and are detected in the right bottom quadrant. (B) Response of primary bone marrow cells from a patient with advanced myeloma to pristimerin 100 nM, dexamethasone 200 nM, or melphalan 1 μM, demonstrating potent selective antimyeloma in vitro efficacy of pristimerin compared with standard antimyeloma agents.