TH9402 is a dibrominated rhodamine derivative and potent photosensitizer and useful in photodynamic therapy. TH9402 preferentially localizes in mitochondria and when exposed to 500- to 600-nm wavelength visible light delivered through the Theralux device, it becomes highly cytotoxic through oxidative damage. TH9402 eradicates multiple myeloma (MM) and BC cell lines, while sparing more than 50% of normal pluripotential blood stem cells from healthy volunteers. TH9402 offers an excellent potential as an ex vivo photodynamic purging agent for autologous transplantation in multiple myeloma and breast cancer treatment. TH9402 photodynamic-cell-therapy process conditions recognized as safe for normal human haematopoietic stem cells and progenitors demonstrated the efficacy of the purging procedure on various leukaemias.