(Reuters) - GlaxoSmithKline said its experimental multiple myeloma treatment showed a meaningful response in patients that have run out of three previous treatment options, in a boost for the British drugmaker's cancer drug business.Two doses of belantamab mafodotin helped subdue the disease in adults who had received three prior treatments for multiple myeloma, a cancer of the white blood cells, GSK said on Friday, adding that it intends to seek market approval and submit data from the trial..