This is a Phase I/II, open-label, 2 arm study to evaluate the safety and clinical activity of GSK2879552 alone, or in combination with azacitidine in subjects with MDS. The study consisted of 2 parts. The objective of Part 1 is to determine the recommended phase 2 dose (RP2D) of GSK2879552 administered alone and in combination with azacitidine in adult subjects with high risk MDS previously treated with HMA. The objective of Part 2 is to evaluate clinical activity after treatment with GSK2879552, alone or in combination with azacitidine, in adult subjects with high risk MDS previously treated with HMA.