PDCD1 and melanoma: 8). Among the most extensively studied enzymes targeting this pathway is IDO1. When used in combination with immune checkpoint inhibitors, IDO1 inhibitors have shown potential to enhance anti-tumour immune responses in animal models (Ref. 9). However, a phase III therapeutic trial using the PD-1 checkpoint inhibitor pembrolizumab in conjunction with the IDO1 inhibitor epacadostat to treat malignant melanoma failed (Ref. 10).