PDCD1 and colorectal carcinoma: Our study described the inhibitory effect of tanshinone IIA sulfonate (STS), a sulfonate derived from tanshinone IIA (TSN), on IDO1/TDO2 and tested the antitumor activity of STS combined with anti-PD1 to treat CRC in vivo, indicating that STS could prevent the progression of CRC and improve the efficacy of anti-PD1 therapy.