FIGURE 2.

The tryptophan depletion/utilisation theory in cancer. (1) Elevated tryptophan transporter expression in tumour cells leads to increased tryptophan absorption, resulting in increased intra‐tumoural tryptophan concentrations. (2) Tryptophan degradation by IDO/TDO enzymes leads to the formation of kynurenine metabolites. (3) As a result, microenvironmental tryptophan concentrations decrease. Both (2) and (3) may promote immune cell death and result in tumour immune escape, which might lead to metastasis. ¹¹ , ⁴¹ , ⁴⁸ , ⁴⁹ , ⁵⁰ , ⁵¹ , ⁵² Image created by Charlise Basson using BioRender (https://biorender.com/).