Revisiting IDO and its value as a predictive marker for anti-PD-1 resistance

Botticelli et al. proposed the activity of indoleamine-2,3-dioxygenase 1 (IDO) as a potential mechanism and predictive marker for primary resistance against anti-PD-1 treatment in the context of non-small cell lung cancer. However, there are a few points for the authors to address in order to strengthen their claims. First, there are many enzymes that modulate the kynurenine to tryptophan ratio, thereby calling into question their use of the ratio as a proxy for IDO activity. Second, the authors could compare IDO to other proposed markers in the literature, providing a better understanding of its predictive value.