Quinoid dihydropteridine reductase (QDPR) deficiency, common in PDAC, disrupts tetrahydrobiopterin homeostasis, reducing tetrahydrobiopterin to <10 nM and driving MDSC accumulation; 20 nM tetrahydrobiopterin supplementation in QDPR-knockout mice reverses MDSC infiltration and restores anti-PD-1 efficacy, with patient-derived PDAC organoids confirming tetrahydrobiopterin levels correlate with CD8+ T cell infiltration (147), highlighting metabolite rescue as a precision strategy for tumor-specific metabolic defects. Here, CD8A is linked to neoplasm.