PAD4 Inhibition + Citrulline

Citrulline supplementation can restore T cell metabolic function, but a metabolically healthy T cell is useless if it is physically excluded from the tumor core by NETs. PADI4 inhibition prevents NETosis, dissolving the physical barrier. T cells, now metabolically fueled by citrulline, can physically infiltrate the tumor bed. The "cold" tumor (immune-excluded) is converted into a "hot" tumor (infiltrated), where the restored cytotoxicity of the T cells can be directed against tumor antigens.

A high Myeloid-Derived Suppressor Cells (MDSC) burden creates an immunosuppressive cytokine environment (TGF-β, IL-10) that dampens T cell function, even when arginine levels are restored.

PADI4 activity is required for the recruitment and accumulation of PMN-MDSCs. Inhibition results in a significant reduction of the MDSC population within the TME.

The reduction in MDSCs reduces the total "arginase load" of the TME, allowing the supplemental citrulline (and any endogenous arginine) to persist longer. Furthermore, the removal of MDSC-derived suppressive cytokines allows the T cells (rescued by citrulline) to maintain a Th1/cytotoxic phenotype rather than drifting toward exhaustion or regulatory phenotypes.

Natural PAD4 Inhibitors

Compound/Extract	Source Plant	Chemical Class	IC50 (μM or μg/mL)	Mechanism of Action	Bioavailability/Stability	Key Preclinical Models	Safety/Toxicity Data
Salvianolic acid A	Salvia miltiorrhiza	Phenolic acid	33.52 μM	Mixed inhibition, hydrogen bonding	Not well characterized	In vitro PAD4 inhibition, cancer models	Low toxicity expected
Berberine	Coptis chinensis Franch	Alkaloid	45.07 μM	Inhibits PADI4 expression, modulates macrophage function	Moderate bioavailability	Lung cancer prevention, RA models	Safe, mild GI side effects
Ephedra Herba extracts	Ephedra spp.	Mixed (alkaloids, flavonoids)	29.11–41.36 μg/mL	Not fully elucidated	Not well characterized	RA, liver failure, antiviral models	Generally safe, needs further study
Cinnamomi ramulus extract	Cinnamomum cassia	Not specified	< 5 μg/mL	Not specified	Not well characterized	PAD4 inhibition assays	Not well documented
Pyrroloquinoline quinone (PQQ)	Synthetic/natural supplement	Quinone	< 4 μM	Not specified	Well absorbed	PAD4 inhibition assays	Safe, used as supplement
Hydroxyanthroquinone, Thymol, Menthol, Salicylic Acid	Computational screen	Various	Not determined	Strong binding to PADI4 active site	Not studied	In silico modeling	Predicted low toxicity