5-A-RU-PABC-Val-Cit-Fmoc is the proagent of 5-A-RU. 5-A-RU, a precursor of bacterial Riboflavin, is a mucosal-associated invariant T (MAIT) cells activator. 5-A-RU forms potent MAIT-activating antigens via non-enzymatic reactions with small molecules, such as glyoxal and methylglyoxal, which are derived from other metabolic pathways.