Benzoic Acid Derivatives Improve Plasma Stability of Diester Butyrophilin Ligand Prodrugs

The potent butyrophilin ligand, (<i>E</i>)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), is a potential cancer immunotherapy agent, but it lacks plasma stability and membrane permeability. Aryl phosphonamidate prodrugs of a key HMBPP analog have improved plasma stability but poor cellular uptake, while aryl phosphonester prodrugs have improved uptake but lack plasma stability. Here, tuning the benzoic acid substructure of a phosphonester prodrug was explored. Twenty-one aryl phosphonester derivatives were prepared in allylic alcohol (<b>8a-k</b>) and allylic acetate (<b>9a-k</b>) forms. Testing revealed that this strategy can provide compounds with high potency for expansion of γ9δ2 T cells (<b>8d</b>, EC₅₀ = 0.86 nM) and interferon γ production in response to loaded K562 cells (<b>8d</b>, EC₅₀ = 2.3 nM). Importantly, these compounds display improved plasma stability (130-fold range; <b>8d</b>, <i>t</i>_1/2 > 24 h), showing the importance of the benzoic acid position for plasma versus cellular metabolism. These findings enable next-generation prodrugs with improved stability and potency.