Enhancing the intestinal absorption of molecules containing the polar guanidino functionality: A double-targeted prodrug approach

A prodrug strategy was applied to guanidino-containing analogues to increase oral absorption via hPEPT1 and hVACVase. L-Valine, L-isoleucine, and L-phenylalanine esters of [3-(hydroxymethyl)-phenyl]guanidine (3-HPG) were synthesized and evaluated for transport and activation. In HeLa/ hPEPT1 cells, Val-3-HPG and Ile-3-HPG exhibited high affinity to hPEPT1 (IC₅₀: 0.65 and 0.63 mM, respectively), and all three L-amino acid esters showed higher uptake (2.6- to 9-fold) than the parent compound 3-HPG. Val-3-HPG and Ile-3-HPG demonstrated remarkable Caco-2 permeability enhancement, and Val-3-HPG exhibited comparable permeability to valacyclovir. In rat perfusion studies, Val-3-HPG and Ile-3-HPG permeabilities were significantly higher than 3-HPG and exceeded/matched the high-permeability standard metoprolol, respectively. All the L-amino acid 3-HPG esters were effectively activated in HeLa and Caco-2 cell homogenates and were found to be good substrates of hVACVase (k _cat/K_m in mM^-1 · s^-1: Val-3-HPG, 3370; Ile-3-HPG, 1580; Phe-3-HPG, 1660). In conclusion, a prodrug strategy is effective at increasing the intestinal permeability of polar guanidino analogues via targeting hPEPT1 for transport and hVACVase for activation.