Covalent conjugation of an alpha amino acid ester to a modulator of the activity of a target intr acellular enzyme or receptor, wherein the ester group of the conjugate is hydrolysable by intracellular carboxylesterase enzyme hCE1 and not by hCE2 or hCE3 to the corresponding acid, leads to selective accumulation of the carboxylic acid hydrolysis product in monocyte/macrophage cell and enables improved or more prolonged enzyme or receptor modulation in those cells relative to the unconjugated modulator.
