this invention herein describes a method of facilitating the entry of drugs into cells and tissues at physiologically protected sites at pharmacokinetically useful levels and also a method of targeting drugs to physiologically protected sites in vivo . also provided are drug conjugates with an amino acid or derivative thereof for facilitating such targeted drug delivery . the conjugates and methods of this invention provide an advance over other drug targeting methods known in the prior art , because the invention provides drug concentrations in such physiologically protected sites that can reach therapeutically - effective levels after administration of systemic levels much lower than are currently administered to achieve a therapeutic dose . this technology is appropriate for use with psychotropic , neurotropic , neurological , antibiotic , antibacterial , antimycotic , antiviral , antiproliferative or antineoplastic drugs , agents and conjugates , for rapid and efficient introduction of such agents across , e . g ., the blood - brain barrier . further , the invention provides means for retention and prolonged enzymatic release of such drugs , agents and conjugates comprising the conjugates of the invention , in the brain and central nervous system and other physiologically - protected sites .