The need to maintain substances in blood is one of the main goals in therapeutic and diagnostic applications, which are often hampered by the short in vivo half-life of administered compounds. Many factors are involved in the removal of substances from the circulation: proteins and peptides are rapidly eliminated by proteolytic degradation, renal filtration, and/or immunogenic and antigenic reactions, while renal excretion and inactivation reactions by blood enzymes (especially the hydrolytic action of esterases) are the main causes for the brief plasma residence time of low-molecular mass (MW) 1 nonprotein drugs.