Several important pharmacological features can be integrated into injected drugs to enhance their therapeutic efficacy following administration. Short-lived peptide/protein drugs should be converted into long-lived species in vivo to avoid multiple injections. Circulating levels of anticancer agents need to be maintained within a narrow therapeutic range for prolonged period. Water-insoluble drugs must be turned into soluble species and blood-brain barrier-impermeable agents need to be modified to cross it following peripheral administrations. The derivatization requiring for achieving those desirable pharmacological features typically result in biologically/pharmacologically inactive products, unless those derivatizations can be carried out in a reversible fashion.