Serum albumin–protamine conjugate for biocompatible platform for targeted delivery of therapeutic macromolecules

A well-defined, one-to-one conjugate between human serum albumin (HSA) and protamine was synthesized and characterized as a biocompatible carrier for macromolecules. In circulation, the conjugate will camouflage drug molecules upon complex formation, while liberating free drug at the desired location using a triggering mechanism. The N-terminus of protamine was thiolated and conjugated with the unpaired Cysteine-34 of HSA, and was purified by ion-exchange chromatography. The molecular weight of the conjugate was 70.8 kDa, confirming one-to-one conjugation between HSA (66.6 KDa) and protamine (4200 Da). Superimposed fluorescence spectra of native HSA and HSA-protamine conjugate indicated no conformational change around the Trp-214. The conjugate had marked reduction in hemolytic and cytotoxic properties compared to protamine. When therapeutic potential was tested using tissue plasminogen activator as a model drug, HSA-protamine conjugate suppressed the enzymatic activity by 65%, which was fully recovered by a triggering agent, heparin. The construct showed binding characteristics with activated platelets upon conjugation with a targeting peptide, demonstrating flexibility to introduce suitable homing moiety on the surface. The camouflaged construct retained triggered release property in human plasma condition. Overall, the conjugate has a good potential to serve as a biocompatible platform for macromolecular drugs. © 2013 Wiley Periodicals, Inc.