Human Serum Albumin as a Pharmaceutical Excipient

INTRODUCTION

Human serum albumin (HSA) is one of the most widely used and characterized proteins in the pharmaceutical field. It occurs naturally in the body, as a plasma protein, with a concentration of 50 mg/mL. At this concentration, HSA regulates the colloidal osmotic pressure of blood. HSA is also responsible for transporting endogenous and exogenous compounds, which might be toxic in the unbound state, but non-toxic as albumin-bound. Human serum albumin purified from plasma is used for therapeutic applications, as a plasma expander, in situations involving severe blood loss. HSA is also widely used as an excipient, especially for biotechnology products. While the albumin used in marketed products is derived from plasma, recombinant versions of the protein are being investigated. Recombinant albumin can also alleviate any theoretical concerns of disease transmissivity associated with the human plasma-derived protein. This article provides a brief review of the use of albumin as a pharmaceutical excipient, and provides an update on the development of recombinant albumin.

USE OF HSA AS AN EXCIPIENT

Human serum albumin is a 66-kD protein, with no glycosylation. The protein has molecular dimensions of 8 nm X 3.8 nm, and a half-life of 15 to 20 days. Due to its high concentration in plasma, HSA is not associated to significant extents with safety or immunogenicity concerns. A 5% albumin solution has an osmolarity of 265 mOsm/kg. Human serum albumin is a remarkably stable protein - it is the only therapeutic protein that is stable as a liquid at room temperature over the shelf life of the product. This is primarily due to the presence of 17 disulfide linkages present in the molecule. The intrinsic stability of the protein also allows it to be heated at 60�C for 10 hours to facilitate virus inactivation during manufacturing. This process has demonstrated elimination of both lipid-enveloped and certain non-lipid-enveloped viruses in validation experiments. The stability of albumin makes its storage and handling easier than typical proteins, thus lending itself well toward the use as an excipient.

Due to its established safety profile and unique properties, HSA is frequently used as a stabilizer for proteins. The protein has amphiphilic properties, which makes it suitable as an additive to inhibit adsorption of the active protein to the container, via competitive adsorption mechanisms. The surface-active character of the protein also makes it suitable for use as a surfactant to prevent protein aggregation. HSA also has a high glass transition temperature, which in combination with its amphiphilic nature, makes it an ideal excipient for cryoprotection. For some proteins, the dual functionality (surfactant and cryoprotectant) results in better cryoprotection for albumin than disaccharides, as was observed by Liu, for Lactate dehydrogenase.1 Table 1 lists representative commercial protein products that contain Albumin as an excipient.

Albumin is also being used as a carrier for microparticles and nanoparticles for sustained-release injectable drugs. A nanoparticulate formulation of paclitaxel containing albumin as the carrier was recently approved by the FDA. A number of researchers have also used albumin for sustained release of small molecules and proteins. Albumin's capacity to adsorb hydrophobic molecules makes it a unique carrier for controlled release because the drug gets released via desorption without significant burst effects. Albumin's adsorption capacity has also been exploited in development of magnetic microparticles. Such particles were used for targeted delivery of chemotherapeutic agents, such as doxorubicin. The particles consisted of albumin for binding of drug and iron for magnetic behavior to facilitate targeting.2 Albumin microspheres have also been used in diagnostic applications to detect intravascular susceptibility.3

In recent years, albumin's long plasma circulation characteristics have been exploited to develop albumin-conjugated protein drugs that have longer half-lives as compared to the unconjugated protein. Albumin-fusion proteins are produced via recombinant techniques, and this concept has been used to extend the half-lives of a number of proteins including interferon-a4, interleukin-2, and G-CSF.5

RECOMBINANT ALBUMIN

Although there has been no case of disease transmission for the use of HSA, a theoretical or perceived risk exists, due to which recombinant human albumin is currently being explored.6 While this recombinant version is currently being explored as a therapeutic, its use as an excipient may be a logical progression, if the product gets approved.

A yeast-derived recombinant version was tested by Bosse and co-workers in a Phase I comparability study with human serum albumin.7 The two proteins were compared side-by-side for both intravenous and intramuscular injections, involving more than 500 volunteers. No serious or potentially allergic events, or immunological response were reported with either product in the IV study. Serum albumin, colloid osmotic pressure changes, and hematocrit ratio were also similar. The authors concluded that rHA and HSA exhibited similar safety, tolerability, and pharmacokinetic/pharmacodynamic profiles, with no evidence of any immunological response. Tarelli and co-workers investigated the use of recombinant albumin as a cryoprotectant for thyroid-stimulating hormone (TSH), interleukin 15 (IL-15), and granulocyte colony-stimulating factor (G-CSF).8 It was observed that the recombinant albumin was equivalent in its functionality to HSA, for stabilization of the proteins as well as binding of fatty acids.

SUMMARY

Albumin is a well characterized protein and serves important needs as a therapeutic, diagnostic agent, as well as an excipient. While use of albumin as an excipient has met some resistance due to perceived risk of disease transmission, recombinant albumin is being developed to address any such concerns. Recombinant albumin may also serve as a useful case study for follow-on biologics.9 However, use of recombinant albumin as an excipient, would depend on the efficiency of the manufacturing process, to allow for reasonable cost of goods.

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REFERENCES

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