Gelatin as a delivery vehicle for the controlled release of bioactive molecules

Abstract

Gelatin is a commonly used natural polymer which is derived from collagen. The isoelectric point of gelatin can be modified during the fabrication process to yield either a negatively charged acidic gelatin, or a positively charged basic gelatin at physiological pH. This theoretically allows electrostatic interactions to take place between a charged biomolecule and gelatin of the opposite charge, forming polyion complexes. Various forms of gelatin carrier matrices can be fabricated for controlled-release studies, and characterization studies have been performed which show that gelatin carriers are able to sorb charged biomolecules such as proteins and plasmid DNA through polyion complexation. The crosslinking density of gelatin hydrogels has been shown to affect their degradation rate in vivo, and the rate of biomolecule release from gelatin carriers has been shown to have a similar profile, suggesting that complexed gelatin/biomolecule fragments are released by enzymatic degradation of the carrier in vivo. This review will emphasize how biomolecules released from gelatin controlled-release systems are able to retain their biological activity, allowing for their use in tissue engineering, therapeutic angiogenesis, gene therapy, and drug delivery applications.

Introduction

For the last few decades, advances in biomaterials science have continued to expand the scope of the field from the original paradigm of designing nonviable materials used in medical devices [1], to a more general approach of studying the physical and biological interactions between any material and the biological environment which surrounds it. By encompassing aspects of biology, chemistry, medicine, and materials science, biomaterials research has been used to address a wide diversity of issues ranging from artificial organ design in tissue engineering, to the fabrication of DNA microarrays in genomics.

More specifically, innovations in biotechnology have helped increase the cost efficiency of producing numerous peptides, proteins, and oligo- and polynucleotides, making them attractive candidates for sustained release applications from both a clinical and financial point of view. Thus, research in biomaterials has also been applied to the classic pharmaceutical challenge of designing systems for the sustained release of bioactive substances. In an effort to develop these controlled-release systems with reproducible and predictable release kinetics, a variety of methods have arisen to address this requirement such as: diffusion-controlled, water penetration-controlled, chemically-controlled, responsive, and particulate systems [2].

Although an extraordinary amount of synthetic (i.e. poly(glycolic acid), poly(l-lactic acid), etc.) and natural materials (i.e. collagen, alginate, fibrin, etc.) have been used as biomaterials in controlled-release applications [3], the purpose of this review is to focus on the contributions of gelatin in this area of research. As will be seen, depending on the fabrication method, variations in the electrical and physical properties of gelatin-based controlled-release systems can be achieved. It is this flexibility in processing that has allowed gelatin-based controlled-release systems to find diverse applications in fields ranging from tissue engineering, to drug delivery and gene therapy.

The significance of this ability to tailor the electrical nature of gelatin relates to the concept of sustained release of proteins from polymer matrices. During the fabrication of a protein-polymer release system, harsh processing conditions may irreversibly denature proteins through exposure to heating, organic solutions, or sonication [4], [5], [6]. This post-fabrication loss of bioactivity has been a significant challenge for protein release technology in the past.

In order to address this issue, mild formulation conditions have been used to fabricate a multitude of polymer hydrogels as matrices for protein release, with the intent of maintaining the bioactivity of a protein drug during production. By incorporating the protein within an inert polymer delivery vehicle, the therapeutic agent is protected against enzymatic degradation and immunologic neutralization in vivo, thus allowing for prolonged release of the protein.

However, the release rate from such hydrogels typically relies on diffusion of the protein through aqueous channels. In general, this diffusion-controlled mechanism reduces the potential for sustained release, and biodegradation of the polymer scaffold decreases the construct's extended therapeutic effect even further [7].

By taking into account these aforementioned characteristics of typical diffusion-controlled designs, a shift away from the paradigm of utilizing a scaffold which is inert towards the protein should be considered. In other words, to achieve a degree of immobilization, molecular interactions between the carrier material and a therapeutic agent should be encouraged so that effective protein release kinetics can be realized [8].

One strategy which has been used to produce such interactions is polyion complexation. Polyion complexes are formed by electrostatic interactions between positively or negatively charged, high molecular weight electrolytes and their oppositely charged partners. Such interactions are quite stable since it would be statistically unlikely for all the ionic interactions between charged residues on the molecules to dissociate simultaneously. As a result, the secondary bonds forming polyion complexes are not dissociated as easily as the bonds between low molecular weight electrolytes [9].

In theory, polyion complexation can be used for the sustained release of numerous charged therapeutic agents such as proteins, polysaccharides, and oligo- and polynucleotides. Two mechanisms of polyion release from a biodegradable, charged polymeric carrier have been described [7]. Initially, a positively charged therapeutic agent is bound to the negatively charged polymeric chains of the carrier through electrostatic interactions. In the first instance, release of the agent from the carrier complex occurs because of an environmental change, such as in high ionic strength conditions. The second mechanism of release is degradation of the polymer carrier itself. However, because biodegradation of the carrier matrix would be the more likely mechanism of release in vivo, designers can control drug release kinetics by adjusting the rate of polymeric carrier degradation.

Considering the factors discussed involving polyion complexation, an ideal polymeric carrier designed for the controlled release of polyion therapeutic agents should allow control over its biodegradability and overall electrical charge, have a proven record of clinical safety, as well as allow for fabrication conditions that preserve the bioactivity of the therapeutic agent to be delivered in vivo.