Protein release from matrices

MECHANISMS AND MODELS FOR PROTEIN RELEASE FROM MATRICES... 

This release is frequently linear with respect to the square root of time (Figure 9.9b), consistent with a diffusive release mechanism (Equation 9-14). To account for the complex structure of the composite matrix material, the diffusion equation incorporates an effective diffusion coefficient for the protein in the polymer matrix (see the section below). This effective diffusion coefficient, which is typically much lower than the diffusion coefficient of the protein in water, provides a quantitative measure of the rate of protein release, decreasing as the rate of protein release from the matrix decreases. 

For a protein incorporated in a matrix, the rate of release (i.e., the effective diffusion coefficient and Ft) depends on the molecular weight of the protein, the size of the dispersed particles, the loading, and the molecular weight of the polymer. For example, matrices of EVAc containing dispersed particles of bovine serum albumin (BSA) release protein for an extended period (Figure 9.11). The rate of release from the matrix depends on BSA loading (the number of BSA particles dispersed per unit volume of matrix) and BSA particle size. The rate of release from these matrices can be characterized by an effective diffusion coefficient or tortuosity, which must depend on the characteristics (i.e., particle size and loading) of the device. Tables 9.2 and 9.3 list the tortuosity values calculated from fits of the desorption equation to available literature data of protein release from EVAc slabs. 

Figure 9.11 Release of bovine serum albumin from EVAc matrices. Controlled release of BSA from an EVAc matrix. Solid particles of BSA either (a) 45 to 75 /rm or (b) 150 to 250 /rm in diameter were dispersed within EVAc by solvent evaporation to achieve final protein loadings from 10, 20, 30, 40, or 50%. In each case five identical slabs, each 70 mg and 1 mm thick, were incubated in cacodylate-buffered water containing 0.02% gentamicin. Periodically, the buffered water was replaced, and the amount of protein released from the matrix was determined by measuring the concentration of protein in the solution that was removed. Each symbol represents the average cumulative fraction of protein released (cumulative mass of protein released/initial mass of protein within the matrix) for the five samples error bars indicate the standard deviation, which in some cases are smaller than the symbols. Data from [16].

To understand how microscopic properties of the material influence these phenomena, it is necessary to develop more complex models of protein release. When detailed information on the microgeometry of the porous network in the polymers is available (as it is for protein-loaded EVAc matrices [37]) or can be estimated accurately, detailed models of protein release can be developed. For example, percolation models of pore network topology (such as those described in Chapter 4, see Figure 4.20) were coupled with analytical models of pore-to-pore diffusion rates to predict the rate of diffusion of proteins from EVAc matrices [16]. Effective diffusion coefficients predicted using this approach agree with those estimated by measuring rates of protein release from the matrix (Figure 9.13). 

For a protein incorporated in the usual slab matrix, the rate of release, and therefore the effective diffusion coefficient and Fx, depend on the molecular weight of the protein, the size of the dispersed particles, the loading, and the molecular weight of the polymer. Tables I and II list the tortuosity values calculated from fits of the desorption equation [Eq. (4) or (5)] to available literature data of protein release from EVAc slabs. 

Tabata, Y. and Ikada,Y., Protein release from gelatin matrixes, Adv. DrugDeliv. Rev., 31,287,1998. 

Cathepsins are intracellular proteinases that reside within lysosomes or specific intracellular granules. Cathepsins are used to degrade proteins or pqffides that are internalised from the extracellular space. Some cathepsins such as cathepsin-G or cathepsin-K may be released from the cell to degrade specific extracellular matrix proteins. All cathepsins except cathepsin-G (serine) and cathepsin-D (aspartyl) are cysteine proteinases. 

PEG/PBT copolymers are also very good matrix materials for the release of growth factors in tissue engineering. Proteins have been delivered from PEG/PBT microspheres with preservation of protein delivery of complete activity. In the case of protein delivery from PLGA and poly(ortho ester) microspheres, the protein activity was significantly reduced. " ... 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

Translocation is believed to occur posttranslation-ally, after the matrix proteins are released from the cytosolic polyribosomes. Interactions with a number of cytosolic proteins that act as chaperones (see below) and as targeting factors occur prior to translocation. 

The catalytic cycle can be divided into three phases, through each of which the three active sites pass in sequence. First, ADP and Pj are bound (1), then the anhydride bond forms (2), and finally the product is released (3). Each time protons pass through the Fo channel protein into the matrix, all three active sites change from their current state to the next. It has been shown that the energy for proton transport is initially converted into a rotation of the y subunit, which in turn cyclically alters the conformation of the a and p subunits, which are stationary relative to the Fo part, and thereby drives ATP synthesis. 

Hydrophobic polymers are often used to deliver biomacromolecules regardless of the route of administration. The rapid transit time of approximately 8 hours limits the time of a device in the gastrointestinal (GI) system, consequently the mechanisms possible for oral drug release are limited. The predominant method of release from hydrophobic polymers has been degradation, or biodegradation, of a polymeric matrix by hydrolysis (Figure 11.1). In fact, all of the hydrophobic polymers described in this chapter for use as oral protein or peptide delivery are hydrolytically unstable. 

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