Evaluation of degradation pathways for plasmid DNA in pharmaceutical formulations via accelerated stability studies.

The stability of highly purified supercoiled plasmid DNA formulated in simple phosphate or Tris-buffered saline solutions has been characterized to establish the overall degradation processes that occur during storage in aqueous solution. Plasmid DNA stability was monitored during accelerated stability studies (at 50 degrees C) by measurements of supercoiled, open-circle, and linear DNA content, as well as the accumulation of apurinic sites and 8-hydroxydeoxyguanosine residues over time. The effects of formulation pH, demetalation, metal ion chelators, and ethanol (hydroxyl radical scavenger) on the supercoiled content of plasmid DNA during storage at 50 degrees C were also determined. The results indicate that free radical oxidation may be a major degradative process for plasmid DNA in pharmaceutical formulations unless specific measures are taken to control it by the addition of free radical scavengers, specific metal ion chelators, or both. The generation of hydroxyl radicals in phosphate-buffered saline was confirmed by examining the hydroxylation of phenylalanine over time by reverse phase high-performance liquid chromatography. Ethanol was found to enhance plasmid DNA stability and to inhibit the hydroxylation of phenylalanine; both observations are consistent with the known ability of ethanol to serve as a hydroxyl radical scavenger. Moreover, the combination of ethylenediamine tetraacetic acid (EDTA) and ethanol had a synergistic enhancing effect on DNA stability. However, the metal ion chelator diethylenetriaminepentaacetic acid (DTPA) was as potent as the combination of EDTA and ethanol for enhancing the stability of plasmid DNA. By controlling free radical oxidation with EDTA and ethanol, the rate constants of plasmid DNA degradation by means of depurination and beta-elimination were then determined, allowing accurate predictions of DNA storage stability as a function of formulation pH and temperature. The ability to predict plasmid DNA storage stability in the absence of free radical oxidation should prove to be a valuable tool for the design of stable pharmaceutical formulations of plasmid DNA.

Enhancement of DNA vaccine potency using conventional aluminum adjuvants.

The immunogenicity and protective efficacy of DNA vaccines have been amply demonstrated in numerous animal models of infectious disease. However, the feasibility of DNA vaccines for human use is not yet known. In order to investigate potential means of increasing the potency of DNA vaccines, conventional adjuvants such as aluminum salts were tested. Coadministration of these adjuvants with DNA vaccines substantially enhanced the ability of these vaccines to induce antibody responses up to 100-fold in mice and guinea pigs, and 5-10-fold in non-human primates. Effective formulations had no demonstrable effect on the levels of antigen expression in situ and consisted of adjuvants that did not form complexes with the plasmid DNA; rather they exerted their effects on antigen after expression in situ. Therefore, the potency of DNA vaccines both in laboratory rodents and in non-human primates can be substantially increased by simple formulation with conventional aluminum adjuvants.

Formulation, stability, and delivery of live attenuated vaccines for human use.

The successful use of live attenuated viral and bacterial vaccines depends not only on the proper choice and delivery of the microorganisms, but also on maintaining the sufficient potency required for an immune response. The inherent lability of live organisms presents a particular formulation challenge in terms of stabilizing and preserving vaccine viability during manufacturing, storage, and administration. This review examines pharmaceutical approaches to the stabilization, formulation, and lyophilization of biological macromolecules in general, as well as the specific applicability of these principles to live attenuated viral and bacterial vaccines. Several formulation development case studies with live vaccines are presented. In addition, comparative stability data are summarized for many other live viral and bacterial preparations. Various pharmaceutical issues with conventional and novel delivery systems for administration of parenteral and oral live vaccines are also discussed.

Degradative covalent reactions important to protein stability.

Commonly observed chemical modifications that occur in proteins during their in vitro purification, storage, and handling are discussed. Covalent modifications described include deamidation and isoaspartate formation, cleavage of peptide bonds at aspartic acid residues, cystine destruction and thiol-disulfide interchange, oxidation of cysteine and methionine residues, and the glycation and carbamylation of amino groups.

Size exclusion HPLC method for the determination of acidic fibroblast growth factor in viscous formulations.

A size exclusion HPLC method has been developed to determine the protein concentration of pharmaceutical formulations of recombinant acidic fibroblast growth factor (aFGF). These topical aFGF formulations not only contain low levels of protein mass (50 micrograms ml-1), but also include buffer ions, polysaccharide polyanions to conformationally stabilize aFGF and 1% hydroxyethylcellulose to increase the solution's viscosity. A cesium chloride mobile phase is utilized during SEC-HPLC to dissociate aFGF from the pharmaceutical excipients and to minimize nonspecific interaction of the protein with the column matrix. The protein content of a viscous aFGF formulation is determined by comparison of aFGF peak areas to standards of known concentration. Fluorescence spectroscopy was utilized to directly demonstrate that the protein remains in its native conformation during sample preparation and analysis.

Size and conformational stability of the hepatitis A virus used to prepare VAQTA, a highly purified inactivated vaccine.

A variety of biophysical techniques have been employed to examine the size and conformational integrity of highly purified hepatitis A virus (HAV) in solution (purified HAV particles are subsequently formalin-inactivated and adsorbed to aluminum salts for use as the vaccine VAQTA). The size of HAV particles was assessed by a combination of electron microscopy, sedimentation velocity, and dynamic light scattering. The effect of ionic strength and temperature on the overall conformational stability of HAV was determined by a combination of intrinsic HAV protein fluorescence, fluorescent probes of both RNA and protein, and UV-visible spectroscopy. A major structural change in HAV occurs near 60 degrees C with the addition of 0.2 M magnesium chloride enhancing the thermal stability of HAV by approximately 10 degrees C. Salt concentrations above 0.2 M, however, decrease the solubility of HAV. The effect of pH on the physical properties of HAV particles was monitored by dynamic light scattering, analytical size exclusion HPLC, and interaction with fluorescent dyes. HAV particles undergo a substantially reversible association/aggregation at pH values below 6 with the concomitant exposure of previously buried hydrophobic surfaces below pH 4. These results are in good agreement with previous studies of HAV thermal stability under extreme conditions in which the irreversible inactivation of the viral particles was measured primarily by the loss of viral infectivity. The wide variety of biophysical measurements described in this work, however, directly monitor structural changes as they occur, thus providing a molecular basis with which to monitor HAV stability during purification and storage.

Analysis of vaccine stability.

The complexity of vaccines creates a unique formulation challenge. Vaccines may consist of one or more types of antigenic component including live attenuated or killed viral or bacterial particles, polysaccharides, proteins, polynucleotides and particle conjugates. In addition, other excipients such as adjuvants may be present. Not only must the chemical and structural integrity of the various components be maintained, but immunogenicity must be ensured. The inherent lability of vaccines can critically limit their distribution, administration, and efficacy in parts of world where it is difficult to maintain a cold chain. Combination with other vaccines and oral administration may also compromise vaccine stability. Successful vaccine stabilization strategies include both empirical efforts to screen and identify appropriate stabilizers and environmental conditions and more rational approaches toward developing an understanding of the causes and mechanisms of vaccine inactivation. In principle, by elucidating the conformational and chemical pathways of macromolecular inactivation, more rational strategies to minimize their occurrence can be adopted. This presentation will review the application of classical techniques such as viral plaque assays to identify vaccine stabilizers by empirical testing. The potential of using various biophysical techniques (both hydrodynamic and spectroscopic methods) to characterize the physicochemical stability of purified vaccine preparations (Hepatitis A and B) is also explored.

Deamidation of polyanion-stabilized acidic fibroblast growth factor.

The deamidation of polyanion-stabilized acidic fibroblast growth factor (aFGF; FGF-1) can be induced by prolonged storage under accelerated conditions of elevated pH and temperature. A urea-isoelectric focusing (urea-IEF) method has been developed to monitor aFGF deamidation in the presence of highly negatively charged polyanions which are required to maintain the conformational stability of the protein. The kinetics of aFGF deamidation have been established by a combination of urea-IEF and an enzymatic ammonia assay. Native, non-deamidated aFGF (complexed with heparin) has a half-life of 16 weeks at pH 7, 30 degrees C, and 4 weeks at pH 8, 40 degrees C. The mitogenic activity and biophysical properties of deamidated aFGF were compared to the non-deamidated protein. These initial deamidation events have no significant effect on the protein's overall conformation, thermal stability, interaction with heparin, or bioactivity. At longer times, however, limited aggregation of the protein was observed after prolonged storage under some conditions. N-terminal protein sequencing of the protein's first 21 amino acid residues have identified one of the deamidation sites in a flexible, peptide-like region of the protein (Asn8-Tyr9).

Interaction of nucleotides with acidic fibroblast growth factor (FGF-1).

A wide variety of nucleotides are shown to bind to acidic fibroblast growth factor (aFGF) as demonstrated by their ability to (1) inhibit the heat-induced aggregation of the protein, (2) enhance the thermal stability of aFGF as monitored by both intrinsic fluorescence and CD, (3) interact with fluorescent nucleotides and displace a bound polysulfated naphthylurea compound, suramin, (4) reduce the size of heparin-aFGF complexes, and (5) protect a reactive aFGF thiol group. The binding of mononucleotides, diadenosine compounds (ApnA), and inorganic polyphosphates to aFGF is enhanced as the degree of phosphorylation of these anions is increased with the presence of the base reducing the apparent binding affinity. The nature of the base appears to have much less effect. Photoactivatable nucleotides (8N3-ATP, 2N3-ATP, 8N3-GTP, and 8N3-Ap4A) were employed to covalently label the aFGF nucleotide binding site. In general, Kd's in the low micromolar range are observed. Protection against 90% displacement is observed at several hundred micromolar nucleotide concentration. Using 8N3-ATP as a prototypic reagent, photolabeled aFGF was proteolyzed with trypsin and chymotrypsin and labeled peptides were isolated and sequenced resulting in the identification of 10 possible labeled amino acids (Y8, G20, H21, T61, K112, K113, S116, R119, R122, H124). On the basis of the crystal structure of bovine aFGF, eight of the prospective labeled sites appear to be dispersed around the perimeter of the growth factor's presumptive polyanion binding site. On residue (T61) is more distally located but still proximate to several positively charged residues, and another (Y8) is not locatable in crystal structures. Using heparin affinity chromatography, at least three distinct photolabeled aFGF species were resolved. These labeled complexes display diminished affinity for heparin and a reduced ability to stimulate mitogenesis even in the presence of polyanions such as heparin. In conclusion, nucleotides bind apparently nonspecifically to the polyanion binding site of aFGF but nevertheless are capable of modulating the protein's activity. Evidence for the presence of a second or more extended polyanion binding site and the potential biological significance of these results in terms of potential natural ligands of aFGF are also discussed but not resolved.

Bacteriophage T7 RNA polymerase and its active-site mutants. Kinetic, spectroscopic and calorimetric characterization.

It has been demonstrated that the amino acids Asp537, Asp812, Lys631, His811 and Tyr639 are involved in bacteriophage T7 RNA polymerase catalysis. In the present paper, we report kinetic, spectroscopic and calorimetric characterization of the wild-type and mutant T7 RNA polymerases generated at these five loci (D537N, E; K631M, R; Y639F, S, A, W; H811Q, A; D812N, E). The wild-type enzyme has a substantial amount of secondary structure as determined by CD analysis (alpha-helix, 43%; beta-sheet, 14%; beta-turn, 25%; unordered, 18%). The CD spectra of 12 mutants at five loci are very similar to that of the wild-type, except for the mutant Y639W. Within experimental error, the thermal transition temperatures measured by CD and DSC as well as the lambda max values of the fluorescence spectra were the same for the wild-type and all of the mutants. Therefore, the overall folding and stability of the mutant enzymes are very similar to those of the wild-type enzyme, although small local conformational changes cannot be excluded. For the synthesis of the pentamer pppGGACU, the mutants D537E and D812E showed an approximately two- to threefold decrease in (kcat)app and an approximately two- to threefold increase in (Km)app, relative to the wild-type, in contrast to the mutants D537N and D812N which exhibited no detectable activity. The mutant K631R showed a sevenfold reduction in (kcat)app and a two- to threefold increase in (Km)app, supporting our earlier observation with the mutant K631M that Lys631 may be involved in phosphodiester bond formation. The mutant Y639S can synthesize the trimer GGA with an approximately 50-fold decrease in (kcat)app and a tenfold increase in (Km)app, relative to the wild-type, underlining the importance of the phenyl ring of Tyr639. The mutant H811A, in which the side-chain at position 811 is incapable of forming a hydrogen bond, can synthesize the trimer GGA with an approximately tenfold decrease in (kcat)app and an approximately 35-fold increase in (Km)app. Thus, either the hydrogen-bonding capacity of this residue is non-essential or some other group can functionally substitute for the His811 side-chain. The wild-type enzyme showed significant effects of the base position in the sequence on the apparent binding constants for the NTPs. The kinetics of GpG-primed trimer, tetramer and pentamer synthesis on three 22 bp templates were investigated for the wild-type and mutant enzymes with measurable activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Origin of the isoelectric heterogeneity of monoclonal immunoglobulin h1B4.

The origin of the microheterogeneity of a highly purified antiinflammatory humanized monoclonal antibody prepared in mammalian cell culture has been investigated. This antibody is an IgG directed toward human CD18 (a subunit of leukocyte integrins). When the IgG preparation is subjected to isoelectric focusing, it is found to contain four major species with pI values ranging from 6 to 7. Although the relative amounts of each form differ and some species are present only in small quantities, each has been isolated by a combination of high-resolution anion-exchange chromatography and isoelectric focusing. Comparative studies reveal no detectable differences in overall secondary (far UV circular dichroism) or tertiary (intrinsic fluorescence) structure, molecular weight (laser-desorption mass spectroscopy), or antigen binding activity. When each of the isolated species is incubated under conditions which favor deamidation, it is converted to forms of lower pI which appear to correspond to naturally observed species. While the isolated light chain is relatively homogeneous, the heavy chain exhibits a pattern of isoelectric focusing bands similar to that of the intact immunoglobulin. These results suggest that in this case, charge microheterogeneity is due to the sequential deamidation of the immunoglobulin heavy chain.

Sucralfate and soluble sucrose octasulfate bind and stabilize acidic fibroblast growth factor.

The actions of the anti-ulcer drug sucralfate have been proposed to be mediated through interaction with fibroblast growth factors (Folkman, J., Szabo, S., Strovroff, M., McNeil, P., Li, W. and Shing, Y. (1991) Ann. Surg. 214, 414-427). We show here that acidic fibroblast growth factor (aFGF; FGF-1) binds in vitro to both the soluble potassium salt and the insoluble aluminum salt of sucrose octasulfate, as demonstrated by a variety of biophysical techniques. Similar to the well-described interaction and stabilization of aFGF by heparin, soluble sucrose octasulfate (SOS) stabilizes aFGF against thermal, urea and acidic pH-induced unfolding as determined by a combination of circular dichroism, fluorescence spectroscopy and differential scanning calorimetry. In addition, SOS also enhances the mitogenic activity of aFGF and partially protects the protein's three cysteine residues from copper-catalyzed oxidation. SOS competes with heparin and suramin for the aFGF polyanion binding site as measured by both fluorescence and light scattering based competitive binding assays. Front-face fluorescence measurements show that the native, folded form of aFGF binds to the insoluble aluminum salt of sucrose octasulfate (sucralfate). Moreover, sucralfate stabilizes aFGF against thermal and acidic pH-induced unfolding to the same extent as observed with SOS. Thus, due to their high charge density, SOS and sucralfate bind and stabilize aFGF via interaction with the aFGF polyanion binding site.

Partially structured self-associating states of acidic fibroblast growth factor.

A combination of near- and far-UV circular dichroism, Fourier-transform infrared spectroscopy, tryptophan fluorescence, size-exclusion chromatography, and a fluorescent extrinsic hydrophobic probe has been employed to characterize partially structured states of human recombinant acidic fibroblast growth factor (aFGF). At low pH, the addition of specific polyanionic ligands or moderate amounts of salts induces states with high secondary but low tertiary structure content. At neutral pH, intermediate amounts of chaotropic agents impose similar partially structured conformational states which also display noncooperative unfolding transitions. Kinetic evidence indicates that similar forms of the protein exist in the first few hundred milliseconds in the refolding pathway of aFGF. The kinetics of their formation appear to be temperature-independent, implying lack of an energy barrier, which is characteristic for further slow folding into the native state. Unlike the native and fully unfolded states, these partially structured conformations exhibit very low solubility, resulting in irreversible aggregation. Potential physiological implications of the existence of such "molten globule" states with regard to the growth factor's transport and biological activity are considered.

Effect of polyanions on the unfolding of acidic fibroblast growth factor.

The urea-induced unfolding of acidic fibroblast growth factor (aFGF) in the presence and absence of various polyanions has been quantitatively examined by fluorescence spectroscopy. In the absence of a stabilizing polyanion, the apparent free energy of unfolding of aFGF is 6.5 kcal mol-1. The presence of equimolar or greater amounts of heparin stabilizes aFGF from unfolding by more than 2.5 kcal mol-1 and slows the rate of unfolding by greater than 2000-fold. The ability of heparin to stabilize aFGF is critically dependent upon many factors including the number of aFGF molecules bound to the heparin chain, ionic strength, temperature, and the extent of sulfation of the polysaccharide. The presence of similar amounts of other polyanions such as sulfated beta-cyclodextrin or heparan sulfate also stabilizes aFGF to a similar extent as heparin. Additional experiments demonstrate that increasing charge density enhances the ability of polyanions such as sulfated beta-cyclodextrins, phosphorylated inositols, and modified heparins to protect aFGF from urea-induced unfolding.

Formulation design of acidic fibroblast growth factor.

The design of an aqueous formulation for acidic fibroblast growth factor (aFGF) requires an understanding of the type of compounds that can either directly or indirectly stabilize the protein. To this end, spectrophotometric turbidity measurements were initially employed to screen the ability of polyanionic ligands, less specific compounds, and variations in solution conditions (temperature and pH) to stabilize aFGF against heat-induced aggregation. It was found that in addition to the well-known protection of aFGF by heparin, a surprisingly wide variety of polyanions (including small sulfated and phosphorylated compounds) also stabilizes aFGF. These polyanionic ligands are capable of raising the temperature at which the protein unfolds by 15-30 degrees C. Many commonly used excipients were also observed to stabilize aFGF in both the presence and the absence of heparin. High concentrations of some of these less specific agents are also able to increase the temperature of aFGF thermal unfolding by as much as 6-12 degrees C as shown by circular dichroism and differential scanning calorimetry. Other compounds were found which protect the chemically labile cysteine residues of aFGF from oxidation. Aqueous formulations of aFGF were thus designed to contain both a polyanionic ligand that enhances structural integrity by binding to the protein and chelating agents (e.g., EDTA) to prevent metal ion-catalyzed oxidation of cysteine residues. While room-temperature storage (30 degrees C) leads to rapid inactivation of aFGF in physiological buffer alone, several of these aFGF formulations are stable in vitro for at least 3 months at 30 degrees C. Three aFGF topical formulations were examined in an impaired diabetic mouse model and were found to be equally capable of accelerating wound healing.

Nature of the interaction of heparin with acidic fibroblast growth factor.

The binding of human acidic fibroblast growth factor (aFGF) to heparin has been analyzed by a variety of different approaches to better elucidate the nature of this protein/sulfated polysaccharide interaction. Static and dynamic light scattering as well as analytical ultracentrifugation analyses indicates that 14-15 molecules of a FGF can bind to a 16-kDa heparin chain, with approximately 10 of these bound relatively uniformly to high-affinity sites. The dissociation constants of these latter sites are estimated to be approximately 50-140 nM on the basis of surface plasmon resonance experiments in which the association and dissociation rates of aFGF interaction with immobilized heparin were measured. The size of the binding site of a FGF on heparin was also determined by heparin lyase digestion of a FGF/heparin complexes followed by isolation and characterization of protected oligosaccharides. The smallest aFGF-protected oligosaccharide comigrated with delta UA2S(1-->4)-alpha-D-GlcNp2S6S(1-->4)-alpha-L-IdoAp-2S( 1-->4)-alpha-D-GlcNp2S6S (where delta UA represents 4-deoxy-alpha-L-threo-hex-4-enopyranosyluronic acid and S is sulfate). Thus, aFGF appears to bind at high density (one molecule every 4-5 polysaccharide units) and with high affinity to heparin. This potentially provides a concentrated, stabilized storage form of the growth factor that can be released for receptor-mediated cellular activation in response to the proper stimuli. It is also possible that close proximity of aFGF molecules on the highly sulfated regions of heparan chains may be involved in the induction of receptor aggregation as suggested by Ornitz et al. [Ornitz, D. M., Yayon, A., Flanagan, J. G., Svahn, C. M., Levi, E., & Leder, P. (1992) Mol. Cell. Biol. 12, 240-247].