The use of microcalorimetry to measure thermodynamic parameters of the binding of ligands to insulin

A Combined LC-MS and Immunoassay Approach to Characterize Preservative-Induced Destabilization of Human Papillomavirus Virus-like Particles Adsorbed to an Aluminum-Salt Adjuvant

During the multi-dose formulation development of recombinant vaccine candidates, protein antigens can be destabilized by antimicrobial preservatives (APs). The degradation mechanisms are often poorly understood since available analytical tools are limited due to low protein concentrations and the presence of adjuvants. In this work, we evaluate different analytical approaches to monitor the structural integrity of HPV16 VLPs adsorbed to Alhydrogel™ (AH) in the presence and absence of APs (i.e., destabilizing m-cresol, MC, or non-destabilizing chlorobutanol, CB) under accelerated conditions (pH 7.4, 50 °C). First, in vitro potency losses displayed only modest correlations with the results from two commonly used methods of protein analysis (SDS-PAGE, DSC). Next, results from two alternative analytical approaches provided a better understanding of physicochemical events occurring under these same conditions: (1) competitive ELISA immunoassays with a panel of mAbs against conformational and linear epitopes on HPV16 VLPs and (2) LC-MS peptide mapping to evaluate the accessibility/redox state of the 12 cysteine residues within each L1 protein comprising the HPV16 VLP (i.e., with 360 L1 proteins per VLP, there are 4320 Cys residues per VLP). These methods expand the limited analytical toolset currently available to characterize AH-adsorbed antigens and provide additional insights into the molecular mechanism(s) of AP-induced destabilization of vaccine antigens.

Molecular interactions between (-)-epigallocatechin gallate analogs and pancreatic lipase

The molecular interactions between pancreatic lipase (PL) and four tea polyphenols (EGCG analogs), like (−)-epigallocatechin gallate (EGCG), (−)-gallocatechin gallate (GCG), (−)-epicatechin gallate (ECG), and (−)-epigallocatechin (EC), were studied from PL activity, conformation, kinetics and thermodynamics. It was observed that EGCG analogs inhibited PL activity, and their inhibitory rates decreased by the order of EGCG>GCG>ECG>EC. PL activity at first decreased rapidly and then slowly with the increase of EGCG analogs concentrations. α-Helix content of PL secondary structure decreased dependent on EGCG analogs concentration by the order of EGCG>GCG>ECG>EC. EGCG, ECG, and EC could quench PL fluorescence both dynamically and statically, while GCG only quenched statically. EGCG analogs would induce PL self-assembly into complexes and the hydrodynamic radii of the complexes possessed a close relationship with the inhibitory rates. Kinetics analysis showed that EGCG analogs non-competitively inhibited PL activity and did not bind to PL catalytic site. DSC measurement revealed that EGCG analogs decreased the transition midpoint temperature of PL enzyme, suggesting that these compounds reduced PL enzyme thermostability. In vitro renaturation through urea solution indicated that interactions between PL and EGCG analogs were weak and non-covalent.

Thermodynamic contributions to the stability of the insulin hexamer

The insulin hexamer is resistant to degradation and fibrillation, which makes it an important quaternary structure for its in vivo storage in Zn(2+)- and Ca(2+)-rich vesicles in the pancreas and for pharmaceutical formulations. In addition to the two Zn(2+) ions that are required for its formation, three other species, Zn-coordinating anions (e.g., Cl(-)), Ca(2+), and phenols (e.g., resorcinol), bind to the hexamer and affect the subunit conformation and stability. The contributions of these four species to the thermodynamics of insulin unfolding have been quantified by differential scanning calorimetry and thermal unfolding measurements to determine the extent and nature of their stabilization of the insulin hexamer. Both Zn(2+) and resorcinol make a significant enthalpic contribution, while Ca(2+) primarily affects the protein heat capacity (solvation) by its interactions in the central cation-binding cavity, which is modulated by the surrounding subunit conformations. Coordinating anions have a negligible effect on the stability of the hexamer, even though subunits shift to an alternate conformation when these anions bind to the Zn(2+) ions. Finally, Zn(2+) in excess of the two that are required to form the hexamer further stabilizes the protein by additional enthalpic contributions.

Calorimetric investigation of protein/amino acid interactions in the solid state

Possible protein/amino acid interactions and the physical states of amino acids after freeze-drying have been studied using isoperibol calorimetry and differential scanning calorimetry (DSC). Good linear correlations (R(2) = 0.99) between the enthalpies of solution and the percentage of antibody in all physical mixtures, as well as unchanging melting temperatures of amino acids for physical mixtures demonstrated that there is no interaction between the antibodies and amino acids studied upon physical mixing. On the other hand, positive deviations for antibody/histidine and antibody/arginine freeze-dried samples obtained from the isoperibol calorimetry results demonstrated that molecular level interactions, such as ion-dipole or electrostatic interactions or hydrogen bonding, occur between antibodies and histidine or arginine. The values of DeltaH(interaction) for antibody/histidine (1:1, w/w) and antibody/arginine (1:1, w/w) lyophilized samples were approximately 8 kJ/mol. These interactions were also confirmed by decreased and/or the disappearance of melting temperatures of the amino acids with DSC measurements. A negative deviation from linearity was detected for antibody/aspartic acid lyophilized samples which indicated partial amorphization of aspartic acid. No deviation from linearity as well as similar melting temperatures of antibody/glycine lyophilized samples indicated the absence of interactions between the antibodies and glycine upon freeze-drying.

Thermodynamic analysis of binding and protonation in DOTAP/DOPE (1:1): DNA complexes using isothermal titration calorimetry

A better understanding of the nature of the interaction between various cationic lipids used for gene delivery and DNA would lend insight into their structural and physical properties that may modulate their efficacy. We therefore separated the protonation and binding events which occur upon complexation of 1:1 DOTAP (1,2-dioleoyl-3-trimethylammonium propane):DOPE (1,2-dioleoylphosphatidylethanolamine) liposomes to DNA using proton linkage theory and isothermal titration calorimetry (ITC). The enthalpy of DOPE protonation was estimated as -45.0+/-0.7 kJ/mol and the intrinsic binding enthalpy of lipid to DNA as +2.8+/-0.3 kJ/mol. The pK(a) of DOPE was calculated to shift from 7.7+/-0.1 in the free state to 8.8+/-0.1 in the complex. At physiological ionic strength, proton linkage was not observed upon complex formation and the buffer-independent binding enthalpy was +1.0+/-0.4 kJ/mol. These studies indicate that the intrinsic interaction between 1:1 DOTAP/DOPE and DNA is an entropy-driven process and that the affinities of cationic lipids that are formulated with and without DOPE for DNA are controlled by the positive entropic changes that occur upon complex formation.

Enhancements in gene expression by the choice of plasmid DNA formulations containing neutral polymeric excipients

Formulations containing maltodextrin (2% w/v) were identified to facilitate intramuscular (im) delivery of plasmid DNA in mice using the reporter genes luciferase and chloramphenicol acetyltransferase (CAT) and the therapeutic gene of erythropoietin (EPO) as monitors of transfection efficiency. Even though considerable variability in gene expression was observed in animals, a 5-8-fold enhancement of reporter gene expression was observed with this excipient compared with saline formulations of DNA. In a therapeutically significant experiment, a single im injection of an EPO plasmid formulation containing 2% (w/v) maltodextrin resulted in a significant and prolonged elevation of the hematocrit levels of mice compared with control DNA in saline. Biophysical studies with Fourier transform infrared (FTIR) spectroscopy, isothermal titration, and differential scanning calorimetry (DSC) suggested a weak interaction between DNA and maltodextrin as well as a thermal stabilizing effect on the DNA. These in vivo and biophysical results with maltodextrin are comparable to those reported previously with other nonionic polymers, such as poly(vinyl pyrrolidone) and poloxamers, and indicate that maltodextrin is an additional nonionic excipient that displays the property of gene expression enhancement.

Effect of ionic strength on the loading efficiency of model polypeptide/protein drugs in pH-/temperature-sensitive polymers

In this report, the effect of ionic strength on the loading efficiency of three model polypeptide/protein drugs, namely angiotensin II, insulin, and cytochrome c, in pH- and temperature-sensitive terpolymers of poly(NIPAAm-co-butylmethacrylate-co-acrylic acid) (poly(NIPAAm-co-BMA-co-AA)) has been investigated. Loading efficiency of polypeptides in pH-/temperature-sensitive beads composed of poly(NIPAAm-co-BMA-co-AA) terpolymer is predominantly governed by hydrophobic interactions, both nonspecific surface interactions and/or specific interactions (binding pockets) between the protein and the polymer molecules. Thus, loading efficiency increases with ionic strength. However, as ionic strength increases further, polymer deswelling (collapse), which is also controlled by hydrophobic forces, becomes more pronounced, and consequently, a higher fraction of water is squeezed out during bead formation and the loading efficiency starts to decrease.

Modulating insulin-release profile from pH/thermosensitive polymeric beads through polymer molecular weight

Stimuli-sensitive statistical terpolymers of N-isopropylacrylamide (NIPAAm) (temperature-sensitive), butyl methacrylate (BMA) and acrylic acid (AA) (pH-sensitive) of various molecular weight (MW) with NIPAAm/BMA/AA feed mol ratio of 85/5/10 were used to modulate release of insulin, a model protein drug, from pH/thermosensitive polymeric beads. Protein drug loading from an aqueous medium into the beads was achieved by preparing a 7 or 10% (w/v) polymer solution with 0.2% (w/v) insulin at low pH and below the lower critical solution temperature (LCST) of the polymer (pH 2.0 and 4 degrees C), and then dropping the solution into an oil bath above the LCST of the solution (35 degrees C). This loading procedure maintained protein stability while achieving high loading efficiency, between 90 and 95% in the beads. Insulin-release studies from beads prepared from terpolymers of the same composition but increasing MW were performed at pH 2.0 and 7.4, at 37 degrees C. It was observed that there was negligible loss of insulin at pH 2.0 from the beads, indicating no burst effect. At pH 7.4, insulin release was seen from all the beads and the release rate was a function of the MW of the polymer. The low MW polymeric beads eroded, dissolved and released most of the insulin within 2 h at pH 7.4 and 37 degrees C, the intermediate MW polymeric beads swelled slightly, dissolved and released most of the insulin within 4 h, whereas the high MW polymeric beads swelled slowly and gradually released the loaded insulin over a period of 8 h. Thus, the release of protein from the low MW polymeric beads is controlled by the rate of dissolution of the polymer, whereas the release from the high MW polymeric beads is controlled by swelling of the beads and drug diffusion. Studies using fluorescein-labeled insulin revealed that insulin was uniformly distributed in the beads regardless of polymer MW. The loaded and released insulin were fully bioactive. Based on the described results, the low MW polymeric beads may be used for immediate delivery of protein drugs in the duodenum, the intermediate MW polymeric beads may be used for lower small intestine targeting, while the high MW polymeric beads may be used to target protein drugs predominantly to the colon.

Physicochemical basis for the rapid time-action of LysB28ProB29-insulin: dissociation of a protein-ligand complex

The rate-limiting step for the absorption of insulin solutions after subcutaneous injection is considered to be the dissociation of self-associated hexamers to monomers. To accelerate this absorption process, insulin analogues have been designed that possess full biological activity and yet have greatly diminished tendencies to self-associate. Sedimentation velocity and static light scattering results show that the presence of zinc and phenolic ligands (m-cresol and/or phenol) cause one such insulin analogue, LysB28ProB29-human insulin (LysPro), to associate into a hexameric complex. Most importantly, this ligand-bound hexamer retains its rapid-acting pharmacokinetics and pharmacodynamics. The dissociation of the stabilized hexameric analogue has been studied in vitro using static light scattering as well as in vivo using a female pig pharmacodynamic model. Retention of rapid time-action is hypothesized to be due to altered subunit packing within the hexamer. Evidence for modified monomer-monomer interactions has been observed in the X-ray crystal structure of a zinc LysPro hexamer (Ciszak E et al., 1995, Structure 3:615-622). The solution state behavior of LysPro, reported here, has been interpreted with respect to the crystal structure results. In addition, the phenolic ligand binding differences between LysPro and insulin have been compared using isothermal titrating calorimetry and visible absorption spectroscopy of cobalt-containing hexamers. These studies establish that rapid-acting insulin analogues of this type can be stabilized in solution via the formation of hexamer complexes with altered dissociation properties.