Maria Miller

Concentrated dispersions of equilibrium protein nanoclusters that reversibly dissociate into active monomers

Stabilizing proteins at high concentration is of broad interest in drug delivery, for treatment of cancer and many other diseases. Herein, we create highly concentrated antibody dispersions (up to 260 mg/mL) comprising dense equilibrium nanoclusters of protein (monoclonal antibody 1B7, polyclonal sheep immunoglobulin G, and bovine serum albumin) molecules which, upon dilution in vitro or administration in vivo, remain conformationally stable and biologically active. The extremely concentrated environment within the nanoclusters (∼700 mg/mL) provides conformational stability to the protein through a novel self-crowding mechanism, as shown by computer simulation, while the primarily repulsive nanocluster interactions result in colloidally stable, transparent dispersions. The nanoclusters are formed by adding trehalose as a cosolute which strengthens the short-ranged attraction between protein molecules. The protein cluster diameter was reversibly tuned from 50 to 300 nm by balancing short-ranged attraction against long-ranged electrostatic repulsion of weakly charged protein at a pH near the isoelectric point. This behavior is described semiquantitatively with a free energy model which includes the fractal dimension of the clusters. Upon dilution of the dispersion in vitro, the clusters rapidly dissociated into fully active protein monomers as shown with biophysical analysis (SEC, DLS, CD, and SDS-PAGE) and sensitive biological assays. Since the concept of forming nanoclusters by tuning colloid interactions is shown to be general, it is likely applicable to a variety of biological therapeutics, mitigating the need to engineer protein stability through amino acid modification. In vivo subcutaneous injection into mice results in indistinguishable pharmacokinetics versus a standard antibody solution. Stable protein dispersions with low viscosities may potentially enable patient self-administration by subcutaneous injection of antibody therapeutics being discovered and developed.

Structural Basis for p300 Taz2/p53 TAD1 Binding and Modulation by Phosphorylation

Coactivators CREB-binding protein and p300 play important roles in mediating the transcriptional activity of p53. Until now, however, no detailed structural information has been available on how any of the domains of p300 interact with p53. Here, we report the NMR structure of the complex of the Taz2 (C/H3) domain of p300 and the N-terminal transactivation domain of p53. In the complex, p53 forms a short alpha helix and interacts with the Taz2 domain through an extended surface. Mutational analyses demonstrate the importance of hydrophobic residues for complex stabilization. Additionally, they suggest that the increased affinity of Taz2 for p53(1-39) phosphorylated at Thr(18) is due in part to electrostatic interactions of the phosphate with neighboring arginine residues in Taz2. Thermodynamic experiments revealed the importance of hydrophobic interactions in the complex of Taz2 with p53 phosphorylated at Ser(15) and Thr(18).

Structural basis of oncogenic activation caused by point mutations in the kinase domain of the MET proto-oncogene: modeling studies.

Missense mutations in the tyrosine kinase domain of the MET proto‐oncogene occur in selected cases of papillary renal carcinoma. In biochemical and biological assays, these mutations produced constitutive activation of the MET kinase and led to tumor formation in nude mice. Some mutations caused transformation of NIH 3T3 cells. To elucidate the mechanism of ligand‐independent MET kinase activation by point mutations, we constructed several 3D models of the wild‐type and mutated MET catalytic core domains. Analysis of these structures showed that some mutations (e.g., V1110I, Y1248H/D/C, M1268T) directly alter contacts between residues from the activation loop in its inhibitory conformation and those from the main body of the catalytic domain; others (e.g., M1149T, L1213V) increase flexibility at the critical points of the tertiary structure and facilitate subdomain movements. Mutation D1246N plays a role in stabilizing the active form of the enzyme. Mutation M1268T affects the S+1 and S+3 substrate‐binding pockets. Models implicate that although these changes do not compromise the affinity toward the C‐terminal autophosphorylation site of the MET protein, they allow for binding of the substrate for the c‐Abl tyrosine kinase. We provide biochemical data supporting this observation. Mutation L1213V affects the conformation of Tyr1212 in the active form of MET. Several somatic mutations are clustered at the surface of the catalytic domain in close vicinity of the probable location of the MET C‐terminal docking site for cytoplasmic effectors. Proteins 2001;44:32–43.

A left-handed crossover involved in amidohydrolase catalysis. Crystal structure of Erwinia chrysanthemi L-asparaginase with bound L-aspartate.

The crystal structure of l‐asparaginase from Erwinia chrysanthemi in the presence and absence of l‐aspartate was determined at 1.8 Å resolution. Conserved residues in a left‐handed crossover (a rare occurrence in protein structures) link pairs of dimers into the catalytically active tetrameric form of the enzyme. The structure of ErA containing bound aspartic acid shows that this unusual strand connectivity is an essential part of the active site architecture, responsible for releasing the product of the enzymatic hydrolysis. The orientation of the bound aspartate indicates for the first time a threonine residue as a catalytic nucleophile.

Two Distinct Motifs within the p53 Transactivation Domain Bind to the Taz2 Domain of p300 and Are Differentially Affected by Phosphorylation

The tumor suppressor p53 functions as a transcriptional activator for many genes, including several key genes involved in cell cycle arrest and apoptosis. Following DNA damage-induced stress, p53 undergoes extensive posttranslational modification, resulting in increased stability and activity. Two critical cofactors for p53-mediated transactivation are the histone acetyltransferase paralogues CREB-binding protein (CBP) and p300. The N-terminal transactivation domain of p53 interacts with several domains of CBP/p300, including the Taz2 domain. Here, we report the effects of specific p53 phosphorylations on its interaction with the Taz2 domain of p300. Using a competitive fluorescence anisotropy assay, we determined that monophosphorylation of p53 at Ser(15) or Thr(18) increased the affinity of p53(1-39) for Taz2, and diphosphorylations at Ser(15) and Ser(37) or Thr(18) and Ser(20) further increased the affinity. In addition, we identified a second binding site for Taz2 within p53 residues 35-59. This second site bound Taz2 with a similar affinity as the first site, but the binding was unaffected by phosphorylation. Thus, p53 posttranslational modification modulates only one of the two binding sites for p300 Taz2. Further investigation of Taz2 binding to p53(1-39) or p53(35-59) by isothermal titration calorimetry indicated that upon complex formation, the change in heat capacity at constant pressure, DeltaC(p), was negative for both sites, suggesting the importance of hydrophobic interactions. However, the more negative value of DeltaC(p) for Taz2 binding to the first (-330 cal/(mol.K)) compared to the second site (-234 cal/(mol.K)) suggests that the importance of nonpolar and polar interactions differs between the two sites.

Flocculated Amorphous Nanoparticles for Highly Supersaturated Solutions

PurposeTo recover polymer-stabilized amorphous nanoparticles from aqueous dispersions efficiently by salt flocculation and to show that the particles redisperse and dissolve rapidly to produce highly supersaturated solutions.MethodsNanoparticle dispersions of itraconazole stabilized by nonionic polymers were formed by antisolvent precipitation and immediately flocculated with sodium sulfate, filtered and dried. The size after redispersion in water, crystallinity, and morphology were compared with those for particles produced by spray drying and rapid freezing.ResultsParticle drug loading increased to ∼90% after salt flocculation and removal of excess polymer with the filtrate. The formation of the flocs at constant particle volume fraction led to low fractal dimensions (open flocs), which facilitated redispersion in water to the original primary particle size of ∼300xa0nm. Amorphous particles, which were preserved throughout the flocculation–filtration–drying process, dissolved to supersaturation levels of up to 14 in pHxa06.8 media. In contrast, both spray dried and rapidly frozen nanoparticle dispersions crystallized and did not produce submicron particle dispersions upon addition to water, nor high supersaturation values.ConclusionsSalt flocculation produces large yields of high surface area amorphous nanoparticle powders that de-aggregate and dissolve rapidly upon redispersion in pHxa06.8 media, for supersaturation levels up to 14.

Flocculated amorphous itraconazole nanoparticles for enhanced in vitro supersaturation and in vivo bioavailability

Rapid flocculation of nanoparticle dispersions of a poorly water soluble drug, itraconazole (Itz), was utilized to produce amorphous powders with desirable dissolution properties for high bioavailability in rats. Antisolvent precipitation (AP) was utilized to form Itz nanodispersions with high drug loadings stabilized with hydroxypropylmethylcellulose (HPMC) or the pH-sensitive Eudragit® L100-55 (EL10055). The HPMC dispersions were flocculated by desolvating the polymer through the addition of a divalent salt, and the enteric EL10055 by reducing the pH. The formation of open flocs by diffusion limited aggregation facilitated redispersion of the flocs at pH 6.8. Upon redispersion of the flocculated nanoparticles at pH 6.8, the particle size was modestly larger than the original size, on the order of 1 μm. High in vitro supersaturation (AUC) of the flocculated nanoparticle dispersions was observed in micellar media at pH 6.8, after 2 hours initial exposure at pH 1.2 to simulate the stomach, relative to the AUC for a commercially available Itz formulation, Sporanox. Greater in vivo bioavailability in rats was correlated directly to the higher in vitro AUC at pH 6.8 with micelles during the pH shift experiment for the flocculated nanoparticle dispersions relative to Sporanox. The ability to generate and sustain high supersaturation in micellar media at pH 6.8, as shown with the in vitro pH shift dissolution test, is beneficial for increasing bioavailability of Itz by oral delivery.

Highly supersaturated solutions of amorphous drugs approaching predictions from configurational thermodynamic properties

Dissolution of pure solid itraconazole in metastable amorphous states was used to produce high supersaturation in low pH media. For a prewet dispersion of particles on the order of 1 microm produced by antisolvent precipitation, an experimental supersaturation of 63 times the crystalline solubility was achieved. This experimental value approached the calculated value of 95 from the configurational free energy, G(conf), which was determined from modulated differential scanning calorimetry measurements. A high fragility, quantitatively determined by the fragility parameter, gamma(cp), is dependent on the configurational heat capacity, C(pconf), favoring a high G(conf) and thus high supersaturation. However, high fragility also increases the driving force for crystallization of the solid during dissolution. The relatively fragile prewet dispersions dissolved rapidly and produced high supersaturation without crystallizing, in contrast with much lower supersaturation values for slowly dissolving particles with low wetted-surface areas formed by spray drying or lyophilization of aqueous dispersions.

Interaction of Macrophage-stimulating Protein with Its Receptor RESIDUES CRITICAL FOR β CHAIN BINDING AND EVIDENCE FOR INDEPENDENT α CHAIN BINDING

Macrophage-stimulating protein (MSP) and hepatocyte growth factor/scatter factor (HGF/SF) are plasminogen-related growth and motility factors that interact with cell-surface protein tyrosine kinase receptors. Each one is a heterodimeric protein comprising a disulfide-linked α chain and a serine protease-like β chain. Despite structural similarities between MSP and HGF, the primary receptor binding site is located on the α chain of HGF/SF but on the β chain of MSP. To obtain insight into the structural basis for MSP β chain binding, β chain structure was modeled from coordinates of an existing model of the HGF β chain. The model revealed that the region corresponding to the S1 specificity pocket in trypsin is filled by the Asn682/Glu648 interacting pair, leaving a shallow cavity for possible β chain interaction with the receptor. Mutants in this region were created, and their binding characteristics were determined. A double mutation of Asn682/Glu648 caused diminished binding of the β chain to the MSP receptor, and a single mutation of neighboring Arg683 completely abolished binding. Thus, this region of the molecule is critical for binding. We also found that at equimolar concentrations of free α and β chains, α chain binding to receptor was detectable, at levels considerably lower than β chain binding. The EC50 values determined by quantitative enzyme-linked immunosorbent assay are 0.25 and 16.9 nm for β and α chain, respectively. The data suggest that MSP has two independent binding sites with high and low affinities located in β and α chain, respectively, and that the two sites together mediate receptor dimerization and subsequent activation.

Templated Open Flocs of Nanorods for Enhanced Pulmonary Delivery with Pressurized Metered Dose Inhalers

PurposeA novel concept is presented for the formation of stable suspensions composed of low density flocs of high aspect ratio drug particles in hydrofluoroalkane (HFA) propellants, and for subdividing (templating) the flocs with aerosolized HFA droplets to achieve high fine particle fractions with a pressurized metered dose inhaler.MethodsBovine serum albumin (BSA) nanorods, produced by thin film freezing (TFF), were added to HFA to form a suspension. Particle properties were analyzed with an Anderson cascade impactor (ACI), static and dynamic light scattering and optical microscopy.ResultsThe space filling flocs in HFA were stable against settling for one year. The pMDI produced high fine particle fractions (38–47%) with an emitted dose of 0.7xa0mg/actuation. The atomized HFA droplets break apart, that is template, the highly open flocs. Upon evaporation of HFA, capillary forces shrink the templated flocs to produce porous particles with optimal aerodynamic diameters for deep lung delivery.ConclusionsOpen flocs composed of nanorods, stable against settling, may be templated during actuation with a pMDI to produce optimal aerodynamic diameters and high fine particle fractions. This concept is applicable to a wide variety of drugs without the need for surfactants or cosolvents to stabilize the primary particles.