Biophysical characterization of a liposomal formulation of cytarabine and daunorubicin.

The biophysical characterization of CPX-351, a liposomal formulation of cytarabine and daunorubicin encapsulated in a synergistic 5:1 molar ratio (respectively), is presented. CPX-351 is a promising drug candidate currently in two concurrent Phase 2 trials for treatment of acute myeloid leukemia. Its therapeutic activity is dependent on maintenance of the synergistic 5:1 drug:drug ratio in vivo. CPX-351 liposomes have a mean diameter of 107 nm, a single phase transition temperature of 55.3 degrees C, entrapped volume of 1.5 microL/micromol lipid and a zeta potential of -33 mV. Characterization of these physicochemical properties led to identification of an internal structure within the liposomes, later shown to be produced during the cytarabine loading procedure. Fluorescence labeling studies are presented that definitively show that the structure is composed of lipid and represents a second lamella. Extensive spectroscopic studies of the drug-excipient interactions within the liposome and in solution reveal that interactions of both cytarabine and daunorubicin with the copper(II) gluconate/triethanolamine-based buffer system play a role in maintenance of the 5:1 cytarabine:daunorubicin ratio within the formulation. These studies demonstrate the importance of extensive biophysical study of liposomal drug products to elucidate the key physicochemical properties that may impact their in vivo performance.

Intra and inter-molecular interactions dictate the aggregation state of irinotecan co-encapsulated with floxuridine inside liposomes.

PURPOSE: The inter/intramolecular interactions between drugs (floxuridine, irinotecan) and excipients (copper gluconate, triethanolamine) in the dual-drug liposomal formulation CPX-1 were elucidated in order to identify the physicochemical properties that allow coordinated release of irinotecan and floxuridine and maintenance of the two agents at a fixed, synergistic 1:1 molar ratio. METHODS: Release of irinotecan and floxuridine from the liposomes was assessed using an in vitro-release assay. Fluorescence, Nuclear Magnetic Resonance spectroscopy (NMR) and UV-Vis were used to characterize the aggregation state of the drugs within the liposomes. RESULTS: Coordinated release of the drugs from liposomes was disrupted by removing copper gluconate. Approximately 45% of the total irinotecan was detectable in the copper-containing CPX-1 formulation by NMR, which decreased to 19% without copper present in the liposomal interior. Formation of higher order, NMR-silent aggregates was associated with slower and uncoordinated irinotecan release relative to floxuridine and loss of the synergistic drug/drug ratio. Solution spectroscopy and calorimetry revealed that while all formulation components were required to achieve the highest solubility of irinotecan, direct drug-excipient binding interactions were absent. CONCLUSIONS: Long-range interactions between irinotecan, floxuridine and excipients modulate the aggregation state of irinotecan, allowing for simultaneous release of both drugs from the liposomes.

The cooperative response of synaptotagmin I C2A. A hypothesis for a Ca2+-driven molecular hammer.

In the current understanding of exocytosis at the nerve terminal, the C2 domain of synaptotagmin (C2A) is presumed to bind Ca2+ and the membrane in a stepwise fashion: cation then membrane as cation increases the affinity of protein for membrane. Fluorescence spectroscopy data were gathered over a variety of lipid and Ca2+ concentrations, enabling the rigorous application of microscopic binding models derived from partition functions to differentiate between Ca2+ and phosphatidylserine contributions to binding. The data presented here are in variance with previously published models, which were based on the Hill approximation. Rather, the data are consistent with two forms of cooperativity that modulate the responsiveness of C2A: in Ca2+ binding to a network of three cation sites and in interaction with the membrane surface. We suggest synaptotagmin I C2A is preassociated with the synaptic vesicle membrane or nerve terminal. In this state, upon Ca2+ influx the protein will bind the three Ca2+ ions immediately and with high cooperativity. Thus, membrane association creates a high-affinity Ca2+ switch that is the basis for the role of synaptotagmin I in Ca2+-regulated exocytosis. Based on this model, we discuss the implications of protein-induced phosphatidylserine demixing to the exocytotic process.

A novel nuclear interactor of ARF and MDM2 (NIAM) that maintains chromosomal stability.

The ARF tumor suppressor signals through p53 and other poorly defined anti-proliferative pathways to block carcinogenesis. In a search for new regulators of ARF signaling, we discovered a novel nuclear protein that we named NIAM (nuclear interactor of ARF and MDM2) for its ability to bind both ARF and the p53 antagonist MDM2. NIAM protein is normally expressed at low to undetectable levels in cells because of, at least in part, MDM2-mediated ubiquitination and proteasomal degradation. When reintroduced into cells, NIAM activated p53, caused a G1 phase cell cycle arrest, and collaborated with ARF in an additive fashion to suppress proliferation. Notably, NIAM retains growth inhibitory activity in cells lacking ARF and/or p53, and knockdown experiments revealed that it is not essential for ARF-mediated growth inhibition. Thus, NIAM and ARF act in separate anti-proliferative pathways that intersect mechanistically and suppress growth more effectively when jointly activated. Intriguingly, silencing of NIAM accelerated chromosomal instability, and microarray analyses showed reduced NIAM mRNA expression in numerous primary human tumors. This study identifies a novel protein with tumor suppressor-like behaviors and functional links to ARF-MDM2-p53 signaling.

Nucleophosmin (B23) targets ARF to nucleoli and inhibits its function.

The ARF tumor suppressor is a nucleolar protein that activates p53-dependent checkpoints by binding Mdm2, a p53 antagonist. Despite persuasive evidence that ARF can bind and inactivate Mdm2 in the nucleoplasm, the prevailing view is that ARF exerts its growth-inhibitory activities from within the nucleolus. We suggest ARF primarily functions outside the nucleolus and provide evidence that it is sequestered and held inactive in that compartment by a nucleolar phosphoprotein, nucleophosmin (NPM). Most cellular ARF is bound to NPM regardless of whether cells are proliferating or growth arrested, indicating that ARF-NPM association does not correlate with growth suppression. Notably, ARF binds NPM through the same domains that mediate nucleolar localization and Mdm2 binding, suggesting that NPM could control ARF localization and compete with Mdm2 for ARF association. Indeed, NPM knockdown markedly enhanced ARF-Mdm2 association and diminished ARF nucleolar localization. Those events correlated with greater ARF-mediated growth suppression and p53 activation. Conversely, NPM overexpression antagonized ARF function while increasing its nucleolar localization. These data suggest that NPM inhibits ARF's p53-dependent activity by targeting it to nucleoli and impairing ARF-Mdm2 association.

Membrane-bound orientation and position of the synaptotagmin C2B domain determined by site-directed spin labeling.

Site-directed spin labeling is used to determine the orientation and depth of insertion of the second C2 domain from synaptotagmin I (C2B) into membrane vesicles composed of phosphatidylcholine (PC) and phosphatidylserine (PS). EPR line shapes of spin-labeled mutants located with the Ca(2+)-binding loops of C2B broaden in the presence of Ca(2+) and PC/PS vesicles, indicating that these loops undergo a Ca(2+)-dependent insertion into the membrane interface. Power saturation of the EPR spectra provides a position for each spin-labeled site along the bilayer normal, and these EPR-derived distance constraints, along with a high-resolution structure of the C2B domain, are used to generate a model for the domain orientation and position at the membrane interface. Our data show that the isolated C2B domain from synaptotagmin I penetrates PC/PS membranes, and that the backbone of Ca(2+)-binding loops 1 and 3 is inserted below the level of a plane defined by the lipid phosphates. The side chains of several loop residues are within the bilayer interior, and both Ca(2+)-binding sites are positioned near a plane defined by the lipid phosphates. A Tb(3+)-based fluorescence assay is used to compare the membrane affinity of the C2B domain to that of the first synaptotagmin C2 domain (C2A). Both C2A and C2B bind PC/PS (75:25) membrane vesicles with a micromolar lipid affinity in the presence of metal ion. These results indicate that C2A and C2B have a similar membrane affinity and position when bound to PC/PS (75:25) membrane vesicles. EPR spectroscopy indicates that the C2B domain has different interactions with PC/PS membranes containing 1 mol % phosphatidylinositol 4,5-bisphosphate.

Membrane-bound orientation and position of the synaptotagmin I C2A domain by site-directed spin labeling.

Site-directed spin labeling was used to determine the membrane orientation and insertion of the C2A domain from synaptotagmin I. A series of single cysteine mutants of the C2A domain of synaptotagmin I was prepared and labeled with a sulfhydryl specific spin label. Upon Ca2+ or membrane binding, the EPR line shapes of these mutants reveal dramatic decreases in label mobility within the Ca2+-binding loops. This loss in mobility is likely due in part to a reduction in local backbone fluctuations within the loop regions. Power saturation was then used to determine the position of each spin-labeled site along the bilayer normal, and these EPR distance constraints were used along with the high-resolution solution structure of C2A to generate a model for the orientation and position of the domain at the membrane interface. This model places the polypeptide backbone of both the first and third Ca2+-binding loops in contact with the membrane interface, with several labeled side chains lying within the bilayer interior. All three Ca2+-binding sites lie near a plane defined by the lipid phosphates. This model indicates that there is some desolvation of this domain upon binding and that hydrophobic as well as electrostatic interactions contribute to the binding of C2A. When compared to the C2 domain from cPLA2 (Frazier et al. (2002) Biochemistry 41, 6282), a similar orientation for the beta-sandwich region is found; however, the cPLA2 C2 domain is translocated 5-7 A deeper into the membrane hydrocarbon. This difference in depth is consistent with previous biophysical data and with the difference that long-range electrostatic interactions and desolvation are expected to make to the binding of these two C2 domains.

Membrane orientation and position of the C2 domain from cPLA2 by site-directed spin labeling.

The C2 domain is a ubiquitous Ca(2+)-binding motif that triggers the membrane docking of many key signaling proteins during intracellular Ca(2+) signals. Site-directed spin labeling was carried out on the C2 domain of cytosolic phospholipase A(2) in order to determine the depth of penetration and orientation of the domain at the membrane interface. Membrane depth parameters, Phi, were obtained by EPR spectroscopy for a series of selectively spin-labeled C2 domain cysteine mutants, and for spin-labeled lipids and spin-labeled bacteriorhodopsin cysteine mutants. Values of Phi were combined with several other constraints, including the solution NMR structure, to generate a model for the position of the C2 domain at the membrane interface. This modeling yielded an empirical expression for Phi, which for the first time defines its behavior from the bulk aqueous phase to the center of the lipid bilayer. In this model, the backbones of both the first and third Ca(2+)-binding loops are inserted approximately 10 A into the bilayer, with residues inserted as deep as 15 A. The backbone of the second Ca(2+)-binding loop is positioned near the lipid phosphate, and the two beta-sheets of the C2 domain are oriented so that the individual strands make angles of 30-45 degrees with respect to the bilayer surface. Upon membrane docking, spin labels in the Ca(2+)-binding loops exhibit decreases in local motion, suggesting either changes in tertiary contacts due to protein conformational changes and/or interactions with lipid.