Electron Microscopy and Theoretical Modeling of Cochleates

Abstract

Cochleates are self-assembled cylindrical condensates that consist of large rolled-up lipid bilayer sheets and represent a novel platform for oral and systemic delivery of therapeutically active medicinal agents. With few preceding investigations, the physical basis of cochleate formation has remained largely unexplored. We address the structure and stability of cochleates in a combined experimental/theoretical approach. Employing different electron microscopy methods, we provide evidence for cochleates consisting of phosphatidylserine and calcium to be hollow tubelike structures with a well-defined constant lamellar repeat distance and statistically varying inner and outer radii. To rationalize the relation between inner and outer radii, we propose a theoretical model. Based on the minimization of a phenomenological free energy expression containing a bending, adhesion, and frustration contribution, we predict the optimal tube dimensions of a cochleate and estimate ratios of material constants for cochleates consisting of phosphatidylserines with varied hydrocarbon chain structures. Knowing and understanding these ratios will ultimately benefit the successful formulation of cochleates for drug delivery applications.

Cochleate characterization by cryogenic electron microscopy methods: Cryo-TEM and Cryo-SEM

“…It was indicated that cochleates have no free water since no change in X-ray diffraction after high-vacuum drying and by the very short lamellar spacing of 54Å that barely leaves a room for interlamellar water [8,16,19,20], but this was challenged recently [21,22].…”

“…Scanning electron microscopy has also been used to visualize cochleates [22,41]. These cochleates were placed on glass slides, air dried and sputter coated before imaging.…”

Micro-spherical cochleate composites: method development for monodispersed cochleate system

“…For the preparation of conventional cochleates, DOPS liposomes were prepared by thin-lipid film hydration method as described elsewhere (Nagarsekar et al, 2014). In brief, DOPS was dissolved in a mixture of chloroform:methanol (3:1) in a round bottom flask and was subjected to a rotary evaporator (R-144 BÜ CHI, Labortechnik GmbH, Essen, Germany) to form a thin-lipid film.…”

“…Cochleates are cylindrical particles featuring a rolled up carpet-like morphology made up of multiple stacked layers of negatively charged phospholipids bound by a positively charged binding agent such as calcium (Nagarsekar et al, 2014). The term ''cochleates'' was introduced by Papahadjopoulous in 1975 while investigating cation-induced phospholipid membrane fusion in great detail Wilschut et al, 1981Wilschut et al, , 1985.…”

“…Cochleates represent a potential delivery system with remarkable advantages being highly stable and with the ability to impart safety to incorporated active ingredients (Zarif, 2002). Cochleates have a hollow central channel and little or no internal aqueous space in the lipid sheets (Nagarsekar et al, 2014;Zarif, 2005). During the past few decades, cochleates were explored for a number of possible applications thanks to their peculiar properties.…”