Mass transport techniques as a tool for a better understanding of the structure of l-Dopa in aqueous solutions

In silico study of levodopa in hydrated lipid bilayers at the atomistic level

This article presents atomistic molecular dynamics and umbrella sampling simulations of levodopa at various concentrations in hydrated cholesterol-free 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol-containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers. Levodopa is the standard medication for Parkinson's disease and is marketed under various trade names; in the context of this article, the levodopa molecule is mostly studied in its zwitterionic form but some results concerning the neutral levodopa are presented as well for comparison purposes. The motivation is to study in detail how levodopa behaves in different hydrated lipid membranes, primarily from the thermodynamic point of view, and reveal aspects of mechanism of its permeation through them. Dependencies of properties on the levodopa concentration are also investigated. Special attention is paid to the calculation of mass density profiles, order parameters and self-diffusion coefficients. Levodopa zwitterions, which form a hydrogen bond network with water and phospholipid molecules, are found to be preferentially located at the water/lipid interface, as well as in the aqueous phase surrounding the cholesterol-free and cholesterol-containing bilayers. This is concluded from the potentials of mean force calculated by umbrella sampling simulations as levodopa is transferred from the lipid to the aqueous phase along an axis perpendicular to the two leaflets of the membranes.

Cyclodextrins in Parkinson's Disease

: Parkinson's disease is a movement disorder characterized by a progressive degeneration of dopaminergic neurons that has been object of study by the scientific community through the last decades. However, nowadays there is still no treatment to cure it, although there are drugs available, with limited efficacy, to relieve the symptoms or replenish the cells with dopamine to supply the lack of dopaminergic neurons. This work was structured in two parts. In the first one, binary aqueous solutions of L-dopa and cyclodextrins were studied. In the second part, ternary aqueous solutions of L-dopa were studied with each of the selected cyclodextrins. In all cases, thermodynamic properties (density, partial molar volume and thermodynamic transfer functions for temperatures between 294.15 ± 0.01 K and 312.15 ± 0.01 K) and transport properties (mutual diffusion coefficients, viscosity, transfer viscosity at 298.15 ± 0.01 K and 310.15 ± 0.01 K) were studied. Using theoretical models to adjust the experimental data obtained for the diffusion coefficients and for the apparent molar volumes, in the ternary aqueous solutions, it was possible to estimate the values to the L-dopa-cyclodextrin association constant. For the aqueous ternary solutes, the partial molar volume of transfer of levodopa in the presence of the cyclodextrins, the partial molar expansibility at infinite dilution and from this, the Hepler constant, were determined. Also, the values of Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰) and entropy (ΔS⁰) were determined. From the obtained information, it was possible to characterize the molecular interactions, as well as to identify some structural characteristics of the controlled drug delivery systems under study and to estimate the influence of the cyclodextrin substituent groups, and, also, the temperature effect in the interaction levodopa-cyclodextrin. It is our intent to attain information about the mechanism of possible new systems for controlled drug delivery systems, throughout an alternative perspective, which could allow to increase its effectiveness in the Parkinson's treatment.

Transport Properties for Pharmaceutical Controlled-Release Systems: A Brief Review of the Importance of Their Study in Biological Systems

The goal of this work was to comprehensive study the transport properties of controlled-release systems for the safe and reliable delivery of drugs. Special emphasis has been placed on the measurement of the diffusion of drugs, alone or in combination with carrier molecules for enhanced solubility and facilitated transport. These studies have provided detailed comprehensive information—both kinetic and thermodynamic—for the design and operation of systems for the controlled release and delivery of drugs. Cyclodextrins are among the most important carriers used in these systems. The basis for their popularity is the ability of these materials to solubilize poorly soluble drugs, generally resulting in striking increases in their water solubilities. The techniques used in these investigations include pulse voltammetry, nuclear magnetic resonance (NMR) and Raman spectroscopy, ultrasonic relaxation, and dissolution kinetics. Transport in these systems is a mutual diffusion process involving coupled fluxes of drugs and carrier molecules driven by concentration gradients. Owing to a strong association in these multicomponent systems, it is not uncommon for a diffusing solute to drive substantial coupled fluxes of other solutes, mixed electrolytes, or polymers. Thus, diffusion data, including cross-diffusion coefficients for coupled transport, are essential in order to understand the rates of many processes involving mass transport driven by chemical concentration gradients, as crystal growth and dissolution, solubilization, membrane transport, and diffusion-limited chemical reactions are all relevant to the design of controlled-release systems. While numerous studies have been carried out on these systems, few have considered the transport behavior for controlled-release systems. To remedy this situation, we decided to measure mutual diffusion coefficients for coupled diffusion in a variety of drug–carrier solutions. In summary, the main objective of the present work was to understand the physical chemistry of carrier-mediated transport phenomena in systems of controlled drug release.

Characterization at 25 °C of sodium hyaluronate in aqueous solutions obtained by transport techniques

Mutual diffusion coefficients, D, were determined for aqueous solutions of sodium hyaluronate (NaHy) at 25 °C and concentrations ranging from 0.00 to 1.00 g·dm(-3) using the Taylor dispersion technique. From these experimental data, it was possible to estimate some parameters, such as the hydrodynamic radius Rh, and the diffusion coefficient at infinitesimal concentration, D0, of hyaluronate ion, permitting us to have a better understanding of the structure of these systems of sodium hyaluronate in aqueous solutions. The additional viscosity measurements were done and Huggins constant, kH, and limiting viscosity number, [η], were computed for interaction NaHy/water and NaHy/NaHy determination.