Effects of Local Anesthetics on the Structure of Phospholipid Bilayers

M. PICKHOLZ; L. F. FRACETO; E. DE PAULA

Porto de Galinhas (Brazil)

Conferencia; International Conference of Synthetic Metals (ICSM); 2008

ICSM

Local anesthetics (LA) are pain-relief drugs. They have a big affinity for the cellular membrane because of their amphiphatic nature. A better understanding of the interaction of local anesthetics with biological membranes could provide insights into the mechanism of anesthesia and could help, for instance, to improve their efficacy and minimize the side effects through the rational design of new local anesthetics or through the use of  mechanisms for drug delivery (i.e. liposomes). In this work, we investigate the interaction of Prilocaine (PLC) – an aminoamide local anesthetic widely used in dentistry – with model membranes by Molecular Dynamics (MD) simulations. PLC has a pKa of 7.9, therefore its charged and uncharged forms are relevant at physiological pH. Although both charged (protonated) and uncharged forms coexist at physiological pH, due to differential partition of charged/uncharged forms into the membranes, a higher ratio of uncharged species is found at pH 7.4. Earlier studies from Malheiros et al. [1] show that the apparent pK of PLC in membranes is 7.6, slightly different from the pKa in water. We have carried out a series of simulations where charged and uncharged PLC, corresponding to different pH, were introduced into a POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) phospholipids bilayer at LA:Lipid molar ratios of 1:3. Simulations for different ionization states of the PLC were able to capture important features of the PLC–phospholipid bilayer interactions: our results show that charged (protonated) PLC is predominantly found at the lipid-water interface, whereas uncharged (neutral) PLC enters more deeply into the hydrophobic tail region. This picture remains when a partition of charged/uncharged are present into the membrane (corresponding to pH 7.4). In presence of PLCs, we found a decrease in the bilayer thickness and an increase in the area per lipid, in respect to plain POPC bilayers, an effect that have been well experimentally described for many local anesthetics. 20 ns runs were sufficiently long to see several neutral PLC hopping from one monolayer to the other. However, no crossing events were observed for protonated PLC during the 20 ns of the simulations, in agreement with Högberg et al. [2] findings for protonated lidocaine in dimyristoyl phosphatidylcholine (DMPC) lipid bilayers. By the other hand, looking at the acyl chain order parameter, we found a decrease of the overall lipid tail organization, both for saturated and unsaturated tails, in good agreement with the experimental results. This lack of orientation of the lipid tails is responsible for the decrease in the bilayer thickness. [1] Malheiros, S.V.P., Gottardo, L., Pinto, L.M.A., Yokaichiya, D.K., Meirelles, N.C. & de Paula, E. “A new look at the hemolytic effect of local anesthetics, considering their real membrane/water partitioning at pH 7.4”, Biophys. Chem. 100:213-221, 2004. [2] Carl-Johan Högberg, Arnold Maliniak, Alexander P. Lyubarstsev, Biophysical Chemistry, 2007, 125, 416 (2007).