Drug-Biomembrane Interaction Studies

The Application of Calorimetric Techniques

Rosario Pignatello

• Publisher: Woodhead Publishing
• Year: 2013
• ISBN:
  9781908818348
• Topic: Medicine
• Subtopic: Pharmacology

The design and development of drugs and new pharmaceutical formulations require a full characterization of the chemical and physicochemical events occurring at the level of the single active ingredients or excipients, as well as their reciprocal interaction. Thermal analysis techniques are among the most widely used methods to achieve this; among them, the Differential Scanning Calorimetry (DSC) technique, in which the thermotropic behaviour of a single substance or mixtures is analyzed as a function of a controlled temperature program. DSC is an accurate and rapid thermo-analytical technique, widely used by the pharmaceutical industry and in drug research to investigate several physico-chemical phenomena, such as polymorphism, melting and crystallization, purity, and drug-excipient interaction; as well as characterizing biomolecules such as genetic material.Drug-biomembrane interaction studies is written by scientists renowned for their work in the field of DSC applications to drug development and delivery, and especially to drug-biomembrane interaction studies. The book combines insights from biochemistry and physiology with those from structural biology, nanotechnology and biothermodynamics, to obtain a complete depiction of cell membranes and their functions.

• Summarizes and updates the recent development in a unique handbook format
• Consists of a combination of scientific updates within the field
• Contains chapters written by some of the highest-level experts in the field of DSC

1

Biological membranes and their role in physio-pathological conditions

R. Pignatello, University of Catania, Italy

Abstract:

This chapter will summarize recent information on cell membranes. Their structure, functions and the role of the various components are discussed, considering both their physiological tasks, such as mechanisms of drug internalization into cells, as well as membrane changes associated with or caused by particular disease states. Later chapters will discuss the possibility of testing biomembrane models to study their interaction with drugs and biological compounds.

Key words

cell membranes

biomembranes

membrane models

lipid rafts

phospholipids

cell uptake

membrane-related diseases

1.1 Importance of drug-biomembrane interactions in biomedical and pharmaceutical research

Organisms have a number of biological membranes with different functions: protecting cells from foreign molecules; controlling the molecules that can enter cells, hence maintaining the biochemical integrity of the cytoplasm; hosting biochemically active molecules such as receptors, ion channels, enzymes and functional proteins, which play a role in cell metabolism, growth, homeostasis and even death; and regulating traffic between the inside of the cell and the surrounding medium, thus participating in the communication and exchange of information between the extra- and intracellular environments.

These and many other functions can be active in physiological conditions or be activated under pathological situations. Moreover, pathological alterations in the architecture or function of a biomembrane can become a cause of disease. Finally, the structure and functions of membranes vary significantly in different parts and tissues in the body.

Upon administration, a drug molecule immediately encounters one or more of these biomembranes, from blood vessel endothelium to circulating macrophages, from absorption membranes to the more complex biological barriers, like the blood-brain or the blood-retinal barriers.

A detailed description of plasma membrane biology and physio-pathology is beyond the aims of this book, and can be easily found in other texts.^1–3 However, a general knowledge of how cell membranes are built and their active components is at least required before commencing a study of the interactions between a drug, or a bioactive compound, and a biological membrane or a biomembrane model, and this will be the topic for this introductory chapter. For instance, a study of these interactions with a drug must take into consideration the physio-pathological landscape in which the drug will be used, the diagnostic or therapeutic aims, whether a local or a systemic administration is foreseen, the dosage (which will be reflected in the concentration of the drug in the plasma or in a specific body compartment), the healthy or damaged condition of the cell membranes on which the drug will act, the eventual controlled release profile or targeting process that will affect its in vivo biodistribution, etc.

The interaction of a drug with a biomembrane is a complex physical and chemical phenomenon. In some cases, such as when the biomembrane is a barrier to drug permeation or is its site of action (for instance, a receptor inserted in the cell membrane), the interaction is the final step. However, in most cases a drug–membrane interaction represents only a preliminary phase of a cascade of chemical and physical processes, like the association with protein receptors or enzymes, before the biological or toxic activity of the active compound is apparent. The drug–membrane interaction can affect the rate of penetration of the biomolecule into the cytoplasm, where it must reach a specific cell organelle or system. Under these circumstances, the partitioning into and binding with cell membranes must be investigated, both for known as well as for newly developed drugs, considering their mechanism of action and/or of toxicity.⁴

Cell and biological membranes essentially consist of a lipid environment into which liposoluble compounds can dissolve and pass through. Therefore, the solubility and partition coefficients are the most relevant physico-chemical features for characterizing the interaction of a small bioactive molecule with living cells.⁵ More precisely, the balance between the hydrophilic and lipophilic character of a molecule, the so-called amphiphilicity, determines the quality and extent of its interaction with cell membranes.

Simple lipophilicity has, in fact, been shown to be unable to describe the overall phase of penetration and passage of a compound through biomembranes. As the following sections will show, the complex structure of the latter indicates that drugs must possess an amphiphilic property able to modulate their movements inside both the lipid domains and the polar areas of the membranes.⁶ This property is called anisotropic lipophilicity:⁴ it merges the hydrophobicity of a compound and also has the capacity of generating electrostatic and hydrogen bonds with plasma membranes.

A drug–membrane interaction can be thus regarded as either a partitioning or a binding process. The events are usually complex and multivalent, but for simplicity we will say that when the cell membrane acts as a barrier to drug penetration, that is, when the pharmacokinetic aspects of the tested drug are emerging, the partitioning phenomena are of paramount importance. In contrast, when the plasma membrane is considered as a site of action for the drug (pharmacodynamic aspects), the drug-binding processes must be investigated.

The forces involved in both kinds of interactions are similar, that is, hydrophobic and polar chemical interactions.⁴ This is useful information because, regardless of the complexity of the biochemical phenomena taking place in living cells, it is relatively easy to simplify and design a valid experimental model for studying or even forecasting drug–biomembrane interactions.

It is noteworthy that these interactions can be mutual: an active molecule (drug) can modify the structure and function of the membrane (model), for example by changing its fluidity and permeability or its charge potential; on the other hand, the structure, stability and pharmacological behaviour of the drug can be in turn affected by its interaction with the membrane constituents, for instance at the level of drug molecular conformation, stereochemistry, duration of the biological activity, etc.^7,8

1.2 The structure of cell membranes

All living cells, prokaryotic and eukaryotic, possess a thin cell or plasma membrane (also known as plasmalemma), which encloses their contents and acts as a semi-porous barrier to the outside environment. It also serves as the communications interface between the cell and its environment. Biological membranes also compartmentalize cellular organelles and their functions. Inside a cell, the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles and vacuoles are surrounded by a single biomembrane sheet. Mitochondria and the nucleus are surrounded by two membrane layers. Finally, the membrane regulates the flow of materials into and out of the cell, mediates intercellular communication, contacts and adhesion, and performs a multitude of other tasks.

Various scientific hypotheses have been use...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Figures
• Tables
• List of abbreviations
• Preface
• About the editor and contributors
• Chapter 1: Biological membranes and their role in physio-pathological conditions
• Chapter 2: Biomembrane models
• Chapter 3: Analytical methods for studying drug–biomembrane interactions
• Chapter 4: Differential scanning calorimetry (DSC): theoretical fundamentals
• Chapter 5: DSC: history, instruments and devices
• Chapter 6: DSC in drug–biomembrane interaction studies
• Chapter 7: DSC applications: macromolecules
• Chapter 8: DSC applications: nucleic acids and membrane interactions
• Chapter 9: Non-steroidal anti-inflammatory drugs
• Chapter 10: Antimicrobial agents
• Chapter 11: Drug delivery systems: drug nanocarriers
• Appendix 1: General experimental set-up of liposomal systems for DSC
• Appendix 2: Journals
• Index