Modern drug design and testing involves experimental in vivo and in vitro measurement of the drug candidate's ADMET (adsorption, distribution, metabolism, elimination and toxicity) properties in the early stages of drug discovery. Only a small percentage of the proposed drug candidates receive government approval and reach the market place. Unfavorable pharmacokinetic properties, poor bioavailability and efficacy, low solubility, adverse side effects and toxicity concerns account for many of the drug failures encountered in the pharmaceutical industry. Authors from several countries have contributed chapters detailing regulatory policies, pharmaceutical concerns and clinical practices in their respective countries with the expectation that the open exchange of scientific results and ideas presented in this book will lead to improved pharmaceutical products.
Experimental and Computational Methods Pertaining to Surface Tension of Pharmaceuticals. In: William Acree, Editor. Toxicity and Drug Testing
Abolghasem Jouyban, Anahita Fathi-Azarbayjani
InTech Publishers. Feb 2012, Chapter 3; pp: 47-70.
The molecules of a fluid experiences attractive forces exerted on it by all its neighboring molecules. In the bulk of the liquid, molecules are attracted equally in all directions resulting in a net force of zero. Molecules at or near the surface experience attractive force which tends to pull them to the interior. Surface chemistry deals with thermodynamic and kinetic parameters that take place between two different coexisting phases at equilibrium. Surface tension, γ is free energy of the surface at any air/fluid interface defined as force per unit length or energy per unit area. The latter term, also called surface energy, is more useful in thermodynamics and it applies to solids as well as liquid surfaces. The surface free energy of a liquid is measured by its surface tension and the surface free energy of a solid can be revealed by contact angle measurements. The surface tension measurement depends very markedly upon the presence of impurities in the liquid, temperature and pressure changes (Buckton, 1988).
Surface tension is a phenomenon that we see in our everyday life. Human biological fluids, e.g. serum, urine, gastric juice, amniotic liquid, cerebrospinal and alveolar lining liquid contain numerous low-and high-molecular weight surfactants, proteins and lipids that adsorb at liquid interface. The physicochemical processes that take place in these interfaces are extremely important for the vital function of body organs and have a great impact on pharmacodynamic parameters of drug molecules (Kazakov et al., 2000; Trukhin et al., 2001). Drug substances are usually administered as part of a formulation in combination with excipients that have varied and specialized pharmaceutical functions. The design of drug formulation is based on the principles of pharmacokinetic, biopharmaceutic and pharmaceutical technology. The pharmaceutical industry has directed its attention mainly to the quality and processability of active pharmaceutical products which is reflected in various physicochemical parameters. The drug and pharmaceutical materials require extensive characterization and testing of their stability, physico-chemical properties, effectiveness, palatability and ease of administration to ensure drug efficacy. Among the important characteristics are the drug dissolution and solubility in gastrointestinal tract, intestinal absorption, drug distribution and drugToxicity and plasma protein binding. By applying special surface treatments such as contact angle and surface tension measurements to pharmaceutical compounds, drug distribution, dissolution behavior and release pattern in various body fluids can be improved (Hancock et al., 1997; Ho et al., 2010).
Surface tension can influence the development, prediction and performance of pharmaceutical products and help to solve industrial problems and improve products quality. Due to the importance of this phenomenon in drug formulations, there is a growing need for specific interfacial consideration that can be used routinely to solve pharmaceutical problems and improve product quality and stability. In order to meet challenges and develop new and better performing pharmaceutical products, knowledge of surface tension and its measurements techniques is of utmost importance. Amongst many techniques used for characterizing the surface energies of pharmaceuticals are the surface tension measurements, contact angle and wettability tests (Buckton, 1988; Chamarthy et al., 2009; Puri et al., 2010). The objective of this chapter is to introduce experimental and computational methods of surface tension measurment in the pharmaceutical industry.