**2. Cell culture system**

fail at phase II and phase III clinical stages due to poor efficacy and safety issues [2]. The high attenuation rates in drug discovery suggest that the main reasons for drug failure are inappropriate preclinical testing methods and *in vitro* models, which do not sufficiently produce information needed for prediction of drug efficacy and safety issues [3]. Hence, one of the main areas expected to improve the success rate of drug development process could be the use of new technologies in preclinical testing and *in vitro* models, in order to

Cell-based assays are crucial in the drug discovery and development process. Mammalian cell culture provides a defined platform for investigating cell and tissue physiology and pathophysiology outside of the organism. For over a century, traditional 2D cell culture was used in drug discovery. In 2D cell culture, cells are grown on flat dishes optimized for cell attachment and growth (**Figure 1**). Nowadays, 2D cell culture models are still used to test cellular drug responses to drug candidates. Although 2D cell culture is generally accepted and has increased understanding of drug mechanisms of action, there are limitations associated with it. The main limitation is that the cells grown as a monolayer on flat petri plates or flasks. This is a stiff platform, offering unnatural growth kinetics and cell attachments. Therefore, natural microenvironments of the cells are not fully represented [4]. Recently, significant work by researchers produced improvements in the form of better *in vitro* cell culture models that resemble *in vivo* conditions. Three-dimensional cell cultures are such products and better mimic tissue physiology in multicellular organisms

While traditional monolayer cultures still are predominant in cellular assays used for highthroughput screening (HTS), 3D cell cultures techniques for applications in drug discovery are making rapid progress [6, 7]. In this chapter, we provide an overview of 2D and 3D cell

culture techniques, and their role in the discovery of new drugs.

**Figure 1.** Simplified sketch of 2D and 3D cell culture.

get better accurate data.

22 Cell Culture

(**Figure 1**) [5].

Cell culture involves the dispersal of cells in an artificial environment that is composed of an appropriate surface, nutrient supply, and optimal conditions of humidity, temperature and gaseous atmosphere [6]. Usually cells are grown for days or weeks in a sterile 37°C humidified incubator with 5% CO2 until a sufficient number of cells are reached. This system allows the study of cellular response to different environmental cues such as physiological stimulants or agonists/antagonists, potential drugs or pathogens.
