ADS’06 Draft Manuscript January 2005 © 2006, UC Regenrs, UCSF BioSystems Group – 1 – Agent-Based Simulations of In Vitro Multicellular Tumor Spheroid Growth Jesse Engelberg 1,2 , Suman Ganguli 2 , and C. Anthony Hunt 1,2 1 Joint Graduate Group in Bioengineering, University of California, Berkeley and San Francisco; 2 The BioSystems Group, University of California, 513 Parnassus Ave., San Francisco, CA, 94143-0446 jae@berkeley.edu sganguli@itsa.ucsf.edu hunt@itsa.ucsf.edu Keywords: Tumor spheroid, agent-based modeling Abstract Multicellular tumor spheroids (MTS) are an in vitro model system of avascular tumors. They are used to study how local microenvironments affect cellular growth, viabil- ity, and therapeutic response. We are developing in silico, agent-based analogues to provide new options for gaining exploitable insight into both the in vitro and in vivo systems. MTS exhibit characteristic phenotypic attributes: initial ex- ponential growth becomes linear and is followed by growth saturation and a stabilization of spheroid size; and a con- centric layered structure, consisting of an outer shell of pro- liferating cells, an intermediate layer of viable but quiescent cells, and an inner necrotic core. Our agent-based, discrete event analogue consists of different spaces for tumor cells, oxygen, nutrient, and toxic inhibitors. It successfully simu- lates the three phases of spheroid growth and achieves the balance of cell gain and loss that is necessary for saturation. When exposed to varying levels of nutrient and oxygen, the model behavior is qualitatively similar to that observed in vitro. A goal is to have increasing overlap between the phe- notype of the analogue in silico systems and the phenotype of their in vitro referents. The development process will necessarily expose gaps in the understanding of tumor sphe- roids and lead experimentalists in new and novel directions. 1. INTRODUCTION Cancer is characterized by improper cellular prolifera- tion. Cancerous cells do not undergo senescence, nor do they listen to regular signals to become apoptotic. In many forms of cancer, this cellular proliferation self-organizes into the growth of solid tumors. Cancer, and in particular the process of tumor growth, is difficult to study in vivo. Hence, much cancer research has focused on developing model systems [1], both in vivo animal models and in vitro cell culture models. Multicellular tumor spheroids (MTS) are an in vitro model system of early stage tumor growth that has been well developed and extensively studied. Here we report on the development and results of an initial agent- based model of MTS growth. What is the motivation for developing such a model? Just as in vitro models provide insight into the in vivo sys- tem, we believe that reliable and robust in silico model sys- tems can provide insight into in vitro systems and by exten- sion into in vivo systems and eventually patients. Our in silico tumor cell agents incorporate aspects of tumor cell biology such as cellular metabolism, the cell cycle, and cel- lular responses to the effects of changing microenviron- mental conditions. One issue that remains unsolved in tumor spheroid research is the action of the necrotic extract. It is unclear whether the necrotic extract slows the progression of cells through the cell cycle or causes more cells to arrest and become quiescent (or both). It is also unclear whether the cells arrest primarily due to exposure to the necrotic ex- tract or if there are other factors, like cellular forces or con- tact inhibition that cause arrest. We aim to use our model to develop and explore hypotheses that have not been exten- sively studied or are difficult or impossible to study in vitro, and thereby generate new insights and avenues for experi- mentation. Moreover, just as the in vitro MTS models were devel- oped in order to improve upon existing animal in vivo (xenograft) models by better reflecting the in vivo situation, in some human cancers in silico models could eventually improve upon existing in vitro models by providing a better model of human in vivo tumor growth. We begin by emu- lating the in vitro model. 2. BIOLOGICAL BACKGROUND Since their development in the early 1970s, researchers have used MTS to study how tumor microenvironments af- fect cellular growth, viability, and therapeutic response. In particular, an initial wave of research on MTS in the 1970s and 1980s extensively explored growth dynamics [2–5]. More recently, researchers have shifted towards using the MTS system as a model to study other topics in oncology, such as cell cycle regulation [9]. An MTS consists of a roughly spherical aggregate of tum- or cells in suspension culture. Researchers have described MTS as exhibiting similar behaviors over a wide range of SpringSim'06 141 ISBN 1-56555-303-9