The complete original paper reference is cited below:

Population-based analysis of methadone distribution and metabolism using an age-dependent physiologically based pharmacokinetic model, Feng Yang, Xianping Tong, D. Gail. McCarver, Ronald N. Hines and Daniel A. Beard, 2006, Journal of Pharmacokinetics and Pharmacodynamics , volume 33, issue 4. PubMed ID: 16758333

The Lotka–Volterra equations, also known as the predator–prey equations, are a pair of first-order, non-linear, differential equations frequently used to describe the dynamics of biological systems in which two species interact, one a predator and one its prey. They were proposed independently by Alfred J. Lotka in 1925 and Vito Volterra in 1926. This model is parameterised as follows: x = 3, y = 6, A = 1, B = 1, C = 1, D = 1.