The simulations of drug–drug interaction between CYP cocktail substrates (including midazolam as CYP3A4 probe substrate [12], omeprazole as CYP2C19 probe substrate [13], and metoprolol as CYP2D6 probe substrate [14]) were conducted for patients with refractory solid tumors using both the default Simcyp® “Healthy Volunteers” and “Cancer” population model files without any further modifications of the baseline model parameters. The gene discussed is CYP2D6; the disease is cancer.