We present CoSBiLab BlenX as a development and analysis platform for modeling pharmacodynamical systems. Together with its supporting tool CoSBiLab KInfer for inferring kinetic parameters, BlenX provides an effective compromise between the differential equations-based and mechanisms-based approaches to modeling of pharmacodynamical systems. In this paper, we support this claim with models that describe the tumor size changes over the time course of chemotherapy in lung cancer patients treated with gemcitabine and carboplatin. Our first simple high-level model relies on a few identifiable and biologically relevant parameters. The estimation of these parameters requires only the tumor growth curve in vivo, typically available in the clinical setting. This model is an essential first step of our ongoing work where we aim at explaining the high-level behavior with our second model. This latter model incorporates the molecular machinery of the drug that determines the cytotoxic effect resulting in the shrinkage of the tumor.