Three nonlinear models of tumour cell growth under continuous delivery of cycle nonspecific anticancer agents are studied. A dynamical optimization problem with the objective of minimizing the final level of tumour cells is posed for these mathematical setups. The simplest setup does not possess toxicity constraints, whereas the other setups contain a dynamical equation describing the therapy burden as a toxicity criterion. In addition, the third setting contains the dynamics of drug resistant cells. A discussion concerning the optimal strategies of the respective models is performed.1733-51