Abstract: One aspect of the invention refers to an oral pharmaceutical composition comprising the MDM2-antagonist of formula I in a dose range of 30 mg to 45 mg for use in the treatment of cancer, wherein this oral pharmaceutical composition is administered in a treatment cycle of once every three weeks (D1 q3w), wherein this treatment cycle of once every three weeks (D1 q3w) may be repeated that many times as considered beneficial for the patient from a medical point of view. Another aspect of the invention refers to an MDM2-antagonist of formula I for use in the first line systemic treatment (primary treatment) of dedifferentiated liposarcoma. A further aspect of the invention refers to the use of the MDM2-antagonist of formula I for the manufacture of a medicament for the treatment of a p53-wildtype and non-MDM2-amplified form of cancer.