PROBLEM TO BE SOLVED: To provide a novel pharmaceutical composition for treating diseases attributed to a deficiency of phosphatase and tensin homolog (PTEN), particularly cancers, such as small cell lung carcinoma, meningioma, head and neck cancer, glioblastoma multiforme, anaplastic astrocytoma, and colorectal cancer.SOLUTION: The invention provides a pharmaceutical composition containing dihydropyrido phthalazinone system compound represented by formula (I) as an active ingredient. (Rto Rare each independently H, halogen, alkenyl, alkoxy, alkyl, or the like； Rand Rare each independently H, alkyl, hydro alkylene, or the like； A and B are each independently H or the like； Y and Z are each independently substitutable phenyl, substitutable heteroaryl, H, alkyl, alkynyl, or the like.)SELECTED DRAWING: Figure 1【課題】ホスファターゼ－テンシンホモログ（ＰＴＥＮ）の欠損に起因する疾患、特に、小細胞肺癌、髄膜腫、頭頸部癌、多形成膠芽腫、退形成性星細胞腫、結腸直腸癌等の癌を治療する新規医薬組成物の提供。【解決手段】式（Ｉ）で表されるジヒドロピリドフタラジノン系化合物を有効成分として含有する医薬組成物。（Ｒ１～Ｒ３は各々独立にＨ，ハロゲン、アルケニル、アルコキシ、アルキル等；Ｒ４及びＲ５は各々独立にＨ、アルキル、ヒドロアルキレン等；Ａ、Ｂは各々独立にＨ等；Ｙ及びＺは各々独立に置換可フェニル、置換可ヘテロアリール、Ｈ、アルキル、アルキニル等）【選択図】図１
