DISEASE CHARACTERISTICS: - Histologically or cytologically confirmed breast cancer meeting the following criteria: - Estrogen receptor-negative - Newly diagnosed metastatic disease to the brain - Patients with histologically or cytologically confirmed other cancers, such as lung cancer, melanoma, etc., with newly diagnosed metastatic disease to the brain allowed (phase I study only) - Measurable disease in the brain, defined as ≥ 1 lesion that can be accurately measured in ≥ 2 dimensions (longest diameter and its longest perpendicular diameter to be recorded) - Tumor HER2/neu status positive or negative - No leptomeningeal metastases - Up to 10 patients with leptomeningeal metastases may be enrolled in order to test the feasibility of this therapy PATIENT CHARACTERISTICS: - Menopausal status not specified - Karnofsky performance status (PS) 70-100% - Recursive Partitioning Analysis (RPA) class I or II - A small feasibility cohort of 10 RPA class III (Karnofsky PS < 70%) allowed, however they will not be included in the efficacy analysis - WBC ≥ 3,000/mm³ - ANC ≥ 1,000/mm³ - Platelet count ≥ 100,000/mm³ - Hemoglobin ≥ 9 g/dL - Total bilirubin normal - AST/ALT ≤ 2.5 times upper limit of normal - Creatinine normal OR creatinine clearance ≥ 60 mL/min - Fertile patients must use 2 forms of effective contraception (i.e., barrier contraception and one other method of contraception) for the duration of study and for ≥ 12 months after treatment - Negative pregnancy test - Not pregnant or nursing - Able to swallow pills - No history of allergic reactions attributed to compounds of similar chemical or biologic composition to gamma-secretase inhibitor RO4929097 - No malabsorption syndrome or other condition that would interfere with intestinal absorption - Not known to be serologically positive for hepatitis A, B, or C, or have a history of liver disease, other forms of hepatitis, or cirrhosis - No uncontrolled hypocalcemia, hypomagnesemia, hyponatremia, hypophosphatemia, or hypokalemia despite adequate electrolyte supplementation - No uncontrolled intercurrent illness including, but not limited to, any of the following: - Ongoing or active infection - NYHA class III or IV symptomatic congestive heart failure - Unstable angina pectoris - Cardiac arrhythmia other than chronic stable atrial fibrillation - Psychiatric illness and/or social situations that would limit compliance with study requirements - Baseline QTcF > 450 msec (male) or QTcF > 470 msec (female) PRIOR CONCURRENT THERAPY: - Any type or number of prior therapies allowed - No prior therapy with Notch inhibitors - No prior cranial radiation - Therapy-naive patients allowed - At least 14 days since any prior experimental therapy, chemotherapy, or radiotherapy and recovered to < grade 2 toxicities - No other concurrent investigational agents - No concurrent combination antiretroviral therapy in HIV-positive patients - No concurrent medications with narrow therapeutic indices that are metabolized by cytochrome P450 (CYP450), including warfarin sodium (Coumadin®) - No concurrent medications that are generally accepted by the QTdrugs.org Advisory Board to carry a risk for Torsades de Pointes, including antiemetics - No concurrent medications that are strong inducers/inhibitors or substrates of CYP3A4 - No concurrent medications or food that may interfere with the metabolism of gamma-secretase inhibitor RO4929097, including ketoconazole and grapefruit juice - No other concurrent anticancer agents or therapies