The purpose of this Phase 2 portion of the study is to use the dose and schedule of RX-5902 identified in the phase 1 to treat subjects with triple negative breast cancer.

In this dose-finding, open-label, single-agent study of RX-5902, subjects will be treated for up to 6 cycles of therapy. RX-5902 will be administered orally daily for 3 weeks followed by 1 week of rest in each cycle. All subjects will be followed for at least 30 days after the last dose of study agent for safety.

Inclusion Criteria:

• Male and female who are 18 yrs or older
• Histologically confirmed triple negative breast cancer that are refractory, intolerant, or ineligible to receive approved standard therapies
• Measurable or evaluable disease per Response Evaluation Criteria in Solid Tumors (RECIST)
• Life expectancy of at least 3 months
• Able to swallow capsules
• Provide written informed consent

Exclusion Criteria:

• Primary brain tumor or active brain metastasis
• Not recovered to less than or equal to Grade 1 toxicities (except Grade 2 alopecia or neuropathy) associated with previous cancer therapies
• Any other cancer treatments within 2 weeks of planned study treatment
• History of any medical or psychiatric condition or addictive disorder, or lab abnormality that in the opinion of the investigator, may increase risks or may interfere with study participation or interpretation of study results
• History of clinically significant GI bleed, intestinal obstruction, or GI perforation within 6 months of study dose
• Uncontrolled diabetes
• History of long QT syndrome or clinically significant cardiac arrhythmias (except stable atrial fibrillation)
• Myocardial infarction within 6 months of study dose
• Active infection requiring IV antibiotics within 2 weeks of study dose
• History of Hepatitis B, C, or HIV
• Use of potent inhibitor or inducer of CYP3A4/3A5 within 14 days of planned study treatment or expected requirement for use of such a drug during study
• Use of a potent inhibitor or inducer of drug transporters or conjugating enzymes within 14 days prior to planned study treatment or expected requirement for use of such a drug during study
• Receiving other investigational agents or not yet completed 30 days since completion of an investigational study
• Pregnant, planning a pregnancy, or breast feeding
• Male or female not willing to use adequate contraceptive precautions during the study period. Females must either be surgically sterile, post-menopausal for 12 months, or use a contraceptive approved by sponsor.
• Unwilling or unable to provide written informed consent, comply with study requirements, or be available for follow-up assessments