Leflunomide in Patients With PTEN-Altered Advanced Solid Malignancies and HER2 Negative Breast Cancer

This study tests leflunomide (a drug currently used for arthritis) as a new anti-cancer treatment in patients with advanced solid tumors that have lost the PTEN tumor suppressor gene, or in patients with HER2-negative advanced breast cancer who have exhausted other treatment options. **You may be eligible if...** - You are 18 or older with advanced or metastatic cancer - Your tumor has confirmed loss of PTEN protein expression or a PTEN gene mutation, OR you have advanced HER2-negative breast cancer - Your cancer has progressed on standard therapies - You are in good health (ECOG 0–2) with adequate blood, liver, and kidney function - You are willing to use contraception (or commit to a prolonged elimination protocol after stopping) **You may NOT be eligible if...** - You have a primary brain tumor - You received chemotherapy or radiation within 4 weeks before starting the study - You have a history of hepatitis B or C (due to leflunomide's effects on the liver) - You are allergic to leflunomide or teriflunomide - You have an uncontrolled infection, serious heart condition, or major psychiatric illness - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.