FdCyd and THU for Advanced Solid Tumors

Background: \- FdCyd (also called 5-fluoro-2'-deoxycytidine) and THU (also called tetrahydrouridine) are experimental cancer treatment drugs. FdCyd may change how genes work in cancer cells. THU helps keep FdCyd from being broken down by the body. FdCyd and THU have been given to people on other cancer treatment trials, usually by vein. Researchers want to give FdCyd and THU by mouth to see if they work against cancers that have not responded to earlier treatments. Objectives: \- To test oral FdCyd and THU on advanced solid tumors that have not responded to earlier treatments. Eligibility: \- Individuals at least 18 years of age who have advanced solid tumors that have not responded to standard treatments. Design: * Participants will be screened with a physical exam and medical history. Blood and urine samples will be collected. Imaging studies and tumor samples will used to study the cancer before treatment. * FdCyd and THU will be given in 21-day cycles. THU should be taken 30 minutes before taking FdCyd. * Participants will take FdCyd and THU by mouth, once a day, for 3 days at the beginning of the first and second weeks of each cycle (days 1 3 and 8 10). The drugs will not be taken during the entire third week of each cycle. * Treatment will be monitored with frequent blood tests and imaging studies. * Treatment will continue as long as the cancer is responding to the drugs and serious side effects do not develop.