The purpose of this study is to determine whether topical nitroglycerin in addition to 5-flourouracil and radiation therapy are effective in the treatment of operable rectal cancer.

This is an open label, non-randomized, multi-cohort, dose escalation trial to evaluate the safety, tolerability, feasibility and maximum tolerated dose (MTD) of topical nitroglycerin in addition to 5-flourouracil and radiation therapy for neo-adjuvant treatment of, T3-T4 or clinically node positive, operable rectal cancer.

Patients that would otherwise be eligible for concurrent neo-adjuvant chemotherapy with continuous infusion 5-FU, for locally advanced operable rectal cancer, will be assigned to 4 sequential cohorts of 3 different dose levels of nitro glycerin patches (0.2; 0.4; and 0.6 mg/hr). Each cohort will consist of 3 patients. All patients will receive radiation therapy, 45-50 Gy in 25-28 fractions to the pelvis along with continuous infusion 5-FU 225mg/M2 for the duration of the radiation therapy. The radiation therapy will be planned and delivered as per institutional standard of care for the Dallas VAMC radiation oncology department.

Inclusion Criteria:

• Biopsy proven Rectal Adenocarcinoma
• Histological diagnosis of operable T3-4, or T1-4 node positive, M0 rectal adenocarcinoma using endorectal ultrasound and/or MRI in addition to Computed Tomography (CT) of the chest, abdomen and pelvis for pre-study staging Acceptable alternatives for systemic staging instead of CT chest , abdomen, pelvis are CT abdomen and pelvis plus a Chest X-ray or a PET/CT
• Ability to give informed consent and willingness to adhere to study protocol
• Age ≥ 18 years and otherwise eligible to receive medical care at the Dallas VA Medical Center.
• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2
• Adequate hematological, hepatic and renal function defined as in protocol.

Exclusion Criteria:

• Any condition that would hamper informed consent or ability to comply with the study protocol
• Significant history of cardiac disease, e.g. uncontrolled hypertension, unstable angina, decompensated congestive-heart failure, myocardial infarction within the last six months or ventricular arrhythmias requiring medication.
• Pregnant and lactating women.
• Patients taking Phosphodiesterase - 5 inhibitors (e.g. Sildenafil, Vardenafil or Tadalafil) and is unable to stop for the duration of the chemoradiotherapy.

From the first clinic visit to 4-12 weeks after surgery or until resolution of all acute side effects from radiation had occurred, approximately 5 years

investigator assessment, regular laboratory testing