The investigators planned a phase I study of preoperative CRT with capecitabine plus temozolomide inpatients with locally advanced resectable rectal cancer: 1) the role of temozolomide as a radiosensitizer has been well established, 2) hypermethylation (or low expression) of MGMT promoter is associated with colorectal carcinogenesis, can be found in 20~40% of colorectal cancer patients, and this proportion could be adequate for validation as its role of predictive biomarker, and 3) temozolomide can be additive or synergistic because radiotherapy is now essential in the treatment of rectal cancer.

Preoperative chemoradiation (CRT) with fluoropyrimidine (5-fluorouracil or capecitabine) is now regarded as a standard treatment option in patients with locally advanced resectable rectal cancer, and pathologic response rates and tumor regression grades after preoperative CRT have been proved to be important prognostic factors for survival outcomes. Several studies of preoperative CRT with fluoropyrimidines plus other agents, such as oxaliplatin, irinotecan, cetuximab, and bevacizumab, have been performed to improve pathologic response rates; however, they have failed to show improved results compared to those with fluoropyrimidine alone. Thus, fluoropyrimidine alone is a standard chemotherapeutic strategy in patients with locally advanced resectable rectal cancer who will be treated with preoperative CRT at present; further studies are needed to find effective combination for improving pathologic responses other than fluoropyrimidines alone in these patient population.

Temozolomide is an oral alkylating agent, and has been proved to be effective in patients with glioblastoma or high grade anaplastic glioma when administered with concurrent radiotherapy either as adjuvant or recurrent settings, and also seemed to be effective in patients with malignant melanoma either as monotherapy or combination chemotherapy. Temozolomide has been known to deplete O6-methylguanine DNA methyltransferase (MGMT), which is one of the DNA repair enzymes, and recent studies have shown that lower expression (by immunohistochemistry) or hypermethylation (by methylation-specific PCR) of MGMT could be a predictive marker of better responses to treatment with temozolomide in patient with glioblastoma, high grade anaplastic glioma or malignant melanoma.

Silencing of MGMT by promoter hypermethylation has been known to involve colorectal carcinogenesis pathway by the association with KRAS mutation and low-CIMP (CpG island methylation phenotype), and hypermethylation of MGMT promoter could be detected in 29% to 46% of tumor tissues from sporadic colorectal cancer patients. Nagasaka et al. showed notable results that MGMT promoter methylation status was associated with microsatellite instability and hypermethylation of MGMT promoter could be a predictive factor of low recurrence in colorectal cancer patients with adjuvant oral fluoropyrimidine chemotherapy after curative surgery. In addition, Shacham-Shmueli et al. showed that temozolomide could be an additional treatment option in a small group of patients with chemotherapy-refractory metastatic colorectal cancer which had lower MGMT expressions.

Inclusion Criteria:

• Histologically confirmed adenocarcinoma of the rectum
• Tumor located within 12cm of anal verge
• Clinical stage of T3-4 or N+ by rectal MRI with or without endorectal ultrasound
• Available tumor samples before study treatment (fresh or paraffin-embedded) for immunohistochemistry (IHC) and methylation-specific PCR (MSP) to investigate MGMT expression and hypermethylation
• Male or female aged over 20 years
• Be ambulatory and have an Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
• No prior systemic treatment (chemotherapy, immunotherapy) or radiation therapy
• Adequate major organ functions as following:
• Be willing and able to comply with the protocol for the duration of the study.
• Give written informed consent prior to study-specific screening procedures, with the understanding that the patient has the right to withdraw from the study at any time, without prejudice.

Exclusion Criteria:

• Histology other than adenocarcinoma or tumor arising from inflammatory bowel disease
• Inadequate tumor sample for MGMT IHC or MSP
• Any evidence of systemic metastasis
• Unresected synchronous colon cancer
• Intestinal obstructions or impending intestinal obstruction, but bypass surgery (colostomy or ileostomy) is permitted before study treatment
• Uncontrolled or severe cardiovascular disease
• Serious concurrent infection or nonmalignant illness that is uncontrolled or whose control may be jeopardized by complications of study therapy.
• Other malignancy within the past 5 years except cured non-melanomatous skin cancer, carcinoma in situ of the cervix, or thyroid papillary carcinoma.
• Organ allografts requiring immunosuppressive therapy.
• Psychiatric disorder or uncontrolled seizure that would preclude compliance.
• Pregnant, nursing women or patients with reproductive potential without contraception.
• Patients receiving a concomitant treatment with drugs interacting with 5-FU such as flucytosine, phenytoin, or warfarin et al.
• Known dihydropyrimidine dehydrogenase (DPD) deficiency.
• Known hypersensitivity to any of the components of the study medications.