A phase i study of bortezomib in combination with standard 5-fluorouracil and external-beam radiation therapy for the treatment of locally advanced or metastatic rectal cancer

Background: Standard therapy for stage II/III rectal cancer consists of a fluoropyrimidine and radiation therapy followed by surgery. Preclinical data demonstrated that bortezomib functions as a radiosensitizer in colorectal cancer models. The purpose of this study was to determine the maximum tolerated dose (MTD) of bortezomib in combination with chemotherapy and radiation. Patients and Methods: Patients with locally advanced rectal adenocarcinomas, as staged by endoscopic ultrasound, were eligible. Bortezomib was administered on days 1, 4, 8, and 11 every 21 days for 2 cycles with 5-fluorouracil at 225 mg/m ²/day continuously and 50.4 Gy of radiation. Dose escalation of bortezomib was conducted via a standard 3 + 3 dose escalation design. A subset of patients underwent serial tumor biopsies for correlative studies. Results: Nine patients in 2 dose cohorts were enrolled. Diarrhea was the principal dose-limiting toxicity and occurred at the 1.0-mg/m² dose level. There was no clear evidence of suppression of nuclear factor-κB target gene expression in biopsy samples. Conclusion: The MTD of bortezomib in combination with chemotherapy and radiation may be below a clinically relevant dose, limiting the clinical applicability of this combination. Performing biopsies before and during irradiation for determining gene expression in response to radiation therapy is feasible.