High SMAD4 Expression is Associated With Better Clinical Outcomes in Patients With Resectable Pancreatic Cancer: An Analysis of NRG Oncology/RTOG 9704

Abstract

SMAD4 (DPC4) is a transcription factor that mediates TGF-beta signal transduction and is mutationally in-activated in about one-third of pancreatic cancer (PC). Studies have suggested that SMAD4 loss is associated with a higher rate of distant metastasis (DM), but other studies have refuted this claim. An assessment of SMAD4 expression was performed in NRG/RTOG 9704, a prospective phase III trial comparing different post-operative chemotherapy regimens combined with 5-fluorouracil-based chemoradiation for resectable PC. It was hypothesized that higher SMAD4 expression is associated with lower incidence of DM and better disease-free and overall survival (DFS & OS).An automated quantitative immunofluorescence SMAD4 analysis was performed in pan-cytokeratin positive tumor cells on a tissue microarray with samples from NRG/RTOG 9704, using a microscope and image analysis software. Total cellular SMAD4 was analyzed as a categorical variable dichotomized by the median. DFS & OS were estimated with the Kaplan-Meier method & groupings compared with the log-rank test. Local-regional recurrence (LRR) & DM were estimated by the cumulative incidence method & groupings compared with Gray's test. Univariate & multivariable Cox proportional hazards (OS & DFS) or Fine-Gray (DM & LRR) regression models were used to evaluate associations between SMAD4 & outcomes. SMAD4 was included in all models; stepwise selection identified significant variables among treatment, age, gender, race, CA19-9, tumor location & diameter, nodes, & surgical margin status.Of 451 eligible patients on NRG/RTOG 9704, tissue from 141 patients was analyzable. There were no significant differences in baseline characteristics/outcomes for patients with & without tissue. There were no statistically significant differences in baseline characteristics between the low & high SMAD4 groupings. High SMAD4 expression was associated with better DFS & OS compared to low SMAD4, with a 32% risk reduction (RRed) of DFS failure [HR (95% CI): 0.68 (0.48, 0.97); P = 0.03] & a 31% RRed of death [HR (95% CI): 0.69 (0.48, 0.99); P = 0.04]. This difference in DFS appeared to be due to a lower incidence of DM in tumors with high SMAD4, with a 31% RRed of failing distantly [HR (95% CI): 0.69 (0.48, 0.99), P = 0.048]. No differences were detected in LRR. After adjusting for CA19-9 on multivariable analysis, high SMAD4 remained associated with better DFS with a 34% RRed of failing [HR (95% CI): 0.66 (0.46, 0.96), P = 0.03]. Tumors with high SMAD4 showed a trend of being associated with less DM [HR (95% CI): 0.68 (0.46, 1.004), P = 0.052], after adjusting for age & surgical margin status.Tumor expression of SMAD4 is significantly associated with better clinical outcome, specifically lower DM and higher DFS and OS, in tumor samples from a large, phase III randomized trial of resectable pancreatic cancer.