Predicting Survival In Gastric Cancer Patients Randomized To Docetaxel With Mass Spectrometric Quantitation Of Tubb3.

4068 Background: No predictive biomarker for chemotherapy has been validated for clinical use. A relationship between resistance to taxanes and expression of class III β-tubulin (TUBB3) protein has been suggested by small clinical studies, but not confirmed in randomized trials. Immunohistochemical definitions of “TUBB3 positive” vary widely between studies and depend on subjective measures of strong vs weak staining. We retrospectively evaluated the relationship between survival and TUBB3 protein quantitated by mass spectrometry in the tumor samples of 247 patients from the Intergroup Trial of Adenocarcinoma of the Stomach (ITACA-S). Patients had been randomized to a docetaxel-containing adjuvant regimen or to monotherapy with fluorouracil and leucovorin (5-FU/LV). Methods: Archived tumor tissues were microdissected and solubilized for proteomic analysis. TUBB3 and 44 other protein biomarkers were quantified with a mass spectrometry-based assay. A TUBB3 protein cutoff of 750 amol/ug was predetermined based on the assay’s limit of detection. The Mantel-Cox log-rank test was used for survival comparisons. Results: Among gastric cancer (GC) patients treated with docetaxel-containing chemotherapy (n = 125), those with TUBB3 levels below the cutoff (750 amol/µg of total protein) had a longer median overall survival (mOS) than patients with TUBB3 levels above the cutoff (1563 vs 886 days, p = 0.04). TUBB3 level made no difference in survival among patients who received 5-FU/LV. Of note, among patients with high TUBB3 levels ( > 750 amol), those treated with 5-FU/LV survived 3 years longer than patients in the docetaxel arm (mOS = 1991 vs 886 days, p = 0.048). Conclusions: Quantitative proteomic analysis of TUBB3 identified a subset of GC patients who benefitted from the addition of docetaxel to adjuvant chemotherapy. GC patients with TUBB3-expressing tumors had worse outcomes on a docetaxel-containing regimen than on 5-FU/LV. An ongoing study of triple-negative breast cancer patients treated with docetaxel suggests that proteomic TUBB3 may be predictive in other cancer indications. Personalized chemotherapy based on quantitated TUBB3 is promising and warrants broader evaluation.