Association between timely targeted therapy initiation and clinical outcomes in patients with advanced HER2+ gastroesophageal adenocarcinoma.

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e16048 Background: Delayed treatment initiation is associated with increased mortality in early-stage cancers. Few studies have examined the impact of timely initiation in advanced cancers. Timely targeted therapy initiation can be especially challenging due to the additional biomarker testing required to identify patients eligible for these treatments. We examined the impact of timely targeted therapy initiation on survival among patients with metastatic HER2+ gastroesophageal adenocarcinoma (GEA). Methods: We performed a retrospective cohort study of patients with metastatic HER2+ GEA treated with first line systemic therapy between 1/2011 and 12/2017 using the nationwide Flatiron Health EHR-derived deidentified database. Timely targeted therapy, defined as initiation of the anti-HER2 monoclonal antibody trastuzumab up to 14 days after first line chemotherapy initiation, was entered as a time-varying exposure on the day of initiation. Patients who initiated trastuzumab after the 14-day cut point or who lacked documentation of having received trastuzumab were classified as having non-timely targeted therapy initiation. Extended Cox regression models were used to compare progression-free (PFS) and overall survival (OS) from the date of first line treatment initiation, adjusted for potential confounders. Pre-planned sensitivity analyses assessed alternative “timeliness” definitions as well as the impact of any trastuzumab administration, regardless of the timing of initiation. Results: We included 459 patients with metastatic HER2+ GEA; 293 (63.8%) received trastuzumab, of whom 223 (76.1%) initiated in a timely manner. The median age was 65.0 years, and patients were predominantly male (82.4%) and non-Hispanic White (78.2%) with an ECOG performance status of 0-1 (85.9%). Relative to non-timely initiation, timely targeted therapy was associated with significantly higher OS (2-year OS 29.6% vs 14.6%; adjusted HR 0.72, 95% CI 0.57-0.91, p = 0.006) and PFS (2-year PFS 8.3% vs 3.9%; adjusted HR 0.78, 95% CI 0.62-0.98, p = 0.030). Results remained similar in sensitivity analyses 1) using alternative “timeliness” definitions (up to 70 days after chemotherapy initiation), and 2) comparing any trastuzumab administration, regardless of the timing of initiation, to no trastuzumab administration (OS - adjusted HR 0.75, 95% CI 0.60-0.95, p = 0.018; PFS - adjusted HR 0.81, 95% CI 0.64-1.02, p = 0.069). Conclusions: OS and PFS were higher among patients with metastatic HER2+ GEA treated with trastuzumab, regardless of the timing of targeted therapy initiation. Although our results provide reassurance to clinicians that delays in targeted therapy may not be detrimental to patient outcomes, efforts should still be made to ensure that all patients with metastatic HER2+ GEA receive first line trastuzumab.