Trastuzumab-dkst versus trastuzumab: Real-world pCR rates in patients with HER2+breast cancer treated with neoadjuvant chemotherapy plus trastuzumab from Alberta, Canada.

e12569 Background: Biosimilars are defined as medical products that contains a version of the active substance with similar biological characteristics, efficacy, and safety as the original product. Differences however, exist between biosimilars and their original comparator. Unlike small-molecule drugs, identical generic versions cannot be mass produced since manufacturers often have proprietary rights to living cell lines and processes involved in producing biologics. Due to this inherent alteration in production, the potential for clinically meaningful differences may exists. These risks are reduced from bench to clinic through a minimum of one phase 1 study to demonstrate comparability of PK and PD between the biosimilar and the original product, and one phase 3 study to demonstrate equivalency for at least one indication. To date, studies have mainly looked at trastuzumab-dkst in the metastatic setting. This retrospective study compared the pathological complete response (pCR) rates of trastuzumab-dkst to trastuzumab in the neoadjuvant setting for HER2+ early breast cancer (EBC) in Alberta. Methods: Neoadjuvant patients with HER2+ EBC treated with trastuzumab from November 2018 -October 2019 and trastuzumab-dkst from December 2019 - September 2020 were identified. There was no crossover between products. Logistic regression was used to control for variables potentially associated with pCR: trastuzumab product (trastuzumab vs trastuzumab-dkst), age ( < 40 vs 40+), pre-operative T (T1/2 vs T3/4) and N stage (negative vs positive), grade (I/II vs III), HR status (ER and/or PR positive vs ER/PR negative), HER2 (3+ vs SISH+), chemotherapy (anthracycline containing vs not), and chemotherapy completion (yes vs no). Results: 136 patients were identified (56% trastuzumab; 43% trastuzumab-dkst) and there were no significant differences in baseline characteristics except more patients in the trastuzumab-dkst group were clinically N negative; 39% vs 14.3% trastuzumab (p = 0.001). pCR was 35.6% for patients treated with trastuzumab-dkst versus 40.3% with trastuzumab (p = 0.598). In the logistic regression model, there was no significant difference in the odds of a pCR for patients treated with trastuzumab-dkst versus trastuzumab after controlling for the variables selected a priori (OR 1.1, 95% CI 0.5-2.4, p = 0.850). There was a trend for decreased odds of pCR for anthracycline use (OR 0.72 95% CI 0.3-1.6, p = 0.417). Conclusions: pCR rates were similar for patients treated with trastuzumab-dkst compared to trastuzumab in our real world study of HER2+ neoadjuvant EBC and comparable to pivotal phase 3 trials. For a 65 kg patient, the estimated cost savings of trastuzumab-dkst therapy is $22,000, and approximately $240-300 for a non-anthracycline chemotherapy backbone.