Circulating tumor DNA in HER2 amplified breast cancer: a translational research substudy of the NeoALTTO phase 3 trial

This translational research project aims to evaluate whether circulating tumor DNA (ctDNA) can be used as a biomarker to assess treatment response in patients with human epidermal growth factor receptor 2 (HER2)-positive early breast cancer treated with neoadjuvant anti-HER2 therapy in the context of the NeoALTTO trial. We have shown that the detection of ctDNA before neoadjuvant anti-HER2 targeted therapies is associated with decreased probability of pCR. Moreover, patients with HER2 enriched subtype tumors and undetectable ctDNA at baseline had the highest pCR rates. These findings support the analysis of ctDNA as a novel biomarker for patient response to anti-HER2 treatment and to assist de-/escalating therapeutic strategies, therefore allowing to avoid over-treatment in patients at lowest risk, and more intensive approaches in patients showing suboptimal responses to therapy. ABSTRACT Purpose: In the neoadjuvant treatment (NAT) setting, dual human epidermal growth factor receptor 2 (HER2) targeted therapy is associated with increased pathological complete response (pCR) rates compared to each therapy alone. Biomarkers allowing to predict treatment response during NAT are needed. We aim to evaluate whether circulating tumor DNA (ctDNA) is associated with response to anti-HER2 targeted therapy. Experimental design: Plasma DNA collected before NAT, at week2 and before surgery from patients enrolled in the NeoALTTO trial was assessed using digital PCR for PIK3CA and TP53 mutation detection. Results: A total of 69 of 455 (15.2%) patients had a PIK3CA and/or TP53 mutation detected in the baseline tumor sample and evaluable ctDNA results from baseline samples. CtDNA was detected in 41%, 20% and 5% patients before NAT, at week 2 and before surgery respectively. CtDNA detection before NAT was significantly associated with older age and ER-negative status. CtDNA detection before NAT was associated with decreased odds of achieving pCR (OR 0.15, 95%CI 0.034-0.7, p=0.0089) but not with event-free survival (EFS). Analyses for EFS were underpowered. Interestingly, the patients with HER2 enriched subtype tumors and undetectable ctDNA at baseline had the highest pCR rates. By contrast, patients with persistent ctDNA detection at baseline and week 2 had the lowest rate of pCR. Conclusions: CtDNA detection before neoadjuvant anti-HER2 therapies is associated with decreased pCR rates. Interestingly, patients with HER2 enriched tumors and undetectable ctDNA at baseline had the highest pCR rates, therefore appearing as the best candidates for treatment de-escalation strategies. The presence of ctDNA was assessed through analysis of PIK3CA and TP53 mutations, these genes being the two most frequently mutated genes in breast cancer (18). A total of 69 of 455 (15.2%) patients had PIK3CA and/or TP53 mutations detected in the baseline tumor samples and evaluable ctDNA results from baseline. Our analyses showed that ctDNA was detected in 41%, 20% and 5% patients before NAT, at week 2 and before surgery respectively. The 41% detection rate prior to any treatment is similar to previous studies reporting a detection rate between 50 and 75% in patients with early stage breast cancer, irrespective of the molecular subtypes (10,11,19,20). Of note, our analysis was focused on the assessment of mutations in only 2 genes, namely PIK3CA and TP53 . Interrogating additional mutations or copy number changes including HER2 amplification may potentially have increased the rate of ctDNA detection. Nonetheless, our study represents the largest analysis to date of early breast cancer patients receiving NAT for HER2-amplified breast cancer to report associations between ctDNA detection and pCR. We have shown that mutant ctDNA detected at baseline prior to NAT was associated with decreased odds of achieving pCR.