Background and importance The use of pertuzumab with trastuzumab in neoadjuvant therapy in breast cancer treatment is supported by two phase II clinical trials (NeosPhere and Tryphaena) that showed better rates of pathological complete response. In addition, Cobleigh et al described how the response to trastuzumab could be conditioned by their plasma levels.

Aim and objectives We analysed the rates of pathological response to neoadjuvant treatment under usual clinical practice conditions.

Material and methods A prospective study was conducted in women diagnosed with HER2 positive (HER2+) breast cancer who completed treatment from 2016 to 2019. To perform the assay, 2 mL of blood, corresponding to the first Cmin of trastuzumab were taken. Determination of the presence of ADA-trastuzumab was carried out with of an ELISA immunoassay. Informed consent was obtained from all patients.

Results A total of 40 patients (women) were studied with a median age of 50.6 years (39–71). The chemotherapy scheme used was adriamycin–cyclophosphamide (AC) followed by taxane with trastuzumab and in some cases pertuzumab.

In the pertuzumab group (n=27), response rates and mean levels of trastuzumab in the first Cmin (μg/mL) were:

• Complete pathological response (RCBO) in 17 (62.9%, n=17), (Cmin=22.30 μg/mL).

• Minimum residual response (RCBI) in 25.9% (n=7) (Cmin=23.50 μg/mL).

• Moderate residual response (RCBII) in 11.1% (n=3) (Cmin=22.30 μg/mL).

In the trastuzumab group (n=13), responses were:

• RCBO in 76.9% (n=10) (Cmin=16.40 μg/mL).

• RCBI in 15.4% (n=2) (Cmin=29.18 μg/mL).

• RCBII in 7.7% (n=1) (Cmin=18.7 μg/mL)

In our study, no difference was found between pathological responses and plasma levels of AD (Pearson 0.033, p=0.840), which supposes a scarce correlation between plasma concentrations of AE and the pathological response obtained. There were no differences between the pathological responses obtained and the plasma concentrations of AD (p=0.639).

Conclusion and relevance Previous studies by our team were unable to identify, under usual clinical practice conditions, differences in the pathology response of neoadjuvant treatment with trastuzumab versus trastuzumab with pertuzumab in patients with infiltrating ductal breast carcinoma HER2+. In the present, we have shown that the plasma levels of trastuzumab do not seem to correlate with this response.

References and/or acknowledgements No conflict of interest.