Breast cancer is the most common female cancer in Hong Kong. The human epidermal growth factor receptor HER2/neu gene is amplified and overexpressed in 15% to 25% of breast cancers.1 The management of human epidermal growth factor receptor 2 (HER2)–positive (HER2+) breast cancer has changed dramatically with the introduction and widespread use of HER2-targeted therapies. The landmark study reported by Slamon et al2 over a decade ago established the combination of trastuzumab with chemotherapy as the standard of care for patients with HER2+ metastatic breast cancer. The later CLEOPATRA trial showed that the combination of pertuzumab with trastuzumab and chemotherapy (specifically, docetaxel) could further improve survival when compared with the standard arm of trastuzumab plus chemotherapy in the first-line setting.3

Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate that incorporates the HER2-targeted antitumour properties of trastuzumab with the cytotoxic activity of the microtubule inhibitor DM1 (which is a derivative of maytansine). The high potency of the cytotoxic DM1 moiety has been suggested as a key factor in the enhanced activity of this compound.4 5 In the second-line setting, the pivotal EMILIA study compared T-DM1 with lapatinib plus capecitabine among patients with HER2+ breast cancer who had previously been treated with trastuzumab and a taxane; T-DM1 showed remarkable activity with an acceptable toxicity profile.6 There is, however, relatively limited real-world information about the effectiveness and safety of T-DM1 in Hong Kong Chinese patients.

In this multicentre retrospective study, we assessed the efficacy and toxicity profiles among local patients with HER2+ advanced breast cancer who had received T-DM1 therapy in the second-line setting and beyond.

This was a retrospective study that involved five local centres that care for over 80% of the local breast cancer population, and included the Pamela Youde Nethersole Eastern Hospital, Prince of Wales Hospital, Queen Mary Hospital, Queen Elizabeth Hospital, and Tuen Mun Hospital between December 2013 and December 2015, the period when the relevant treatment was first started. The institutional ethics committee of each participating centre approved the study.

Inclusion criteria included patients who had recurrent or metastatic histologically confirmed HER2+ breast cancer who either had progressed during trastuzumab with chemotherapy in the first-line treatment setting, or had developed progressive disease after at least one line of anti-HER2 agent including trastuzumab. Patients who had received endocrine therapy for recurrent or metastatic disease were included. Exclusion criteria included patients who received T-DM1 as first-line treatment for recurrent or metastatic HER2+ breast cancer.

Patient charts were reviewed for background information, T-DM1 response, and toxicity data by medical staff who were not blinded to the study objectives. Biochemical and haematological profiles were extracted from patient charts. Tumour response assessments were recorded according to the Response Evaluation Criteria in Solid Tumors Committee.7 Adverse events were graded according to the National Cancer Institute’s Common Terminology Criteria for Adverse Events (version 3.0). Adverse events were also documented during chemotherapy and 28 days after the last dose of study medication.

Outcomes in terms of tumour response, progression-free survival (PFS), and overall survival (OS) were determined. The PFS was assessed from day 1 of treatment cycle 1 to the date when objective disease progression was observed, and OS was calculated from day 1 of treatment cycle 1 to the date of death. Death was regarded as a progression event in those subjects who died before disease progression. Subjects without documented objective progression at the time of the final analysis were censored at the date of their last tumour assessment; data cut-off was on 31 August 2016. Survival curves were constructed using the Kaplan-Meier method.

Patient characteristics are shown in Table 1. Of a total of 37 patients, 33 (89.2%) had an Eastern Cooperative Oncology Group performance status of 0 or 1.

Of the 37 patients, tumour biology studies at initial disease presentation showed that 15 (40.5%) patients were oestrogen receptor (ER)–positive, 10 (27.0%) were progesterone receptor (PR)–positive, and 31 (83.8%) had HER2+ breast cancer. Overall, 21 patients had tumour re-biopsy at the time of developing metastatic disease, 10 (47.6%) patients were ER-positive, nine (42.9%) PR-positive, and 21 (100%) had HER2+ (which included six patients who were found to have HER2+ tumours only when anti-HER2 therapy was considered for metastatic disease).

At the time of initiating T-DM1 therapy, 21 patients had three or more disease sites involved; the most common sites included lymph nodes (n=27, 73.0%), lungs (n=20, 54.1%), and bones (n=19, 51.4%).

Prior treatments that patients received are listed in Table 1. With regard to adjuvant treatments, 13 (35.1%) patients had prior adjuvant trastuzumab, 21 (56.8%) had adjuvant chemotherapy, eight (21.6%) had adjuvant endocrine therapy, and 19 (51.4%) had adjuvant radiotherapy.

With regard to treatment for recurrent/ metastatic disease, nine (24.3%) patients had one line of prior trastuzumab with chemotherapy including three who had trastuzumab in combination with pertuzumab and chemotherapy; 11 (29.7%) had two lines while 17 (45.9%) had three or more lines of anti-HER2 therapy. Overall, 22 (59.5%) patients had received prior lapatinib, and five (13.5%) had received pertuzumab beyond the first-line setting.

A total of 26 (70.3%) patients had received two or more lines of palliative chemotherapy, with the majority having received taxanes (n=33, 89.2%), capecitabine (n=23, 62.2%) and vinorelbine (n=17, 45.9%). Nineteen patients had received one or more lines of palliative endocrine therapy, these included eight (21.6%) with tamoxifen, 15 (40.5%) with aromatase inhibitors, and seven (18.9%) with ovarian ablation.

The median number of days from last anti-HER2 therapy to the first dose of T-DM1 was 32 days (range, 14-274 days).

The median number of cycles was six (range, 1-43). The follow-up data were frozen on 31 August 2016. The median follow-up period was 15.6 months (95% confidence interval [CI], 8.1-20.4 months). Overall, 33 patients were started on the standard dose of 3.6 mg/kg, given once every 3 weeks; 13 patients had dose delay, 10 patients had dose reduction for subsequent cycles, and six patients had both dose delay and dose reductions for subsequent cycles. A total of 326 cycles were administered; 44 (13.5%) cycles were delayed, 11 (3.4%) cycles had further dose reductions in the subsequent cycles, and 51 (15.6%) cycles had both dose delay and dose reductions.

At the time of data cut-off, 28 had discontinued T-DM1 treatment: 20 (71.4%) due to progressive disease, four (14.3%) were lost to follow-up, one (3.6%) due to patient withdrawal, and three (10.7%) due to unspecified causes. No patient discontinued treatment due to intolerable toxicities.

Among the 37 patients, there were three (8.1%) complete response (CR), eight (21.6%) partial response (PR), 11 (29.7%) stable disease (SD), and 12 (32.4%) progressive disease; three patients could not be assessed (ie they did not have response assessment documented during their treatment). The median duration of response was 17.3 months (interquartile range, 9.4-24.5; 95% confidence interval, 8.4-24.8 months). The clinical benefit rate, defined as CR, PR, or SD of 12 weeks or longer, was 37.8% (95% CI, 22.2%-53.5%).

Overall, based on the Kaplan-Meier method, the median PFS was 6.0 (95% CI, 3.3-9.8) months; the 6-month and 12-month PFSs were 51.6% and 23.1%, respectively (Fig a). The median duration of follow-up for PFS was 5.0 (interquartile range, 2.2-10.3) months. The median OS was not reached; the 6-month and 12-month OSs were 82.1% and 74.4%; respectively (Fig b).

Haematological and non-haematological toxicities are listed in Table 2. Grade 3 or 4 toxicities that occurred in one or more patients included thrombocytopenia (n=5, 13.5%), raised alanine transaminase (n=3, 8.1%), anaemia (n=2, 5.4%), and hypokalaemia (n=1, 2.7%). Apart from these, other toxicities that occurred in more than 10% of patients included raised alkaline phosphatase, hyponatraemia, neutropenia, leukopenia, fatigue, raised serum creatinine, and diarrhoea. There was no cardiac toxicity and no patients died as a result of toxicities.

During the past decade, the treatment of HER2+ breast cancer has rapidly evolved, and patients with HER2+ metastatic breast cancer have experienced a remarkable improvement in clinical outcomes in terms of OS.8

The efficacy of T-DM1 was well demonstrated in the pivotal EMILIA study that compared T-DM1 with lapatinib plus capecitabine among HER2+ breast cancer patients in the second-line setting. The studied patients had previously been treated with trastuzumab and a taxane. For the T-DM1–treated patients, the objective response rate was 44%, the median PFS was 9.6 months, and the median OS was 30.9 months.6

In the current multicentre retrospective study among the Chinese patients with breast cancer, over 70% were heavily pretreated with anti-HER2 agents as well as cytotoxic chemotherapy. The efficacy results are consistent with previous findings from the TH3RESA study.9 The latter involved over 600 HER2+ patients with advanced breast cancer who had received two or more anti-HER2–containing regimens, including trastuzumab and lapatinib, and previous taxane therapy. At a median follow-up of 6.5 months, the TH3RESA study reported that among the T-DM1–treated patients, the objective response rate was 31%, the median duration of response was 9.7 months, the median PFS was 6.2 months, and the median OS was not reached.9 Similarly, the safety profile in the current study was consistent with the reported clinical trials, where grade 3 or worse thrombocytopenia was the most commonly reported adverse event (13.5%), followed by raised alanine transaminase (8.1%), anaemia (5.4%), and hypokalaemia (2.7%). Notably there was no grade 3 or worse neutropenia, no febrile neutropenia, and no cardiac toxicity noted in the current study.

In heavily pretreated patient populations, two studies, namely the TH3RESA study9 and the EGF104900 study10 (which assessed combination of trastuzumab and lapatinib in the absence of chemotherapy), have shown that even after a median of four prior regimens, the use of anti-HER2 therapy can lead to meaningful clinical benefits. In the TH3RESA study, the PFS benefit with T-DM1 was observed in subgroups including hormone receptor–positive tumours and non-visceral disease, as well as asymptomatic or treated brain metastases. An exploratory analysis conducted in the present study revealed that the median PFSs for patients with hormone receptor–positive disease and hormone receptor–negative disease were 7.5 and 6.0 months, respectively. Owing to small patient numbers, the finding was not significant (P=0.78) but nonetheless lends support to the published data.

Among the 37 patients in the current study, five had prior pertuzumab therapy in addition to trastuzumab (including one who also had lapatinib). One of these patients achieved PR and had a total of eight cycles of T-DM1 treatment. The efficacy of T-DM1 among patients previously treated with trastuzumab and pertuzumab has recently been reported in a retrospective study.11 Although the response rate was relatively low at 18%, 30% of the patients had received prolonged T-DM1 therapy, defined as treatment duration of 6 months or longer.

It has to be noted that despite the efficacy shown in the second-line and beyond setting among HER2+ patients with advanced breast cancer, the MARIANNE study, which tested three different anti-HER2 regimens in the first-line setting, did not show T-DM1 to be superior to standard treatment.12 In that study, previously untreated patients with HER2+ metastatic breast cancer were randomised to one of the three arms: control (trastuzumab plus taxane), T-DM1 alone, or T-DM1 plus pertuzumab. Although the results revealed that grade 3 or higher adverse events were lower in the T-DM1 arm, efficacy data on PFS were similar in all three arms, at 13.7 months, 14.1 months, and 15.2 months, respectively. In another exploratory analysis in the present study, the PFS of those patients who had undergone only one line of prior anti-HER2 therapy was compared with those who had two or more lines of anti-HER2 therapy revealed corresponding figures of 8.2 and 5.1 months, respectively (P=0.34).

In addition, cost-effective analysis has been conducted in a number of countries with regard to the use of T-DM1. For patients with HER2+ metastatic breast cancer, the Canadian analysis demonstrated that utilising T-DM1 could lead to substantial savings for the public health care system when the costs of treatment-related adverse events incurred by other anti-cancer agents were taken into account.13 Nonetheless, analyses based in the United Kingdom and the United States have not supported such findings.14 15 16

The identification of an appropriate patient population for the utilisation of T-DM1 may enable better resource allocation. Yet to date, no biomarkers have been identified that can predict better outcome among patients with HER2+ advanced breast cancer treated with T-DM1. Based on the biomarker analyses from EMILIA and TH3RESA studies, T-DM1 was similarly effective in the presence of PI3K wild-type or mutated tumours, and the benefit with T-DM1 was seen irrespective of HER2 mRNA, HER3 mRNA, or PTEN protein level.17 18

The current study is limited by its retrospective design, possible information bias during data retrieval/extraction/coding, as well as the small number of patients (especially for subgroup analysis) and inadequate follow-up period for OS. Although the results could not be compared directly with reported prospective trials, patients were representative, and treatment and outcomes reflect routine clinical practice. The T-DM1 therapy provided a meaningful PFS with a favourable toxicity profile among heavily pretreated patients with HER2+ advanced breast cancer. Research is needed to identify biomarkers that will predict sensitivity and resistance to individual anti-HER2 agents, and thereby enable identification of those patients most likely to respond to T-DM1 and appropriate treatment to optimise patient benefit, reduce excessive toxicities, and minimise costs.

The T-DM1 therapy has a tolerable toxicity profile among local patients with recurrent or metastatic HER2+ breast cancer. For patients who responded to T-DM1 therapy, there was a durable response. In our study, T-DM1 is associated with a PFS of 6 months and an OS that has not been reached. Further biomarker study is needed to enable appropriate patient selection for this treatment.

We thank Dr Vicky TC Chan of the Department of Clinical Oncology, Prince of Wales Hospital, and Drs Carol Kwok and Raymond KY Wong of the Department of Oncology, Princess Margaret Hospital, for their support in this study.

This study has been supported by the Hong Kong Breast Oncology Group. W Yeo has received honoraria for expert opinion from Novartis and Pfizer and has received a research grant from Mundipharma in relation to breast cancer research over the past 12 months. The funder had no role in study selection, quality assessment, data analysis, or writing the manuscript. All other authors have disclosed no conflicts of interest.

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