Real-World Outcomes of Trastuzumab Deruxtecan in Patients With HER2+ Metastatic Breast Cancer: The DE-REAL Study

Abstract Background Trastuzumab deruxtecan (T-DXd) demonstrated unprecedented efficacy in patients with pretreated HER2+ metastatic breast cancer (mBC). However, few data are available about its efficacy in routine clinical practice. In this multicenter retrospective study, we examined effectiveness and safety of T-DXd in a real-world population. Methods Clinico-pathological information about patients with HER2+ mBC who received T-DXd were collected from 12 Italian hospitals. HER2 status was determined locally. Patients who received at least one administration of T-DXd, as any therapy line for advanced disease were included in the analysis. The primary endpoint was real-word PFS (rwPFS). Results One hundred and forty-three patients were included. Median age was 66 (range: 37-90), and 4 men were included. Hormone receptor (HR) status was positive in 108 (75%) patients and negative in 35(25%). T-DXd was administered as first, second, third, or subsequent lines in 4 (3%), 16 (11%), 42 (29%), and 81 (57%) patients, respectively. Among 123 patients with measurable disease, the ORR was 68%, and the DCR was 93% (9 CRs, 74 PRs, and 30 SD). Nine (7%) patients had a primary resistance to T-DXd. With a median follow-up of 12 months, the median rwPFS was 16 months. RwPFS was 84%, 59%, and 39% at 6, 12, and 18 months, respectively. A favorable trend in rwPFS was reported in patients receiving T-DXd as I/II line versus further lines (17 vs. 15 months; P = .098). Any-grade toxicity was registered in 84 patients (59%). Most common adverse events (AEs) reported were nausea (33%), neutropenia (21%), and asthenia (21%). Liver toxicity and diarrhea were uncommon (5% and 1%). Severe toxicities was registered in 18% of patients, and the most frequent were neutropenia, nausea/vomiting, and ILD observed in 15, 2, and 3 patients. AEs led to dose reduction in 37 patients (26%). Dose reduction and AEs do not affect patients’ response and survival outcomes. Conclusions Efficacy and safety of T-DXd were confirmed in an unselected real-world population of HER2+ mBC. These results are consistent with the results of known findings, and no new safety concerns were reported.


Background
Breast cancer (BC) is the most common malignancy among women, and the second cause of cancer-related mortality worldwide. 1Up to 20% of BCs harbor overexpression of Human Epidermal growth factor Receptor 2 (HER2) protein and/or amplification of the HER2 gene.Of note, HER2positive (HER2+) BC is an especially aggressive disease, and it is characterized by poor prognosis and more frequent relapses. 2,35][6] In the advanced disease setting, the CLEOPATRA trial demonstrated a consistent PFS and OS benefit by adding pertuzumab to trastuzumab and docetaxel in previously untreated patients with HER2+ BC. 5 According to these data, the combination of pertuzumab, trastuzumab, and taxane-based chemotherapy has become the first-line standard of care for advanced HER2+ BC.For a decade, the standard second-line therapy has been represented by the antibody-drug conjugate (ADC) trastuzumab emtansine (T-DM1), which was associated with PFS and OS improvement when compared to capecitabine plus lapatinib in patients with metastatic HER2+ pretreated with trastuzumab and a taxane. 6ecently, the therapeutic scenario for patients with HER2+ mBC has been revolutionized with the introduction of a new ADC, namely Trastuzumab deruxtecan (T-DXd), which has been specifically engineered to combine an anti-HER2 monoclonal antibody (MoAb) function with the delivery of a potent topoisomerase inhibitor, namely DXd, to cancer cells. 7n the phase II trial DESTINY-Breast01 (DB-01), T-DXd demonstrated unprecedented efficacy in heavily pretreated patients with HER2+ mBC.Median PFS registered was 19.4 months associated with a median response duration of 20 months. 8,9Moreover, the phase III trial DESTINY-Breast03 (DB-03) demonstrated that T-DXd is also associated with superior PFS and OS when compared to T-DM1 as a second-line therapy for patients with HER2+ mBC progressing on first-line taxane-trastuzumab-pertuzumab. 10,11ased on results of the DB-03 trial, T-DXd will become the standard-or-care second-line therapy for patients with HER2+ mBC.More recently, T-DXd demonstrated a statistically significant and clinically meaningful improvement in PFS and OS versus treatment of physician's choice in patients with HER2+ unresectable and/or mBC previously treated with T-DM1 (DESTINY-Breast02 trial). 12owever, despite this impressive clinical progress and the rapidly emerging role of T-DXd in HER2+ BC treatment, real-world data about safety and efficacy of T-DXd in routine clinical practice are lacking.Real-world information are urgently needed to better understand the management of this new treatment and its real activity in unselected patients.
In this analysis, we reported the results of a retrospective multicenter study, the DE-REAL study, which investigated the safety, antitumor activity, and efficacy of T-DXd in an unselected real-world population of patients with HER2+ mBC.

Patient Population and Enrollment Criteria
The medical records of 12 Italian referral hospitals were reviewed to identify patients with HER2+ mBC treated with T-DXD.Eligible patients were aged 18 years or older and were required to have a diagnosis of unresectable/metastatic HER2-positive BC.HER2 positivity was determined locally, and defined as 3+ immunohistochemical (IHC) staining or 2+ IHC staining and positive fluorescence in situ hybridization test (FISH). 13Patients who received at least one administration of T-DXd as any therapy line for advanced disease were included in the analysis.

Planning Schedule
Patients received T-DXd (5.4 mg per kilogram of body weight) every 3 weeks, until progression or unacceptable toxicity.

Study Objectives and Endpoints
The primary objective of the study was to evaluate the efficacy of T-DXd in a real-world population of patients with HER2positive mBC.Radiological response was locally evaluated and the tumor assessment was performed every 3 to 4 months according to national and local guidelines. 14All patients were evaluated with full body CT scan.Bone scan, brain MRI, and PET scan were included if clinical indicated.In presence of visceral, measurable disease, RECIST 1.1 criteria were applied locally to evaluate the radiological response. 15Considering the real-world data collected in our study, we selected the real-world PFS (rwPFS) as primary endpoint. 16wPFS was defined as the time from start of treatment to evidence of disease progression or death, whichever occurred first, in a clinical practice context, with no strict protocols for radiological evaluations and centralized review of images. 17,18atients without an rwPFS event were censored at the last time they were known to be alive and free from progression.
Secondary end-points were overall response rate (ORRdefined as complete plus partial responses), disease control rate (DCR), and overall survival (OS).
The proportion of patients that achieved complete response, partial response, and stable disease defined the DCR.
OS was defined as the time from start of T-DXd treatment to death; patients without an OS event were censored at the last time they were known to be alive.Milestone rwPFS and OS were evaluated at 6, 12, and 18 months, respectively.
Furthermore, adverse events (AEs) and safety profile were evaluated.Treatment-related AEs collected in the referral centers were categorized and graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE), version 5.0. 19

Statistical Analysis
Mean ± standard deviation and number (percentages) were used for summarizing data.Continuous variables were analyzed using independent t-test, while categorical variables were studied using chi square or Fisher's exact test.Univariate and multivariate analysis were performed to identify independent factors that had an impact on real-world progression-free survival (rwPFS) and overall survival (OS).The Kaplan-Meier method was used to estimate the distribution of rwPFS and OS.Differences among groups in time-to-event endpoints were tested with unstratified log-rank tests.Hazard ratios (HR) with their relative 95% confidence intervals (95% CI) were estimated with Cox regression analyses.

Study Population
Between April 2021 and October 2022, 143 patients were enrolled in the De-REAL study.Median patient age was 66 (range 37 to 90), and 4 patients were male (3%).Patients' characteristics are reported in Table 1.Across the patient population, 108 (75%) patients had hormone receptor positive (HR+/HER2+) tumors, whereas the remaining 35 (25%) patients had HR-negative/HER2-positive (HR−/HER2+) BC.T-DXd was administered as first or second-line in 20 patients (14%), while 123 patients (86%) received treatment as third or subsequent lines.The median number of previous lines of therapy for metastatic disease was 4 (range 1 to 11).At the time of data cutoff, 78 of 143 patients (54%) were still receiving T-DXd.

Discussion
We reported the results of the multicenter, retrospective Italian study DE-REAL, which investigated the safety and efficacy of T-DXd in a real-world population of patients with HER2+ mBC.We showed that T-DXd is safe, active and effective in patients with HER2+ mBC.Of note, T-DXd was similarly effective when used in different lines of therapy, and regardless of HR status.Overall, our findings support the efficacy of T-DXd in a real world setting, thus providing additional evidence of clinical relevance of this novel drug for patients with HER2+ mBC.
][10][11][12] Although T-DXd has massively entered our daily practice based on excellent results of prospective clinical trials, the safety, feasibility, and antitumor efficacy of T-DXd in a real-world population of patients with HER2+ mBC remains unknown.

Overall response rate (ORR) 68
[10][11][12] Notably, safety and efficacy results of T-DXd in our realworld cohort of patients treated with T-DXd are comparable to the results of clinical trials.In more detail, the single-arm DB-01 and the randomized phase III DB-02 clinical trials showed median PFS of 19.4 months and 17.8 months and ORR of 61.4% and 70%, respectively, in heavily pretreated patients with HER2+ mBC receiving T-DXd. 8,9,12Moreover, a significant improvement in PFS was shown with T-DXd versus TDM-1 for patients with HER2-positive mBC pretreated with trastuzumab and taxane in DB-03 trial (28.8 vs. 6.9 months, 95% CI, 22.4-37.9). 10,11Patients enrolled in DB-01 had received a median number of 6 systemic treatments for advanced disease before being treated with T-DXd. 8,91][12] In our study, the median number of prior lines was 4, and median rwPFS was 16 months, which is in line with results of DB-01 and DB-02 trials, where T-DXd was administered after a different number of prior systemic lines of therapy.In the DE-REAL study, only a small fraction of population (20 patients, 14%) received T-DXd in a first/second line setting and could be compared with the population of the DB-03 trial. 10,11Of note, among patients receiving T-DXd as early treatment, only one has progressed before the 12th month.Fifteen patients had a follow-up longer than 12 months with no progression, and 4   had a follow up shorter than 12 months.Collectively, in this subgroup, the 12-month rwPFS rate was 75%, 12-months OS rate 89% and ORR 70%.Although the small percentage of patients receiving T-DXd in early settings in our cohort, these findings were comparable to DB-03 results (12-months PFS of 75.2%, 12-months OS of 94.1%, and ORR 79% reported, respectively). 10,11hese results, together with our data showing that T-DXd is similarly effective in patients treated in first/second versus more advanced treatment lines, suggests that HER2+ mBC sensitivity to T-DXd is not significantly affected by the number of prior lines of anti-HER2 therapies, but it could reflect intrinsic tumor sensitivity or resistance to this agent.This, in turn, could be a function of the capability of tumor cells to bind T-DXd (based on HER2 expression on tumor cell plasma membranes) and/or the intrinsic sensitivity of tumor cells to the payload DXd.
In our study, the observed ORR of 68% was also promising, and in line with results of known findings.These results confirm consistent antitumor activity of T-DXd also in later lines of treatment and in patients with comorbidities and high disease burden.As expected according to literature, no difference was shown in outcomes regarding to HR status. 8,10edian OS in DB-01 and in DB-02 was 24.6 and 39.2 months, respectively, while the median OS in our cohort was 20 months. 8,9,12However, OS data in our study are still immature, and a longer follow-up is needed to draw more definitive conclusions about the impact of T-DXd on OS in a real-world population of patients with HER2+ mBC.
Differences in the study population that might have influenced the results needs to be critically discussed.First, patients enrolled in DB-01 had a higher median number of previous treatment regimens when compared to patients included in our real-world cohort; on the other hand, patients enrolled in DB-02 were less heavily pretreated.In addition, patients enrolled in our real-world cohort had older age than patients enrolled in both clinical trials (66y vs. 55y in DB-01 and 54y in DB-02, respectively). 8,9,12These differences may explain the similar PFS outcomes, but potentially lower OS in the DE-REAL cohort as compared patient cohorts included in DESTINY-Breast trials.
11][12]20 Indeed, despite the fact that we enrolled patients with older median age, we reported lower bone marrow toxicity (ie, anemia, decreased neutrophil count) when compared to the observed rates of bone marrow toxicities in published clinical trials.This finding may depend on several factors: (1) retrospective analyses, such as the DE-REAL study, are limited by an under-reporting of the incidence and grading of AEs, which reflects the retrospective collection of safety data; (2) the increasing experience in the prevention or management of T-DXd-related toxicities in the clinical practice may have reduced the incidence of some severe AEs that are typical of T-DXd, such as nausea and ILD; (3) in real-world analyses, dose reductions are more frequent, mostly due to less stringent protocols; this, in turn, may result in lowered toxicities, including hematological and non-hematological ones.Lower non-hematologic toxicity (ie, diarrhea, nausea, fatigue) was also observed in our cohort with low rates of grade ≥3 events.In particular, we reported a lower incidence of nausea/vomiting if compared to registration trials.][10][11][12] Furthermore, subjective toxicity assessment in real-world cohorts may be affected by underreporting in medical records, as already shown in literature. 21,22oth treatment-related adverse events (AEs) and dose reduction were studied in relation to their impact on the patients' survival outcomes and treatment response.Our retrospective analysis showed that neither treatment-related toxicities nor dose reduction appeared to significantly affect patients' rwPFS and OS.Moreover, ORR was similar in patients that experienced toxicities of any grade compared to population without AEs.
Of note, the occurrence of AEs during T-DXd or the decision to reduce the treatment dosage did not seem to have a substantial negative impact on the patients' chances of survival in clinical practice, at least based on the data analyzed.
Similar real-world experiences have been conducted in other countries.In the TREX-Old retrospective registry, colleagues evaluated the toxicity of T-DXd in elderly patients (≥70 years).Overall, any-grade treatment-related AEs occurred in 19 patients (70%).Among them, nausea was the most common AE (37%), followed by asthenia (18%) and 2 out of 14 patients developed grade 3 or 4 AEs (14%).Eight patients (30%) started with a dose-reduction, and 8 patients (30%) had secondary dose adjustment. 23he results were similar to those in our study with 37% nausea and 18% asthenia, as common toxicities.Another single-center experience reports 13% severe AEs. 24A UK real-world data collection reported an higher number of discontinuation but a similar efficacy compared to DB-02. 25 Clinically relevant efficacy was confirmed in all these unselected, real-world populations.A strength of our study consist in the multicenter effort to collect data about the activity and the safety of T-DXd in a heavily pretreated, realworld cohort of patients with HER2+ mBC who are also older than patients typically included in randomized clinical trials.Moreover, the population of our study is greater compared to other real-world experiences, increasing the reliability of our findings.The following are potential study limitations: rwPFS data may have been over-estimated as a result of longer time intervals between subsequent radiological assessments in real-world assessments as compared to clinical trials, in which tumor re-evaluation typically occurs every 6-8 weeks, at least during the first 1-2 years of treatment. 22In addition, other limitations of DE-REAL study consist in limited sample size, the lack of centralization of radiological evaluations, the different timing of imaging assessment among centers and the possibility of an underestimation of AEs.

Conclusions
In conclusion, we confirm the excellent efficacy and safety outcomes of T-DXd in a real-world population of Italian patients with heavily pretreated HER2-positive mBC.Real-world multicenter studies including higher numbers of patients and with longer follow-up are needed to confirm findings of our study, as well as to identify patient-and tumor-related variables associated with the efficacy of T-DXd in patients with HER2+ mBC.

Figure 1 .
Figure 1.Best response in the overall population.Response distribution according to RECIST 1.1 criteria.

Figure 2 .
Figure 2. PFS in the whole study population.Kaplan-Meier estimates of PFS in the overall population.mPFS was 16 months.The gray area represents the confidence interval.Tick marks represent data censored at the last time the patient was known to be alive.

Figure 3 .
Figure 3. PFS according to T-DXd treatment line.Kaplan-Meier estimates of PFS in the I/II line versus subsequent lines.mPFS was 17 versus 15 months (P = .098).The colored area represents the confidence interval.Tick marks represent data censored at the last time the patient was known to be alive.

Figure 4 .
Figure 4. PFS according to HR status.Kaplan-Meier estimates of PFS in HR-versus HR+.mPFS was 17 versus 15 months (P = .75).The colored area represents the confidence interval.Tick marks represent data censored at the last time the patient was known to be alive.

Figure 5 .
Figure 5. OS in overall population.Kaplan-Meier estimates of OS in the overall population.mOS was 20 months.The gray area represents the confidence interval.Tick marks represent data censored at the last time the patient was known to be alive.