Trastuzumab-dttb (Ontruzant) is now officially available for clinical use in the United States.
Trastuzumab-dttb (SB3; Ontruzant), is now officially available for clinical use in the United States, according to Merck (MSD), which co-develops the trastuzumab (Herceptin) biosimilar with Samsung Bioepis.1
There are 2 FDA-approved doses of trastuzumab-dttb: single-dose 150-mg vials, and multiple-dose 420-mg vials. The US list price, or wholesaler acquisition cost, is estimated at $1325 and $3709 for the single- and multiple-dose vials, respectively. According to Merck, these figures translate to a 15% discount compared with the cost of trastuzumab. The company also noted in a press release, that ,“Wholesaler acquisition costs do not include discounts to payers, providers, distributors, and other purchasing organizations.”
The FDA approved the 420-mg multidose vial of trastuzumab-dttb on March 24, 2020.2 Trastuzumab-dttb was initially approved in January 2019 as a 150-mg single-dose vial for the treatment of patients with HER2-overexpressing breast cancer or metastatic gastric or gastroesophageal junction adenocarcinoma.3,4
Phase III data on the biosimilar were published in January 2018, in which the biosimilar inducted a rate of breast pathologic complete response (bpCR) similar to trastuzumab in patients with HER2-positive breast cancer, and also demonstrated comparable safety outcomes.
From April 2014 to August 2015, 800 patients were randomized to 8 cycles of trastuzumab-dttb (n = 402) or trastuzumab (n = 398) concurrently with 4 cycles of docetaxel followed by 4 cycles of fluorouracil, epirubicin, and cyclophosphamide. Patients then underwent surgery, followed by 10 cycles of adjuvant trastuzumab-dttb or trastuzumab.
The treatment groups were well balanced for demographics and baseline disease characteristics, with no statistical differences between the 2 arms. The median page age was 51 years (range, 22-65). Most patients (52.8%) had T2 disease and clinically involved lymph nodes (79.5%). Estrogen receptor (ER) and progesterone receptor (PR) were negative in 40.9% of tumors. The mean left ventricular ejection fraction level at baseline was 65.24%.
Based on the study design, trastuzumab-dttb was considered equivalent for bpCR if the 95% CI of the ratio was within 0.785 to 1.546, or the 95% CI of the difference was within ±13%.
At the time of data cutoff, the median follow-up was 337 days (range, 94-489) in the trastuzumab-dttb arm and 338 days (range, 24-475) in the trastuzumab arm. Additionally, there were no relevant differences between the 2 arms in the mean values for the relative dose-intensity of both investigational products and noninvestigational products.
Results showed that, in the per-protocol population, 51.7% of patients achieved bpCR in the trastuzumab-dttb arm versus 42.0% in the trastuzumab group. The adjusted bpCR ratio was 1.259 (95% CI, 1.085-1.460), which was within the predefined equivalence margins. Moreover, the adjusted difference was 10.70% (95% CI, 4.13-17.26), with the lower margin contained within and the upper margin outside the predefined equivalence margins.
In the intent-to-treat population, the bpCR rate was 49.0% in the trastuzumab-dttb arm and 39.7% in the trastuzumab arm. The adjusted ratio of the bpCR rate was 1.243 (95% CI, 1.070-1.444), and the adjusted difference in the bpCR rate was 9.59% (95% CI, 3.26-15.91). In the intent-to-treat population, total pathologic complete response rate was 45.8% in the trastuzumab-dttb arm and 35.8% in the trastuzumab arm.
The bpCR rates were higher in estrogen receptor (ER)— and progesterone receptor (PR)–negative patients compared with ER- and/or PR-positive patients. The event-free survival and overall survival data had not matured at the data cutoff date.
Regarding safety, ≥95% of patients in both arms experienced treatment-emergent adverse events (TEAEs) during the neoadjuvant period.
In the trastuzumab group, the most common (≥20%) TEAEs were neutropenia (63.7%), alopecia (63.2%), nausea (30.4%), and leukopenia (24.4%). The most common (≥20%) TEAEs in the trastuzumab-dttb group were neutropenia (67%), alopecia (67%), nausea (31.1%), leukopenia (27.9%), and diarrhea (20.1%). Four deaths occurred on the study, but none were determined to be related to the study drug.
Outcomes were similar between the 2 arms for TEAEs of special interest. In the trastuzumab-dttb arm, the rate of infusion-related reaction was 8.2%, the rate of asymptomatic left ventricular systolic dysfunction was 0.9%, and rate of congestive heart failure was 0.5%. Those rates in the trastuzumab group were 10.0%, 0.7%, and 0%, respectively.
Additionally, in the pharmacokinetics population, there were 161 patients in the trastuzumab-dttb arm and 152 in the trastuzumab arm. Mean Ctrough profiles from cycle 3 to cycle 8 were similar between the 2 treatment groups. A total of 99.2% of patients in the trastuzumab-dttb group and 97.3% of patients in the trastuzumab group had Ctrough values >20 µg/mL at predose cycle 8. At cycle 8, the geometric least squares mean ratio of Ctrough was 110%, 52.535 µg/mL in the trastuzumab-dttb arm and 47.816 µg/mL in the trastuzumab arm. The 90% CI was 102% to 119%, which was contained within the predefined equivalence margins.
Three patients in the trastuzumab-dttb arm expressed antidrug antibodies (ADAs) up to cycle 9 versus 0 in the trastuzumab group. One of those 3 obtained bpCR, and none presented significant TEAEs related to immunogenicity. Investigators said the overall incidence of ADAs was too low to perform a statistical analysis of the relationship between the ADA status and efficacy or safety, but there was no significant difference between patients who were ADA positive and negative.
The European Commission approved trastuzumab-dttb in November 2017.
The approval was based on data sets from 7 clinical trials, which demonstrated similarity in survival outcomes and safety between trastuzumab-dttb and reference trastuzumab in patients with HER2-positive adjuvant and metastatic breast cancer, as well as HER2-positive metastatic gastric cancer.