Metastatic HER2 + breast cancer is an expanding area of drug development and research, with three new drugs approved in 2020 alone. While first-line therapy is well-established for metastatic HER2 + breast cancer, the standard of care for second-line therapy will likely be changing soon based on the results of the DESTINY-Breast03 trial. In the third-line setting, many options are available. Considerations in choosing between regimens in the third-line include resistance to trastuzumab, the presence of brain metastases, and tolerability. High rates of resistance exist in this setting particularly due to expression of p95, a truncated form of HER2 that constitutively activates downstream signaling pathways. We suggest a tyrosine kinase inhibitor (TKI)-based regimen because of the activity of TKIs in brain metastases and in p95-expressing tumors. Attempts to overcome resistance to anti-HER2 therapies with PI3K inhibitors, mTOR inhibitors, and CDK 4/6 inhibitors are an active area of research. In the future, biomarkers are needed to help predict which therapies patients may benefit from the most. We review the many new drugs in development, including those with novel mechanisms of action.

3032 Background: Though in theory ADCs should deliver high-dose chemotherapy directly to target cells with few systemic effects, in clinical practice numerous side effects have been observed. We hypothesized that ADCs with cleavable linkers would have more systemic toxicities than those with non-cleavable linkers due to the increased free payload released systemically. To compare their side effect profiles, we conducted a meta-analysis of adverse events (AEs) of commercially available ADCs. Methods: Systematic review yielded 12 phase II/III clinical trials that led to the FDA approval of commercially available ADCs. Polatuzumab vedotin was not included because it was only studied in combination with other agents. Any grade AEs and grade ≥3 AEs occurring in at least 5% of patients in each study were recorded. The estimated inverse variance weighted absolute average risk and 95% confidence interval (CI) were estimated for each AE. Absolute risk differences and 95% CIs were estimated by linker type. Results: Data from 2,417 patients treated with 9 ADCs were pooled. 7 ADCs had cleavable linkers (N = 1,082), and 2 had non-cleavable linkers (N = 1,335). At least half of studies reported thrombocytopenia, neutropenia, anemia, increased AST and ALT, nausea, vomiting, diarrhea, hypokalemia, headache, and fatigue, as well as rates of all grade and grade ≥3 AEs. AEs ≥ grade 3 occurred in 43% of patients overall, 47% in the cleavable linker arms and 34% in the non-cleavable arms. This was significantly different (weighted risk difference -12.9%; 95% CI -17.1% to -8.8%). There was also a significant difference favoring non-cleavable linkers for ≥ grade 3 neutropenia (-9.1%; 95% CI -12% to -6.2%) and ≥ grade 3 anemia (-1.7%; 95% CI -3.3% to -0.1%). Cleavable linkers were significantly associated with increased AST all grade (3.9%; 95% CI 0.3% to 7.5%) and increased ALT all grade (3.7%; 95% CI 0.2% to 7.3%), though notably the CI approached 0 on the low end of difference for each. There was no significant difference in rates of all grade AEs or in rates of discontinuation due to AEs. There was no significant difference in rates of all grade nausea, vomiting, diarrhea, hypokalemia, or headache. Finally, there was no significant difference in rates of grade ≥3 thrombocytopenia, increased AST/ALT, or fatigue. Conclusions: Cleavable linkers appear to have significantly higher rates of ≥ grade 3 AEs and neutropenia within the limitations of this non-randomized comparison and treatment of heterogeneous malignancies. The increased payload in the circulation likely accounts for this; however, it may also make them more efficacious, as suggested by the results of the DESTINY-Breast03 trial. In the final analysis, we will compare the efficacy of cleavable vs non-cleavable ADCs indirectly using the standard of care for each tumor and line of therapy, with the exception of breast cancer.

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