Tucatinib Demonstrates Potent Activity in HER2-Mutant Tumor Models

Scott Peterson

Tucatinib (Tukysa) was found to be a potent inhibitor of HER2-mutant signaling in vitro, according to results from an analysis presented during the 2020 AACR Virtual Annual Meeting II.¹

“Tucatinib is an orally available, reversible small molecule kinase inhibitor that’s highly selected for the kinase domain of HER2 without significant inhibition of EGFR. The agent has demonstrated activity in multiple HER2-amplified preclinical tumor models, either alone or in combination with trastuzumab (Herceptin),” said lead study author Scott Peterson during the poster session. “[Data from this analysis] demonstrate that tucatinib is effective in patient-derived xenograft models harboring mutations either as a single agent or in combination with trastuzumab, and these data support the evaluation of tucatinib in clinical studies in HER2-mutant cancers.”

With regard to the agent’s mechanism of action, the small molecule is thought to diffuse into cells, selectively bind to the kinase domain of HER2, inhibit activation of downstream signaling cascades. By decreasing the HER2 signaling, the agent reduces tumor cell proliferation and induces tumor cell death.

For the research presented during the meeting, investigators evaluated tucatinib in several HER2-mutant cancer models. In addition to gene amplification, somatic mutations occurring in the extracellular domain, the juxtamembrane domain, and the kinase domain of HER2 have all been shown to stimulate kinase activity and drive tumorigenic signal transduction, according to Peterson. “These HER2 mutations are found across multiple cancers and represent an important unmet medical need in patients with cancer,” Peterson said.

Data from the analysis showed that tucatinib is a potent and selective inhibitor of exon 20 HER2-mutant kinase signaling. In an MCF10A cell line clone expressing the HER2 exon 20 insertion mutation G776insV_G/C, investigators evaluated the dose response of tucatinib. HER2 phosphorylation was identified via the Luminex assay which used total tyrosine phosphorylated HER2 as an end point with the agent titrated from 1-2000 nM in four-fold dilutions, the authors wrote in their poster.

Investigators also evaluated the impact of tucatinib on NCI-H1781 downstream signaling by calculating phosphorylation of HER2, HER3, AKT, ERK1/2, and MEK1 through the use of a multiplexed antibody capture assay, also utilizing Luminex technology. To do this, investigators measured HER2, HER3 (total Tyr), ERK1/2 (Thr185 and Tyr187), MEK1 (Ser222) and AKT (Ser473) via cell extracts following 2-hour tucatinib treatment. Here, the agent was titrated through the use of 3-fold dilutions that began at 0.08 nM to 5000 nM.

Lastly, investigators examined the impact of the agent on NCI-H1781 cells using the Cell Titer-Glo Luminescent Cell Viability Assay; for this analysis, tucatinib was titrated using 3-fold dilutions that began from 0.0001 µM to 10,000 nM. “Here, we showed that in exon 20 insertion–mutant non­–small cell lung cancer cells, tucatinib blocks downstream signaling and inhibits cell proliferation,” noted Peterson.

Tucatinib was also found to be effective in HER2-L755S–mutant patient-derived xenograft (PDX) tumor models. To do this, investigators evaluated the dose response of the agent in an MCF10A cell line clone that expressed the HER2-L755S mutation. They identified the HER2 phosphorylation through the use of the Luminex assay with total tyrosine phosphorylated HER2 serving as an end point and the agent being titrated from 1-2000 nM in 4-fold dilutions. The activity of the agent in combination with trastuzumab was also analyzed in L755S-mutant patient-derived xenograft tumor models. Here, the agent was given at 50 mg/kg twice daily for the duration of the study and trastuzumab was administered at a weekly dose of 20 mg/kg.

“We showed that tucatinib in combination with trastuzumab is capable of inducing regressions in two of three PDX models containing the HER2-L755S mutation, which is associated with resistance to trastuzumab in clinical samples and also drives resistance to the HER2 inhibitor lapatinib (Tykerb),” explained Peterson.

In an analysis of dose response of the agent in an MCF10A cell line clone expressing the HER2-V777L mutation, investigators determined HER2 phosphorylation through the use of the Luminex assay. Specifically, total tyrosine phosphorylated HER2 served as an end point with tucatinib titrated from 1-2000 nM in 4-fold dilutions. The activity of the agent was also observed in a HER2-V777L–mutant PDX tumor model of colorectal cancer (CRC). The agent was given at 50 mg/kg twice daily for the duration of the study, and trastuzumab was administered at a weekly dose of 20 mg/kg.

“We also showed that tucatinib alone or in combination with trastuzumab can induce regression in a HER2-V777L–mutant patient-derived xenograft tumor model of CRC,” stressed Peterson.

Lastly, investigators were able to demonstrate the activity of tucatinib in S310Y-mutant HER2 PDX models of gall bladder and gastric cancer. Again, the agent was given at 50 mg/kg twice daily for the duration of the study and trastuzumab was given at 20 mg/kg weekly.

Based on these data, tucatinib alone and in combination with trastuzumab can inhibit the growth of PDX models harboring an extracellular domain activating mutation in HER2, according to Peterson.

“These data demonstrate that tucatinib can potently inhibit the activity of HER2 that is activated by mutations in the kinase domain by either exon 20 insertion mutations or HER2-L755S or V777L,” said Peterson. “With regard to exon 20 insertion mutations, we demonstrated that tucatinib treatment blocks downstream signaling and induces inhibition of cell proliferation in vitro. Importantly, we saw that tucatinib can inhibit the growth of tumors harboring mutations in the kinase domain or in the extracellular domain. Tucatinib also induced regression in both the L755S patient-derived xenograft models and in the V777L patient-derived models either alone or in combination with trastuzumab.”

In April 2020, tucatinib was approved by the FDA for use in combination with trastuzumab and capecitabine (Xeloda) for the treatment of patients with unresectable locally advanced or metastatic HER2-positive breast cancer, including those with brain metastases, following at least 1 prior anti–HER2-based regimen in the metastatic setting.

The regulatory decision was based on data from the pivotal HER2CLIMB trial, which demonstrated that treatment with the tucatinib triplet led to a 34% reduction in the risk of death compared with trastuzumab and capecitabine alone in this heavily pretreated patient population.^2,3

Specifically, the median overall survival (OS) with the triplet was 21.9 months (95% CI, 18.3-31.0) compared with 17.4 months (95% CI, 13.6-19.9) with trastuzumab/capecitabine alone (HR, 0.66; 95% CI, 0.50-0.88; P =.0048). The 1-year OS rate was 76% with the triplet versus 52% with the doublet, and the 2-year OS rates were 45% and 27%, respectively. Notably, the OS benefit was found to hold true across all prespecified subgroups analyzed in the trial.

Updated data from the trial were presented at the 2020 ASCO Virtual Scientific Program and showed that the tucatinib triplet significantly improved response rates and central nervous system (CNS) progression-free survival (PFS) in patients with HER2-positive metastatic breast cancer brain metastases.⁴ Specifically, the triplet let to a 68% reduction in the risk of CNS-PFS (HR, 0.32; 95% CI, 0.22-0.48; P <.0001) at 1 year. The median CNS-PFS was 9.9 months with the triplet versus 4.2 months with the doublet.

In the overall brain metastases population, the triplet led to a 42% reduction in the risk of death (HR, 0.58; 95% CI, 0.40-0.85; P =.005). The median OS was 18.1 months and 12 months in the triplet and doublet arms, respectively.

References

1. Peterson S, Rosler R, Klutcher K, et al. Tucatinib, a selective small molecule HER2 inhibitor, is active in HER2-mutant driven tumors. Presented at: 2020 AACR Virtual Annual Meeting II; June 22-24, 2020. Abstract 4222. bit.ly/3hYkE0T.
2. Murthy R, Loi S, Okines A, et al. Tucatinib vs placebo, both combined with capecitabine and trastuzumab, for patients with pretreated HER2-positive metastatic breast cancer with and without brain metastases (HER2CLIMB). Presented at: 2019 San Antonio Breast Cancer Symposium; December 10-14; San Antonio, TX. Abstract GS1-01.
3. Murthy RK, Loi S, Okines A, et al. Tucatinib, trastuzumab, and capecitabine for HER2-positive metastatic breast cancer [published online December 11, 2019]. N Engl J Med. doi: 10.1056/NEJMoa1914609.
4. Lin NU, Murthy RK, Anders CK, et al. Tucatinib versus placebo added to trastuzumab and capecitabine for patients with previously treated HER2+ metastatic breast cancer with brain metastases (HER2CLIMB). J Clin Oncol. 2020;38(suppl 15):1005. doi:10.1200/JCO.2020.38.15_suppl.1005