Proteolysis-targeting chimeras (PROTACs) represent an emerging class of protein degraders that could transform treatment options for patients with estrogen receptor-positive, HER2-negative breast cancer who have developed resistance to standard therapies. The novel PROTAC vepdegestrant (ARV-471) has demonstrated encouraging early efficacy signals in heavily pretreated patients, particularly those harboring ESR1 mutations.
VERITAC Study Results Show Promise in ESR1-Mutant Disease
The phase 2 VERITAC study (NCT04072952) evaluated vepdegestrant in 71 patients with heavily pretreated ER-positive, HER2-negative advanced breast cancer. Patients received either 200 mg or 500 mg of vepdegestrant once daily. The combined analysis revealed a clinical benefit rate (CBR) of 38.0% (95% CI, 26.8%-50.3%).
More significantly, the drug showed enhanced activity in patients with ESR1 mutations. Among 41 patients with ESR1-mutant disease, the CBR reached 51.2% (95% CI, 35.1%-67.1%), compared to just 20% in patients with ESR1 wild-type tumors. This differential efficacy aligns with the drug's mechanism of action and the clinical need for therapies targeting acquired resistance mutations.
"There's an important unmet clinical need to develop better hormonal agents such as PROTACs to prove—for example, in the ESR1-mutant population and in patients with other mutations that might drive resistance to hormonal agents—that there may be better options moving forward," said Seth A. Wander, MD, PhD, assistant professor of Medicine at Harvard Medical School.
Progression-Free Survival and Safety Profile
The median progression-free survival (PFS) for the overall cohort was 3.7 months (95% CI, 1.9-8.3), extending to 5.7 months (95% CI, 3.6-9.4) in patients with ESR1-mutant disease. Among patients treated with the 200-mg dose who had ESR1 mutations, median PFS was 5.5 months (95% CI, 1.8-8.5).
The safety profile proved manageable, with grade 1, grade 2, and grade 3/4 treatment-related adverse events reported in 34%, 31%, and 7% of patients, respectively. The most common adverse events were fatigue, nausea, arthralgia, hot flush, and increased aspartate aminotransferase levels, predominantly grade 1 or 2 in severity. Only three patients in the 500-mg arm required dose reductions, and treatment discontinuation due to adverse events occurred in just three patients across both dose levels.
Addressing Critical Treatment Gap
The development of vepdegestrant addresses a significant gap in treatment options for patients who progress on CDK4/6 inhibitor therapy. Currently, fulvestrant monotherapy represents the standard of care for many of these patients, but its efficacy is limited.
"We know from [data of] multiple studies that the median PFS with fulvestrant alone is approximately 2 months so not a great option and most clinicians are not using fulvestrant after a CDK4/6 inhibitor," noted Rinath M. Jeselsohn, MD, assistant professor of medicine at Harvard Medical School and director for ER+ Translational Discovery Research at Dana-Farber Cancer Institute.
Understanding ESR1 Mutations and Resistance
ESR1 mutations typically occur in the ligand-binding domain of the estrogen receptor and allow the receptor to remain constitutively active even without estrogen. These mutations develop as an acquired resistance mechanism, particularly after treatment with aromatase inhibitors.
"These mutations typically occur in the ligand-binding domain and allow the [ER] to be constitutively active, even in the absence of the estrogen ligand. Drugs such as aromatase inhibitors that work by dropping the level of estrogen or dropping the level of the ligand are not effective against a mutant form of the receptor that's constantly active," Wander explained.
The median ER degradation achieved with vepdegestrant 200 mg was 69% (range, 28%-95%), with a mean ER degradation of 71%, demonstrating the drug's ability to effectively target and degrade the estrogen receptor protein.
Phase 3 VERITAC-2 Trial Launches
Building on these promising phase 2 results, investigators have initiated the phase 3 VERITAC-2 trial (NCT05654623). This randomized study will compare vepdegestrant 200 mg orally once daily to fulvestrant 500 mg administered intramuscularly in patients who have progressed on CDK4/6 inhibitor therapy.
The trial's primary endpoint is progression-free survival by blinded independent review, assessed in both the ESR1-mutant population and the intention-to-treat population. Eligible participants must have received prior CDK4/6 inhibitor therapy, with no prior fulvestrant or chemotherapy for locally advanced or metastatic disease permitted.
"Understanding what to do in the second-line setting after patients progress on standard hormonal therapy, typically with a CDK4/6 inhibitor in the first-line setting, is the largest unmet need in hormone receptor–positive metastatic breast cancer," Wander emphasized.
Post-Hoc Analysis Reveals Encouraging Signals
A post-hoc analysis of eight patients meeting the planned phase 3 trial enrollment criteria showed particularly encouraging results. These patients, who had not received prior fulvestrant or chemotherapy in the metastatic setting, achieved a CBR of 62.5%. As of the November 2022 data cutoff, five patients continued therapy with treatment durations ranging from 8 to 14 months, and median PFS had not been reached.
Expanding PROTAC Development Pipeline
Beyond the VERITAC-2 trial, vepdegestrant is being evaluated in combination studies through the umbrella TACTIVE-U study. This includes combination with abemaciclib in substudy A (NCT05548127) and with ribociclib in substudy B (NCT05573555), exploring the potential for enhanced efficacy through combination approaches.
The development of PROTACs represents a significant advancement in targeting protein degradation rather than just protein inhibition. "PROTACs are an exciting type of agent moving forward in the clinic," Wander noted, highlighting their potential to overcome resistance mechanisms that limit current therapies.
As the field continues to evolve, the success of vepdegestrant in targeting ESR1-mutant disease could establish a new treatment paradigm for patients with hormone receptor-positive breast cancer who have exhausted current therapeutic options.
