Expert Highlights Durable Responses Induced by BLU-667 in RET+ Thyroid Cancers

July 30, 2019
Danielle Ternyila

Vivek Subbiah, MD,&nbsp;discussed the activity and tolerability of BLU-667 in patients with advanced&nbsp;<em>RET</em>-altered MTC and PTC. He also discussed the next steps planned for this ongoing trial and how this agent can impact the treatment landscape of patients with&nbsp;<em>RET</em>-altered thyroid cancers.

Vivek Subbiah, MD

BLU-667, a potent RET inhibitor, induced durable responses in patients with advanced thyroid cancers with aRETalteration, according to findings from the phase I/II ARROW study (NCT03037385) presented at the 2019 ASCO Annual Meeting. This drug was granted a breakthrough therapy designation by the FDA for patients withRET-mutant medullary thyroid cancer (MTC) and previously treatedRETfusion—positive non–small cell lung cancer (NSCLC).

Patients were enrolledin the dose-escalation phase of the ARROW studyand received varying doses of BLU-667 between 30 mg and 600 mg once daily or twice daily. The 400 mg daily dose was moved to the second part of the trial.

Part 2 is currently ongoing, accruing patients withRET-positive MTC, papillary thyroid cancer (PTC), and NSCLC. As of the data cutoff of December 19, 2018, 60 patients with advancedRET-altered MTC and 5 patients withRET-altered PTC were treated. Responses from 49 patients in this cohort were evaluable and presented at this year’s meeting.1

The objective response rate (ORR), 1 of the coprimary endpoints of the study, was 47%; 2 patients had a complete response (CR) while 21 had partial responses (PRs). The disease-control rate (DCR) in this cohort was 98%, while the median duration of response had not yet been reached.

Of the patients with MTC who had prior treatment with a RET inhibitor, either cabozantinib (Cabometyx) or vandetanib (Caprelsa), the ORR was 63%, and 16 patients had a CR. The DCR in these patients was 94%. For the 9 patients withRET-altered PTC, the ORR was 83%; responses are pending for 2 patients, and 8 patients are still receiving treatment.

In terms of safety, a coprimary endpoint, the most common treatment-emergent adverse events (AEs) included hypertension, constipation, and neutropenia in patients withRET-mutant MTC. The investigators noted that treatment-related AEs were generally low grade and reversible, including hypertension, neutropenia, constipation, and leukopenia. No patients with MTC discontinued treatment due to treatment-related toxicity.

The clinical trial sponsor intends to submit BLU-667 to the FDA for accelerated approval in 2020 for patients withRET-altered MTC who had previously been treated with either cabozantinib or vandetanib.

In an interview withTargeted Oncology, Vivek Subbiah, MD, assistant professor in the Department of Investigational Cancer Therapeutics, Division of Cancer Medicine, at The University of Texas MD Anderson Cancer Center, discussed the activity and tolerability of BLU-667 in patients with advancedRET-altered MTC and PTC. He also discussed the next steps planned for this ongoing trial and how this agent can impact the treatment landscape of patients withRET-altered thyroid cancers.

TARGETED ONCOLOGY: How frequently areRET-altered thyroid cancers seen? What sort of challenges currently exist in the treatment of these patients?

Subbiah:RETalterations are targetable oncogenic drivers in about 90% of advanced MTC and about 20% of PTC. To date, the FDA-approved options for MTC have included multikinase inhibitors that have been repurposed to treat patients withRETalterations. These drugs, like vandetanib and cabozantinib, were originally designed to target other kinases, such as VEGFR2. This leads to significant adverse-effect profiles—diarrhea, rash, and hypertension—that limit use in certain patients or limit the dose that patients can tolerate.

TARGETED ONCOLOGY: What was the rationale for investigating BLU-667 in this patient population?

Subbiah:BLU-667 is a next-generation small-molecule RET inhibitor specifically designed for highly potent and selective targeting of oncogenicRETalterations, including the most prevalentRETfusions (eg, KIF5B—RETand CCDC6—RET) andRET-activating mutations (eg, C634W, M918T). BLU-667 was also designed to inhibit predicted resistance mutations (eg, V804L/M) with the goal of providing durable benefit. BLU-667 demonstrated increasedRETpotency and selectivity relative to multikinase inhibitors bothin vitroand inin vivo models ofRET-driven thyroid, lung, and colorectal cancers.

TARGETED ONCOLOGY: Could you go into the methods of design for this trial?

Subbiah:ARROW is a phase I/II clinical trial designed to evaluate the safety, tolerability, and efficacy of BLU-667 in multiple ascending doses in adults withRET-altered cancers, including MTC. The trial consists of 2 parts: a dose-escalation portion, which is now complete, and an expansion portion, in which enrollment is ongoing.

The expansion portion consists of 7 defined cohorts of patients treated with BLU-667 at the recommended phase II dose of 400 mg once daily: (1) Patients withRET-fusion NSCLC who were previously treated with a platinum-based chemotherapy, (2) those withRET-fusion NSCLC who have not previously received a platinum-based chemotherapy, (3) those withRET-mutant MTC who were previously treated with cabozantinib or vandetanib, (4) patients withRET-mutant MTC who have not previously received cabozantinib or vandetanib, (5) patients with otherRET-fusion tumors, (6) patients with otherRET-mutant tumors, and (7) those withRET-altered solid tumors who were previously treated with a selective RET inhibitor.

Trial objectives include assessing response, pharmacokinetics, pharmacodynamics and safety. The trial is enrolling patients at multiple sites in the United States, European Union, and Asia.

TARGETED ONCOLOGY: What were the results that were presented at ASCO this year?

Subbiah:Updated safety and efficacy data were presented at ASCO this year.As of the data cutoff date, 32 patients withRET-mutant MTC who initiated treatment at the recommended phase II dose of 400 mg once daily were evaluable for response assessment, including 16 patients previously treated with the multikinase inhibitors cabozantinib or vandetanib. The ORR was 63% (9 confirmed PRs, 1 PR pending confirmation) and the DCR was 94% in patients withRET-mutant MTC who were previously treated with cabozantinib or vandetanib. Across all patientswithRET-mutant MTC, the median duration of response was not reached, and all responders remained on treatment as of the data cutoff date, with treatment durations up to 15.6 months for patients receiving a starting dose of 400 mg once daily.

TARGETED ONCOLOGY: What are the most significant findings to come from these data? And what is the impact of these findings?

Subbiah:RETis an established oncogene found in a wide range of cancers including NSCLC, MTC, and other tumor types. RET has now been shown to be an actionable target and that selective RET inhibitors are here to stay. In addition, based on the data generated to date, the trial sponsor has announced plans to seek FDA approval of BLU-667 next year.

TARGETED ONCOLOGY: Could you also speak to the efficacy and toxicity profile that has been associated with this drug so far?

Subbiah:As of the data cutoff date, 226 patients received a starting dose of 400 mg once daily and were evaluable for safety. Across all patients, BLU-667 was well-tolerated and most adverse events reported by investigators were grade 1 (mild) or 2 (moderate). There were no patients with MTC that discontinued treatment due to treatment-related adverse events, and only 4% discontinued due to treatment-related adverse events across the entire study.

In addition to MTC, 6 patients with papillary thyroid cancer were evaluable for response assessment by RECIST version 1.1. In these patients, the ORR was 83% (3 confirmed PRs, 2 PRs pending confirmation).

TARGETED ONCOLOGY: What are the next steps for this agent?

Subbiah:The FDA has granted a breakthrough therapy designation to BLU-667 for the treatment of patients withRET-fusion positive NSCLC that has progressed following platinum-based chemotherapy, andRET-mutation positive MTC that requires systemic treatment and for which there are no acceptable alternative treatments. The clinical trial sponsor plans to submit a new drug approval application for accelerated approval to the FDA in the first half of 2020 for patients withRET-mutant MTC previously treated with cabozantinib or vandetanib.

TARGETED ONCOLOGY: How do you see this agent potentially impacting patients withRET-altered thyroid cancers?

Subbiah:I think the selective RET kinase inhibitors will supersede the multikinase inhibitors based on adverse effects alone. We will wait for the duration of response and progression-free survival data from the trials.

TARGETED ONCOLOGY: Is there anything else that is important to note about BLU-667 orRET-altered thyroid cancer?

Subbiah:We have a highly potent and targeted drug in our hands. Key is to sequence all thyroid cancer patients forRETaberration and enroll them on selective RET trials.


Taylor MH, Gainor JF, Hu MI-N, et al. Activity and tolerability of BLU-667, a highly potent and selective RET inhibitor, in patients with advanced RET-altered thyroid cancers.J Clin Oncol.2019;37(suppl 15; abstr 6018). doi: 10.1200/JCO.2019.37.15_suppl.6018.