Chloe E. Atreya, MD, PhD
The combination of dabrafenib (Tafinlar), trametinib (Mekinist), and panitumumab (Vectibix) demonstrated an objective response rate (ORR) of 26% and a median progression-free survival (PFS) of 4.1 months in pretreated patients with BRAFV600E
-mutated metastatic colorectal cancer (mCRC), according to findings from a phase I/II study presented at the 2015 ASCO Annual Meeting.1
“Triplet therapy with dabrafenib, panitumumab, and trametinib appears to be more active in BRAF
-mutated colorectal cancer than dabrafenib plus panitumumab or dabrafenib plus trametinib,” lead investigator Chloe E. Atreya, MD, PhD, assistant clinical professor in the Department of Medicine, University of California, San Francisco, said during her presentation. "Dermatologic toxicities are significant, resulting in dose reductions, interruptions, and delays. A nuance to this is that dabrafenib may lessen skin-related toxicity due to panitumumab and trametinib."BRAF
mutations are found in approximately 8% of patients with mCRC and are associated with a poor prognosis. The median overall survival in patients with BRAF
-mutant mCRC is 13.4 months compared with 37.1 months in those with BRAF
wild-type disease, Atreya explained.
Prior studies have shown that single-agent inhibition of BRAF inhibition was largely ineffective in CRC, with MEK plus BRAF inhibition only moderately more effective. The rationale behind the study was that the addition of panitumumab to dabrafenib and trametinib would provide a more effective blockade of phospho-ERK signaling.
In the first arm of the phase I/II study, 20 patients received dabrafenib at 150 mg twice daily and panitumumab at 6 mg/kg every 2 weeks. Thirty-five patients received dabrafenib, trametinib, and panitumumab, with 24 receiving the phase II recommended dose of dabrafenib at 150 mg twice daily, panitumumab at 6 mg/kg every 2 weeks, and trametinib at 2 mg daily. Additionally, in two separate arms panitumumab plus trametinib was administered to patients with BRAF
-mutant mCRC (n = 12) and those with acquired resistance to EGFR inhibition (n = 2).
Following progression on the doublet arms, patients were allowed to cross over to receive triplet therapy. At the data cutoff of March 16, 2015, six patients had crossed over from the doublet arms to receive the triplet therapy. However, data on the outcomes for these patients were not yet available.
In the triplet arm, the majority of patients were female (63%) with an ECOG performance status of 0 (57%) or 1 (43%). As is common with BRAF
-mutant CRC, the majority of tumors had originated in the right colon (71.4%). All patients were pretreated, with 54% having received three or more prior lines of therapy. The primary endpoint of the study was ORR, with secondary measures focused on PFS.
In the dabrafenib plus panitumumab arm the ORR was 10%, comprised of one complete response (CR) and one partial response (PR). In the triplet arm, the ORR was 26%, with one CR and eight PRs. Additionally, 21 patients (60%) had stable disease with the triplet, for an overall disease control rate of 86%. The unconfirmed response rate with the triplet was 34%, Atreya added.
In the dabrafenib doublet arm, 25% of patients remained on treatment for >6 months. In the triplet arm, 26% of patients were on treatment >6 months and 11% of patients remained on therapy for longer than a year. The median duration of response with the triplet therapy was 5.4 months.
“Triplet therapy has produced more durable disease control than the dabrafenib plus panitumumab doublet, with four triplet patients on study for over a year and 4 patients receiving ongoing treatment,” said Atreya.
The median PFS in the dabrafenib and trametinib doublet arms, respectively, were 3.4 months (95% CI 2.6-5.8) and 4.1 months (95% CI, 2.6-4.5). In the triplet arm, the median PFS was 4.1 months (95% CI, 2.8-5.5).
In the triplet arm, patients in the dose escalation portion of the study who received panitumumab at a lower dose of 4.8 mg/kg every 2 weeks experienced better outcomes. At this lower dose, the ORR was 67% and the median PFS was 10.1 months in those who received 1.5 mg daily of trametinib with dabrafenib (n = 2). In those who received trametinib at 2 mg daily (n = 2) with the lower dose panitumumab in the triplet, the ORR was 50% and the median PFS was 9.0 months.
“Although numbers are very small, we note that patients in this cohort achieved higher panitumumab drug compliance and were exposed to fewer lines of prior therapy. This will be further investigated,” Atreya explained.