Atebimetinib Plus Modified Gemcitabine/Nab-Paclitaxel Shows Encouraging Efficacy, Tolerability in First-Line Metastatic PDAC

The novel, pulsatile MEK1/2 inhibitor atebimetinib demonstrated a favorable tolerability profile and overall survival (OS) outcomes when combined with modified gemcitabine and nab-paclitaxel (Abraxane; mGnP) in patients with first-line metastatic pancreatic ductal adenocarcinoma (PDAC), according to results from a phase 2a study (NCT05585320) presented at the 2026 ASCO Annual Meeting.¹

Among response-evaluable patients receiving atebimetinib plus mGnP (n = 50), the confirmed objective response rate (ORR) was 36% (95% CI, 23%-51%) and the disease control rate (DCR) was 82% (95% CI, 69%-91%). At a median follow-up of 17 months, the median OS in the initial cohort (n = 34) was 17.3 months. Across all patients in the expanded cohort (n = 55), the median OS remained consistent at 17.3 months (95% CI, 11.2-not reached [NR]), with a median follow-up of 11.6 months. The median progression-free survival (PFS) was 8.3 months (95% CI, 5.9-9.6).

Moreover, activity was consistent across RAS mutation subtypes. Among the 43 participants with a detected RAS mutation by circulating tumor DNA (ctDNA), the ORR was 40% and the DCR was 86%, supporting the all-comer approach carried forward into phase 3.

"The combination showed promising efficacy across ORR, DCR, PFS, and OS. [Atebimetinib will now advance to the] global randomized phase 3 trial, MAPKeeper 301, [(NCT07562152) which] is now recruiting." - Peter Vu, MD, MHA, presenting author

Vu is a medical oncologist, associate professor of medicine, and medical director of Cancer Quality at the University of California San Diego Moores Cancer Center in La Jolla.

What is the mechanistic rationale for pursuing atebimetinib in pancreatic cancer?

MEK inhibitors have been considered an attractive therapeutic approach in PDAC because they act downstream of RAS, rendering them agnostic to codon-specific RAS mutations. However, chronic MEK inhibition has been largely unsuccessful in this disease, with prior agents undermined by adaptive feedback resistance and tolerability challenges, Vu explained.

Atebimetinib is differentiated by its mechanism of action. It is a potent, highly selective allosteric inhibitor of MEK1/2 that is resistant to RAF bypass and has a short half-life of 2 to 3 hours. That short half-life produces cyclic, pulsatile MEK suppression, cycling between near-complete daily inhibition of MAPK signaling and a daily pathway reset. This pharmacokinetic profile was designed to limit the adaptive feedback mechanisms that have historically undermined continuous MEK blockade while simultaneously improving tolerability compared with prior-generation MEK inhibitors.

How was the phase 2a study designed, and what were the baseline patient characteristics?

This open-label, single-arm trial enrolled all-comer patients with first-line metastatic or locally advanced PDAC. Key eligibility criteria included no prior systemic therapy for advanced disease, ECOG performance status (PS) of 0 or 1, at least one measurable lesion per RECIST 1.1 criteria, and adequate organ function. All RAS/KRAS mutation subtypes were permitted to enroll.

The trial enrolled in 2 stages: an initial cohort of 34 participants followed by expansion to 55 total participants, all receiving atebimetinib 320 mg once daily plus mGnP (gemcitabine 1,000 mg/m² plus nab-paclitaxel 125 mg/m²) on days 1 and 15 of each 28-day cycle. The primary end point was ORR per RECIST 1.1 criteria. Secondary end points included DCR, duration of response (DOR), PFS, OS, safety, patient weight, and quality of life (QOL).

The data cutoff for the current analysis is April 24, 2026. Baseline characteristics in the expanded cohort are as follows:

• Median age: 68 years (range, 42-85); 62% of patients were aged 65 or older
• Disease status: 96% metastatic, 4% locally advanced
• ECOG PS: 0 in 56%, 1 in 44%
• CA 19-9 elevated (≥37 U/mL): 89% of evaluable participants
• Presence of liver metastasis: 51%
• Presence of peritoneal metastasis: 36%
• Presence of lung metastasis: 27%
• De novo metastatic disease: 51%
• Mutation status: KRAS-mutant disease: 87%; RAS mutations unknown: 13%

Of note, 60% of patients who came off study treatment received second-line treatment.

What additional efficacy outcomes from the expanded cohort were reported?

At the data cutoff, 11 patients in the expanded cohort were still on treatment and 44 had come off treatment. Of those 44, twenty-one went on to receive subsequent cancer therapy, 14 did not, and 9 were unknown. Of the patients who received subsequent therapy, 20 went on to receive chemotherapy and/or radiation and 1 to receive a RAS inhibitor.

Furthermore, approximately half of patients were still alive at the data cutoff. Vu noted that the OS Kaplan-Meier curve showed gradual early decline followed by separation from historical comparators that was maintained throughout the observation period, suggesting a subset of participants deriving durable benefit rather than a uniform shift in the survival distribution. He cautioned that no head-to-head trial has been conducted comparing atebimetinib with other candidates, and cross-trial comparisons are hypothesis-generating only.

What was the safety profile of atebimetinib plus mGnP?

The tolerability profile of atebimetinib plus mGnP was notable for the absence of significant MEK inhibitor class toxicities at higher grades. Grade 3 or higher anemia and neutropenia each occurred in over 10% of patients, but both were attributed to the chemotherapy backbone. Treatment-related adverse effects (TRAEs) occurring in 20% or more of patients (any grade; grade 3 or higher) included fatigue (53%; 2%), nausea (45%; 0%), anemia (42%; 16%), rash (36%; 5%), diarrhea (36%; 0%), vomiting (29%; 2%), alopecia (27%; 0%), dysgeusia (27%; 0%), peripheral edema (27%; 2%), neutropenia (25%; 18%), peripheral neuropathy (22%; 0%).

The only MEK inhibitor class effect reaching grade 3 or higher was rash, occurring in 5% of participants and resolving with medical management. Ocular, cardiac, and diarrhea events, when observed, were grade 1/2 and did not result in any discontinuations. Only 1 patient in the expanded cohort discontinued atebimetinib over the course of the study, and that discontinuation was not due to toxicity; the participant continued chemotherapy. Notably, 84% of participants were weight stable or gained weight at 3 months — a finding the investigators highlighted as meaningful in a disease defined by cachexia.

Vincent Chung, MD, a medical oncologist at City of Hope Cancer Center in Duarte, California, and a co-investigator on the study, reflected on this tolerability signal in a Q&A session following the presentation. "[Atebimetinib] has a short half-life, so it's out of the system very quickly. This is why we're not seeing the same toxicities that we've seen with first- and second-generation MEK inhibitors," he explained.

When asked about patient QOL with atebimetinib in the ensuing Q&A, Vu noted that, "In my experience, treating a lot of patients on this study, patients have done incredibly well from a QOL perspective. It's been remarkable to see how they have felt very well throughout the study period. I was talking to Dr. Chung and he also has patients with a similar experience, where they're on study for a few years and still feeling very minimal AEs."

What study limitations and unanswered questions were acknowledged? How will MAPKeeper 301 seek to address them?

As a single-arm trial without a control group, efficacy and tolerability of the combination cannot be directly compared with standard of care (SOC), Vu emphasized. He noted that follow-up in the expanded cohort is still maturing and the OS upper confidence bound has not yet been reached. Moreover, the second-line treatment rate of 60%, while encouraging, cannot be definitively attributed to atebimetinib's tolerability without a randomized comparator. Additionally, the modified GnP schedule used in the study, which is administered on days 1 and 15 rather than the standard weekly regimen, differs from the full standard-dose approach, leaving the independent contribution of that dose modification to this outcome unknown.

These are all the questions that the MAPKeeper 301 trial is designed to answer. MAPKeeper 301 is a randomized, global study comparing atebimetinib plus mGnP against SOC gemcitabine and nab-paclitaxel at the full weekly schedule in patients with first-line metastatic PDAC.^1,2 The trial is currently recruiting with a target population of 225 participants per arm. The primary end point is OS, with secondary end points of PFS, DCR, ORR, and QOL. Stratification factors include geography (United States vs rest of world), liver metastasis (present vs absent), ECOG PS (0 vs 1), and disease presentation (de novo vs recurrent /metastatic).

Disclosures: Vu disclosed no relevant relationships with companies.

References

1. Chung V, Vu P, Ma VT, et al. Results from a phase 2a study of atebimetinib in combination with mGnP in first-line metastatic pancreatic cancer. J Clin Oncol. 2026;44(16 suppl):4013. doi:10.1200/JCO.2026.44.16_suppl.4013
2. Atebimetinib + GnP as a first line treatment in patients With metastatic pancreatic adenocarcinoma (MAPKeeper 301). ClinicalTrials.gov. Updated May 15, 2026. Accessed June 1, 2026. https://clinicaltrials.gov/study/NCT07562152