Background
Phase II randomized multi-cohort open-label trial. 117 patients with chemotherapy-refractory, HER2-positive (IHC3+ or IHC2+/ISH+), RAS wild-type unresectable or metastatic colorectal cancer after ≥2 prior lines. Cohorts A+B received tucatinib + trastuzumab; cohort C received tucatinib monotherapy. HER2 amplification/overexpression occurs in ≈3–5% of mCRC.
Interventions and follow up
Arm A+B (Combination): Tucatinib 300 mg PO bid + trastuzumab (8 mg/kg IV loading, then 6 mg/kg q3wk) until progression or unacceptable toxicity
Arm C (Monotherapy): Tucatinib 300 mg PO bid alone; internal comparison only, not a pre-specified control arm
Primary endpoint: Confirmed ORR per BICR (cohorts A+B combined)
Median follow-up: 20.7 months
Arm C (Monotherapy): Tucatinib 300 mg PO bid alone; internal comparison only, not a pre-specified control arm
Primary endpoint: Confirmed ORR per BICR (cohorts A+B combined)
Median follow-up: 20.7 months
Results
ORR (cohorts A+B, n=84): 38.1% (95% CI 27.7–49.3%; 3 CR + 29 PR)
mDoR: 12.4 months
mPFS: 8.2 months
mOS: 24.1 months
ORR (cohort C, tucatinib monotherapy): 3% (1/30)
mDoR: 12.4 months
mPFS: 8.2 months
mOS: 24.1 months
ORR (cohort C, tucatinib monotherapy): 3% (1/30)
Adverse events
Most common any-grade: diarrhea 64% (cohorts A+B), plus nausea, fatigue, rash
Most common grade ≥3: hypertension 7%; tucatinib-related serious AEs (acute kidney injury, colitis, fatigue) 3% each
Safety: no treatment-related deaths; cardiac toxicity uncommon
Most common grade ≥3: hypertension 7%; tucatinib-related serious AEs (acute kidney injury, colitis, fatigue) 3% each
Safety: no treatment-related deaths; cardiac toxicity uncommon
Conclusions
Tucatinib + trastuzumab achieved an ORR of 38.1% with mPFS 8.2 months and mOS 24.1 months in heavily pretreated HER2+ RAS wild-type mCRC — the first FDA-approved anti-HER2 regimen for mCRC. Tucatinib monotherapy produced only 3% ORR, confirming that dual blockade is required for meaningful activity.
Key Limitations
Non-randomized design between combination and control (cohort C was not a pre-specified randomized comparator). Restricted to HER2+ RAS wild-type population — excludes RAS-mutant patients who may also harbor HER2 amplification. Small sample size (N=84 primary analysis set). No head-to-head comparison with T-DXd (trastuzumab deruxtecan). Accelerated approval based on ORR — confirmatory OS data pending from MOUNTAINEER-03 (phase III).
Clinical Context
Tucatinib + trastuzumab received FDA accelerated approval in January 2023 for previously treated HER2+ RAS wild-type mCRC — the first targeted therapy approval for HER2-amplified colorectal cancer. DESTINY-CRC02 (T-DXd) represents the other FDA-approved HER2-directed option in mCRC; no head-to-head trials exist. ESMO recognizes anti-HER2 therapy for HER2+ mCRC in the refractory setting. MOUNTAINEER-03 is evaluating tucatinib + trastuzumab + mFOLFOX6 in first-line HER2+ mCRC.