Intracranial pharmacokinetics and resistance mechanisms of tucatinib in patients with HER2+/mutant solid tumors and brain metastasis: Results from the WinHER2 study (NCT05892068).

2031 Background: Tucatinib, a potent HER2-selective tyrosine kinase inhibitor approved for the treatment of HER2-positive (+) metastatic breast cancer (MBC) and colorectal cancer (CRC), has demonstrated activity in stable and active brain metastasis (BrM). However, intracranial pharmacokinetics and specific mechanisms of resistance driving BrM progression remain unknown. Methods: This window-of-opportunity, prospective, non-randomized study evaluated tissue and serum levels of tucatinib ([Tt], [Ts]) in patients undergoing BrM resection for HER2+/mutant (mut) MBC, NSCLC, CRC or gastroesophageal cancer. HER2+ MBC patients were enrolled in 2 parallel cohorts: those with progressing BrM while on tucatinib (A) and tucatinib-naïve patients (B); cohort C was comprised of patients with GI malignancy, NSCLC and mutHER2 MBC. All patients received tucatinib 300 mg BID for 4 days preoperatively. [Tt/s] were measured by mass spectrometry in resected BrM and serum at screening and 3 intraoperative timepoints; CSF [T] was collected when available by optional lumbar puncture. BrM tissue was used for correlative studies including WGS, scRNA-seq, and electron microscopy. The primary endpoint compared BrM-to-serum tucatinib ratios between cohorts A and B with lower ratios hypothesized in cohort A. [T] was reported as median and interquartile range (IQR). Results: 9 female patients enrolled between 7/2023 and 12/2025: 3 in cohort A, 5 in cohort B, and 1 HER2+ CRC patient in cohort C. Median age was 48.5 years (43-65). One patient from cohort B was unevaluable due to lack of viable recurrent disease on pathologic examination. Total tucatinib levels in resected BrMs ranged from 2.22 to 192.87 ng/mg, (median=30.82; IQR=16.52-111.84) in cohort A and from 1.5 to 4.21 ng/mg (median=3.04; IQR=1.93-4.05) in cohort B. The median tucatinib serum concentration was 1158.3 ng/mL (range=229.2-3440; IQR=693.78-2299.17) and 365.52 ng/mL (range=167.23-731.03; IQR=176.47-596.37), in cohorts A and B, respectively. The BrM/serum ratio did not differ between cohorts A and B (median=9.7; range=9.0-166.5; IQR=9.43-102.77] vs 7.36 [range=3.77-22.28; IQR=5.26-12.30]). The single cohort C patient had tucatinib BrM level of 1.66 ng/mg and BrM/serum ratio of 7.41. CSF [T] was available in two cohort A patients: it was undetectable in one and 35.71 ng/mL in the other (BrM/CSF ratio: 5401). Conclusions: Patients with progressing BrM while receiving tucatinib had BrM/serum tucatinib ratios similar to those of patients receiving tucatinib for the first time in the trial. These data indicate that intratumoral drug concentration alone is unlikely to account for intracranial acquired resistance. Correlative analyses investigating potential genomic and microenvironmental mechanisms of resistance are ongoing, and results will be presented at the meeting. Clinical trial information: NCT05892068 .