Tucatinib With Brain and/or Spinal XRT in Patients With HER2+ Metastatic Breast Cancer and LMD

The proposed study will evaluate the safety and efficacy of XRT followed by systemic therapy among patients with HER2+ metastatic breast cancer and LMD

Breast cancer is the most common cancer among women worldwide, and the second leading cause of brain metastases (BrM). Despite recent treatment advances, the prognosis of patients with breast cancer BrM remains poor, and the prognosis among those with leptomeningeal disease (LMD) is particularly dire with a median survival of only 2-4 months.

Patients with HER2+ metastatic breast cancer have a high life-time risk of central nervous system (CNS) metastases, with an approximately 50% lifetime risk. Although the cause of death among patients with breast cancer BrM is challenging to ascertain, approximately 50% of patients with HER2+ BrM are thought to die from central nervous system (CNS) disease involvement. Among patients with LMD specifically, the cause of death is most commonly related to CNS disease.

In an analysis of 430 patients (96 of whom had breast cancer) treated for LMD in the National Cancer Database between 2005 and 2014, those patients treated with chemotherapy plus radiotherapy had a longer median overall survival of 5 months (3.5 - 6.5 months) compared to patients treated with XRT or chemotherapy alone. In addition, a majority (n=18/26, 69%) of observational studies irrespective of primary tumor site have shown an "improvement or likely improvement" in survival with the use of XRT for LMD, either alone or in combination with systemic therapy. Hence, this proposed study will evaluate the safety and efficacy of XRT followed by systemic therapy (which is considered standard of care) among patients with HER2+ metastatic breast cancer and LMD.

Eligibility Criteria:
Inclusion Criteria: Phase 1

1. Men or women with HER2+ metastatic breast cancer.
2. Evidence of LMD in the brain and/or spine
3. Age 18+ at time of consent;
4. ECOG ≤ 2;
5. If applicable, the last dose of prior chemotherapy, immunotherapy, endocrine therapy therapy must have been completed 14 days prior to study enrollment.
6. More than 14 days or 5 half-lives from the last dose of any experimental agent is required, whichever is greater;
7. All toxicity related to prior cancer therapies must have resolved to ≤ Grade 1 prior to enrollment, except for alopecia; neuropathy, must have resolved to ≤ Grade 2.

Phase 2: Inclusion Criteria

1. Left ventricular ejection fraction (LVEF) must be within institutional limits of normal as assessed by ECHO or MUGA documented within 2 weeks prior to starting systemic therapy on the study;
2. Adequate hematologic, liver, and renal function within 2 weeks prior to phase 2 enrollment, as follows:

   1. Hemoglobin ≥ 9 g/dL
   2. ANC ≥ 1 x109/L
   3. Platelets ≥ 100 x109/L
   4. Total bilirubin ≤ 1.5 X upper limit of normal (ULN)
   5. AST and ALT ≤ 2.5X ULN
   6. International normalized ratio (INR) and activated partial thromboplastin time (aPTT) ≤ 1.5 X ULN
   7. Creatinine clearance (CrCL) ≥ 50 mL/min

Exclusion Criteria: Phase 1

1. Prior WBRT for brain metastases
2. Prior therapy specifically directed at LMD
3. Inability to comply with MRI-based surveillance of CNS disease.
4. Inability to swallow pills or any significant gastrointestinal diseases such as inflammatory bowel disease.
5. Presently known dihydropyrimidine dehydrogenase deficiency;
6. Diagnosed with Hereditary fructose intolerance;
7. Diagnosed with Gilbert's disease;
8. Prior history of other cancer with evidence of disease within the last 5 years;
9. Prior use of tucatinib at any time prior to enrollment.

Phase 2:

1. Currently pregnant or breastfeeding;
2. Use of a strong cytochrome P450 (CYP)2C8 inhibitor within 5 half-lives of the inhibitor or use of a strong CYP3A4 or CYP2C8 inducer within 5 days prior to the first dose of systemic therapy
3. Myocardial infarction or unstable angina within 6 months prior to the first dose of systemic therapy.
4. Blood product transfusions in order to meet eligibility criteria