Disease control with disitamab vedotin in HER2-low polymorphous adenocarcinoma of the tongue base after chemotherapy failure: A case report

Polymorphous adenocarcinoma (PAC) is a distinct subtype of salivary gland carcinoma (SGC), and its occurrence at the tongue base is uncommon. Effective systemic treatment options for patients with advanced disease remain limited. Although HER2-low expression has been identified in a subset of SGCs, evidence supporting the use of HER2-targeted antibody-drug conjugates (ADCs) in HER2-low PAC is currently lacking. CASE PRESENTATION: A 72-year-old female with HER2-low (immunohistochemistry 1+) PAC of the tongue base presented with cervical nodal metastases (cT3N2cM0, Stage IVA). The tumor progressed after two cycles of first-line induction chemotherapy with paclitaxel plus cisplatin. The patient declined radical surgery and radiotherapy due to concerns regarding functional morbidity. She subsequently received disitamab vedotin combined with cisplatin, followed by disitamab vedotin maintenance therapy. This regimen resulted in rapid symptomatic improvement and radiologic regression of the primary lesion, achieving sustained disease control for over eight months at the time of reporting, with only grade 1-2 peripheral neuropathy observed. CONCLUSIONS: To our knowledge, this is the first reported case of HER2-low PAC achieving durable disease control with a HER2-targeted ADC following chemotherapy failure. The response to disitamab vedotin, despite prior taxane progression, suggests the potential critical role of HER2-mediated drug delivery and indicates that low antigen density may be sufficient for ADC efficacy. This case indicates the importance of routine HER2 testing with explicit HER2-low categorization in advanced SGCs and offers preliminary evidence that may help expand therapeutic options for this treatment-refractory population.