Immune checkpoint inhibitor-associated autoimmune encephalitis (ICI-AIE) is an increasingly recognized immune-related adverse event. To date, these cases have been reported with drugs targeting programmed cell death protein 1 (PD1) and its ligand (PDL1) and cytotoxic T-lymphocyte–associated antigen 4 (CTLA4).^1,2

Case Report

A 55-year-old woman with metastatic breast cancer presented with new onset neurologic symptoms. She first developed breast cancer in 2008 (ER+/PR+/HER2+), was treated with lumpectomy, radiation, chemotherapy (docetaxel, carboplatin, epirubicin, cyclophosphamide, traztuzumab) and hormonal therapy with no evidence of disease from 2009-2021. She represented 4/2021 with facial pain and swelling and was found to have bone and lymph node metastatic recurrence. She received radiation to the orbit from 4/2021-6/2021 and started palbociclib 6/2021. PET scan 8/25/21 showed improvement in metastases with no new lesions. On 9/7/21, she developed blurry vision followed by subacute encephalopathy, vertical nystagmus, ataxia, constipation and urinary retention. Work-up revealed positive serum/cerebrospinal fluid AMPAR antibodies. She was treated with steroids, IVIg and rituximab with marked improvement in her neurologic symptoms.

AMPAR encephalitis has been well-described as a paraneoplastic syndrome associated with breast (among other) cancers.³ Of interest in our patient’s case, her neurologic symptoms developed while her cancer was responding to chemotherapy, whereas most paraneoplastic syndromes are described in the setting of tumor progression or recurrence.⁴ On the other hand, ICI-AIE typically occurs within the first 3 months of therapy and is more likely to occur when the cancer is responding to therapy, consistent with our patient’s presentation.^1,5 We report this case to highlight a potential association between palbociclib and ICI-AIE so other providers may consider it on the differential in a patient with new-onset neurologic symptoms while on this therapy.