Supplementary Material for: Abscopal Effect Induced by LAG-3/PD-1 Inhibition and Radiation Therapy in Metastatic Acral Melanoma: A Case Report

Introduction: Acral melanoma (AM) is a rare and biologically distinct subtype of melanoma that exhibits poor responsiveness to immune checkpoint inhibitors (ICIs) and limited treatment options. Relatlimab, a LAG-3 inhibitor, in combination with nivolumab, a PD-1 inhibitor, has recently emerged as a dual ICI strategy targeting T-cell exhaustion. Case Presentation: We report the first documented case of an abscopal effect induced by LAG-3 inhibition and radiation therapy in a man in his 40’s with metastatic amelanotic AM of the right foot. The patient progressed after four cycles of relatlimab plus nivolumab, prompting palliative radiation therapy to a bulky, bleeding foot lesion. Remarkably, the tumor began to regress within one week of initiating radiation. Two months later, the lesion had nearly resolved, and follow-up imaging demonstrated complete disappearance of pulmonary metastases, consistent with a systemic abscopal response. Conclusion: This case illustrates the potential synergistic effect of sequential LAG-3/PD-1 inhibition followed by radiation therapy in eliciting systemic immune activation in AM. It represents the first reported instance of an abscopal effect associated with relatlimab-based immunotherapy, suggesting that strategic sequencing of immunotherapy and radiation may be critical in overcoming immune resistance in this challenging melanoma subtype.