Clinical activity of pembrolizumab in refractory MDM2-amplified advanced intimal sarcomas.

ABSTRACT

Intimal sarcoma (InS) is an ultra-rare and aggressive subtype of soft tissue sarcoma (STS). It usually arises in large mediastinal arteries and the heart. In the advanced setting, sequential cytotoxic chemotherapy is often used, mainly based on retrospective studies and case series but with modest benefit. The use of immune checkpoint inhibitors is a promising strategy for some STS, but identifying biomarkers of response remains challenging due to disease rarity and heterogeneity. A reactive and pro-inflammatory tumor microenvironment (TME) is believed to be associated with better outcomes for patients receiving anti-PD-1-based regimens, generating the rationale to explore this strategy in malignancies with this characteristic, such as InS. We report three cases of advanced InS patients experiencing partial response to pembrolizumab-based therapy despite low tumor mutational burden and absence of mismatch-repair deficiency. We hypothesize that TME-related characteristics such as PD-L1 expression and the presence of tertiary lymphoid structures might explain this phenomenon.

Pembrolizumab in advanced intimal sarcomas Intimal sarcomas are ultra-rare and highly aggressive malignant tumours that most frequently arise in mediastinal arteries or the heart. Besides arising in challenging areas of the thoracic cavity, their rarity directly interferes with the development of new therapies, which negatively impacts patients' prognosis, especially after disease progression on chemotherapy regimens. We reported three cases successfully treated with immunotherapy (represented by pembrolizumab-based regimens), as well as a potential explanation for their outcomes.