Lenvatinib Plus Pembrolizumab in Well Differentiated G3 Neuroendocrine Tumors

This is the first study to be done in a newly described class of neuroendocrine tumors known as well-differentiated grade 3 neuroendocrine tumors (WD G3 NET). First described in the pancreas in 2017, the classification was broadened to include gastrointestinal tract tumors in 2019. Recent data suggest an equivalent subtype exists in the lungs (NEC with carcinoid morphology). WD G3 NETs can occur de novo as well as the result of grade progression over time. This is a single arm, multi-site, Phase II study in biomarker "unselected" participants. This study will also incorporate serial blood samples, tumor biopsies, and special imaging to better understand the impact of therapy on the tumor and microenvironment. Hyperpolarized (HP) 13C-pyruvate magnetic resonance imaging (MRI) - a novel non-radioactive imaging modality able to provide in vivo measurements of the pyruvate-to-lactate conversion rate (kpl).