Addition of Peptide Receptor Radiotherapy to Immune Checkpoint Inhibition Therapy Improves Outcomes in Neuroendocrine Tumors.

Neuroendocrine tumors (NETs) are often diagnosed in advanced stages. Despite the advances in treatment approaches, including somatostatin analogs and peptide receptor radionuclide therapy (PRRT), these patients have no curative treatment option. Moreover, immunotherapy often yields modest results in NETs. We investigated whether combining PRRT using [Lu]DOTATATE and immune checkpoint inhibition therapy improves treatment response in NETs. A gastroenteropancreatic NET model was generated by subcutaneous implantation of human QGP-1 cells in immune-reconstituted NOD.Cg- /SzJ mice engrafted with human peripheral blood mononuclear cells ( = 96). Mice were randomly assigned to receive pembrolizumab (anti-PD1), [Lu]DOTATATE (PRRT), simultaneous anti-PD1 and PRRT (S-PRRT), anti-PD1 on day 0 followed by PRRT on day 3 (delayed PRRT [D-PRRT]), PRRT on day 0 followed by anti-PD1 (early PRRT [E-PRRT]), or vehicle as control ( = 12/group). Human granzyme-B-specific [Ga]NOTA-hGZP PET/MRI was performed before and 6 d after treatment initiation, as an indicator of T-cell activation. Response to treatment was based on tumor growth over 21 d and on histologic analyses of extracted tissues on flow cytometry for T cells, hematoxylin and eosin staining, and immunohistochemical staining. [Ga]NOTA-hGZP PET/MRI showed significantly increased uptake in tumors treated with E-PRRT, S-PRRT, and anti-PD1 on day 6 compared with baseline (SUV: 3.36 ± 0.42 vs. 0.73 ± 0.23; 2.36 ± 0.45 vs. 0.76 ± 0.30; 2.20 ± 0.20 vs. 0.72 ± 0.28, respectively; < 0.001), whereas no significant change was seen in PET parameters in the D-PRRT, PRRT, or vehicle groups ( > 0.05). Ex vivo analyses confirmed the PET results showing the highest granzyme-B levels and T cells (specifically CD8-positive effector T cells) in the E-PRRT group, followed by the S-PRRT and anti-PD1 groups. Tumor growth follow-up showed the most significant tumor size reduction in the E-PRRT group (baseline to day 21, 205.00 ± 30.70 mm vs. 78.00 ± 11.75 mm; = 0.0074). Tumors showed less growth reduction in the PRRT, D-PRRT, and S-PRRT groups than in the E-PRRT group ( < 0.0001). The vehicle- and anti-PD-1-treated tumors showed continued growth. Combination of PRRT and anti-PD1 shows the most robust inflammatory response to NETs and a better overall outcome than immune checkpoint inhibition or PRRT alone. The most effective regimen is PRRT preceding anti-PD1 administration by several days.