Paving the Way for CCK2R-Targeted Peptide Receptor Radionuclide Therapy with [177Lu]Lu-DOTA-MGS5 in Patients with Small Cell Lung Cancer

<b>Background/Objectives</b>: Peptide receptor radionuclide therapy (PRRT) is an established treatment for neuroendocrine tumors (NETs), enabling targeted radiation delivery via radiolabeled peptides. Small cell lung cancer (SCLC) remains a major therapeutic challenge due to its aggressive nature and poor prognosis. Despite advances, relapse rates are high and effective therapies are limited. We previously demonstrated the diagnostic potential of the cholecystokinin-2 receptor (CCK2R)-targeting minigastrin analog [⁶⁸Ga]Ga-DOTA-MGS5 in PET/CT imaging of different NETs. Building on this, we developed and evaluated [¹⁷⁷Lu]Lu-DOTA-MGS5 as a therapeutic PRRT agent. <b>Methods</b>: Preclinical studies investigating the receptor-mediated cellular internalization and intracellular distribution over time in A431 cells with and without CCK2R expression were performed using the fluorescent tracer ATTO-488-MGS5. Short- and long-term cytotoxic effects of [¹⁷⁷Lu]Lu-DOTA-MGS5 were evaluated on the same cell line using trypan blue exclusion and clonogenic survival assays. CCK2R expression was assessed by immunohistochemistry in 42 SCLC tissue specimens. In addition, the first PRRT with [¹⁷⁷Lu]Lu-DOTA-MGS5 was conducted in a patient with extensive disease SCLC (ED-SCLC) after confirming CCK2R-positive uptake in [⁶⁸Ga]Ga-DOTA-MGS5 PET/CT. <b>Results</b>: Rapid binding and internalization into A431-CCK2R cells, with progressive accumulation in intracellular compartments, was observed for ATTO-488-MGS5. Short-term irradiation effects of [¹⁷⁷Lu]Lu-DOTA-MGS5 were comparable for 4 h and 24 h incubation and were between the effects obtained with 2 and 4 Gy of external beam radiotherapy (EBRT). Clonogenic survival of A431-CCK2R cells incubated with increasing activity of [¹⁷⁷Lu]Lu-DOTA-MGS5 decreased in a dose-dependent manner. Immunohistochemistry on SCLC specimens confirmed moderate to high CCK2R expression in 16 out of 42 SCLC samples. In the first patient with SCLC treated with four cycles of [¹⁷⁷Lu]Lu-DOTA-MGS5 with a total activity of 17.2 GBq, an improvement in clinical symptoms was observed. <b>Conclusions</b>: The preclinical and clinical results confirm the feasibility of [¹⁷⁷Lu]Lu-DOTA-MGS5 PRRT in patients with SCLC and support further clinical studies investigating the therapeutic value and clinical applicability of this new CCK2R-targeted theranostic approach in larger patient cohorts.