Enhancing [177Lu]Lu-DOTA-TATE therapeutic efficacy in vitro by combining it with metronomic chemotherapeutics.

Background: Peptide receptor radionuclide therapy (PRRT) uses [¹⁷⁷Lu]Lu-[DOTA⁰-Tyr³]octreotate ([¹⁷⁷Lu]Lu-DOTA-TATE) to treat patients with neuroendocrine tumours (NETs) overexpressing the somatostatin receptor 2A (SSTR2A). It has shown significant short-term improvements in survival and symptom alleviation, but there remains room for improvement. Here, we investigated whether combining [¹⁷⁷Lu]Lu-DOTA-TATE with chemotherapeutics enhanced the in vitro therapeutic efficacy of [¹⁷⁷Lu]Lu-DOTA-TATE.

Results: Transfected human osteosarcoma (U2OS + SSTR2A, high SSTR2A expression) and pancreatic NET (BON1 + STTR2A, medium SSTR2A expression) cells were subjected to hydroxyurea, gemcitabine or triapine for 24 h at 37^oC and 5% CO₂. Cells were then recovered for 4 h prior to a 24-hour incubation with 0.7-1.03 MBq [¹⁷⁷Lu]Lu-DOTA-TATE (25 nM) for uptake and metabolic viability studies. Incubation of U2OS + SSTR2A cells with hydroxyurea, gemcitabine, and triapine enhanced uptake of [¹⁷⁷Lu]Lu-DOTA-TATE from 0.2 ± 0.1 in untreated cells to 0.4 ± 0.1, 1.1 ± 0.2, and 0.9 ± 0.2 Bq/cell in U2OS + SSTR2A cells, respectively. Cell viability post treatment with [¹⁷⁷Lu]Lu-DOTA-TATE in cells pre-treated with chemotherapeutics was decreased compared to cells treated with [¹⁷⁷Lu]Lu-DOTA-TATE monotherapy. For example, the viability of U2OS + SSTR2A cells incubated with [¹⁷⁷Lu]Lu-DOTA-TATE decreased from 59.5 ± 22.3% to 18.8 ± 5.2% when pre-treated with hydroxyurea. Control conditions showed no reduced metabolic viability. Cells were also harvested to assess cell cycle progression, SSTR2A expression, and cell size by flow cytometry. Chemotherapeutics increased SSTR2A expression and cell size in U2OS + SSTR2A and BON1 + STTR2A cells. The S-phase sub-population of asynchronous U2OS + SSTR2A cell cultures was increased from 45.5 ± 3.3% to 84.8 ± 2.5%, 85.9 ± 1.9%, and 86.6 ± 2.2% when treated with hydroxyurea, gemcitabine, and triapine, respectively.

Conclusions: Hydroxyurea, gemcitabine and triapine all increased cell size, SSTR2A expression, and [¹⁷⁷Lu]Lu-DOTA-TATE uptake, whilst reducing cell metabolic viability in U2OS + SSTR2A cells when compared to [¹⁷⁷Lu]Lu-DOTA-TATE monotherapy. Further investigations could transform patient care and positively increase outcomes for patients treated with [¹⁷⁷Lu]Lu-DOTA-TATE.