Combination treatments to enhance peptide receptor radionuclide therapy of neuroendocrine tumours

The incidence of neuroendocrine tumours (NETs) is increasing, but curative therapeutic options are limited because diagnosis is often delayed until the tumour has metastasized. Peptide receptor radionuclide therapy (PRRT) is among the most effective therapeutic options for metastatic NETs because of targeted delivery of radioactivity to the tumour via the somatostatin receptor (SSTR) and relatively low systemic toxicity. However, current PRRT regimes result in palliation rather than cure, and higher doses of PRRT that might achieve remission would also be too toxic to the patients. Therefore, there is a need to improve PRRT of NETs by combining it with other agents to achieve maximum benefits from the internal radiation therapy, while sparing non-target organs from radiation toxicity. Here we review various current and potential combination strategies to improve Lu-177-octreotate-based PRRT of NET, some of which could also apply to other radionuclide therapies. These strategies include co-administered drugs that improve delivery of the radiopharmaceutical via increased tumour perfusion or through increased SSTR density at tumour surface. Other combinations are aimed at enhancing the biological effects of the radiation-induced DNA damage in tumour cells or generating additional DNA damage burden to effectively increase the cytotoxicity of PRRT. We also propose an algorithm for stratifying NET patients to receive or not combination therapies with PRRT. Considering that PRRT and many of these combination agents are already used for treating patients with NET and other cancers, the proposed strategies to improve the efficacy of PRRT could be rapidly translated into the clinic.