Novel in situ forming depot-based brachytherapy enables intratumoral release of 177Lu and tumor growth delay

235 Objectives: Brachytherapy (BT) is a safe and effective form of radiotherapy that can deliver a high focal radiation dose to solid tumors while limiting damage to the surrounding tissue. However, to date, the clinical application of BT has been limited due to invasive administration of BT sources, uncontrolled BT source migrations after administration and only allowing the use of photon-emitting radionuclides. This study aims to develop an injectable BT source material to provide a minimal invasive administration of the beta-emitter 177Lu, and investigate the intratumoral release of 177Lu in order for 177Lu to provide a short-range local radiation dose within the tumor. Methods: Water-insoluble in situ forming depot systems embedding 177Lu, referred to as BrachyGels, have been designed. These optimize both the release of 177Lu and its subsequent retention in the tumor region. Hydrophobic ionophores (IOs), 8-hydroxyquinoline and its derivatives, are used to form hydrophobic complexes with 177Lu. The hydrophobicity and viscosity of the BrachyGel formulations were modified and the hydrophobicity of the IOs was varied. The distribution of 177Lu(IO)x within the BrachyGel and tumor tissue was quantified by SPECT/CT imaging after intratumoral injection of BrachyGel in a CT26 murine tumor model. A proof-of-principle study was performed to evaluate the in vivo therapeutic efficacy of the optimized 177Lu-BrachyGel in terms of tumor growth and survival rate. Results: The hydrophobic Lu(IO) complexes were initially detected by UV-VIS spectroscopy and the cell toxicity and cellular uptake of Lu(IO) were characterized in the CT26 cell line. Lu(8HQ) and Lu(8HQ-2I) were found to be taken up by CT26 cells and show potential cytotoxicity. The in vitro release results suggested that the release of 177Lu could be tuned by changing the type and amount of sugar esters in the BrachyGels and varying the hydrophobicity of the IOs, resulting in gels with either non-releasing and slow-releasing profiles of 177Lu. Furthermore, the in vivo release profiles of 177Lu derived from SPECT/CT images were similar to the release results observed in vitro for the same type of BrachyGel. A maximum intratumoral release of 177Lu(8HQ) of 21 ± 4% after six days was observed for the non-releasing BrachyGel, whereas the released 177Lu(8HQ) was 86 ± 1% after six days for the slow-releasing BrachyGel. The use of 8HQ-2I for the complexation with 177Lu resulted in a significant decrease in the released 177Lu (58 ± 6% after six days). The prolonged tumor-retention of 177Lu(8HQ) (75 ± 2% after six days) was obtained for the non-releasing BrachyGel when compared to other types of slow-releasing BrachyGels (40 ± 3% after six days). The in vivo evaluation of the efficacy of slow-releasing 177Lu(8HQ)-BrachyGel indicated that the tumor growth was delayed after its single intratumoral injection compared to that of non-radioactive BrachyGel. Conclusions: 177Lu-BrachyGel treatment can be considered as a promising brachytherapy approach offering easy and safe administration of 177Lu. Therapeutic optimization of the BrachyGels including careful injection of the BrachyGel or using other beta emitters (e.g., high energy 90Y) is needed to improve the tumor control.