Gold Nanoparticles Functionalised with 197mHg/197Hgfor Receptor-IndependentNanobrachytherapy of Triple-Negative Breast Cancer and Glioblastoma

<ns3:p>&lt;br&gt;&lt;b&gt;Introduction:&lt;/b&gt; This study aimed to develop a novel therapeutic strategy for treatment- -resistant cancers based on nano-brachytherapy, using gold nanoparticles as carriers for the Auger-electron–emitting radionuclides &lt;sup&gt;197m&lt;/sup&gt;Hg and &lt;sup&gt;197&lt;/sup&gt;Hg. &lt;br&gt;&lt;b&gt;Methods:&lt;/b&gt; Five-nanometre PEGylated gold nanoparticles were functionalised with &lt;sup&gt;197m&lt;/sup&gt;Hg/&lt;sup&gt;197&lt;/sup&gt;Hg via surface amalgamation and evaluated in vitro in triple-negative breast cancer (MDA-MB-231) and glioblastoma (T98G) cell lines. Cellular internalisation and subcellular distribution were assessed by uptake studies and fractionation. Cytotoxicity was evaluated using MTS assays, while therapeutic efficacy in three-dimensional models was investigated in tumour spheroids. Flow cytometry was employed to analyse apoptosis and cell-cycle distribution. DNA double-strand breaks were quantified by γ-H2AX phosphorylation. Ex vivo biodistribution studies compared intratumoural and intravenous administration routes.&lt;/br&gt; &lt;br&gt;&lt;b&gt;Results:&lt;/b&gt; The radio-conjugate underwent straightforward synthesis, exhibited very high radio-labelling efficiency at low mercury loading, and maintained excellent colloidal stability. Efficient cellular uptake and pronounced nuclear accumulation were observed in both cell lines. In vitro studies demonstrated strong, time- and dose-dependent cytotoxicity predominantly mediated by apoptosis, with minimal necrosis, accompanied by significant induction of DNA double-strand breaks. In three-dimensional cultures, MDA-MB-231 spheroids underwent rapid, dose-dependent disintegration, whereas T98G spheroids displayed increased resistance. Ex vivo biodistribution revealed high tumour retention following intratumoural administration, minimal systemic exposure and predominantly renal clearance.&lt;/br&gt; &lt;br&gt;&lt;b&gt;Conclusions:&lt;/b&gt; &lt;sup&gt;197m&lt;/sup&gt;Hg/&lt;sup&gt;197&lt;/sup&gt;Hg-functionalised gold nanoparticles represent a promising receptor-independent platform for localised nano-brachytherapy and warrant further preclinical evaluation in aggressive and therapy-resistant tumours.&lt;/br&gt;</ns3:p>