High spatial resolution imaging of cisplatin and Texas Red cisplatin in tumour spheroids using laser ablation isotope dilution inductively coupled plasma mass spectrometry and confocal fluorescence microscopy

Oncology research uses different imaging techniques to provide information about the spatial distribution of the chemotherapy drugs used for the targeted tissues. Among them, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is increasingly being used to track the spatial distribution of metal-based chemotherapeutics in different tissue samples. In this investigation, instrumental parameters were optimized for the bioimaging of Pt in HT29 tumour spheroids treated with cisplatin (CDDP) or Texas Red cisplatin (TR-CDDP) using LA-ICP-MS. A high spatial resolution, using pixel dimensions of 2.0 μm × 2.5 μm, and a high sensitivity, with the limits of detection (LOD) better than 0.78 mg kg^-1 Pt, was achieved. Matrix-matched gelatine standards and/or isotope dilution (ID) analyses were used to quantify the amount of Pt. Differences between the results of the Pt concentrations determined by the two quantification were less than 4%. The results of the LA analysis revealed that the Pt in the CDDP-treated tumour spheroids was localized primarily in the outer rim of the spheroids and to a lesser extent in the intermediary layer and the necrotic core. Due to the steric effects, significantly lower Pt concentrations were accumulated in the spheroids treated with TR-CDDP (2.2 times lower than in CDDP-treated spheroids, normalized to the spheroid volume), while the Pt was mostly distributed in the areas of the outer rim. Finally, imaging with confocal fluorescence microscopy, which is commonly used in oncology research, was compared with that by LA-ICP-MS. The results of the two complementary techniques demonstrated good agreement in terms of the spatial distribution of the TR-CDDP, while the intensity of the fluorescence matched well with the concentrations of Pt determined with LA-ICP-MS.