Comparative Physicochemical Characterization and Biological Effects of Magnetite- and Maghemite-Based Magnetic Colloidal Suspensions in A704 Renal Cancer Cells

Background/Objectives: Magnetic iron oxide nanoparticles are investigated in cancer research; however, the direct biological effects of unloaded magnetic colloidal suspensions in renal cancer remain insufficiently characterized. This study compared two double-oleic-acid-coated formulations prepared from precursors: magnetite-based MCS 1 and maghemite-based MCS 2. Methods: The suspensions were characterized by vibrating-sample magnetometry, dynamic light scattering, bright-field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Their effects on A704 renal adenocarcinoma cells were evaluated after 24 h of exposure to 1–5 µg/mL using MTT, neutral red uptake, JC-1, Hoechst 33342/MitoTracker Red CMXRos, acridine orange/propidium iodide staining, and a 7-day clonogenic assay. Results: MCS 2 showed higher volumetric saturation magnetization (1.541 vs. 1.059 Gs), a smaller Z-average hydrodynamic diameter (79.65 vs. 103.9 nm), and a more uniform volume-weighted distribution. Both formulations significantly reduced metabolic activity and neutral red uptake and induced mitochondrial depolarization, cell-death-associated morphological changes, and impaired clonogenic capacity. MCS 1 generally produced a moderate response with an apparent plateau, whereas MCS 2 showed a pronounced concentration-related effect at 4–5 µg/mL. At 5 µg/mL, MCS 2 reduced metabolic activity to approximately 49%, neutral red uptake to 50.04%, the JC-1 aggregate-to-monomer ratio to 46.02%, and colony formation to 31.44% of the control. Conclusions: The suspensions exhibited distinct physicochemical and biological profiles. MCS 2 produced greater effects at the highest concentrations, potentially related to its smaller hydrodynamic size and more uniform particle-size distribution. Further studies should establish tumor selectivity, cellular uptake, and the underlying molecular mechanisms.