Material-Driven Design of Mixed-Metal Nanoparticles for Enhanced Heating Efficiency and Cancer-Selective Thermal Response

Abstract

Multifunctional, biocompatible magnetic materials, such as iron oxide nanoparticles (Fe-oxide NPs), hold great potential for biomedical applications, including diagnostics (e.g., MRI) and cancer therapy. In particular, they can play a crucial role in advancing cancer thermotherapy by generating heat when administered intratumorally and exposed to an alternating magnetic field. This heat application is often combined with radio(chemo)therapy and/or imaging. Designing materials for such a multimodal approach requires hybrid nanoparticles that retain their magnetic properties while integrating additional functionalities. However, while pursuing additional properties, the safety of treatment should also be considered. For instance, reducing the amount of Fe-oxide NPs used for the treatment may minimize safety concerns related to dose-related toxicity, while focusing on tumor treatment selectivity may help avoid unpredictable damage to adjacent healthy tissues caused by intratumoral injection. The mechanisms of thermotherapy by means of magnetic NPs and the associated safety considerations are described in Chapter 1.

In Chapter 2, the synthesis and investigation of magnetically enhanced nanoparticles with a palladium core (envisioned for future radiolabeling with therapeutic 103Pd) and a magnetic iron oxide shell containing paramagnetic manganese (Pd/Fe|(nMn)-oxide, n = 0.25 and 0.5) were introduced. Doping the iron oxide lattice with Mn significantly increases magnetic saturation (MS), boosting specific loss power (SLP) up to 1.7 times compared to undoped analogues. Interestingly, higher Mn content in Pd/Fe|(0.5Mn)-oxide leads to a pronounced Mn outer rim, enhancing the heating efficiency at 346 kHz and 23 mT and contributing to the water exchange on the surface of the paramagnetically doped nanoparticles, resulting in additional T1-weighted MRI contrast. The enhanced magnetic properties of the hybrid Pd/Fe|Mn-oxide NPs enable effective therapeutic outcomes with injection of only small quantities of the material, offering great potential for effective cancer treatment strategies that combine hyperthermia/thermal ablation with radiotherapy, while allowing for real-time monitoring via MRI.

In Chapter 3, holmium (envisioned for future radiolabeling with therapeutic 166Ho) was doped into iron oxide nanoparticles (xHo/Fe-oxide NPs, x = 0.1 - 5%) via modified co-precipitation method and investigated their influence on MS and SLP. It was shown that the Ho-doping ratio up to 0.5% was not sufficient to increase MS and SLP values, which mainly depend on the size of the resulting nanoparticles. Starting at 0.5% Ho-doping, both MS and SLP values increased as the Ho-doping ratio rose, reaching their maximum at 2.5%. Further increases in the doping ratio caused MS and SLP to decrease due to larger particle size and insufficient lattice substitution. This explanation was supported by another batch synthesized at room temperature, which also showed poor Ho lattice distribution along with reduced MS and SLP. These findings indicated the possibility of improving magnetic and heating properties by tuning Ho-doping, which provided the basis for developing ¹⁶⁶Ho-labeled Fe-oxide NPs for combined magnetic hyperthermia and radiotherapy (MHT-RT).

Subsequently, in Chapter 4, Ho3+ in 2.5%Ho/Fe-oxide NPs was replaced by therapeutic 166Ho to investigate their potential for synergistic MHT-RT. In vitro experiments using both single and combined treatments were performed on U87 and U2OS cell lines by evaluating their viability after incubation with 165/166Ho/Fe-oxide NPs with/without AMF. For both cell lines, the lower viability of treated with MHT+RT was observed compared to cells treated with either treatment alone, clearly demonstrating the synergistic potential of 166Ho/Fe-oxide NPs. When applying both MHR and RT, the effective concentration could be reduced to 2 mg mL⁻¹ and 2.5 mg mL⁻¹, and the radioactivity to 1.6 MBq and 0.3 MBq for U87 and U2OS cells, respectively. This shows that the synergistic effect allows for a reduction in the required concentration and activity of NPs to avoid the dose-related toxicity on healthy tissues.

Chapter 5 presents the concept of pH-responsive Fe-oxide nanocomposites (Fe-ZIF NCs) to improve the tumor selectivity of MHT. ZIF-8 was coated onto Fe-oxide NPs to induce their aggregation, and therefore, reduce the overall heating performance at physiological pH 7.4. In the tumor microenvironment, known to be more acidic, the heating performance was recovered, and a temperature difference was observed. By optimizing the AMF frequency, synthesis condition, NPs concentration and particle size, the maximum heating temperatures were tuned to the therapeutic ranges of 42 - 45 ºC reached only at acidic pH. In vitro experiments with U87 and HEK-293t cells further demonstrated the tumor-selective MHT effect. Using AFM at 710 kHz and 10 mT for 45 min, Fe-ZIF NCs showed selective cytotoxicity for U87 cells within the concentration range of 2.3–3 mg mL-1. This work demonstrates a promising approach for improving MHT selectivity through modulation of the aggregation state of Fe-oxide NPs.

Finally, Chapter 6 provides a brief overview of the relevance of hyperthermia in oncology and the role of nanomaterials in its advancement. The objectives of this thesis are then contextualized in relation to developments in the field, and the main limitations are discussed. Ultimately, this critical perspective crystallizes into recommendations regarding the main findings.