Fe(III)-Chelated Polydopamine Nanoparticles for Synergistic Tumor Therapies of Enhanced Photothermal Ablation and Antitumor Immune Activation

Both the low energy density of near-infrared (NIR) photothermal conversion during treatment and the recurrence and metastasis after local treatment have been the main obstacles and conundrums in polydopamine-mediated tumor photothermal therapy (PTT). Herein, On the basis of the enhancement of NIR absorption by ligand to metal charge transfer (LMCT) in transition-metal complexes and the activation of antitumor immunity by an appropriate concentration of Fe(III) ions, Fe(III)-chelated PDA nanoparticles (Fe-PDA NPs) with high loading and responsive release of iron ions were synthesized through a prechelation-polymerization method. First, Fe(III) chelated with the catechol groups in DA to form a mono-dopa-Fe(III) chelate, and then the polymerization of dopamine was initiated under alkaline conditions. The results revealed that the mono-dopa-Fe(III) chelate was still the main form of the Fe ion existing in Fe-PDA and was able to greatly enhance the light absorption behaviors of PDA in NIR, resulting a superior photothermal conversion ability (η = 55.5%). Moreover, the existence of Fe(III) also gave Fe-PDA a T₁-weighted MRI contrast-enhancement performance (r₁ = 7.668 mM^-1 s^-1) and it would enable the accurate ablation of primary tumors in vivo with Fe-PDA under NIR irradiation by means of the guidance of MRI and thermal imaging. Furthermore, Fe-PDA exhibited better H₂O₂-responsive biodegradability in comparison to PDA and easily released Fe ions in tumors, which could effectively promote the tumor-associated macrophage (TAM) repolarization to the M1 mode. TAM repolarization combined with the immunogenic cell death (ICD) induced by PTT could effectively enhance the efficacy of immunotherapy, preventing tumor recurrence and metastasis. The design of Fe-PDA nanoparticles should provide more inspiration for structural and functional improvements of melanin-based materials in tumor suppression.