**1. Introduction**

Cancer ranks as a leading cause of death and an important barrier to increasing life expectancy in every country of the world [1]. Cancer is the first or second leading cause of death before the age of 70 years in a vast majority of countries [2], underlining the urgent need to address unmet needs in oncology. According to the type and stage of cancer, various approaches can be employed. While surgery is usually the first line of treatment, other strategies based on chemotherapy and radiotherapy can also be performed. Even if all these strategies can be combined, the desired success rate in cancer treatment has not yet been achieved, especially due to the iatrogenic disorders they induce. As a consequence, many therapies have been developed to specifically and safely target cancers.

**Citation:** Dias, A.M.M.; Courteau, A.; Bellaye, P.-S.; Kohli, E.; Oudot, A.; Doulain, P.-E.; Petitot, C.; Walker, P.-M.; Decréau, R.; Collin, B. Superparamagnetic Iron Oxide Nanoparticles for Immunotherapy of Cancers through Macrophages and Magnetic Hyperthermia. *Pharmaceutics* **2022**, *14*, 2388. https://doi.org/ 10.3390/pharmaceutics14112388

Academic Editor: Xiangyang Shi

Received: 22 July 2022 Accepted: 25 October 2022 Published: 5 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Among these targeted strategies, cancer immunotherapies have now revolutionized the field of oncology by prolonging the survival of more and more patients suffering from aggressive and fatal cancers [3]. In immunotherapy, the agents are designed to induce an immune response against cancer cells and can be used in combination, strengthening their central role as a first-line therapy for many cancers in the future. Immunotherapies can be divided into several classes: (i) immune checkpoint inhibitors (ICIs) [4]; (ii) chimeric antigen receptor (CAR) cell therapies (e.g., CAR Natural Killers (CAR NK) [5]; CAR Macrophages (CAR M) [6] and CAR T-Cells [7]; (iii) cytokines-based immunotherapy [8]; (iv) agonistic antibodies against costimulatory receptors [9]; (v) cancer vaccines [10] and (vi) bispecific antibody therapy.

Another very exciting field to specifically and safely target tumors for diagnosis and therapy relies on the use of nanoparticles (NPs), also called nanomedicines. Their size typically ranges between 1 and 100 nm, they can be made from different materials and have various physicochemical properties (e.g., size, shape, surface features, magnetism, etc.). According to their chemical composition, NPs can be classified into organic (e.g., liposomes, polymeric micelles, dendrimers, etc.), inorganic (e.g., super paramagnetic iron oxide NPs (SPIONs), gold nanorods, carbon nanotubes, etc.), or hybrid (e.g., lipidpolymer NPs, organic-inorganic NPs, etc.) NPs [11]. In addition to their intrinsic properties due to the material they are made of, NPs can be modified with a lot of targeting ligands, affecting their biological behavior accordingly.

Even if it is always discussed, it is commonly accepted that NPs target tumors via two main mechanisms. The first one is passive targeting (enhanced permeability and retention (EPR)). There are a few points about the EPR effect that should be made clear. Despite the fact that the EPR effect is frequently described as a process that enables the delivery and retention of drugs at cancerous sites thanks to structural and architectural abnormalities (such as abnormal fenestrations and structural disorganization), the truth is that this increase in permeability and retention is not yet fully understood and may have other explanations. For instance, it is currently understood that this effect, is also influenced by the impairment of lymphatic drainage and permeability-enhancing factors, including nitric oxide, bradykinin, or vascular endothelial growth factors [12]. Moreover, additional phenomena, such as vascular transcytosis-based nutritional pathways (mediated by caveolae, clathrin-coated pits, and macropinocytotic vesicles), may potentially play a role in NP uptake and, subsequently, the EPR effect, especially for NPs with a size between 50 and 100–150 nm [13]. A second transcytosis pathway, known as the vesiculo-vascular organelle (VVO), has also been identified in normal endothelial cells and may potentially contribute significantly to the EPR effect. This system is made up of a vast network of grouped and connected cytoplasmic vesicles and vacuoles. Therefore, more investigation is required to understand exactly the biophysical and metabolic mechanisms that result in the extravasation of NPs into the tumor and, ultimately, the EPR effect [12].

The second mechanism by which NPs target tumors is the active targeting through an ad hoc surface functionalization (e.g., targeting peptide) of the NPs [14]. Through these mechanisms of targeting, NPs are well-known for their capabilities to release encapsulated or conjugated bioactive agents within tumors. NPs make it possible to improve the bioavailability of drugs, to combine therapeutic agents with imaging (i.e., nanotheranostics) techniques, or to boost antitumor effects [15]. Over the last 20 years, around 80 nanomedicine products have been approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) in various indications including cancers [16]. Unfortunately, in spite of considerable technological success, nanomedicines have demonstrated modest effects on survival and in some examples, less than other approved therapies [17].

Since the majority of patients do not benefit from the currently available immunotherapies, and they can experience severe adverse events, immunotherapeutic nanomedicines might enhance efficacy while mitigating certain life-threatening toxicities [18]. Moreover, aiming for pharmacological synergy, it is also possible to design new combinations

associating classical immunotherapies and nanomedicines to overcome their respective weaknesses [19]. In this respect, iron oxide nanoparticles (IONPs) such as SPIONs may be an appealing class as they combine many features that allow targeting of the immune system and tumors for theranostic purposes [20]. SPIONs are typically made up of magnetite (Fe3O4) or maghemite (γ-Fe2O3) with a core radius ranging from 5 to 15 nm and a hydrodynamic radius (i.e., core with shell and water coat) ranging from 20 to 150 nm [21]. These SPIONs have already been demonstrated to act as advanced platforms for drug delivery and contrast agents in magnetic resonance imaging (MRI) and magnetic hyperthermia (MHT) [22]. Very recently, the theranostic potential of IONPs in cancer immunotherapy has been reported, emphasizing their ability to perform tumor imaging for early assessment of the efficacy of immunotherapy and their capability to alter macrophage polarization [20]. Moreover, more and more studies have demonstrated that SPIONs exhibit the intrinsic capability to stimulate systemic antitumor immune responses through MHT, paving the way for new immunotherapeutic strategies [19].

In this review, we will discuss the multiple applications of SPIONs in cancer immunotherapy, focusing on their intrinsic theranostic properties to target tumor-associated macrophages (TAMs) and to generate MHT in light of their effects on anticancer immunity. The first section of this review will briefly describe immune targets for NPs. The following sections will deal with the overall properties of SPIONs, including the development of MHT. Next, we will see how SPIONs can induce an immune response through the targeting of TME, with a more in-depth focus on TAMs and MHT.
