Immunocytokines (ICKs) are based on the fusion of a cytokine with an antibody in a single molecule. This type of approach combines the immunostimulatory properties of the cytokine with the targeting specificity of the antibody, something that can concentrate the activity of the cytokine on the desired tissue, increasing its activity and potentially avoiding off-target effects. Although several cytokines have been employed to construct ICKs, this review will focus only on those derived from interleukin-12 (IL-12), a cytokine with remarkable antitumor properties whose clinical use has been restricted due to toxicity (Lasek, Zagożdżon, & Jakobisiak, 2014).

IL-12 is produced mostly by activated antigen presenting cells (APCs), B cells, and neutrophils in response to pathogens (Del Vecchio et al., 2007). This cytokine has a broad spectrum of activity inducing both innate and adaptive responses. Structurally, IL-12 is composed of two subunits, p35 and p40, that are covalently linked to form the p70 heterodimer which activates immune cells through binding to its receptor (IL-12R), composed of low affinity (IL-12Rβ1) and high affinity (IL-12Rβ2) subunits. IL-12R is expressed on the surface of natural killer (NK) cells and cytotoxic T lymphocytes, being able to boost interferon-gamma (IFN-γ) production upon IL-12 binding (Tait Wojno, Hunter, & Stumhofer, 2019). Consequently, IL-12 exhibits potent antitumor activity by increasing the inflammation in the tumor microenvironment (TME), with anti-angiogenic and anti-metastatic properties. These effects include re-programming tumor-associated macrophages and increasing expression of major histocompatibility class I and II molecules (MHC-I/MHC-II), thus positioning it as a promising cytokine for cancer immunotherapy. Nevertheless, systemic administration of IL-12, particularly when delivered as a recombinant protein, is highly toxic due to severe systemic side effects, which have so far restricted its use in cancer patients (Tugues et al., 2015).

Monoclonal antibodies (mAbs) originated from Kohler and Milstein’s development of the hybridoma technique in 1975, followed by Greg Winter’s humanization of mAbs in 1988 (Bayer, 2019), which led to substantial progress in the research, diagnosis, and treatment of various diseases, including cancer (Shiravand et al., 2022). Conventional mAbs consist of two heavy and two light polypeptide chains connected by disulfide bonds, resembling the structure of endogenous immunoglobulin G (IgG). The success of mAbs in therapy has driven ongoing research towards the creation of new antibody types, including antibody fragments, like antigen-binding fragments (Fab), single-chain variable fragments (scFv), and monodomain camelid-derived nanobodies (Nb), as well as engineered antibody-based molecules, such as bispecific mAbs, diabodies, T-cell engagers, and immunocytokines (Silva-Pilipich et al., 2023b). The high specificity and affinity of mAbs and antibody fragments for their targets make them effective vehicles for delivering therapeutic agents, such as cytokines, directly to tumors, thereby reducing the side effects associated with systemic exposure.