Chloroquine and hydroxychloroquine were widely used at the beginning of the pandemic. They seemed to be promising drugs for the treatment of COVID-19 due to their multiple pharmacological properties. They inhibit the synthesis of cytokines, such as IL-1 and IL-6, but they also enhance the production of other inflammatory mediators [44]. Therefore, chloroquine and hydroxychloroquine have anti-inflammatory effects. Moreover, they showed antiviral activity against a broad spectrum of microorganisms, including SARS-CoV-1. Preliminary in vitro and in vivo findings seemed to confirm their beneficial effect against SAR-CoV-2 infection [45,46]. Nevertheless, data from randomized controlled trials (RCTs) and subsequent metanalysis demonstrated that they have little or no impact on COVID-19 prevention or treatment. Furthermore, they were associated with an increased risk of adverse events [47].

Colchicine is an anti-inflammatory drug used to treat gout, pericarditis, and familial Mediterranean fever, which acts by inhibiting activation and migration of neutrophils and by interfering with the inflammasome complex, essential for IL-1β and IL-18 production [52]. An RCT showed the potential benefit in the treatment of SARS-CoV-2 infection, but further research did not confirm these data [53,54,55]. Therefore, colchicine is not recommended by current guidelines.

Based on the hypothesis that the RAAS can contribute to the genesis of the COVID-19-related cytokine storm, ACE inhibitors and sartans have been studied in patients with COVID-19, but their efficacy is still uncertain. While several retrospective studies have shown a potential protective effect, an RCT found no clinical benefit from the use of these drugs in patients with COVID-19 [56,57,58,59,60,61]. Further randomized clinical trials are currently underway and could help clarify the issue (NCT04335786, NCT04311177, NCT04328012). Another possible way to target the RAAS, although not yet tested, could be the use of a direct renin inhibitor, such as the Food and Drug Administration (FDA)-approved aliskiren, to reduce the production of AT1 (Figure 1).

With the emergence of biotechnological pharmacology, numerous cytokine inhibitors have been developed and successfully used in immune-rheumatological pathologies, including conditions characterized by cytokine storm syndrome (CSS). Hence, as the cytokine storm could also play a crucial role in the pathogenesis of COVID-19, researchers experimented with the use of several of these drugs in patients infected by SARS-CoV-2. IL-1 is produced following the activation of the innate immune system; it is one of the most important pro-inflammatory molecules involved in the cytokine storm. Anakinra is a recombinant form of the IL-1 receptor antagonist, which, similarly to its endogenous analog, is able to competitively inhibit the binding of IL-1β to its receptor IL1R. This drug has been used successfully in numerous autoimmune and auto-inflammatory conditions, including cytokine release syndrome (CRS) secondary to CAR-T cell therapy and HLH [62,63,64]. These characteristics have led to the experimentation of anakinra in COVID-19 patients with conflicting results. A series of cohort studies recorded a clear benefit from the use of this drug in severe forms of COVID-19 [65,66,67]. On the other hand, a recent RCT conducted in France was suspended prematurely for futility, as no advantage was appreciated in the treatment group [68]. However, the results of the latter study were contested because the sample was composed of patients with mild to moderate forms of the disease, characterized by relatively low levels of C reactive protein, thus excluding the patients who could most benefit from the inhibition of IL-1 (i.e., severe forms characterized by signs and symptoms of CSS). 

Rather than targeting a single molecule, Janus kinase (JAK) inhibitors can down-modulate the signaling pathways of multiple cytokines simultaneously: in fact, these agents act by inhibiting JAK, essential components of the Janus kinase-signal transducers and activators of the transcription (JAK-STAT) transduction system which mediate the cellular effects of several cytokine receptors, including IL-2, IL-6, IL-10, IFN-γ, and GM-CSF. JAK inhibitors have aroused particular interest following the positive results of two multicenter, double-blind, placebo-controlled trials. 

The role of free radicals and oxidative stress in the pathogenesis of COVID-19 has been relatively denied. However, it is known that the recruitment and activation of immune cells in the context of numerous respiratory infections leads to the production of reactive oxygen and nitrogen species. These molecules contribute to the escalation of the inflammatory reaction, stimulating the synthesis of pro-inflammatory cytokines and damaging cell membranes through oxidative mechanisms, thus contributing to the onset of endothelial damage and thrombotic phenomena. In this sense, antioxidant agents could represent an intriguing adjunctive therapy in severe COVID-19 patients. Among others, particularly favorable properties are possessed by methylene blue, a tricyclic phenothiazine, approved as a therapy for methemoglobinemia and malaria, but also tested with success in other pathological conditions, including septic shock [81]. In the hyperinflammatory context of the cytokine storm, methylene blue may be able to simultaneously inhibit the production of reactive oxygen species (ROS), reactive nitrogen oxide species (RNOS), and cytokines themselves, thanks to its ability to counteract the generation of superanions by the xanthine oxidase pathway, to inhibit the synthesis of nitrous oxide (NO) by NO synthase, and to attenuate the nuclear factor-kB (NF-kB) signaling. In addition, in an in vitro study, methylene blue was shown to inhibit cellular infection by SARS-CoV-2 [82]. Currently, methylene blue has only been clinically tested in COVID-19 patients in a small, open-label, randomized trial in Iran. 

An alternative, non-pharmacological approach to CSS is represented by blood purification therapies. Previous studies demonstrated that these techniques could remove cytokines from patients’ blood, and therefore, these techniques have been used in the treatment of various immune diseases [84]. In particular, therapeutic plasma exchange (TPE) was applied with promising results in several cytokine-storm-related conditions, such as HLH and sepsis [85,86,87,88]. Clinical results from many case reports, case series, and retrospective studies support TPE as an effective therapy in selected COVID-19 patients [89,90,91,92,93,94], but data from more structured RCTs are still lacking.

It has been suggested that COVID-19 vaccines may help prevent the cytokine storm through the early control of SARS-CoV-2 infection [95]. However, precise data concerning the prevalence of cytokine storms among vaccinated COVID-19 patients are not available. In the elderly, the “inflammaging” phenomenon may enhance cytokine release in COVID-19, but it may also reduce the immune response to the vaccine, as happens with other vaccines [96,97]. Therefore, the efficacy of COVID-19 vaccines in preventing the cytokine storm may be decreased in older patients [98]. To date, specific research concerning the actual effects of COVID-19 vaccines on the development of the cytokine storm has not been carried out or published. In vaccine clinical trials, researchers usually use disease severity, mortality, infection, transmission, or other surrogate endpoints to evaluate the vaccine efficacy, and the cytokine storm incidence is not studied [99]. Supposedly, the vaccine-induced immune response to the virus should prevent the cytokine storm by reducing the initial inflammatory reaction to the infection. Vaccine efficacy trials demonstrated high percentages of protection against severe disease [100]. However, new virus variants may decrease the vaccine’s effectiveness [100].