Pathophysiology of SARS-CoV-2-associated ARDS
Refractory progressive respiratory failure has been the primary cause of death in the COVID-19 pandemic. In patients who died from COVID-19–associated or influenza-associated respiratory failure, the histologic pattern of the peripheral lung was diffuse alveolar damage with perivascular T-cell infiltration. The lungs from patients with COVID-19 also showed distinctive vascular features, consisting of severe endothelial injury associated with the intracellular virus and disrupted cell membranes. Histologic analysis of pulmonary vessels in patients with COVID-19 showed widespread thrombosis with microangiopathy. These observations indicated pulmonary vascular disease in patients with COVID-19 ARDS.
By studying moderately and severely ill COVID-19 patients, we found active NLRP3 in peripheral blood mononuclear cells and postmortem patient tissue. Inflammasome-derived products, such as active caspase-1 and IL-18, were observed in the sera, as well as interleukin-6 (IL-6) and LDH, markers of COVID-19 severity. Higher IL-18 and caspase-1 are associated with disease severity and poor clinical outcome. It appears that IL-6 levels in COVID-19 patients are not exceptionally high in the broader context of ARDS.
COVID-19 ARDS onset can present with relatively preserved aeration on chest CT imaging despite severe respiratory hypoxemia. In some patients, this early, high-compliance phenotype evolves into a low-compliance phenotype with poor aeration. We described the first clinical presentations as low elastance, high compliance, and preserved aeration (L-type) and the second as high elastance, low compliance, and poor aeration (H-type). The inhomogeneous phenotypes of COVID-19-associated ARDS illustrate important clinical descriptions and may have diverse therapeutic implications.
Transient bulbar palsy is the origin of ARDS in COVID-19
Extrapulmonary manifestations have been reported, including neurologic complications, such as stroke and encephalopathy; anosmia; cardiac complications, such as acute coronary syndrome, myocarditis, arrhythmia, and Takotsubo cardiomyopathy; renal complications, such as acute kidney injury; hepatic complications, such as transaminitis; and gastrointestinal complications, such as diarrhoea, nausea, vomiting, and anorexia. With the data on these diverse complications, the relationship of the virus, its direct cytotoxic effects, its effects on the renin-angiotensin-aldosterone systems, its impact on the endothelium, and the various manifestations of COVID-19 disease was explored.
In vitro human brain organoid experiments and in vivo SARS-CoV-2 mouse model hybrid experiments revealed possible evidence for the neuroinvasive capacity of SARS-CoV-2 and an unexpected consequence of direct infection of neurons by SARS-CoV-2. In hybrid experiments, no consensus on the evidence for CNS infections has been reached. (Fig. 1)
Brainstem involvement, especially pre-Bötzinger complex involvement[24, 25], could explain the respiratory failure and sudden high death rate of COVID-19 ARDS patients. It could also explain the sudden recovery of cases 1, 2, 3, and 4 (within 24-48 hours) after taking dapsone in the ARDS-onset and aggravated stages in the second phase. Dapsone treats and prevents transient bulbar palsy through the medulla oblongata. Dapsone also has a therapeutic effect on COVID-19-associated ARDS, as evidenced by the recovery of several patients within 24 - 48 hours of taking dapsone. Therefore, the causal relationship is clear.
The solubility of dapsone varies over an extensive range depending on the solvent used (e.g., water, 0.2 mg/mL, methanol, 52 mg/mL). Dapsone has been considered a difficult-to-handle compound for experimental investigations, especially using living cell assays. According to inflammasome competitor theory, the treatment's preventive effect will not appear if dapsone does not reach the proper concentration. After the ingestion of a single 50- to a 300-mg dose of dapsone, maximal serum concentrations are reached between 0.63 and 4.82 mg/L. A doctor should administer dapsone after thoroughly testing it.
An inflammasome competitor theory
After progressive dyspnoea and worsening desaturation, we tend to see a pattern portending fatal outcome, with no further standard treatments to offer. According to an inflammasome competitor theory, we started oral dapsone 100 mg to target the NLRP3 inflammasome.
Furthermore, the dapsone dose was increased to 200 mg PO daily because the theory is that SARS-CoV-2-activated inflammasomes and dapsone compete for DNA binding at the molecular level. We have used dapsone so far in a total of 19 patients. We established objective criteria for improvement, such as a reduction in the FIO2 requirement and a decrease in the progression of hypoxia.
As in the debate between the inflammatory hypothesis[28, 29] and the regression hypothesis for Alzheimer's disease, in 17 out of 19 patients receiving treatment targeting the NLRP3 inflammasome, the progression of ARDS was blocked, whereas in 2 patients, no response to dapsone treatment was observed, and their conditions progressed to requiring mechanical ventilation. Dapsone passes the BBB very well. Dapsone is a newly proven drug for the treatment and prevention of AD.
The findings for dapsone are important. SARS-CoV-2 might gain entry to the CNS through the olfactory bulb to invade the brainstem. Moreover, SARS-CoV-2 CNS access might also occur from the peripheral circulation through BBB compromise. Another possible SARS-CoV-2 CNS entry route could be its dispersal from the lungs into the vagus nerve via pulmonary stretch receptors, eventually reaching the brainstem[32, 33]. Brainstem involvement, especially pre-Bötzinger complex involvement, could explain the respiratory failure and high death rate of COVID-19 patients and the sudden recovery of patients after taking dapsone.
Inflammasome treatment mechanism
The receptor-binding domain of the S protein on the surface of SARS-CoV-2 interacts with the ACE2 receptor (ACE2) in host cells. It is now well‑established that the entry of SARS‑CoV‑2 into host cells is facilitated by its neuropilin‑1 (NRP1), a transmembrane receptor that lacks a cytosolic protein kinase domain. NRP1 is also expressed in the CNS, including olfactory‑related regions such as the olfactory tubercles and paraolfactory gyri. Furthermore, NRP1 is a host factor for the entry of SARS‑CoV‑2 into the brain through the olfactory epithelium. SARS-CoV-2 evokes a response that needs strong induction of a subclass of cytokines, including type I and, obviously, type III interferons and a few chemokines, such as the response to influenza A virus and, specifically, respiratory syncytial virus[14, 38]. SAMHD1 links to the NF-kB pathway. Now, we can correlate ACE2, NRP1, and SAMHD1 with the neurologic complications of COVID-19.
The Korea Centers for Disease Control and Prevention (KCDC) and the Korean Hansen Welfare Association (KHWA) supply dapsone to Hansen's disease (HD) patients. The KCDC provided dapsone free to all HD patients (dapsone 100 mg–1000 tablets/bottle to 3814 of 9134 HD patients). Of note, the KHWA reported that HD patients in Korea had no reports of any outbreaks of respiratory infectious diseases between 2002 and 2021.04.30.
Dapsone is a small molecule with anti-inflammatory and immunosuppressive properties as well as antibacterial and antibiotic properties. Dapsone passes through the BBB[39, 40], and high-dose sulfadiazine results in an effective CSF concentration in humans.
Dapsone binds to myeloperoxidase and regulates the production of hypochlorite. It reduces the inflammatory response of cells. The nucleophilic/electrophilic region of DDS interacts with amino acids by molecular bonding. Neurotoxicity, aggregation, and free radical formation are initiated by the methionine (Met) residue at position 35 in the Aβ C-terminal domain[42-44]. Two-electron oxidation of bicarbonate is mediated by hydrogen peroxide after the generation of peroxymonocarbonate (HCO4−). The bicarbonate/carbon dioxide pair stimulated one-electron oxidation. Carbonate radical anions (CO3●ㅡ) mediate one-electron reactions to promote one-electron oxidation to efficiently oxidize Met residue thioesther sulfur to sulfur radical cations (MetS●+). DDS has a structure that can competitively reduce the positively charged sulfur radical production rate because it has a similar structure to methionine sulfoxide. Control for reversing protein ubiquitylation was the subject of our study. The reversibility of ubiquitination by the action of deubiquitinating enzymes (DUBs) serves as a significant regulatory layer within the ubiquitin system. The human genome encodes approximately 100 DUBs, and DUBs have implicated pathologies, including neurodegeneration and cancer. The conjugation of ubiquitin can be reversed by DUBs, which reflect additional regulation of ubiquitin. The covalent attachment of ubiquitin to substrates has generated a repurposed drug (dapsone) capable of competing with pathogenic targets in Ub-conjugating targeting chimaeras[3, 48]. The nucleophilic properties of dapsone compete with ubiquitin (Ub), similar to DUBs. Before loading Ub onto the substrate, the Ub-activating (E1)/Ub-conjugating (E2)/E3 ligase acts at each stage of the ubiquitination process. Enzymes can carry Ub via a thioester linkage, which allows vigorous favourable attack of the substrate nucleophile. Dapsone can compete with the ubiquitination cascade. The identical mechanism can potentially ubiquitinate cysteine thiols and hydroxyls on serines, threonines, leucines, and tyrosines[3, 46]. Dapsone noncovalently binds/interacts with the minor groove of DNA. The DDS-DNA interaction/binding relative binding energy is −6.22 kcal mol-1, estimated using in silico studies. Docking analysis further revealed that dapsone preferentially binds to the AT-rich region of DNA. The nucleophilic properties of DDS also compete with NLRP3. ORF8b activates NLRP3 through the interaction of the AT-rich repeat domain of NLRP3. (Fig. 2)
Dapsone was added in the desperate attempt to stop the decline in these patients’ conditions. Excluding the patients whose conditions progressed to requiring mechanical ventilation, the rapid recovery of ARDS patients within 24 hours of being treated with dapsone provides evidence supporting the use of this very new, meaningful clinical treatment in the ICU. Activated microglia were found adjacent to neurons by pathologic studies, which suggests neuronophagia in the olfactory bulb, substantia nigra, a dorsal motor nucleus of the vagal nerve, and the pre-Bötzinger complex in the medulla.
Transient global amnesia is a rare clinical syndrome in which a sudden onset of anterograde amnesia recovers within 24 hours. Although the underlying pathophysiology is uncertain, focal hippocampal ischaemia, venous congestion, migraine-related mechanisms, hypoxic-ischaemic events, epilepsy, and metabolic stress may be involved. Serum neurofilament light chain (NFL) is a specific biomarker of neuronal injury. NFL was higher in patients with COVID-19 than in the comparator groups. Higher NFL levels were associated with short-term outcomes, indicating that neuronal injury is common in critically ill patients. Brainstem involvement could explain sudden respiratory failure.
An alternative way to help us prevent or treat SARS-CoV-2-associated ARDS is to use an inflammasome competitor to reduce the prevalence rate. The molecular properties of dapsone, including electron density and its Laplacian delocalization index, have provided a specific mechanism for chemical bonding and atomic and molecular details. The redox properties of DDS dependent on amine and sulfone moieties explain the oxidation mechanism of DDS by electron transfer. As it applies to electrons, we can understand the various neuropathologic findings of COVID-19, including its mysterious sensory manifestations.
Early indications of dapsone’s effects show that it may have the potential to alleviate the course of COVID-19. There are enough theoretical clinical data to warrant a pilot study in deteriorating patients with COVID-19[3, 55-57]. Among patients hospitalized with onset to aggravated COVID-19-associated ARDS, the use of dapsone will improve clinical status within one day compared with standard care alone.