key: cord-0780613-jthqqtw6
authors: Szabo, Csaba; Martins, Vanessa; Liaudet, Lucas
title: Poly(ADP-Ribose) Polymerase Inhibition in Acute Lung Injury. A Reemerging Concept
date: 2020-11-03
journal: Am J Respir Cell Mol Biol
DOI: 10.1165/rcmb.2020-0188tr
sha: c8509c7344c0b3689388c5c652810f4bed3d0e59
doc_id: 780613
cord_uid: jthqqtw6

PARP1, the major isoform of a family of ADP-ribosylating enzymes, has been implicated in the regulation of various biological processes including DNA repair, gene transcription, and cell death. The concept that PARP1 becomes activated in acute lung injury (ALI) and that pharmacological inhibition or genetic deletion of this enzyme can provide therapeutic benefits emerged over 20 years ago. The current article provides an overview of the cellular mechanisms involved in the pathogenetic roles of PARP1 in ALI and provides an overview of the preclinical data supporting the efficacy of PARP (poly[ADP-ribose] polymerase) inhibitors. In recent years, several ultrapotent PARP inhibitors have been approved for clinical use (for the therapy of various oncological diseases): these newly-approved PARP inhibitors were recently reported to show efficacy in animal models of ALI. These observations offer the possibility of therapeutic repurposing of these inhibitors for patients with ALI. The current article lays out a potential roadmap for such repurposing efforts. In addition, the article also overviews the scientific basis of potentially applying PARP inhibitors for the experimental therapy of viral ALI, such as coronavirus disease (COVID-19)–associated ALI.

mono-or poly-ADP-ribosyl-transfer reactions (4) . In the current article, we focus on PARP1 because this enzyme is responsible for the vast majority of cellular PAR formation, and this is the enzyme that has been implicated in the pathogenesis of acute lung injury (ALI).

One of the first recognized roles of PARP1 was the so-called "guardian-angel" function (i.e., its role in the regulation of DNA repair). Although PARP1 is not a DNA-repair enzyme per se, it plays a role in the maintenance of genome integrity, in significant part through the recruitment of DNA-repair enzymes to the sites of DNA damage (single-and double-strand breaks in the DNA, as a result, for instance, of oxidative/nitrative stress, ionizing radiation, or genotoxic drugs) (1) (2) (3) (4) . The nuclear concentration of PAR may also provide an "energetic role": it may be metabolized to ATP, which, in turn, is used by DNArepair enzymes (5, 6) . The translational consequence of these observations was the emergence of a novel therapeutic concept emerged in the field of oncology: via PARP inhibition, DNA repair may be suppressed and, thereby, cancer cell death may be therapeutically induced (4, 7, 8) . Subsequent work has discovered that the cytotoxic anticancer effect of PARP inhibitors is most pronounced when the cancer cells have mutations in their HRR (homologous recombination DNA repair) system because, in such instances, the PARP1-dependent DNA-repair system becomes the "last man standing" in the process of DNA repair; elimination of this system, in turn, produces remarkable antitumor efficacy in such tumors (7, 8) . Accordingly, ultrapotent (or thirdgeneration) PARP inhibitors (such as olaparib, rucaparib, niraparib, and talazoparib) have recently been clinically approved in many countries); the approved therapeutic indications are typically HRRdeficient tumors (e.g., tumors with BRCA1 or BRCA2 mutations) (7, 8) .

Although PARP1's role in DNA repair is not directly relevant in the pathophysiological context, as an active effector of ALI, this aspect of the enzyme is nevertheless very important because if therapeutic PARP inhibition suppresses or delays DNA repair, this must be considered as a potential risk factor or side effect of such therapy (see below).

Because the substrate of PARP1 is NAD 1 , the suggestion was already raised in the 1980s by Nathan Berger's group that the activation of PARP may, in turn, have consequences for the cell's bioenergetic and metabolic status (9) . Although the redistribution of PAR into the nuclear compartment is beneficial for DNA repair (see above), it can also result in a significant depletion of NAD 1 in the cytosolic compartment. This, in turn, has been shown to result in the depletion of cellular ATP concentrations. The "Berger Hypothesis" was originally developed on the basis of studies in cells subjected to ionizing radiation or genotoxic carcinogens such as N-methyl-N9-nitro-Nnitrosoguanidine. However, follow-up studies have demonstrated that PARP It also contains the WGR domain, which is one of the domains involved in the RNA-dependent activation of PARP1. Below the domains, on the right side, the structure of NAD 1 is presented, with the nicotinamide part highlighted. The middle part of the figure shows the sequences of the PARylation process catalyzed by PARP, starting with recognition of the DNA-strand breaks by the DNA-binding domain (gray ovals depicting the zinc fingers binding to the DNA breaks), followed by the catalytic activation of the enzyme and the cleavage of NAD 1 , the production of nicotinamide, and the generation of PAR polymers, which, in turn, PARylates various acceptor proteins as well as PARP itself. The consumption of NAD 1 has metabolic and bioenergetic effects. PARP inhibitors prevent the binding of NAD 1 to the active site of PARP and inhibit the catalytic activity of the enzyme. On the left side, the effect of PARG (PAR glycohydrolase) and ARH3 (ADP-ribosylhydrolase 3) is shown; these enzymes break down the PAR polymers, leading to the liberation of free PAR. Reprinted by permission from Reference 17. BRCT = BRCA1 C-terminal; NLS = nuclear localization signal; PARylation = poly-ADP-ribosylation; WGR = tryptophan-glycine-arginine-rich.

activation can also develop in response to endogenous production of hydroxyl radical or peroxynitrite (which can also create DNA single-strand breaks). PARP overactivation, and the subsequent bioenergetic "catastrophe" was subsequently demonstrated to induce a regulated form of cell necrosis (10) (11) (12) (13) . Moreover, the various components of the "Berger Pathway" (i.e., DNA damage, PARP activation, cellular energetic and mitochondrial deficits, and, most importantly, the beneficial effect of PARP inhibitors) have been demonstrated in various animal models of disease, ranging from reperfusion injury to various forms of local and systemic inflammation and various types of critical illness (14-17); many of these processes are also relevant for the pathogenesis of ALI (see below). Although the "critical care condition" → "PARP overactivation" → "cell necrosis" scheme is attractive in its simplicity, studies over the last two decades revealed that, in fact, there are several different (often interacting, other times complementary) pathways involved in the pathophysiological aspect of PARP activation ( Figure 2 ). According to the "classic pathway" (Figure 2A ) (in reperfusion injury, circulatory shock, various forms of inflammation [as well as in a variety of other diseases]), reactive oxidants and free radicals are formed, for instance, as a consequence of a multitude of biochemical pathways, including reduced NAD 1 phosphate oxidase, and/or infiltration of the tissues with activated immune cells and the consequent release of various reactive species. These species, in turn, produce DNA damage (primarily in the form of single-strand breakage), which is, in turn, recognized by the zinc fingers of PARP1, which, in turn, activate this enzyme. When the DNA damage is widespread (because the oxidant burden is high), the extent of PARP1 activation can be so pronounced, that a subsequent cellular energetic deficiency (NAD 1 depletion, followed by mitochondrial inhibition and depletion of cellular ATP) can produces cell dysfunction (1, (14) (15) (16) (17) .

For instance, in a rat model of endotoxic shock, peritoneal macrophages exhibit reduced NAD 1 and ATP concentrations and suppressed mitochondrial respiration; these effects are suppressed by inhibition of PARP by nicotinamide (18) . Moreover, PARP activity (assessed by the rate of NAD 1 consumption in various tissues) were found to be significantly higher during shock in nonsurvivors compared with survivors in a porcine hemorrhagic-shock model (19) . In addition, administration of the PARP inhibitor PJ-34 led to decreased serum HMGB1 concentrations (an indicator of cell necrosis) in mice subjected to cecal ligation and puncture (CLP) (20) . Figure 2B depicts an alternative mechanism, which is generally known as the "parthanatos concept." According to this mechanism (which has initially been demonstrated in neuronal models but has subsequently been also extended to a variety of other cells, tissues, and disease conditions), the poly-ADP-ribosylation (PARylation) of various acceptor proteins (as a result of PARP activation, as in Figure 2 . Mechanisms responsible for the cytoprotective and antiinflammatory effects of PARP inhibitors in nononcological diseases. (A) PARP activation and consequent NAD 1 depletion (the "Berger Hypothesis"). These processes can lead to a cellular energetic deficit and cell dysfunction; inhibition of PARP prevents these processes and exerts cytoprotective effects (inhibition of cell necrosis). (B) Role of PARP activation and free PAR polymers in inducing mitochondrial release of AIF (apoptosis-inducing factor), which, in turn induces cell death (parthanatos). Inhibition of PARP suppresses these processes and inhibits parthanatos. (C) The role of PARP in liberating free PAR polymers, which, on their own, exert cytotoxic effects; inhibition of PARP prevents free PAR polymer formation and suppresses cell death. (D) PARylation contributes to activation of the proteasome through an interaction with RNF146; PARP inhibitors suppress these processes. (E) Role of PARP in contributing to proinflammatory signal transduction via enhancing JNK-mediated (left sequence) and NF-kB-mediated (right sequence) activation of multiple genes and gene products. By inhibiting PARP, these processes are attenuated and inflammatory signaling can be attenuated. (F) PARP regulates the activation of the cytoprotective Akt pathway. Under normal conditions, PARylation anchors the ATM-NEMO complexes, which are retained in the nucleus. However, after PARP inhibition, the ATM-NEMO complex translocates to the cytoplasm, where Akt and mTOR are recruited to form the ATM-NEMO-Akt-mTOR cytoprotective signalosome, which, in turn, activates various mitochondrial protective and cell-survival pathways. Adapted by permission from Reference 17. ARH3 = ADP-ribosylhydrolase 3; ATM = ataxia telangiectasia mutated; NEMO = NF-kB essential modulator; P = phosphate group; PAAN = PARP-1-dependent AIF-associated nuclease; Ub = ubiquitin group; UPS = ubiquitin-proteasome system. Figure 2A) , is followed by the removal and degradation of PAR. Free PAR, in turn, leaves the nucleus and translocates into the cytosolic compartment of the cell. It also reaches the mitochondria, where it binds to specific mitochondrial receptors, resulting in the release of AIF (apoptosis-inducing factor). AIF, in turn, diffuses back to the nucleus, where it induces large-scale nuclear fragmentation and cell death (parthanatos). This process has been primarily indicated in central-nervoussystem pathologies (21) ; its potential role in critical illness remains to be explored. In addition, free PAR polymer can have independent roles as a pathogenetic factor ( Figure 2C ) because it can bind to various protein acceptors intracellularly (or even, in some cases, extracellularly). These PARylation reactions (a form of posttranslational modifications) have been shown to contribute to various pathophysiological processes ranging from neurodegeneration to vascular injury (21) .

Another aspect of PARP relates to the fact that it can also regulate ubiquitylation-mediated protein-degradation reactions ( Figure 2D ). The process of ubiquitin-mediated protein degradation is involved in various cellsignaling and protein-quality-control processes. Iduna/RNF146, a constitutively expressed ubiquitin E3 ligase is activated by PARylation. In turn, proteosomally mediated degradation of various proteins can ensue, which may have various adverse effects on the cells affected by it (22, 23) .

A significant further role of PARP1 relates to its regulatory role on gene transcription ( Figure 2E ). A general mechanism involved in this process relates to the modulation of chromatin structure (in principle, due to the fact that the PAR polymer is negatively charged), affecting the availability of the DNA to the enzymes involved in gene transcription. A secondary mechanism relates to transcriptional coregulation, and a third mechanism relates to the modulation of DNA methylation. In this context, PARP1 does not "need" to be activated by DNA-strand breaks; resting (constitutive) PARP can confer these actions, in some cases because of its scaffolding (protein-protein interaction) roles. Pharmacological PARP inhibitors in many (but not all) instances can significantly modulate the above processes, with the end result being the suppression of gene transcription in a semispecific manner. In many experimental models, the generation of proinflammatory cytokines and chemokines can be suppressed by PARP inhibitors, producing an antiinflammatory and/or immunomodulatory result (15, 23) . However, in complex situations (e.g., in vivo models of disease), the antiinflammatory effects of PARP inhibitors may also be related to the interruption of various positive-feedback cycles of disease (1) (Figure 3 ). In the context of critical illness, PARP inhibitors have been demonstrated to suppress the activation of NF-kB and the subsequent production of various proinflammatory cytokines (e.g., TNF-a) (24) and various chemokines (e.g., MIP-1a and MIP-2) (25). The models used initially employed murine models of endotoxemia (24, 25) but were subsequently extended into in various rodent and large-animal models of sepsis, septic shock (16, 17) , and ALI (see below).

Finally, PARP inhibition has been shown to activate the cytoprotective Akt pathway ( Figure 2F ). The first step in this process is that PARP inhibition increases the interaction between p-ATM and NEMO proteins, thereby facilitating the translocation of this complex from the nucleus into the cytoplasmic compartment. In turn, a cytoprotective signalosome (p-ATM-NEMO-Akt-mTOR) is formed, which induces the activation of Akt. Akt is a "master regulator" of various cellsurvival pathways; its activation produces a cytoprotective phenotype (26) .

Multiple lines of in vivo experiments have demonstrated that pharmacological PARP inhibitors or PARP1 deficiency can significantly improve the outcomes of various animal models of acute and chronic lung injury, including endotoxin-or sepsis-induced lung injury, pancreatitis-induced lung injury, lung inflammation elicited by various agents (e.g., zymosan, carrageenan or elastase), ventilator-induced lung injury, environmental agent-or drug-induced lung injury, or lung fibrosis and allergy/asthma-associated lung inflammation and dysfunction . Table 1 focuses on the findings related to the effect of PARP inhibitors in various forms of ALI and lung inflammation.

Generally, the effects of PARP inhibitors include the correction of the hyperinflammatory response (i.e., suppression of cytokine and chemokine production), reduction of the infiltration of the lung tissue and the alveolar space with inflammatory cells, reduced oxidative and nitrosative stress (most likely due to the interruption of the positive-feedback cycles outlined in Figure 3 ), improved pulmonary gas exchange, and improved histological status of the lung tissue. Importantly, the beneficial effects of PARP inhibition not only have been shown in rodent models but also hve been extended to several clinically more-relevant large-animal models of ALI (31, 40, 49, 50, 65) .

However, it must be emphasized that the studies summarized in Table 1 have a number of limitations. For instance, many studies (especially the studies in the 90s) used first-generation PARP inhibitors, such as 3-aminobenzamide or nicotinamide; these agents have many additional pharmacological actions in addition to PARP inhibition, including antioxidant effects and inhibition of mono-ADP ribosylation (85) (86) (87) . Although the use of such agents was acceptable when the available tools were limited, current and future work should use third-generation inhibitors, preferably in combination with PARP1-deficient animal models, as these animals are viable and commercially available. Another common limitation of many of the published studies is that they used only one sex of animals (typically male). Since the mid-2000s, it has become more and more obvious that the effect of PARP inhibitors in various rodent models of shock, inflammation, and reperfusion injury is sex-specific; in most (but not all) cases, male animals benefit more (as well as aged females and ovariectomized females), whereas the protective effect of PARP inhibitors in females is less pronounced (88, 89) . However, it should also be mentioned that in some models (e.g., burn/smoke inhalation-associated lung injury), PARP inhibitors show significant therapeutic benefit in female large-animal models as well (e.g., References 40, 48) . Moreover, in pancreatitis and pancreatitis-associated lung injury, both male and female mice were found to respond comparably well to PARP inhibition (56, 63, 90) . We recommend that future studies, especially ones that focus on translationally relevant (i.e., clinically approved) PARP inhibitors, In inflammatory states, various proinflammatory pathways are stimulated in response to autoimmune responses and/or proinflammatory microbial components. The corresponding isoforms of NOS (nitric oxide [NO] synthase; brain NOS in the central nervous system, endothelial NOS in the cardiovascular system, and inducible NOS under inflammatory conditions) produce NO (but under conditions of L-arginine depletion, NOS can also produce superoxide). Under low-pH conditions (such as tissue hypoxia/acidosis), nitrite can also be converted to NO. Superoxide (which is produced from various cellular sources, including mitochondria) and NO react to yield peroxynitrite. Peroxynitrite and hydroxyl radical induce single-strand breaks in DNA, which, in turn, activate PARP. This can deplete the cellular NAD 1 and ATP pools. Cellular energy exhaustion triggers the further production of reactive oxidants. PARP activation leads to cellular dysfunction via the energetic mechanism as well as via several other pathways outlined in Figure 2 . Oxidative and nitrative stress can cause endothelial-cell dysfunction, at least in part though the depletion of NADPH concentrations, which, in turn, leads to reduced endothelial NO formation. The cellular dysfunction is further enhanced by the promotion of proinflammatory gene expression by PARP, through the promotion of NF-kB, AP1 (activator protein-1), and MAP (mitogen-activated protein) kinase activation. PARP can also promote complement activation. The oxidant-induced proinflammatory-molecule and adhesion-molecule expression, along with the endothelial dysfunction, induce neutrophil recruitment and activation, which initiates positive-feedback cycles of oxidant generation, PARP activation, and cellular injury. For instance, tissue-infiltrating mononuclear cells produce additional oxidants and free radicals. PARP is also involved in triggering the release of mitochondrial cell-death factors, such as AIF. There are many oxidative and nitrosative injury pathways that are triggered by oxygen-and nitrogen-centered oxidants and free radicals, which act in parallel or in synergy with PARP-mediated pathways of cell injury. Although most of the pathways shown in the figure have been demonstrated in acute lung injury (ALI), the relative contribution of cell necrosis versus inflammatory cell injury, as well as the relative role of the various pathways shown in the figure, depends on the specific form of ALI and the stage of the disease. Reprinted by permission from Reference 1. 

Cardiopulmonary bypass-induced ALI Piglet [J]: Efficacy parameters were studied only; no investigation of the safety of the inhibitor (e.g., on DNA integrity or repair) was conducted. *Inhibitory effect of pharmacological agents on tissue nitrotyrosine immunoreactivity was frequently equated with reduced production of the ROS peroxynitrite, which (similar to hydroxyl radical) is an endogenous trigger of DNA-strand breakage and subsequent PARP activation. However, subsequent studies demonstrated that other biochemical reactions (including reactions involving MPO) may also produce nitrotyrosine. † Inhibitory effect of pharmacological agents on tissue MPO concentrations (in fact, the assay most commonly used measures MPO enzymatic activity and not MPO content) may also indicate effects on neutrophil degranulation, and/or neutrophil extracellular net formation and/or direct effects on the activity of the MPO enzyme. In the table, therefore, we distinguish "MPO content" from "neutrophil lung count" (evidenced by histological or flow cytometric analysis). ‡ TUNEL positivity in tissue slides is often equated to apoptosis, but in fact it actually shows DNA-strand breakage, which can occur in conjunction of various forms of cell death or cell dysfunction.

should include separate animal groups of both sexes. (70, 72, 80, 83) and, importantly, also exerts beneficial effects against the ALI-associated centralnervous-system dysfunction (e.g., cognitive defects) (83) . Olaparib also exerts beneficial effects in other animal models that have a significant pulmonary-injury component and/or systemic hyperinflammatory component, including asthma models (72, 74) , a pulmonary inflammation/emphysema model (80) , and murine models of acute burn injury and pancreatitis (84, 90) . In a murine model of sepsis (induced by CLP), olaparib improved survival and exerted beneficial effects on inflammatory-mediator production and immune-cell balance, but in this model, no significant pulmonary injury was noted, and olaparib had no effect on the various pulmonary parameters investigated (81) . As discussed previously (17, 81) , the effective dose of the clinically approved PARP inhibitors in nononcological models is significantly lower than the doses of the same agents in oncology, most likely due to the fact that in nononcological models a partial inhibition of PARP is sufficient to exert therapeutic effects. Thus, it will be possible to find effective doses of olaparib for the therapy of nononcological conditions (in particular, in disease conditions in which the patients do not have any baseline defect in DNA-repair pathways, such as ALI) in which a partial inhibition of PARP exerts beneficial effects without interfering with DNA repair and DNA integrity. Indeed, in a CLP model, in which the efficacy and safety of olaparib were simultaneously assessed, olaparib (at the effective dose range of 6-30 mg/kg/d), olaparib beneficially modulated the cytokine and immune status of the animals and improved survival but did not exert any adverse effects on mitochondrial or nuclear DNA integrity in various tissues, including the lung (81) . In a second study, in which the effect of PARP inhibition in a critical illness was assessed not only on efficacy parameters but also on DNA injury, the PARP inhibitor used (INO-1001, 4 mg/kg) provided hemodynamic stabilization in a porcine model of thoracic aortic crossclamping-induced ischemia/reperfusion, without worsening the DNA damage in peripheral blood lymphocytes (91) . It must be reiterated, however, that the abovementioned models did not induce a significant degree of lung injury as a function of the sepsis or cross-clamping procedure (81, 91) , and the safety of PARP inhibitors in models that are relevant for ALI remain to be evaluated in future studies.

The current coronavirus disease (COVID-19) world epidemic is associated with a form of ALI, the clinical management of which remains challenging (92) (93) (94) . Could PARP inhibitors possibly be effective against COVID-19-associated inflammatory responses and/or ALI? There are, unfortunately, no published studies evaluating the potential efficacy of PARP inhibitors in coronavirus-associated ALI, nor in other forms of viral pneumonia or ALI. Nevertheless, PARP inhibitors are effective in a variety of ALI models (Table 1) , and, at the later stage of the disease, many of these models (especially the endotoxin-and sepsis-associated models) share many pathophysiological pathways and features with viral ALI. In fact, therapeutic modulation of IL-1b and IL-6 overproduction has been suggested as a potentially effective therapy in COVID-19-associated ALI (92) (93) (94) , and PARP inhibitors have also been shown to be effective in downregulating the production of these mediators in various models of ALI (Table 1) .

It must also be stressed that endothelial damage and dysfunction, as well as thrombosis and intravascular coagulation, are important components of COVID-19-associated diseases (95) (96) (97) . Although PARP1 does not appear to play a major role in the regulation of thrombocyte function (nor does it directly regulate coagulation factors), PARP1 is known to play an active role in the pathogenesis of endothelial dysfunction, as shown in various disease models, including ALI, circulatory shock, diabetes, and heart failure (31, 34, 98, 99) . Thus, we can hypothesize that a PARP inhibitor-associated protection against endothelial dysfunction may have a protective effect against COVID-19-associated thrombotic events.

A key question to be raised in this context is whether PARP is also involved in viral replication; what would be the expected effect of PARP inhibitors on viral replication and viral release in various forms of viral pneumonia or viral ALI (including COVID-19-associated lung diseases)? Some viruses express vPIP (PARP1-interacting protein), which has been implicated in facilitating viral replication (100) . However, the mechanism of vPIP involves protein-protein interaction with PARP1 (and not catalytic PARP activation and subsequent PAR formation) (100). Protein-protein interactions would not be expected to be significantly affected by a pharmacological PARP inhibitor.

Importantly, PARylation responses in the host mammalian cells have been implicated in the replication of polyomavirus, and 3-aminobenzamide (3-AB; a first-generation PARP inhibitor) and 3,4-dihydropiperidine-isoquinoline (a second-generation PARP inhibitor) as well as genetic PARP1 deficiency have been shown to inhibit viral capsid protein expression and virion release in fibroblasts in vitro (101). 3-AB was also found to exert cytoprotective effects and inhibit herpes simplex virus 1 and John Cunningham virus replication and virion release (102) (103) (104) , whereas N-(5,6-dihydro-6-oxo-2-phenanthridinyl)-2-acetamide (a secondgeneration PARP inhibitor) has suppressed the replication of human coronavirus OC43, although this effect is likely related to the action of the inhibitor on a molecular target other than PARP1 (105) .

In contrast, the replication of the coronavirus JMHV (mouse hepatitis virus strain) was reported to be enhanced rather than inhibited by 3-AB (106); however, the mode of the regulation of this response was linked to another member of the PARP family (PARP14) rather than to PARP1 (106) . Another coronavirus, the mouse hepatitis virus, was recently reported to induce the upregulation of another member of the PARP family, the TiPARP (TCDDinducible PARP) family (107) . The abovelisted coronavirus effects are independent of PARP1 and are unlikely to be affected by the currently clinically approved PARP1 inhibitors, such as olaparib. Although one can speculate whether or not a PARP1 inhibitor may influence severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication or release, the fact remains that there are currently no direct studies evaluating the role of PARP1 and/or the effect of the clinically approved PARP inhibitors on SARS-CoV-2 in vitro or in vivo. Nevertheless, it should be emphasized that in several models of viral infection, PARP1 inhibitors are effective in counteracting the viral-associated cellular energetic disturbances, can exert cytoprotective effects in vitro (108).

Although the field of PARP and critical illness was born more than two decades ago, and it has steadily evolved and advanced over the years, the fact remained that these basic scientific advances could not be turned into translational efforts because no clinically approved PARP1 inhibitor was available to be used in patients. Major advances in the field of oncology have changed this situation, and now at least four clinically approved PARP inhibitors are available, with several additional PARP inhibitors in advanced clinical trials. As outlined in Reference 17, the possibility has, therefore, emerged that such inhibitors may be repurposed for various nononcological diseases. The evidence presented in the current article indicates that ALI may well be one of these indications, as there is a significant unmet clinical need, the available therapeutic options are limited, and there is a significant body of preclinical data (including translationally predictable large-animal models) showing preclinical efficacy of various PARP inhibitors in multiple models of ALI. Nevertheless, many of these models have significant limitations, as detailed in Table 1 , necessitating further preclinical work in this area. The likely effective doses of PARP inhibitors in ALI are expected to be lower than the oncological doses, and they therefore are expected to be well tolerated. Moreover, the expected duration of a PARP inhibitor's administration in ALI is expected to be relatively short, which should improve the expected safety of such trials. Biomarkers (e.g., the measurement of PARylated proteins in the circulating blood cells or in cells obtained from the BAL fluid) may be suitable to confirm therapeutic-target engagement (i.e., confirm that the dose used is sufficient to inhibit PARP's enzymatic activity in the patients).

With respect to COVID-19-associated ALI (or viral lung diseases in general), the available preclinical data are very limited. We can reasonably assume that a PARP inhibitor may suppress the cytokine storm, may attenuate the cytotoxic effects of oxidants and free radicals produced in the late stage of viral ALI, and may improve endothelial function, but preclinical efficacy data in coronavirus-associated ALI models are currently not available. The current article should encourage direct studies to assess the effect of clinically approved PARP inhibitors in animal models of coronavirus-induced ALI, and if positive, may, in turn, encourage subsequent clinical testing of the concept. n

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Role of poly (ADP) ribose synthetase in lung ischemiareperfusion injury

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Niacinamide abrogates the organ dysfunction and acute lung injury caused by endotoxin

Inflammatory and transcriptional roles of poly (ADP-ribose) polymerase in ventilator-induced lung injury

Nicotinamide abrogates acute lung injury caused by ischaemia/reperfusion

Ischemic and inflammatory lung impairment by extracorporeal circulation: effect of PARP-inhibition by INO1001

Post-allergen challenge inhibition of poly(ADP-ribose) polymerase harbors therapeutic potential for treatment of allergic airway inflammation

Therapeutic treatment with poly(ADP-ribose) polymerase inhibitors attenuates the severity of acute pancreatitis and associated liver and lung injury

Effects of PARP-1 deficiency on airway inflammatory cell recruitment in response to LPS or TNF: differential effects on CXCR2 ligands and Duffy Antigen Receptor for Chemokines

Administration of poly(ADP-ribose) polymerase inhibitor into bronchial artery attenuates pulmonary pathophysiology after smoke inhalation and burn in an ovine model

Inhibition of poly (adenosine diphosphate-ribose) polymerase attenuates lung-kidney crosstalk induced by intratracheal lipopolysaccharide instillation in rats

PARP-1 inhibitor, DPQ, attenuates LPS-induced acute lung injury through inhibiting NF-kB-mediated inflammatory response

The poly(adenosine diphosphate-ribose) polymerase inhibitor PJ34 reduces pulmonary ischemia-reperfusion injury in rats

ADPribose) polymerase inhibition with HYDAMTIQ reduces allergeninduced asthma-like reaction, bronchial hyper-reactivity and airway remodelling

PARP inhibitor, olaparib ameliorates acute lung and kidney injury upon intratracheal administration of LPS in mice

The role of poly(ADP-ribose) polymerase-1 inhibitor in carrageenan-induced lung inflammation in mice

PARP is activated in human asthma and its inhibition by olaparib blocks house dust mite-induced disease in mice

Necroptosis and parthanatos are involved in remote lung injury after receiving ischemic renal allografts in rats

Potential of inducible nitric oxide synthase as a therapeutic target for allergen-induced airway hyperresponsiveness: a critical connection to nitric oxide levels and PARP activity

HYDAMTIQ, a selective PARP-1 inhibitor, improves bleomycininduced lung fibrosis by dampening the TGF-b/SMAD signalling pathway

Pharmacological reconditioning of marginal donor rat lungs using inhibitors of peroxynitrite and poly (ADP-ribose) polymerase during ex vivo lung perfusion

Poly-ADP-ribose polymerase inhibition provides protection against lung injury in a rat paraquat toxicity model

PARP inhibition treatment in a nonconventional experimental mouse model of chronic asthma

Neutrophil transfer of miR-223 to lung epithelial cells dampens acute lung injury in mice

PARP-1 inhibition ameliorates elastase induced lung inflammation and emphysema in mice

The PARP inhibitor olaparib exerts beneficial effects in mice subjected to cecal ligature and puncture and in cells subjected to oxidative stress without impairing DNA integrity: a potential opportunity for repurposing a clinically used oncological drug for the experimental therapy of sepsis

Treatment with 3-aminobenzamide during ex vivo lung perfusion of damaged rat lungs reduces graft injury and dysfunction after transplantation

Pharmacological inhibition of poly (ADP-ribose) polymerase by olaparib, prevents acute lung injury associated cognitive deficits potentially through suppression of inflammatory response

The clinically used PARP inhibitor olaparib improves organ function, suppresses inflammatory responses and accelerates wound healing in a murine model of thirddegree burn injury

ADP ribosylation: knowledge and perspectives

The therapeutic potential of poly(ADP-ribose) polymerase inhibitors

Nicotinamide: a jack of all trades (but master of none?)

Ischemic nitric oxide and poly (ADP-ribose) polymerase-1 in cerebral ischemia: male toxicity, female protection

Gender differences in the endotoxin-induced inflammatory and vascular responses: potential role of poly(ADP-ribose) polymerase activation

Effects of the poly(ADP-ribose) polymerase inhibitor olaparib in cerulein-induced pancreatitis

The parp-1 inhibitor ino-1001 facilitates hemodynamic stabilization without affecting DNA repair in porcine thoracic aortic cross-clamping-induced ischemia/reperfusion

The role of cytokines including interleukin-6 in COVID-19 induced pneumonia and macrophage activation syndrome-like disease

COVID-19: what should anaethesiologists and intensivists know about it?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and coronavirus disease-2019 (COVID-19): the epidemic and the challenges

Endothelial cell infection and endotheliitis in COVID-19

The case of complement activation in COVID-19 multiorgan impact

COVID-19 and thrombosis: what do we know about the risks and treatment?

Endothelial dysfunction in a rat model of endotoxic shock: importance of the activation of poly (ADP-ribose) synthetase by peroxynitrite

Diabetic endothelial dysfunction: the role of poly(ADP-ribose) polymerase activation

Structurebased mechanism of action of a viral poly(ADP-ribose) polymerase 1-interacting protein facilitating virus replication

PARP-1 interaction with VP1 capsid protein regulates polyomavirus early gene expression

Herpes simplex virus 1 infection activates poly(ADP-ribose) polymerase and triggers the degradation of poly(ADP-ribose) glycohydrolase

Suppressive effect of PARP-1 inhibitor on JC virus replication in vitro

The dual action of poly(ADP-ribose) polymerase -1 (PARP-1) inhibition in HIV-1 infection: HIV-1 LTR inhibition and diminution in Rho GTPase activity

Structural basis for the identification of the N-terminal domain of coronavirus nucleocapsid protein as an antiviral target

The coronavirus macrodomain is required to prevent PARPmediated inhibition of virus replication and enhancement of IFN expression

Murine coronavirus infection activates the aryl hydrocarbon receptor in an indoleamine 2,3-dioxygenase-independent manner, contributing to cytokine modulation and proviral TCDD-inducible-PARP expression

PARP-1 mediated cell death is directly activated by ZIKV infection

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