Aspirin as a potential treatment in sepsis or acute respiratory distress syndrome

Initially used by the ancient Greeks in the form of willow leaf tea and later refined
in Germany in the 19th century, by Felix Hoffman, aspirin has become one of the most
commonly used drugs today 43]. Aspirin is a non-selective inhibitor of the enzyme cyclooxygenase (COX), has a half-life
of approximately 20 minutes and is subject to significant first-pass metabolism, and
most of its action occurs in the portal circulation of the liver 43]. Aspirin has previously been used in high doses for the treatment of rheumatic fever,
but currently low-dose aspirin continues to be used in both primary and secondary
prevention in cardiovascular medicine.

There are several mechanisms in which aspirin can manipulate the processes involved
in both sepsis and ARDS (Fig. 1): 1) inhibition of COX 43]; 2) inhibition of nuclear factor kappa B (NF?B) 44]; 3) production of nitric oxide (NO) 45]; and 4) lipoxin production 46].

Fig. 1. Mechanisms in which aspirin can manipulate the process in sepsis and acute respiratory
distress syndrome: Inhibit the enzyme COX, preventing the formation of pro-inflammatory
thromboxane and prostaglandins. Inhibit the release of NF?B from its inhibitor IkB,
preventing the formation of pro-inflammatory cytokines and chemokines. Production
of aspirin triggered lipoxin, which induces the release of NO, inhibits production
of IL-8 and MPO, restores neutrophil apoptosis and promotes resolution. Increase production
of NO, resulting in reduced migration and infiltration of neutrophils and reduced
permeability of endothelium. 15-epi-ATL, aspirin-triggered 15-epi-lipoxin A4, AA arachidonic acid, COX cyclooxygenase, eNO endothelial nitric oxide, IKK IkB kinase, IL-8 interleukin 8, MPO myeloperoxidase, NF?B nuclear factor kappa B, NO nitric oxide, PGE ₂
prostaglandin E
₂
, TXA ₂
thromboxane

Inhibition of cyclooxygenase

The most obvious mechanism is irreversible inhibition of both COX I and COX II enzymes
43]. The inhibition results from the direct acetylation and obstruction of the active
portion of the enzyme, thus preventing interaction with the substrate. This inhibition
prevents the conversion of membrane phospholipid-derived arachidonic acid to thromboxane
(TXA
₂
) and prostaglandins, including the pro-inflammatory prostaglandin E
₂
(PGE
₂
) 47]. As the platelet is anucleated, it has limited ability to replicate new proteins
or enzymes, thus resulting in irreversible inhibition of the enzyme for the life span
of the platelet, namely 7–10 days. Aspirin is significantly more potent at inhibiting
COX I, especially at the lower 75 mg dose, than COX II. COX I is responsible for normal
haemostatic processes, including platelet activation and aggregation through TXA
₂
production, which is a feature of both sepsis and ARDS 41]. COX II undergoes increased expression following stimulation from IL-1, TNF? and
lipopolysaccharide (LPS) and results in increased production of prostaglandins, including
PGE
₂48]. PGE
₂
is required for the production of pro-inflammatory cytokines and mediates the formation
of oedema 49].

Inhibition of nuclear factor kappa B production

As well as direct inhibition of COX, aspirin has been shown to downregulate the production
of pro-inflammatory cytokines. NF?B is an important transcription factor required
for production of pro-inflammatory interleukins and cytokines. Aspirin prevented NF?B
production and ultimately leucocyte adhesion in a stimulated human epithelial cell
model 44] and it does this by preventing the release of NF?B from its cytosolic inhibitor I?Ba
50]. However, this specific effect was demonstrated only after treatment with high-dose
aspirin of 10 or 20 mM, which is higher than the serum therapeutic concentration.
Inflammation leads to an acidic environment and an acidic extracellular interstitial
environment. This can enhance salicylate accumulation 51] because of local ion trapping and lead to higher local concentrations than serum
concentrations.

Production of nitric oxide

Low-dose aspirin reduced inflammation within the vascular endothelium and led to the
development of smaller atherosclerotic lesions with less macrophages in low-density
lipoprotein receptor-deficient mice 52]. In a study using dissected porcine coronary arteries, aspirin was shown to directly
acetylate the endothelial nitric oxide synthase protein, thus releasing NO from the
coronary artery endothelium. NO acts as an anti-adhesive, inhibiting the migration
and infiltration of leucocytes through the endothelium as well as regulating vascular
tone and micro-thrombi formation in the septic state 53]. Importantly, this was independent of COX inhibition as demonstrated by a lack of
effect with indomethacin, another non-steroidal anti-inflammatory drug (NSAID), or
with an aspirin metabolite 45].

Lipoxin production

Recent evidence has also demonstrated anti-inflammatory properties with aspirin not
seen in other NSAIDs. Aspirin can induce the production of a type of lipoxin called
aspirin-triggered 15-epi-lipoxin A4 (ATL) 46] and can do so at the lower 75 mg dose 54]. Once the active site of the COX enzyme is blocked by the acetylation action of aspirin,
the arachidonic acid is converted to ATL via 15-R-hydroxyeicosatetraenoic acid 55]. The anti-inflammatory effects of ATL have been extensively demonstrated in the pre-clinical
septic models and LPS models of ARDS. ATL inhibits the production of IL-8 through
inhibition of NF?B, thus reducing inflammation and leucocyte migration 56], and can independently trigger the release of NO 49]. ATL suppresses the anti-apoptotic effects of myeloperoxidase via inhibition of the
B2 integrin signalling pathway, thus restoring the natural cell cycle of the polymorphonuclear
neutrophils (PMNs), leading to effective resolution of inflammation 46], 57]. In addition, lipoxins can stimulate phagocytosis of apoptotic neutrophils by macrophages,
possibly through enhancement of macrophage neutrophil adhesions permitting efficient
resolution of inflammation 58]. Persistent inflammation and delayed apoptosis of PMNs are features of ARDS and are
associated with worse outcomes 59].

Furthermore, in two experimental murine models of ARDS, one with intra-tracheal LPS
and the second a transfusion-related acute lung injury, ATL significantly reduced
the concentration of neutrophil platelet aggregates via antagonism of the lipoxin
A4 receptor, resulting in decreased neutrophil migration, pulmonary oedema and vascular
permeability 41]. Finally, ATL significantly improved 48-hour survival and decreased BAL concentrations
of TNF? and macrophage inflammatory protein-2 following LPS-induced lung injury in
mice 60].