Activated protein C for H1N1 influenza? More work to do!

An animal model of H1N1 influenza demonstrates that this infection is associated with pulmonary and systemic activation of coagulation and impairment of fibrinolysis in addition to systemic inflammation and intense neutrophil influx into the lung. Activated protein C attenuates coagulation activation and restores fibrinolytic capacity but has little effect on inflammation or survival from this infection. This animal model points to a profound inflammatory state developing in H1N1 infection that impacts mortality. Additional modifications to the model and the type and amount of activated protein C dosing will provide the data to determine the possible use of activated protein C as a therapy in human H1N1 infection.

In 2009 the seasonal infl uenza virus was replaced with a pandemic H1N1 infection strain (swine fl u). Since that time, numerous reports have surfaced of severe disease occurring and resulting in acute lung injury and mortality. Treatment of this infection has involved oseltamavir and supportive care. A logical next step would be to fi nd an adjuvant agent that could be of benefi t in severe disease. To discover this agent, one must fi rst understand the pathogenesis of this unique virus.
In the previous issue of Critical Care, Schouten and colleagues attempt to build on the knowledge gained about the pathogenesis of H1N1 in a lethal mouse model [1]. Th ese authors ask two questions. Does viral pneumonia due to H1N1 cause sytemic and pulmonary activation of coagulation and inhibition of fi brinolysis in the lungs similar to what is known to occur in communityacquired bacterial pneu monia and acute respiratory distress syndrome? If so, does activated protein C (APC) -a molecule with anti coagulant, anti-infl ammatory and profi brinolytic proper ties -abrogate this response and improve outcome, as is suggested by its eff ects in patients with sepsis due to community-acquired pneumonia [2]?
Th e results of Schouten and colleagues' study indicate that activation of coagulation and impairment of fi brinolysis does in fact occur during H1N1 infection. Th ey also corroborate the fi nd ings of intense neutrophil infl ux in the lung, pro longed cytokine storm and diff use alveolar damage as key components of the pathogenesis of the infection [3]. APC was able to decrease coagulation activation and restore normal fi brinolysis compared with placebo but had marginal eff ects on cytokine levels, pulmonary neutrophil infl ux and outcome.
Th e results from this animal study add to the evidence that coagulation inhibition per se does not improve outcome in acute lung injury. A randomized, placebocontrolled trial of recombinant human APC in 75 patients with acute lung injury without sepsis demonstrated no benefi t of APC with respect to ventilator-free days, mortality or lung injury score [4]. A trial of a recombinant tissue factor pathway inhibitor in patients with severe community-acquired pneumonia demonstrated no benefi t (Wunderiak R, et al.,unpublished data).
A possible downside to thrombin inhibition by anticoagulation agents is the loss of the ability to wall off infection through fi brin formation. Fortunately, APC led to lower viral load in the lungs. Additionally, inhibition of thrombin formation could prevent the activation of thrombin activatable fi brinolysis inhibitor. Activated throm bin activatable fi brinolysis inhibitor inhibits the chemotactic factors C3a and C5a, which could be important for prevention of infl ux of leukocytes into the lung [5]. As the authors of the current study mention, mutant variants of APC with anti-infl ammatory properties and little anticoagulant activity could be examined in future animal studies.
Th e lack of eff ect of APC on cytokine production and neutrophil infl ux that is prominent in H1N1 merits discussion. Th e current study's authors showed that APC had no eff ect on cytokine elaboration or pulmonary Abstract An animal model of H1N1 infl uenza demonstrates that this infection is associated with pulmonary and systemic activation of coagulation and impairment of fi brinolysis in addition to systemic infl ammation and intense neutrophil infl ux into the lung. Activated protein C attenuates coagulation activation and restores fi brinolytic capacity but has little eff ect on infl ammation or survival from this infection. This animal model points to a profound infl ammatory state developing in H1N1 infection that impacts mortality. Additional modifi cations to the model and the type and amount of activated protein C dosing will provide the data to determine the possible use of activated protein C as a therapy in human H1N1 infection. neutro phil infl ux in a Pseudomonas aeruginosa pneumonia model and in an endotoxin challenge model in rats [6,7]. In vitro models have demonstrated an inhibitory eff ect of APC on cytokine eff ect with much higher concentrations of APC relative to the levels achieved in this study [8]. In both a human and a sheep pulmonary endotoxin study, recombinant human APC given as a continuous infusion of 24 μg/kg/hour was able to decrease the infi ltration of neutrophils into the lung [9,10]. In human septic patients and in an intravenous endotoxin challenge model in healthy human volunteers, no anti-infl ammatory eff ects were observed with this dosing strategy [11,12]. Th ese data would suggest that the anti-infl ammatory eff ects of APC vary by species, by type of infectious challenge, by means of APC dosing and by blood con centrations, such that more information needs to be learned with respect to optimized dosing in H1N1 infection. Future animal studies with the previously men tioned APC variants with minimal anticoagulant eff ects would allow the authors to push the blood concentration for determination of the maximal anti-infl ammatory eff ect.
Th e absence of a benefi t in terms of survival with APC treatment in this murine model of H1N1 infection does not necessarily predict a lack of benefi t in human H1N1 infection. Th is model was quite severe with 100% lethality, while mortality in human H1N1 infection is less than 20% in severe cases [13,14]. Th e mice were young, healthy and of normal weight, which does not mimic the clinical situation in humans. Oseltamavir was not given in this model, which could aff ect the treatment response to APC. Upwards of 30% of human patients with H1N1 develop bacterial pneumonia and severe sepsis in which recombinant human APC may still be benefi cial [3]. Severe human H1N1 infection is complicated by shock in 30 to 60% of cases and by renal failure in 22% of cases [13,14]. Th is animal model did not monitor organ dysfunction, which APC may prevent through PAR-1 signal ing [15].
In summary, the jury is still out regarding whether APC could potentially play a future role in the management of H1N1 infection. Future experiments will need to include mice and other species of diff erent ages, diff erent infecting doses of H1N1, concomitant oseltamavir treat ment, monitoring and evaluation of hemodynamic and nonpulmonary organ function, and diff erent dosages and means of administration of APC and APC variants.

APC, activated protein C.
Competing interests SPL is a former employee of Eli Lilly and company, the maker of recombinant human activated Protein C.