Nitric oxide synthase as a therapeutic target in sepsis--more questions than answers?

Nitric oxide synthase inhibition as therapy for sepsis: a decade of promise

BACKGROUND

Inhibition of nitric oxide synthase (NOS) has held promise as a novel and important therapeutic target for sepsis for over a decade. However, the question as to whether an inhibitor of NOS will prove to be efficacious in human septic shock remains unanswered.

METHODS

Review of the pertinent English-language medical literature.

RESULTS

Observations of the induction of NOS (iNOS, NOS2) by proinflammatory cytokines led to the hypothesis that nitric oxide (NO) relaxes vascular smooth muscle, thereby producing vasodilation and hypotension in sepsis. Other effects of NO in vascular tone may be manifested by refractoriness to vasopressors or myocardial dysfunction. However, such negative effects of NO are balanced by the role of NO in maintaining microvascular perfusion and host defenses against invading pathogens. Initial animal studies of septic shock modulated by the administration of competitive inhibitors of NOS demonstrated that NOS inhibition might be beneficial therapy for clinical septic shock. However, subsequent animal studies have produced variable results, and a phase III clinical trial of a nonselective NOS inhibitor has failed to show benefit for therapy of septic shock.

CONCLUSION

A better understanding of the effects of NOS and its inhibitors is needed as is an understanding of the underlying pathophysiology of sepsis. Moreover, a nontoxic, short-acting, titratable, specific inhibitor of NOS2 has yet to be identified and tested. Until then, efforts should be designed to describe more completely the role of NO in the pathophysiology of sepsis.

Nitric oxide in the pathogenesis of sepsis

In sepsis and septic shock, inflammatory mediators result in the production of increased concentrations of nitric oxide (NO) from the enzymatic breakdown of the amino acid L-arginine. The increased amounts of NO are responsible for changes in vasomotor tone, decreased vasopressor responsiveness, and decreased myocardial function, characteristic of septic insult. Therapeutic strategies designed to reduce the concentration of NO by inhibiting the action of the nitric oxide synthase enzyme, or by scavenging the excess NO, offer the potential to treat directly the vasomotor abnormalities and myocardial depression seen in sepsis and other inflammatory states. This article reviews the biology of NO in sepsis and discusses strategies for neutralization of the increased NO production, in the setting of severe sepsis and septic shock.