To minimize the loss of blood during surgical interventions at the liver e.g., after severe traumata or during liver transplantation, it is necessary to interrupt the blood supply of parts of the organ or the organ in total for a certain period of time. The concurrent clamping of Arteria hepatica and Vena portae, known as Pringle maneuver, is still widely used to completely interrupt the blood flow through the liver. The main disadvantage is the often appearing warm I/R injury followed from the transient hypoxia.
To reduce or to exclude the post surgical liver injury one tries to improve the surgical options on the one hand and to selectively interact with the above described pathophysiological cascade on the other hand.
Surgical options
Aim of the surgical options is the increasing abdication of the Pringle maneuver with its long lasting organ wide ischemic period. This is feasible by clamping single arterial branches, prevention of bleeding by coagulating injured blood vessels with an argon laser, or the application of supersonic assisted tissue sections [Lentsch et al., 2000; Teoh et al., 2003a].
Interruption of pathophysiological signal transduction pathways
Hepatic preconditioning The concept of ischemic preconditioning is based on the biological principle that tissue primed by various types of sublethal stress develops tolerance to subsequent lethal injury [Kang, 2002].
During ischemic preconditioning liver is exposed to short intervals (5-10 min) of ischemia and reperfusion, whereby negative consequences of warm I/R can be clearly reduced. The underlying mechanisms of the protective effect are so far not completely understood, altered TNFα release, involvement of adenosine A2 receptors
and NO, maintenance of the microcirculation as well as an enhanced cytoprotection mediated by an accelerated entrance into cell cycle are discussed in literature [Lentsch et al., 2000; Kang, 2002; Koti et al., 2003; Teoh et al., 2003a; Teoh et al., 2003b].
Aside it is reported about positive effects towards oxidative stress caused by Kupffer cells that were activated by hormonal preconditioning with atrial natriuretic peptide (ANP) [Bilzer et al., 2000; Kang, 2002].
Pharmacological interventions There are certain clinical situations e.g., acute liver traumata, where a time frame for preventive strategies as described above is not given. In these cases pharmacotherapeutic approaches are necessary to attenuate or to avoid hepatic I/R injury. The application of particular pharmaceuticals may be carried out before or during ischemia and reperfusion. The first is described as pharmacological preconditioning [Kang, 2002].
Arising from the numerous illustrated factors leading to liver injury (chapter 3.1.2) several points of attack are imaginable based on antioxidative strategies, gentherapeutical approaches, receptor blockade, and anti-inflammatory or immunosuppressive mediators.
In the beginning ROS formation plays the main role in the pathological events [Teoh et al., 2003a]. Thus different antioxidative strategies that should diminish the outcome of the excessive ROS production are described in this context. Apart from the application of antioxidative active low molecular weight substances like vitamin E [Serracino-Inglott et al., 2001; Jaeschke, 2002] or the endogenous glutathione [Bilzer et al., 2000; Kang, 2002], the reduction of ROS is aimed for by the administration of enzymes. From initially built peroxides superoxide anions are produced within hepatocyte mitochondria which in turn are degraded by the superoxide dismutase (SOD). This rapidly proceeding reaction circumvents the oxidation of NO to peroxynitrite explaining the cytoprotective effect of SOD [Jaeschke, 2002]. Beside the direct administration of the modified enzyme [Yabe et
al., 2001] a combined genetherapeutic and antioxidative approach is described. To amplify the enzymatic potency its gene is transferred into hepatocytes via an adenoviral vector where it is subsequently expressed [Jaeschke, 2002]. A further adenoviral mediated gene expression is utilized for the application of hemoxygenase (HO). As oxidative events foster the natural transcription of HO an advanced protection from ischemia induced oxidative stress by an enhanced HO expression is suggested which could already be proven in several models [Fondevila et al., 2003].
Within the inflammatory cascade activation of Kupffer cells play the main role. In an animal model it could be shown that a gadolinium chloride induced suppression of Kupffer cell activity lead to an attenuated hepatic I/R injury [Mosher et al., 2001; Giakoustidis et al., 2003]. Augmentation of Kupffer cell activity by latex beads in comparison worsens the state of the liver after ischemia and reperfusion [Shiratori et al., 1994]. Modulation of Kupffer cell activity and therefore the impact on numerous released inflammatory mediators is thus a potential target for pharmacotherapeutical interventions [Mosher et al., 2001]. So it could be shown that blocking TNFα and IL- 1 as well as subsequent mediators like VCAM or certain chemokines clearly reduce hepatic I/R injury [Lentsch et al., 2000]. Most of the mentioned proinflammatory mediators have the regulation of their transcription via the inducible transcription factor NF-κB in common [Morishita et al., 1998; Serracino-Inglott et al., 2001]. This is crucial for the inflammatory processes followed from Kupffer cell activation and has to be taken into consideration as potential target during anti-inflammatory therapy [Banafsche et al., 2001].
A further point of attack within the pathophysiological mechanism of I/R injury is the maintenance of the microcirculation. Microcirculation is controlled by the interaction of the functional antagonists’ nitric oxide and endothelin as described in chapter 3.1.2. The administration of exogenous nitric oxide via an NO donor (FK 409) improves the hepatic microcirculation, suppresses the production of endogenous NO and therewith the formation of peroxynitrite as well as the activation of neutrophils [Nozaki et al., 2003]. On the other hand endothelin receptor antagonists and inhibitors of the endothelin converting enzyme also contribute to an improved microcirculation after ischemia and reperfusion [Ricciardi et al., 2001; Uhlmann et al., 2001; Witzigmann et al., 2002].
Further more it was reported about hepatoprotective effects against ischemia and reperfusion for the immunosuppressive substances azathioprine and cyclosporine as well as tacrolimus (FK 506) proved in transplantation-medicine [Kawano et al., 1993; Baron et al., 2002].
3.1.4 Therapeutic strategy – Gelatin nanoparticles for targeted delivery of an