Anti-inflammatory Agents: Present and Future

Abstract

Inflammation involving the innate and adaptive immune systems is a normal response to infection. However, when allowed to continue unchecked, inflammation may result in autoimmune or autoinflammatory disorders, neurodegenerative disease, or cancer. A variety of safe and effective anti-inflammatory agents are available, including aspirin and other nonsteroidal anti-inflammatories, with many more drugs under development. In particular, the new era of anti-inflammatory agents includes "biologicals" such as anticytokine therapies and small molecules that block the activity of kinases. Other anti-inflammatories currently in use or under development include statins, histone deacetylase inhibitors, PPAR agonists, and small RNAs. This Review discusses the current status of anti-inflammatory drug research and the development of new anti-inflammatory therapeutics.

Figure 1. Inflammation and Points of Inhibition by Anti-inflammatory Agents

Shown is the inflammatory pathway associated with ischemia-induced tissue damage and the anti-inflammatory drugs that can be used to block inflammation at various points in the pathway. During ischemia, mesenchymal-derived cells undergo hypoxic damage, lose membrane integrity, and release their cytoplasmic contents. This is followed by the release of biologically active proinflammatory cytokines such as IL-1 and VEGF. These cytokines bind to and activate their cognate receptors on nearby macrophages. The activation of the cytokine receptors triggers tissue-resident macrophages to express an extensive portfolio of proinflammatory genes. Some gene products such as the IL-1β and IL-18 precursor proteins require processing by caspase-1 and secretion as active cytokines. There is also chemokine production by activated tissue macrophages. A concentration gradient of secreted cytokines and chemokines forms between the macrophage and the endothelium of the microvasculature. Activation of cytokine and chemokine receptors on the endothelium takes place resulting in the opening of endothelial junctions. This leads to a capillary leak and the passage of plasma proteins into the ischemic area. During the ischemic event, there is activation of complement with induction of cytokines and chemokines, as well as a direct effect of C5a on chemoattraction. As part of the cytokine activation of the endothelium, there is expression of the adhesion molecule VCAM-1, which facilitates the rolling of innate immune cells such as monocytes and neutrophils (myeloid cells) and their adherence to the endothelium. These inflammatory cells then leave the circulation and enter the tissue space by diapedesis. With increasing numbers of infiltrating myeloid cells, the healthy tissue surrounding the ischemic area (called the penumbra) is subjected to damage from the cellular infiltrate due to the release of cytokines and chemokines into the penumbral tissue. Release of damaging proteases from neutrophils contributes to the loss of tissue in the penumbra. Adherence of platelets to the endothelium causes further endothelial activation by increasing expression of receptors for platelets. During the development of an inflammatory infiltrate in the ischemic tissue, a cascade of clotting factors is initiated by cytokine-induced endothelial tissue factor. In addition, the cytokine-activated endothelium releases inhibitors of fibrinolysis. With increased coagulation in the microvasculature and decreased fibrinolysis, tissue perfusion slows and hypoxia and tissue damage worsen.

Figure 2. Production and Release of IL-1β

(Top left) Primary blood monocytes, tissue macrophages, or dendritic cells are activated by the cytokine IL-1β (blue oval) resulting in the formation of the IL-1 receptor complex (composed of IL-1RI and IL-1RAcP). The intracellular TIR domains of the IL-1 receptor complex recruit MyD88 and other signaling components leading to translocation of the transcription factor NF-κB into the nucleus. This results in transcription of the gene encoding the precursor of IL-1β and its translation into protein (blue dumb-bell) (Bocker et al., 2001). ATP released from activated monocytes (or from dying cells) accumulates outside the cell. As the extracellular levels of ATP increase, the P2X7 receptor is activated, triggering the efflux of potassium ions from the cell. Low intracellular potassium ion levels enable the components of the caspase-1 inflammasome to assemble. Assembly results in the conversion of procaspase-1 to active caspase-1. There is evidence that the components of the inflammasome localize to secretory lysosomes together with the IL-1β precursor protein and lysosomal enzymes (Andrei et al., 2004). In the secretory lysosome, active caspase-1 cleaves the IL-1β precursor, generating active mature IL-1β. Mature IL-1β is released along with the IL-1β precursor and the contents of the secretory lysosomes (Andrei et al., 2004). There is evidence that secretion of active IL-1β requires an increase in intracellular calcium ion levels (Kahlenberg and Dubyak, 2004). Processing of the IL-1β precursor can also take place in the cytosol independently of caspase-1 and the inflammasome (Brough and Rothwell, 2007). Studies have also shown that Rab39a, a member of the GTPase family, contributes to the secretion of mature IL-1β by helping IL-1β traffic from the cytosol into the vesicular compartment (Becker et al., 2009) from where it is secreted by exocytosis (Qu et al., 2007) (not shown). Mature IL-1β can also exit the cell through a loss in membrane integrity (Laliberte et al., 1999) or exocytosis via small vesicles (MacKenzie et al., 2001).