Drug Targeting Organ-Specific Strategies

instance [46]. Large quantities of NO produced by the inducible NO-synthase enzyme
(iNOS) prevent further NFκB activation either by S-nitrosylation of cysteine 62 of the p50
subunit of NFκB or by stabilization of the inhibitory IκB-α protein [47,48]. As described
above, the anti-apoptotic proteins Bcl-2,A20 and Bcl-x L play an important role in controlling
inflammation, as well as in endothelial cell activation. This is partly due to their ability to inhibit
NFκB activation in endothelial cells. In vitro, these proteins block the induction of proinflammatory
genes such as cytokines, pro-coagulant and adhesion molecules, and hence
serve as a regulatory mechanism to restrain activation and injury [49,50].
Class I and II cytokine receptors that do not have direct tyrosine kinase activity mediate
signal transduction in cells via JAK (janus kinase) and STAT (signal transduction and activator
of transcription) molecules. Among these are receptors for IL-15 and GM-CSF, cytokines
involved in T cell recruitment to rheumatoid arthritis lesions and pro-angiogenic responses
of endothelium, respectively [17]. Each cytokine activates a pre-defined set of JAKs
through interaction between one of the receptor subunits and a JAK molecule. The JAKs
then become activated by reciprocal transphosphorylation and in turn phosphorylate substrates
such as receptor subunits (Figure 7.3). This creates a docking site for signalling molecules
leading to subsequent signal transduction cascades. Downstream of JAK activation lies
a variety of targets, including the low molecular weight G protein Ras and its targets PI3-kinase
and the serine/threonine kinase Akt or protein kinase B. In parallel, members of a family
of STAT transcription factors sharing a central DNA-binding domain, can become phosphorylated.
Their subsequent dimerization leads to nuclear localization and DNA-binding,
although this is not sufficient for their transactivation. Through physical and functional interactions
between STATs and other transcription factors gene transcription can take place
to modulate cellular functions [51]. It has been reported that the c-Jun N-terminal kinase
group of MAP kinases plays a role in endothelial cell signal transduction and activation upon
exposure to inflammatory cytokines, in a manner similar to that described above. Due to
space limitations, this will not be discussed further: a detailed description of the role of this
regulatory pathway in various inflammatory conditions can be found in reference [52].
7.4 Targeting Drugs to the Endothelial Cell
7.4 Targeting Drugs to the Endothelial Cell 179
In most, if not all, chronic inflammatory diseases endothelial cells are prominently involved
in the disease process. This is demonstrated by an increased expression of adhesion molecules
and production of cytokines, and their pro-angiogenic behaviour.This leads to continuous
recruitment of leucocytes into the inflamed area, without (detectable) antigen present in
the affected tissue, resulting in a vicious circle of tissue damage and leucocyte recruitment.
Targeting inhibitory agents (in)to the endothelial cell may interrupt in this process by controlling
the activation status of this cell type.
The advantage of endothelial cell targeting is the localization of the endothelial cells. Since
they are in direct contact with the blood, the ‘homing’ devices do not have to cross the endothelial
barrier to find their targets [53].The expression of adhesion molecules and epitopes
involved in angiogenesis are, at least in theory, suitable target epitopes for drug targeting purposes.
In this respect it is important to distinguish between potential carriers that only bind