The specificity of selectins for selectin ligands affords the opportunity to target these molecular entities to effectively alter the progression of inflammatory responses and cancer metastasis. As depicted in , several modes of putative intervention have been developed and are being evaluated as anti-inflammatory/metastatic agents at present. Results from clinical trials involving therapeutics targeting selectin interactions, may be found in the following references [
10,
252].
A recombinant soluble form of PSGL-1 may competitively inhibit the interaction between PSGL-1 expressed natively on circulating leukocytes and tumor cells and E- and P-selectin expressed on endothelial blood vessels. Recombinant PSGL-1-immunogloblulin (PSGL-1-Ig) can be sulfated and glycosylated if expressed in cell lines prior to purification [
253]. This bioactive, soluble form of PSGL-1 consists of the first 47 N-terminal amino acids of human PSGL-1 linked to the Fc portion of human IgG1 [
254]. Recombinant PSGL-1-Ig directly inhibits rolling of murine leukocytes mediated by all three selectins in assays involving intravital microscopy [
255] and prevents inflammation in a number of animal models [
256-
260]. Interestingly, the concentration of PSGL-1-Ig that dampens overall inflammation is 30-fold lower than the amount that inhibits selectin-mediated rolling [
255], suggesting that PSGL-1-Ig may inhibit inflammation by mechanisms other than and/or in addition to leukocyte rolling. PSGL-1 is similar to chemokine receptors in its requirement for post-translational sulfation and glycosylation [
261]. In fact, PSGL-1-Ig binds the CXC chemokine, KC [
255], and has been shown recently to bind the CC chemokine, CCL21 [
262]. Titration of chemokines with PSGL-1-Ig inhibits chemotaxis of mouse neutrophils in response to KC [
255]. Production of PSGL-1-Ig for clinical trials is cost-prohibitive due to its production in mammalian cells co-transfected with FT and C2GlcNAcT-I. The high production costs, in combination with inadequate efficacy, may help explain why clinical trials have been discontinued [301].
Inhibitors targeting the α1,3- and α1,4-FTsthatgenerate sLe
X and sLe
a, respectively, are in development. Most such inhibitors are modeled after the structures of acceptor, donor and transition-state analogs of FT reaction components [
263,
264]. At least in theory, FT antagonists should be efficacious, given that mice with targeted deletions of FT7 exhibit a near absence in functional ligands of E- and P-selectin and display reduced leukocyte rolling in postcapillary venules [
180]. Unfortunately, chemical synthesis of such small-molecule inhibitors has been hampered by a number of technologic, pedagogic, or design issues. First, it is essential that FT antagonists penetrate two membranes: cellular and Golgi. Second, there is no available structural information about FTs, as evidenced by the absence of X-ray crystallographic or nuclear magnetic resonance models. Third, FTs have a low affinity for GDP-fucose and acceptor substrates, calling into question the practicality in designing inhibitors that are of sufficient affinity and potency [
265-
267]. At least one molecule, compound 24, was identified from a library of 85 different GDP-triazole compounds screened against FT6 [
268]. Compound 24 is a non-competitive inhibitor of the
N-acetyllactosamine acceptor molecule with potency in the nanomolar range [
268,
269]. Targeting only one FT alone may be insufficient in controlling disease as observed in one patient carrying a missense mutation of the FT7 gene. In this individual, FT4 compensated for the inactivity of FT7 in the synthesis of active selectin ligands [
270]. In such a scenario, the use of compound 24 may be inadequate because it does not appear to potently inhibit FTs other than FT6, namely FT3 and FT5, both of which share high sequence identity with FT6 [
268]. To this end, the authors and other co-workers have circumvented the problem of FT compensation with a novel fluorinated analog of
N-acetylglucosamine: peracetylated-4-fluorinated-D-glucosamine (4-F-GlcNAc). 4-F-GlcNAc is a metabolic inhibitor of lactosamine (Galβ1,4GlcNAc) biosynthesis, which involves a series of steps that precedes and is even independent of FTs. The hydroxyl that is normally present in the 4′ position of GlcNAc has been replaced by fluorine in the 4-F-GlcNAc mimetic. Fluorine leads to a block in the addition of galactose and results in premature termination of lactosamine elongation and absence of sLe
X [
176]. Radiolabeled 4-F-GlcNAc is incorporated directly into lactosamine natively expressed on purified T cells and abrogates expression of sLe
X [
176]. 4-F-GlcNAc does not incorporate into some E-selectin ligands, including glycolipids, which may explain why E-selectin is a less sensitive target than P-selectin in response to 4-F-GlcNAc intervention [
271]. Nonetheless, 4-F-GlcNAc is effective at inhibiting allergic contact hypersensitivity responses dependent on T cell E- and P-selectin ligand expression in mice [
272].
Pan selectin competitive inhibitors, including sLe
X oligosaccharides [
273], sLe
X mimetics [
274], multivalent sLe
X ligands [
275] or diverse molecular weight species of heparin have been developed. A few such carbohydrates have shown some success in the treatment of psoriasis (Bimosiamose or TBC-1269 and Efomycine M) or asthma (Bimosiamose) [
276-
278]. Unfractionated heparin and low molecular weight heparins have been shown to inhibit lung metastasis in experimental mouse models, presumably by inhibiting tumor cell binding of L- and P-selectin [
279,
280]. An alternative approach to competitive inhibition, is to divert native glycosylation away from the lactosamine backbone of Lewis antigens with carbohydrate decoys that compete with the backbone as substrates for FTs. Thus, structural derivatives of Galβ1,3GlcNAc or Galβ1,4GlcNAc added exogenously to cells can efficiently enter the Golgi compartment and downregulate the synthesis of sLe
X -bearing ligands [
281-
283]. These studies underscore several promising modes of novel intervention in the modulation of selectin or selectin ligand activity in inflammation and cancer.