AMD3100 is a potent and selective antagonist of the CXCR4 receptor; it has been shown to block the route of entry of HIV into host T-cells. This compound and its analogues have since been found to act as haematopoietic stem cell mobilisation agents and, more recently, as anti-cancer agents. Here, we have examined a fluorescent derivative of AMD3100, L(1), which offered the potential to assess the behaviour of AMD3100 at the cell surface by using optical imaging modalities. The binuclear Zn(II) , Cu(II) and Ni(II) complexes of L(1) have also been investigated as these metals have been previously shown to enhance the binding properties of AMD3100. Furthermore, Zn(II) and Cu(II) are known to enhance and quench, respectively, the fluorescence of similar anthracenyl-based ligands. Whilst L(1) demonstrates an ability to inhibit the binding of the anti-CXCR4 monoclonal antibody 12G5 (IC(50) =0.25-0.9 μM), the incorporation of an anthracenyl moiety resulted in a significantly reduced affinity for CXCR4 compared to AMD3100 (IC(50) =10 nM). We observed no significant increase in fluorescence intensity following incubation with murine pre-B cells overexpressing CXCR4 compared to a control cell line. This limits the usefulness of L(1) as a fluorescent imaging probe. Interestingly, the Zn(II) complex, which carries an overall +4 charge, revealed marginally higher specificity and reduced toxicity in vitro compared to the free ligand, albeit with reduced affinity for CXCR4 (IC(50) =1.8-5 μM). We suggest that the incorporation of an anthracenyl group contributes to the lipophilic character of the free ligand, thereby resulting in transport across the plasma membrane. This effect is seemingly diminished when the ligand is complexed to charged metal ions.
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