elucidate the aggregate architecture and the drug solubilisation
mechanism of 7 under physiological conditions.
Conclusions
In summary, a multivalent folate conjugate (7) in which four
folate units are clustered by means of a calix[4]arene platform has
been designed and synthesized. The presence of multiple folate
homing moieties and a good water-solubility at physiological pH,
in addition to the capability to increase indomethacin water-
solubility are prerequisites that make 7 potentially appealing
for targeted drug delivery. The efficiency of compound 7 in FR
binding and cell uptake as well as targeted drug delivery has to be
demonstrated and it will be matter of a future work.
Fig. 2 Phase–solubility diagram of indomethacin in PBS, pH 7.4, at
increasing concentration of folate–calix[4]arene (7).
The calix[n]arene family offers a variety of oligomers which
might be engineered to meet the specific needs of novel targeted
drug delivery systems with controlled characteristics. Therefore,
compound 7, as the first example of a calixarene-based folate
conjugate, may open the way to a novel class of multivalent and
multifunctional targeting systems in which drugs and/or imaging
agents are covalently or non-covalently conjugated to multiple
homing moieties by means of a calix[n]arene scaffold.
4
2
than one suggesting, as for other calixarene–drug complexes, the
formation of 1 : 1 stoichiometry complex in solution.
43
But, considering the amphiphilic nature of compound 7 and
the presence of folate moieties, whose self-assembling properties
are known, aggregate formation in PBS medium can easily be
imagined. This was confirmed by DLS measurements showing
that in the concentration range 2–0.04 mM, compound 7 formed
large aggregates, in any case 99% of the mass of the sample
had an average hydrodynamic radius of 5 nm. Loftsson et al.
reported that in the presence of aggregate species in solution, the
stoichiometry of a complex cannot be derived from simple phase
type diagram does not necessarily indi-
cate formation of 1 : 1 host/guest inclusion complex. Therefore,
as for other amphiphilic calixarene aggregates, it is likely that
drug solubilisation by compound 7 is based on drug–aggregate
interaction.
44
45
We thank Dr N. Micali for DLS experiments and Dr M.
D’Agosta for the contribution during his thesis work.
39
Notes and references
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H NMR experiments evidenced that compound 7–drug inter-
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A very broad H NMR spectrum of compound 7 in PBS medium,
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ordered NMR spectroscopy (DOSY), which provided a diffusion
-
10
2
-1
coefficient (D) of 1.69 ± 0.03 ¥ 10 m s corresponding to
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congruent with a medium radius (rcalc) of 1.29 nm, calculated by
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494 | Org. Biomol. Chem., 2011, 9, 6491–6495
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