Xyloglucan is a polysaccharide derived from tamarind seeds
and it is composed of a (1→4)-β-D-glucan backbone which has
(1→6)-α-D-xylose branches that are partially substituted by
(1→2)-β-D-galactoxylose (Fig. 1J).
Since native xyloglucan is unable to gel, it has been partially
degraded by β-galactosidase in order to eliminate galactose
residues by 44%, making it suitable for studies on drug release.
Degraded, the polymer turns into a hot gelation, i.e. a sol–gel
transition that occurs when the system is heated up. The
temperature of gel transition is concentration dependent as it
increases by lowering the polymer concentration and it is related
to the galactose removal percentage. Although the xyloglucan
gelation has been often compared with that of Pluronic®, since
both polymers undergo the phase transition by heating,
nevertheless the gelation mechanism appears to be rather
different. A packing of micelles is detected in the case of
Pluronic®, while for xyloglucan, a lateral stacking of the rodlike
chains is responsible for the network assembly [166]. While
a relevant number of publications and patents are reported for
the use of xyloglucan as a food additive, much less has been
published about its use in the field of pharmaceutics. However,
the few examples reported below indicate its potential for a
wide variety of applications with numerous advantages
compared with other macromolecular systems.
Xyloglucan has been proposed for the formulation of
sustained release indomethacin suppositories that were compared
both in vitro and in vivo with the corresponding
commercial products [167]. No difference in bioavailability
between the two formulations was detected while the drug was
effective for a longer period of time in the case of xyloglucan
gel. Placing the gel in the lower section of the rectum can allow
the administration of drugs subject to first-pass metabolism.
Additionally, when the same drug was orally administered a
constant plasma level for several hours was detected [168].
The xyloglucan gels have also been tested for intraperitoneal
administration giving a broad profile of drug plasma concentration,
the gel was still present after several hours [169].
In comparison with gellan and alginate, in the oral
administration of cimetidine, xyloglucan gels appear to be the
system with the widest application because its gelation does not
require the presence of cations, as in the case of alginate, and its
use is not restricted by the charged nature of the drug, as in the
case of gellan [44].
Xyloglucan gels have also been investigated for ocular
delivery of pilocarpine and timolol [170,171]. The in situ gelling
formulations were consistently more effective than aqueous
buffer solutions while the rapid gelation was essential in
preventing the loss of drug by drainage from the eye.