Chemical Modifications of Hyaluronan for Therapeutic Applications - Hyaluronic Acid for Biomedical and Pharmaceutical Applications - page 151

8.3 Chemical Modifications of Hyaluronic Acid
To develop HA-based drug delivery systems, several chemical
modifications have been performed. For this purpose, most of the
research has been focussed on modifications of the carboxylic acid
functionality of glucuronic acid or the C-6 hydroxyl group [15, 16].
There are also a few examples of selective deprotection of the acetyl
group from N -acetylglucosamine and use of the amino residue for
bioconjugation. These types of chemical modifications also increase
the half-life of HA, which is an essential aspect for designing targeted
drug delivery systems. We summarise below the general chemical
coupling strategy that has been used so far.
8.3.1 Modification of Carboxyl Group
The most common modifications of the carboxyl group for HA
derivatisation are shown in Figure 8.1 . Carboxyl groups of HA
are commonly modified by carbodiimide-mediated reactions,
esterification and periodate oxidation.
Carbodiimide coupling chemistry under aqueous conditions is
performed using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC) which was first performed by Danishefsky and Siskovic in 1971
[17]. Generally this reaction is performed in aqueous conditions at
pH 4.75 without requiring any catalysts [18]. This method, however
works efficiently with alpha-effect nucleophiles such as hydrazides
and aminooxy derivatives as these nucleophiles have pKa between 2
and 3.5 [19]. When amines are used as nucleophiles, such reactions
do not take place and EDC by-product, N -acylurea, is formed as
the major product. To overcome this problem, nucleophilic catalysts
such as N -hydroxybenzotriazole (HOBt) or N -hydroxysuccinimide
(NHS) are used which allow the reaction to occur at higher pH
(6 to 7). Under these conditions, coupling reaction takes place without
the formation of any EDC by-product. This is due to the fact that
highly reactive carbodiimide activates carboxylate group with HOBt
or NHS to form a more stable non-rearrangeable intermediate that