Design and Synthesis of Cellulose Ether Derivatives for Oral Drug Delivery

Chemical modification of naturally occurring cellulose into ester and ether derivatives has been of growing interest due to inexhaustible cellulose resources, and to excellent properties and extremely broad applications of these derivatives. However, traditional esterification and etherification involve relatively harsh conditions (strongly acidic or strongly alkaline), greatly limiting the content and range of functional groups that may be installed onto the cellulose backbone. Amorphous solid dispersion (ASD) is an effective method to promote oral delivery of poorly-soluble drugs by dispersing crystalline drugs in a polymer matrix, creating drug supersaturation upon release. Cellulose 𝜔-carboxyesters have been proven to be effective ASD matrices for many different drugs; however, synthesis of such polymers involves protecting-deprotecting chemistry and one synthetic route only leads to one structure. Developing a new generation of cellulosic polymers for oral drug delivery such as ASD matrices requires new synthetic techniques and powerful tools.

Olefin cross-metathesis (CM) is a mild, efficient and modular chemistry with extensive applications in organic, polymer, and polysaccharide chemistry. Successful CM can be achieved by appending olefin “handles” from cellulose esters and reacting with electron-deficient olefins like acrylic acid. Cellulose ethers have much better hydrolytic stability compared to esters and are also commercially very important. The overarching theme of this dissertation is to investigate modification of cellulose ether derivatives, and to design and synthesize effective ASD polymers by olefin CM. We first validated the strategy of performing CM by appending metathesis “handles” through etherification and then subjected these terminal olefins to various partners (acrylic acid and different acrylates). After demonstration of the concept, we applied different starting materials (e.g. ethyl cellulose, methyl cellulose, microcrystalline cellulose, and hydroxypropyl cellulose) with distinctive hydrophobicity/hydrophilicity balance. Furthermore, α,β-unsaturated CM products tended to undergo radical crosslinking through abstraction of 𝛾-protons and recombination of polymer radicals. In order to resolve this issue, we first applied post-CM hydrogenation and then explored a thiol-Michael addition to the α,β-unsaturation, which also incorporates an extra functional group through the thioether. We have successfully prepared a collection of cellulose 𝜔-carboxyether derivatives through the above-mentioned method and preliminary drug induction experiments also revealed that these derivatives hold high promise for ASD application.

We also explored the possibility of conducting CM in a reverse order: i.e. appending electron-deficient acrylate groups to the polymer, then subjecting it to electron-rich small molecule terminal olefins. The failure of this metathesis approach was speculated to be due mainly to low acrylate reactivity on an already crowded polymer backbone and the high reactivity of rapidly diffusing, small molecule terminal olefins. Last but not least, we further utilized olefin CM to conjugate bile salt derivatives (e.g. lithocholic acid and deoxycholic acid) to a cellulose backbone by converting bile salts into acrylate substrates. Successful CM of bile salt acrylates to cellulose olefin “handles” further demonstrated the great versatility, excellent tolerance, and very broad applicability of this strategy.

Overall, we have founded the strategy for performing successful olefin CM in many cellulose ether derivatives with acrylic acid and a variety of different acrylates. Post-CM hydrogenation efficiently removes the α,β-unsaturation and provides stable and effective cellulose 𝜔-carboxyether derivatives for ASD application. Tandem CM/thiol-Michael addition not only eliminates the crosslinking tendency but also enables an even broader library of polymer structures and architectures for structure-property investigations. We anticipate these methods can be readily adapted by polysaccharide chemists and applied with numerous complex structures, which would greatly broaden the range of cellulose and other polysaccharide derivatives for applications including ASDs, P-glycoprotein inhibition, antimicrobial, coating, and other biomedical applications.