Hyaluronic acid graft copolymers as potential vehicles for intravitreal drug delivery

Water-soluble polymers are promising drug carrier materials. Especially polysaccharide graft copolymers are desirable for this purpose as they combine the favorable features of biopolymers, such as biocompatibility and biodegradability, with the controlled structure and functionality of synthetic polymers. This thesis examines water-soluble hyaluronic acid (HA) graft copolymers with cleavable arms as potential vehicles for sustained intravitreal drug delivery. Retinal diseases are the leading cause of visual impairment in the aging Western societies, but drug delivery to the back of the eye is complicated by multiple barriers. Intravitreal injections so far yield the highest bioavailability of drugs, however they need to be repeated frequently due to the rapid clearance of the therapeutics. Sustained delivery of drugs over extended periods of time is a promising strategy to prolong the injection intervals. Macromolecular drug delivery vehicles can help to reduce the clearance rate due to their high molecular weight and low diffusivity. The studied graft copolymers are based on a high molecular weight HA backbone and poly(glyceryl glycerol) (PGG) side chains attached via hydrolysable linkers. HA is a natural constituent of the vitreous and used to prolong the vehicle’s retention time in the eye. PGG is a multihydroxyfunctional polyether featuring a poly(ethylene glycol) (PEG) backbone and pendant 1,2-diol moieties in every repeating unit. As such, PGG possesses similar biocompatibility and antifouling properties as PEG, while being amenable to the conjugation of multiple drugs, probes and targeting moieties. HA was functionalized with hydrolysable alkynyl linkers for use in click grafting. The effect of modifications (i.e. amidation, esterification and click reaction) on HA properties was studied and the reaction conditions were optimized to minimize degradation while achieving efficient derivatization. Azido-functional PGG was prepared by ring-opening polymerization of epoxide monomers. Functionalization of PGG hydroxyl groups was explored to establish strategies for conjugation of drugs, probes and targeting molecules. For example, PGG could be efficiently labeled with rhodamine B boronic acid, due to formation of reversible boronic esters with the pendant 1,2-diol moieties. HA-PGG graft copolymers were prepared by copper-mediated azide-alkyne cycloaddition (CuAAC). The synthesized materials were studied under simulated physiological conditions to determine their stability and the cleavage of hydrolysable bonds. The HA backbone was stable during one month of incubation in buffer or vitreous liquid. The polymer-from-polymer release of PGG grafts from the HA-PGG ester copolymer was investigated and the hydrolysis rates were quantified. Hydrolytic cleavage of PGG chains from HA was significantly slower than cleavage of the small molecular weight alkynyl linkers, and was attributed to steric crowding at the ester bond. Hence, graft copolymers with cleavable arms have the potential to achieve longer lasting release than polymeric prodrugs with drugs attached directly to the backbone. The biocompatibility of PGG and HA-PGG copolymers was tested in cell cultures. The materials exhibited similar levels of cell viability as polyvinyl alcohol, which is FDA-approved for ocular applications.Ikääntymiseen liittyvien silmäsairauksien määrä on nousussa väestön keski-iän noustessa. Erityisesti verkkokalvon rappeumasairaudet aiheuttavat enenevässä määrin näön heikkenemistä ja sokeutumista, ja vaikuttavat maailmanlaajuisesti yli 100 miljoonaan ihmiseen. Näitä sairauksia hoidetaan nykyisin kerran kuukaudessa tai kahdessa kuukaudessa tehtävillä injektioilla silmän lasiaiseen. Taajaan tehtävät injektiot lisäävät komplikaatioiden riskiä ja rasittavat potilaita sekä kuormittavat hoitojärjestelmää. Tässä väitöskirjatyössä on tutkittu vesiliukoisten polymeerien käyttöä injektoitavina lääkekuljettimina. Suuren kokonsa vuoksi polymeerikantaja mahdollistaisi lääkeaineiden hitaan ja kontrolloidun vapautumisen ja vähentäisi siten tarvittavien injektioiden määrää. Lääkekuljettimeksi valittu polymeeri, hyaluronihappo, on luonnostaan silmän lasiaisessa esiintyvä polysakkaridi. Työssä hyaluronihappoa muokattiin reaktiivisemmaksi, siihen kiinnitettiin erilaisia toiminnallisia molekyylejä ja lisäksi tutkittiin näiden vapautumista pyrkien samalla säilyttämään hyaluronihapon luontaiset ominaisuudet. Muokattujen hyaluronihappojohdosten ominaisuuksia tutkittiin olosuhteissa, jotka imitoivat olosuhteita silmän lasiaisessa. Hyaluronihappojohdoksilla havaittiin lupaavia ominaisuuksia, kuten bioyhteensopivuus, stabiilisuus testiolosuhteissa ja kiinnitettyjen malliaineiden hidas vapautuminen. Paitsi silmälääkinnässä, kehitetyillä menetelmillä voidaan nähdä sovelluksia myös muiden sairauksien hoidossa