Oral Delivery of Therapeutic Proteins Exhibiting High Isoelectric Points in pH-Responsive Hydrogels

Oral Delivery of Therapeutic Proteins Exhibiting High Isoelectric Points in pH-Responsive Hydrogels

Michael C. Koetting and Nicholas A. Peppas, The University of Texas at Austin, Department of Chemical Engineering, Austin, TX

Introduction: Therapeutic delivery of proteins via intravenous injection is an invasive and often painful method which has been shown to cause low patient compliance with doctor-recommended treatment.  Delivery via the oral route is strongly preferred to avoid the pain and embarrassment of injection and therefore increase patient compliance and improve quality of life.  Environmentally-responsive hydrogel systems have shown great promise as drug delivery vehicles to overcome the obstacles of oral delivery.  Our group has previously developed copolymer hydrogel delivery vehicles—notably poly(methacrylic acid) grafted with poly(ethylene glycol), or P(MAA-g-EG)—for the oral delivery of insulin.  These hydrogels protect the protein through the gastrointestinal tract and release the protein in the small intestine by swelling at the higher pH found in the intestine.

            Our current work is focused on creating a pH-responsive copolymeric hydrogel delivery platform for high isoelectric point-exhibiting proteins, specifically salmon calcitonin which is used to treat osteoporosis and Paget's disease.  Calcitonin displays very different properties than insulin, primarily a high isoelectric point of 8.86 compared to 5.39 for insulin.  Since roughly half of the human proteome consists of proteins with similarly high isoelectric points, the ability to orally deliver these many compounds will be a significant accomplishment toward making protein therapy cost effective, minimally invasive, and clinically desirable.  In this work, we investigate potential hydrogel delivery systems to develop an effective oral delivery platform for achieving high bioavailability with salmon calcitonin.

Methods and Results: pH-Responsive hydrogels were synthesized by UV-initiated free radical polymerization in 0.7 mm thick films.  The hydrogels were composed of either methacrylic acid (MAA) or itaconic acid (IA) copolymerized with either N-vinyl pyrrolidone (NVP) or poly(ethylene glycol) methyl ether methacrylate (PEGMMA).  The hydrogels' swelling characteristics were determined in equilibrium and dynamic swelling studies, with all gels demonstrating a collapsed network at low pH (pH 2.0) and a significantly swelled network at neutral pH (pH 7.4) that is suitable for protection of encapsulated protein through the acidic, proteolytic stomach environment and for diffusive release in the neutral environment of the small intestine.  Protein delivery capabilities were tested in vitro by loading 90-150 μm microparticles (from crushing/sieving of dried films) with salmon calcitonin by imbibition in solutions of varying ionic strength.  Calcitonin was then released in a pH 7.4 solution of 0.150 M PBS.  Loading and release levels were determined by HPLC or MicroBCA assay as appropriate.  Hydrogels displayed significantly improved delivery capability when loaded in low ionic strength solutions, achieving up to 15.8 times as much calcitonin released per unit of hydrogel compared to when loaded in standard 0.150 M PBS.  Additionally, hydrogels based on methacrylic acid, specifically poly(methacrylic acid-co-N-vinyl pyrrolidone) or P(MAA-co-NVP), achieved improved delivery levels compared to gels based on itaconic acid.  Together, the platform achieves delivery levels that would suggest its suitability as a therapeutic oral delivery platform for salmon calcitonin.

Conclusions:  pH-Responsive hydrogels were synthesized and screened as candidates for oral delivery of the therapeutic protein salmon calcitonin, as well as similar high isoelectric point-exhibiting proteins.  Loading and release studies demonstrated that hydrogels based on methacrylic acid as the pH-responsive moiety offer greater delivery capabilities than itaconic acid-based hydrogels.  Importantly, it was also shown that the ionic strength of the loading solution strongly affects both loading and release levels, with low ionic strength loading solutions offering significantly improved delivery capabilities in vitro.  Therefore, we have shown improved delivery capability achieving levels sufficient for clinical therapy. 

Acknowledgment:     This work was supported by grant 1R01-EB000246-19 from the National Institutes of Health.