Complex coacervation, the self-assembly of oppositely charged polyelectrolytes into spherical droplets, has been extensively studied. This process is driven by entropic counterion release and results in liquid-liquid phase separation. While variations exist, most research focuses on uniformly charged polyelectrolytes. Classical coacervates are most stable within a pH range allowing maximal charge on both polyelectrolytes. However, controlling release characteristics using pH as a trigger is desirable, especially for gastrointestinal (GI) drug delivery. The significant pH gradient in the GI tract (acidic stomach, neutral/basic intestines) presents an opportunity to design systems that protect drugs from gastric degradation and release them in the absorption-efficient small intestine. This study proposes a novel approach using polyzwitterions, which possess both positive and negative charges within the same molecule. The theory behind the approach focuses on the interplay of electrostatic interactions and entropy gain from counterion release. By selecting a polyzwitterion (e.g., poly(2-methacryloyloxyethyl phosphorylcholine), PMPC) with a pH-dependent charge and a polyelectrolyte (e.g., poly(acrylic acid), PAA), the researchers aimed to create a system that complexes at low pH (stomach) and dissociates at higher pH (intestines), thereby protecting and releasing the drug payload at the desired location.

The paper extensively reviews the existing literature on complex coacervation, highlighting the traditional understanding of the process driven by electrostatic interactions and counterion release. It notes the existing limitations of using traditional coacervates for oral drug delivery due to their sensitivity to the pH environment of the stomach. It references studies on various modifications to coacervates such as bio-inspired monomers, block copolymers and chiral patterning, but emphasizes the lack of work done using polyzwitterions for targeted drug delivery. The authors highlight the need for new drug delivery technologies capable of withstanding the harsh acidic conditions of the stomach while releasing their payload in the more alkaline small intestine to enhance drug absorption.

The researchers synthesized a 22 kDa poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) using a free-radical polymerization method. They used commercially available 50 kDa poly(acrylic acid) (PAA). The phase behavior of PMPC-PAA mixtures was investigated by varying the stoichiometric ratios of the two polymers and measuring the turbidity of the resulting solutions at different pH values (2, 3) and temperatures (20-40°C). Turbidity measurements were conducted using a Tecan Infinite 200 Pro instrument at 550 nm. Optical microscopy (Leica DM 2700P) with a single polarizer was used to visualize the morphology of the formed complexes. NMR spectroscopy was employed to confirm the presence of both PMPC and PAA in the complexes. To simulate the physiological conditions of the duodenum, a setup was devised that allowed the controlled introduction of a basic solution (NaOH) to a droplet of the PMPC-PAA complex. Video microscopy (Phantom PCC 3.5 software) was used to monitor the dissolution process. Protein encapsulation was tested using fluorescently labeled bovine serum albumin (BSA). Centrifugation and absorbance measurements were used to assess the protein's localization within the complexes at different pH values. The researchers also performed titrations to determine the effect of pH on complexation and dissociation.

The study found that PMPC and PAA undergo liquid-liquid phase separation at low pH, forming spherical complexes (pZCs). The turbidity measurements revealed a sharp peak at a 30:70 PMPC:PAA ratio at pH 2, indicating that an excess of PAA is needed for complexation. At pH 3, the peak shifted to a 60:40 ratio, suggesting a decrease in PMPC's availability for complexation at higher pH. Microscopy confirmed the spherical morphology of the complexes at low pH, with fewer complexes observed at pH 3 compared to pH 2. NMR experiments confirmed the presence of both PMPC and PAA within the complexes, ruling out self-complexation of either polymer. Importantly, the complexes exhibited pH-dependent behavior, remaining stable at low pH (pH 2) and disassembling at higher pH values (pH > 4). This orthogonal phase behavior was attributed to the pH-dependent ionization of the phosphoryl group in PMPC, which controls the net charge on the molecule. The researchers developed a quantitative model illustrating how at pH < 4 the charge-dipole interaction between the positively charged PMPC and the dipolar PAA promotes complex formation. As the pH increases and the PMPC becomes neutral and dipolar, the weak dipole-dipole interaction cannot support complex stability. The researchers demonstrated the pH-triggered release of the encapsulated BSA protein upon the addition of NaOH in the controlled system mimicking the duodenal environment and confirmed this finding by optical density measurements. This confirmed the potential of pZCs as a pH-sensitive drug delivery platform. Temperature stability studies (20-40°C) showed that the pZCs remained stable over the relevant physiological temperature range.

The findings demonstrate a novel approach to creating pH-responsive drug delivery systems. The orthogonal phase behavior of the pZCs is a key advantage, enabling protection of the drug payload in the acidic stomach and release in the more alkaline intestines. The mechanism of phase separation, based on the interplay of electrostatic interactions and counterion release, is consistent with the behavior of traditional polyelectrolyte complexes but extends this understanding to polyzwitterionic systems. The successful encapsulation and pH-triggered release of a model protein (BSA) showcase the practicality of this approach. This work provides a foundation for designing tailored polyzwitterionic systems with adjustable pH responsiveness by modifying the chemical structure of the constituent monomers.

This study successfully demonstrates the formation of pH-responsive polyzwitterionic complexes (pZCs) from PMPC and PAA, exhibiting orthogonal phase behavior, suitable for gastrointestinal drug delivery. The pH-triggered release of an encapsulated protein validates the system's potential. Future research should explore the use of different polyzwitterions to fine-tune the pH responsiveness and investigate the encapsulation of various drugs and therapeutic agents. Further investigations into the impact of other physiological factors (e.g., ionic strength) on the stability and release of the complexes are also recommended.

The study used a model protein (BSA) for encapsulation. Further research is needed to investigate the encapsulation and release of a wider range of drugs with varying properties and sizes. The in vitro simulation of the duodenal environment may not perfectly represent the complexities of the in vivo GI tract. Additional in vivo studies are necessary to confirm the effectiveness of pZCs as a drug delivery system. The focus was on the pH responsiveness, while other factors such as ionic strength could also play a role in pZC behavior in the GI tract.