(C-116) Development of an Environmentally Responsive Selenium-Derived Selenoketal Polymer for Drug Delivery Applications

Recently, there has been a growing interest in developing environmentally responsive biopolymers for drug delivery applications. Among these, thioketals (TK) have emerged as a promising class of polymers due to their facile synthesis and unique sensitivity to Reactive Oxygen Species (ROS), enabling triggered drug release in response to oxidative conditions. However, despite their advantages, TK polymers still suffer from incomplete chain scission during oxidative degradation, limiting their applicability in precise drug release scenarios.

To address this limitation, we focused on selenium-derived selenoketals (SK), which present a potential advancement in oxidative-sensitive biopolymers. Selenium's superior sensitivity to oxidation compared to sulfur offers the prospect of achieving complete polymer chain scission, potentially leading to enhanced drug release kinetics and therapeutic efficacy. Building on the previous research on TK polymers, we investigated the design and synthesis of a novel SK-based polymer system with increased oxidative responsiveness.

In this study, we present the synthesis process and the characterization of the SK-based polymer. We also evaluate its oxidative degradation behavior and compare it with traditional TK-based polymers. The results obtained from this investigation contribute to a deeper understanding of the potential advantages of SK polymers for drug delivery applications, bringing us closer to the development of advanced and precise drug delivery systems.

Polymer Synthesis: The environmentally responsive selenium-derived selenoketal (SK) polymer, known as poly(selenoketal β-amino amide) (PSK-BAA) polycation, was synthesized using a modified thioketal (TK) synthesis protocol. The precursor, selenocysteamine hydrochloride, was protected to obtain SeCTFA (trifluoroacetate-protected selenium) and thiolated CTFA for control purposes. Reduction with tris(2-carboxyethyl)phosphine (TCEP) facilitated polymerization, yielding protected selenoketal and thiolated diamines. Subsequent deprotection with sodium hydroxide (NaOH) prepared the monomers for polymerization, with acryloyl chloride introducing acrylate groups to form the final PSK-BAA and CTFA-based polymers.

Cell Toxicity Assay: The cytotoxicity of SeCTFA and CTFA was assessed using the MC3T3-E1 cell line. Cells were exposed to varying concentrations of the agents dissolved in dimethyl sulfoxide (DMSO), with controls including DMSO alone and no DMSO. The LD50 assay determined cell viability, providing valuable insights into the biocompatibility and safety of these polymer systems for drug delivery applications.

Degradation Study: To evaluate stability and degradation kinetics, nuclear magnetic resonance (NMR) spectroscopy was employed on SeK Linker and TK Linker. Each polymer was placed in separate NMR tubes with deuterated water (D2O) as the solvent, and 10 mM peroxide induced oxidative degradation. NMR measurements were taken immediately after the start point and subsequently every 2 hours until 12 hours after the study began then at 24 and 48 hours to monitor the degradation behavior.

In the degradation study, SeK Linker exhibited greater degradation when exposed to 1 mM peroxide, with a final value of 70% bond persistence compared to TK Linker's 84% bond resistance. Surprisingly, other peroxide concentrations showed the opposite trend, with TK Linker displaying lower percent bond resistance compared to SeK Linker at the final 48-hour time point.

The LD50 cytotoxicity assay results indicated that SeCTFA's cell viability remained comparable to CTFA up to 500 µM concentration. However, at 500 µM, a notable drop in cell viability was observed for SeCTFA, with luminescence from the plate reader decreasing from an average of 1307 to 629 lumens. In contrast, CTFA showed a steady and nearly linear decrease in viability until reaching 2.5 mM concentration.

The LD50 cytotoxicity assay demonstrated that SeCTFA and CTFA initially had similar effects on cell viability. However, the notable drop in cell viability observed for SeCTFA at 500 µM concentration reveals its potential of greater toxicity at higher concentrations. On the other hand, CTFA maintained a more gradual decrease in cell viability. Further investigation is warranted to understand the underlying reasons for the divergent toxicity patterns observed in SeCTFA and CTFA.

In the degradation study, the unexpected trend of SeK Linker displaying greater degradation at 1 mM peroxide, while showing lower degradation at other peroxide concentrations compared to TK Linker, requires careful consideration. The higher susceptibility of SeK Linker to degradation at 1 mM peroxide could be attributed to its unique selenium-based structure, offering a potential advantage for targeted drug delivery under specific oxidative conditions. However, the exact mechanism underlying this phenomenon necessitates in-depth exploration.

Overall, the findings provide valuable insights into the cytotoxicity and degradation behavior of the novel selenium-derived selenoketal polymer (SeCTFA) compared to the control (CTFA) and the sulfur-based thioketal (TK linker). The results indicate promising avenues for future investigation, particularly in exploring the applications of SeCTFA in drug delivery systems and optimizing its synthesis to harness its unique properties effectively. Further research is essential to unlock the full potential of SeCTFA in precise drug delivery and enhance its therapeutic applications while ensuring biocompatibility and efficacy.