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In, Sohyun, et al. Scientific Reports 9.1 (2019): 13507.
This case study highlights the use of polyquaternium-51 (PQ) as a hydrophilic polymeric zwitterionic counter ion to investigate ion-pairing effects on the skin penetration behavior of the zwitterionic active compound CAR. Zwitterionic molecules typically exhibit limited transdermal transport due to strong polarity and weak lipid affinity. Counter-ion pairing offers an effective strategy to modulate physicochemical properties and improve permeability.
In a controlled formulation study, CAR was combined with PQ at an equimolar ratio (4 wt% CAR basis) and compared with two additional counter ions-betaine (BT) and HSC-representing small hydrophilic and amphiphilic zwitterions, respectively. PQ, with its polymeric structure and strong hydration capability, provided a unique platform to evaluate how macromolecular zwitterions influence ion-pair formation and penetration enhancement.
Experimental results demonstrated that PQ-CAR complexes significantly altered the hydration interface within the stratum corneum, modulating ion-pair stability and affecting diffusion kinetics. Unlike small-molecule BT, PQ contributed steric and hydration effects that reduced lipophilicity enhancement, thereby providing mechanistic contrast across zwitterionic systems. Comparison with amphiphilic HSC further revealed the importance of molecular architecture-particularly hydrophilicity versus amphiphilicity-in overcoming transdermal barriers.
Overall, polyquaternium-51 serves as a valuable hydrophilic zwitterionic counter ion for probing transport mechanisms of zwitterionic drugs in vitro and in vivo. Its use enables deeper understanding of ion-pair-mediated penetration modulation, supporting the rational design of advanced dermal delivery systems.
He, Qidi, et al. Toxics 13.11 (2025): 948.
This case study explores the role of polyquaternium-51 (PQ-51) as a polymeric conditioning agent within mixed-surfactant cleansing systems, particularly its impact on irritation when combined with sodium lauryl ether sulfate (SLES). Surfactant combinations are widely used in cosmetic formulations to synergistically enhance mildness, improve dispersibility, and reduce the overall surfactant load. However, the interaction between polymers and anionic surfactants critically determines the system's dermatological performance.
In a controlled evaluation using a 5% SLES model system, PQ-51 was incorporated at 0.007% to assess its effect on ocular irritation and cellular responses. Animal-model irritation scoring revealed that the SLES-PQ-51 mixture maintained a slightly irritating classification, indicating that PQ-51 neither exacerbated nor substantially mitigated SLES-induced irritation under these conditions. This contrasted with other co-surfactants such as panthenol, MG-60, and Tween-28, which successfully reduced irritation to a non-irritating level.
Complementary SIRC cell viability assays further supported the mild physiological profile of PQ-51. At an equivalent low-level concentration (0.00007% in vitro), the SLES-PQ-51 combination did not significantly alter cellular viability relative to SLES alone, suggesting that PQ-51 does not negatively affect epithelial cell tolerance. Unlike DCG, which reduced viability markedly, PQ-51 preserved cellular metabolic activity.
Overall, polyquaternium-51 demonstrates stable, non-aggravating behavior in SLES cleansing systems, making it a suitable conditioning polymer in formulations where irritation neutrality-rather than irritation reduction-is required.
