Stimuli-responsive star poly(ethylene glycol) drug conjugates for improved intracellular delivery of the drug in neuroinflammation

N-Acetyl cysteine (NAC) is a vital drug currently under clinical trials for the treatment of neuroinflammation in maternal–fetal applications. The free sulfhydryl groups in NAC lead to high plasma protein binding, resulting in low bioavailability. Preparation and activity of conjugates of NAC with thiol terminated multi-arm (6 and 8) poly(ethylene-glycol) (PEG) with disulfide linkages involving sulfhydryls of NAC are reported. Multiple copies (5 and 7) of NAC were conjugated on 6 and 8-arm-PEG respectively. Both the conjugates released 74% of NAC within 2 h by thiol exchange reactions in the redox environment provided by glutathione (GSH) intracellularly (2–10 mM). At physiological extracellular glutathione concentration (2 µM) both the conjugates were stable and did not release NAC. MTT assay showed comparable cell viability for unmodified PEGs and both the PEG–S–S–NAC conjugates. The conjugates were readily endocytosed by cells, as confirmed by flow cytometry and confocal microscopy. Efficacy of 6 and 8-arm-PEG–S–S–NAC conjugates was evaluated on activated microglial cells (the target cells, in vivo) by monitoring cytokine release in lipopolysaccharide (LPS) induced inflammatory response in microglial cells using the reactive oxygen species (ROS), free radical nitrile (NO), anti-inflammatory activity and GSH depletion. The conjugates showed significant increase in antioxidant activity (more than a factor of 2) compared to free drug as seen from the inhibition of LPS induced ROS, NO, GSH and tumor necrosis factor-alpha (TNF-α) release in microglial cells.

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