(M1330-07-39) Photochemical Reaction Mechanism and Kinetics of Nitric Oxide (NO) Release from Solid S-nitrosothiol

Purpose: Nitric oxide (NO) is a very potent pulmonary vasodilator, and inhalation NO (iNO) is used in standard care for persistent pulmonary hypertension of the newborn (PPHN). NO is also an effective antimicrobial agent and an FDA-approved orphan drug for treating cystic fibrosis (CF) and helps treat a variety of other pulmonary diseases (chronic obstructive pulmonary disease, chronic rhinosinusitis, and acute respiratory distress syndrome). Its wider application is hindered by its low storage stability and the costly and bulky gas cylinder-based delivery systems. On-demand low-cost NO generation is needed to expand its therapeutical use. The stable solid S-nitrosothiol (RSNO) can liberate NO when exposed to suitable wavelengths of light. Thus, RSNO can be used as a molecular reservoir for NO and potentially exploited to establish tank-less and very portable inhalable NO delivery. We have devised a controlled, light-induced NO generation technology from solid-state RSNO doped gas permeable polymer substrate [1,2]. However, the mechanism of the photolytic release of NO from the solid RSNO is not well understood, which motivated this study.

Methods: The photo-chemical NO release kinetics from S-nitroso-N-acetyl penicillamine (SNAP) and the yield of NO release were investigated by exposing solid-state RSNO to the light of narrow-band LEDs with different principal wavelengths (340, 385, 470 nm) and light intensity in the presence and absence of O₂. The light penetration depth experiments were examined using optical microscopy after the photolysis experiment. Decomposition products of SNAP were analyzed using NMR, UPLC-MS, UV-Vis spectroscopy, FTIR spectroscopy, and EPR spectroscopy. The changes in the morphology of SNAP were examined using PXRD and SEM.

Results: Among the assessed wavelengths, the 340 nm induced the fastest initial release rate. A maximal light penetration depth (200 µm) through the solid layer of SNAP was achieved with 470 nm light (Figure 1). The solid-state decomposition product was observed to be a stable thiyl radical (RS^·) along with the disulfide (RSSR) (Figure 2). This solid thiyl radical (RS^·) can react with the O₂ content of air (Figure 3) to form several sulfonyl and sulfonyl peroxyl radicals {RS(O)_xO^·, x=0 to 7} but it did not react with NO to reform SNAP.

Conclusion: The wavelength of light used for photochemical liberation of NO from solid RSNO has a critical role in the achievable penetration depth in the solid state, and thus on the available NO payload. The released NO molecule leaves after photolysis of solid S-nitroso-N-acetyl penicillamine (SNAP) matrix without recombination with the stable solid thiyl radicals. However, these thiyl radicals can readily react with molecular oxygen to form sulfonyl and sulfonyl peroxyl radicals. These findings are important for the design of the novel photochemically based on-demand NO controlled-release technology.

References: [1] Lautner, G.; Stringer, B.; Brisbois, E. J.; Meyerhoff, M. E.; Schwendeman, S. P. Nitric Oxide - Biol. Chem. 2019, 31–37.
[2] Lautner, G.; Lautner-Csorba, O.; Stringer, B.; Meyerhoff, M. E.; Schwendeman, S. P. J. Control. Release 2020, 264–269.

Acknowledgments: We thank the funding agency- the National Institute of Health (NIH), NIH Grant No. 1R01EB028775-01 for financial support.

Figure 1. Light penetration depth in SNAP tablet in air- (A) 340 nm, 40 mW/cm2; (B) 340 nm, 80 mW/cm2; (C) 385 nm, 40 mW/cm2; (D) 385 nm, 80 mW/cm2; (E) 470 nm, 40 mW/cm2; and (F) 470 nm, 80 mW/cm2.



Figure 3. Characterization of the photolyzed products of SNAP using UPLC-MS.



Figure 2. Characterization of the photolyzed products of SNAP using electron spin resonance (EPR) spectroscopy.