An evaluation of hyaluronic acid and poly(L-lysine) polyelectrolyte complexes for intraocular nicotinamide adenine dinucleotide (NAD+) delivery.

Purpose: The treatment of ocular diseases before or at the early onset of their pathogeneses is typically overlooked. Ocular supplementation with NAD+, a co-factor that plays a vital role in cellular longevity, may render cells less susceptible to the deleterious effects of aging. We hypothesis that NAD+-laden polyelectrolyte complexes (NAD+PECs), composed of hyaluronic acid and poly(L-lysine), may be a suitable candidate for supplementary ocular drug delivery application.

Methods: Non-stoichiometric NAD+PECs were formulated via polyelectrolyte complexation. PECs were characterised using dynamic light scattering, Fourier-transform infrared spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. Reverse-phase high performance liquid chromatography was used to quantify both NAD+ complexation efficiency (% CE) and cumulative in vitro release of NAD+ from the PECs (n=3). PEC stability was evaluated via changes in size parameters during storage in various release media over the in vitro release period (n=2). Two-tailed Student’s t-tests and one-way analysis of variance (ANOVA) analyses, coupled with post-hoc comparison tests were used for statistical analyses.

Results: PECs exhibited a size of 235.1 ± 19.0 nm, a zeta potential value of – 38.0 ± 1.1 mV and a % CE of 77.2 ± 9.5%. 10.60 ± 0.99 mg of NAD+ was released from the PECs over 8 hours under infinite sink conditions whereas a longer release duration was demonstrated under physiological ocular flow rates (~ 3 µL/min) using a microfluidic platform (n=3). In vitro NAD+ release from the PECs was statistically slower than that in the control group (p = 0.005). A significant increase in size between the PECs in 0.01 M PBS (pH 7.4) (458.5 ± 10.1 nm) and those in ultrapure water (pH 7.4) (176.8 ± 2.6 nm) was observed after 8 hours (p = 0.001).

Conclusions: Consistent with our hypothesis, NAD+PECs exhibiting optimal size parameters and % CE values were successfully formulated for the first time. Extended in vitro release of NAD+ from the PECs was also demonstrated under ocular volumetric flow rates, further highlighting their suitability for ocular delivery. However, based on the significant increase in PEC size in ionic medium, further modification will be required to enhance intraocular stability and prolong ocular retention at the administration site.