*4.8. Stability Studies*

Table 7 shows the results of stability studies of optimized rivastigmine-loaded NLC formulations, where it can be seen that although the NLC exhibited sustained drug release (Section 4.7), after 90 days of storage at different temperatures the particle size, PDI, and ZP values showed slight increases, while the EE value showed a slight decrease. These results suggest that both optimized rivastigmine-loaded NLC formulations are stable during storage and fulfil the QTPP for nasal administration. This high stability is related to the presence of Tween® 80 and Phospholipon® 90 G, which stabilize NLC via distinct mechanisms (steric and electrostatic, respectively) and also to the presence of vitamin E, which has antioxidant activity that provides chemical stability to rivastigmine and prevents oxidation of the lipid matrix [31,34,56,57,63]. Besides, it has been described that NLC formulations with ZP values close to |30| mV show high long-term stability [105]. Nonetheless, stability studies should be performed for longer periods to confirm these data.

Some authors have reported similar results for the long-term stability of NLC formulations. For example, Huang et al. developed three NLC formulations containing co-encapsulated quercetin and linseed oil, which showed high stability over 90 days of storage at 25 ◦C. The initial particle size values were 89.2, 91.3, and 95.6 nm, while the initial EE values were 95.9%, 94.5%, and 93.6%. After 90 days, small increases in the particle size (<7 nm) and EE (<3%) were observed. In addition, the NLC co-encapsulated with quercetin and linseed oil showed sustained drug release [106]. Gadhave et al. developed a NLC formulation for intranasal delivery of teriflunomide, with a particle size of 99.82 nm, ZP of −22.29 mV, and EE of 83.39%. Accelerated stability studies performed over 6 months at 40 ◦C and 75% relative humidity showed that the evaluated parameters were within acceptable limits, without suffering significant changes, indicating the good stability of the NLC formulation. In addition, teriflunomide-loaded NLC also showed a sustained drug release profile [31]. Garg et al. carried out stability studies on an optimized aceclofenac-loaded NLC formulation over 90 days of storage. On the production day, the aceclofenac-loaded NLC showed a particle size of 230 nm and PDI of 0.16. After storage at three different temperatures (2–8 ◦C, 2 ◦C and 60% relative humidity, and 40 ◦C and 75% relative humidity), no significant changes were observed in these values, with the respective particle sizes being 228.3, 239.8, and 251.1 nm; and with respective PDI values of 0.21, 0.27, and 0.33. This NLC formulation also showed sustained release of about 80% aceclofenac after 48 h [104]. Cavalcanti et al. optimized a zidovudine-loaded NLC, which showed high stability and sustained drug release. The optimized formulation had a particle size of 266 nm, PDI of 0.168, and ZP of −29 mV. After 45 days of storage at 4 ◦C, the formulations maintained their physical stability, without showing significant changes. Additionally, in vitro release studies showed 100% drug release from the NLC after 45 h [107]. Jojo et al. prepared a NLC formulation for intranasal delivery of pioglitazone, which had a particle size of 211.4 nm, PDI of 0.257, ZP of 14.9 mV, and EE of 70.18%. Stability studies were performed for 90 days at 4 and 25 ◦C, and no significant changes were observed in the investigated

parameters. In addition, the pioglitazone-loaded NLC also showed a biphasic pattern, with an initial fast drug release followed by sustained drug release, reaching about 50% release after 24 h [108].


**Table 7.** Results of the stability studies of rivastigmine-loaded nanostructured lipid carriers (NLC) formulations prepared by ultrasound technique and high-pressure homogenization (HPH) method.

Results are presented as mean ± SD (*n* = 3); NLCS: rivastigmine-loaded NLC produced by ultrasound technique; NLCHPH: rivastigmine-loaded NLC produced by HPH; <sup>1</sup> Temperature; <sup>2</sup> Volume distribution: D50 and D90; <sup>3</sup> Z-Ave: mean particle size; <sup>4</sup> PDI: polydispersity index; <sup>5</sup> ZP: zeta potential; <sup>6</sup> EE: encapsulation efficiency.
