**3. Results**

#### *3.1. pH Value*

The pH value was examined in both undiluted and diluted products, while the goblet cell cultures were treated with diluted products (Figure 1). Of the undiluted eye drops (Figure 1a), Monoprost® had the highest pH value of 6.84 ± 0.032, while Latanest® had the lowest pH value of 6.33 ± 0.003. The pH value of the remaining eye drops was 6.34 ± 0.004 (Gaap Ofteno®), 6.70 ± 0.003 (Xalmono®), and 6.71 ± 0.000 (Xaloptic® Free). Significant differences (*p* ≤ 0.0001) were observed between all eye drops, except between Latanest® and Gaap Ofteno®, and between Xalmono® and Xaloptic® Free. As presented in Figure 1b, the pH value of the diluted formulations was 7.62 ± 0.002 (Monoprost®), 6.89 ± 0.007 (Latanest®), 7.34 ± 0.004 (Gaap Ofteno®), 7.34 ± 0.009 (Xalmono®) and 7.37 ± 0.002 (Xaloptic® Free). Significant differences were observed between the eye drops with *<sup>p</sup>* ≤ 0.001 or *<sup>p</sup>* ≤ 0.0001. Gaap Ofteno® and Xalmono® were not significantly different.

**Figure 1.** The pH value characterization of preservative-free 0.005% latanoprost products Monoprost®, Latanest®, Gaap Ofteno®, Xalmono®, and Xaloptic® Free: (**a**) pH value of the undiluted eye drops; (**b**) pH value of the diluted eye drops (1:7, *v*/*v*). Values are listed as mean ± SD, and *n* = 3. A one-way ANOVA with a Tukey multiple-comparison test (*p* = 0.05) was performed. ns = not significant, with *p* ≥ 0.05, \*\*\* *p* ≤ 0.001, \*\*\*\* *p* ≤ 0.0001.

#### *3.2. Osmolality*

The osmolality was examined in both undiluted and diluted products, while the goblet cell cultures were treated with diluted products (Figure 2). The osmolality of the undiluted Gaap Ofteno® was 325.9 ± 2.9 mosmol/kg and was significantly higher, compared to the four other products (*p* ≤ 0.0001) (Figure 2a). The osmolality of the other eye drops was 275.8 ± 0.7 mosmol/kg (Monoprost®), 278.9 ± 1.3 mosmol/kg (Latanest®), 261.0 ± 1.2 mosmol/kg (Xalmono®) and 261.2 ± 0.3 mosmol/kg (Xaloptic® Free), as visualized in Figure 2a. There were no significant differences between Monoprost® and Latanest® or between Xalmono® and Xaloptic Free®.

**Figure 2.** Osmolality characterization of preservative-free 0.005% latanoprost eye drops Monoprost®, Latanest®, Gaap Ofteno®, Xalmono® and Xaloptic® Free: (**a**) osmolality of the undiluted eye drops; (**b**) osmolality of the diluted eye drops (1:7, *v*/*v*). Values are listed as mean ± SD, and *n* = 3. A oneway ANOVA with a Tukey multiple-comparison test (*p* = 0.05) was performed. ns = not significant with *p* ≥ 0.05, \*\* *p* ≤ 0.01, \*\*\*\* *p* ≤ 0.0001.

Of the diluted eye drops (Figure 2b), Latanest® had the lowest osmolality of 281.9 ± 4.2 mosmol/kg, and Xalmono® had the highest osmolality of 292.7 ± 2.1 mosmol/kg. Monoprost® had an osmolality of 285.3 ± 2.2 mosmol/kg, Xaloptic® Free, osmolality of 288.1 ± 3.5 mosmol/kg, and Gaap Ofteno®, osmolality of 288.9 ± 0.5 mosmol/kg (Figure 2b). Significant differences (*<sup>p</sup>* ≤ 0.01) were observed between Latanest® and Xalmono®. No significant differences were observed among the other eye drops.
