A comprehensive literature review in section 2.8.4 showed that a high pH environment can degrade some pharmaceuticals. Results from the study by Yin and co-workers (2017) indicated that pharmaceuticals such as fluoxetine experienced the highest degradation in a high pH environment, while other pharmaceuticals were degraded more in acidic and neutral environments (Yin et al., 2017). Furthermore, the analysis from Table 1 indicated that the degradation of the pharmaceuticals (chosen for this work) varies across the pH spectrum.
The addition of calcium hydroxide to human urine caused a high pH (>12) which degraded each of the selected pharmaceuticals for this work at varying degrees (shown in Figure 12).
The recommended calcium hydroxide dosage of 10 mg mL-1 of fresh urine was an overestimate to account for urine with different compositions, while ensuring there were excess hydroxide ions available for the degradation of the pharmaceuticals (Randall et al., 2016). The exposure of the pharmaceuticals to the high pH environment was for at least 75 days. Since some of the antiretrovirals (ARVs) showed complete degradation after 75 days, the over the counter (OTC) common pharmaceuticals were left for more than 75 days to see
whether complete degradation would occur. Even though the samples were left for a total of 112 days (refer to Annexure B.1.1), the degradation of the NSIADs did not improve over time.
The OTCs showed a degradation range of 8 - 44%, with paracetamol and chlorpheniramine maleate experiencing the lowest and highest degradation, respectively. The range of degradation for the ARVs due to the high pH was 0 - 100%. Abacavir sulfate and nevirapine experienced no degradation, while stavudine, lamivudine, zidovudine and tenofovir experienced complete degradation. The detailed experimental results are provided in Annexure B1. As such, the degradation of the pharmaceuticals investigated for this work due to a high pH (˃12) was not the same for all eleven pharmaceuticals. The difference in the degradation of the pharmaceuticals is attributed to the level of deprotonation of the acid and basic functional groups (such as carboxylic acid, hydroxyl groups and amines) which are present in the structure of the pharmaceuticals (Hapeshi et al., 2010). Thus, the degradation of a pharmaceutical due to a change in pH was dependent on the molecular structure of the pharmaceutical (Yin et al., 2017).
Paracetamol recorded the least degradation of 8% for the OTCs. The amide bond in the structure of paracetamol (shown in Figure 6) is more resistant to hydrolysis with a base, than the carboxylic acid group in the structure of salicylic acid and diclofenac. This is because it could form resonant structures which reinforces the stability of paracetamol. Furthermore, Yang and co-workers (2008) mentioned that there is a significant decrease in degradation for paracetamol in a strong alkaline environment. The study revealed that the degradation of paracetamol decreased significantly when the pH was greater than 9.5 (Yang et al., 2008).
Calcium hydroxide dissociates into calcium and hydroxide ions in an aqueous solution, which means more hydroxide ions would be available at a higher pH (Copeland and Greenberg, 1960). It can be assumed that the low degradation of paracetamol was also attributed to the absence of a catalyst, despite an increase in the concentration of available hydroxide ions which are formed from the dissociation of calcium hydroxide.
Figure 12: High pH results: OTCs sample 1 (A), OTCs sample 2 (B), ARVs sample 1 (C), ARVs sample 2 (D). The respective abbreviations of the pharmaceuticals labelled under each graph are as follows: paracetamol (PARA), salicylic acid (SALI), chlorpheniramine maleate (CHL), diclofenac (DIC), clopidogrel (CLO), stavudine (STA), lamivudine (LAM), zidovudine (ZID), abacavir sulfate (ABA), nevirapine (NEV) and tenofovir (TEN).
Salicylic acid and diclofenac both have carboxylic acids in their groups (shown in Figure 6).
However, salicylic acid is stabilized by the positioning of the carboxylic acid (COOH) group which is close to the phenol group. The carboxyl group is stabilized by resonance, making it resistant to hydrolysis. Contrary, the extra carbon chain between the hydroxyl group and the ring in diclofenac decreases the potential for the ring and the nitrogen to provide resonance, thereby making diclofenac susceptible to hydrolysis. As a result, diclofenac experienced more degradation than salicylic acid. The nitrogen-sulphur ring and the ester in the structure of clopidogrel (refer to Figure 6) are close to each other, resulting in steric hindrance, thereby
providing hydrolysis resistance for clopidogrel. Thus, less degradation was observed for clopidogrel when compared to diclofenac.
The highest degradation for the OTCs was observed from chlorpheniramine maleate (44%).
Although the amine group in chlorpheniramine maleate (shown in Figure 6) is resistant to hydrolysis, it is less resistant than the carboxyl, amide and ester group, which is the cause for chlorpheniramine maleate to experience the most degradation. Lv and co-workers (2015) found that there was an increase in the degradation of chlorpheniramine maleate when the pH was elevated from 8 to 9. This was attributed to an increase in the formation of hydroxide ions. However, further analysis showed that the degradation from the hydroxide ions was less effective compared to the degradation from ozonation (Lv et al., 2015). This may explain why less than 50% of chlorpheniramine maleate was degraded, even though the formation of the hydroxide ions increased when the pH increased. Therefore, the difference in the structures of the pharmaceuticals caused each pharmaceutical to react differently with the hydroxide ions. None of the OTCs experienced a degradation of more than 45%, while most of the ARVs were completely degraded after the 75-day period.
Stavudine, zidovudine and lamivudine have similar structures (shown in Figure 7), thus they behave in an analogous way in a high pH environment. Stavudine and zidovudine both have the furan ring, thiamine ring and the alcohol group. Additionally, zidovudine has an azide group in its structure. Dunge and co-workers (2004) found that the hydrolysis of stavudine and zidovudine is enhanced in an acidic condition rather than an alkaline condition. As a result stavudine did not experience immediate degradation in an alkaline condition. Furthermore, the absence of epoxide formation for zidovudine suggests that the degradation of zidovudine is a result of normal hydrolysis (Dunge et al., 2004). Therefore, it was deduced that the degradation of stavudine and zidovudine observed from the current work was caused by hydrolysis. The structure of lamivudine is similar to that of stavudine and zidovudine, however it has a cytosine ring, oxathiolane ring and an alcohol group. Wang and co-workers (2019) mentioned that the degradation of lamivudine is independent from the initial pH of the solution. It was inferred that the degradation of lamivudine was also due to normal hydrolysis.
Further analysis showed that the degradation of tenofovir (an antiretroviral drug) was almost immediate due to the high pH. This was expected since Golla and co-workers (2016) showed that the degradation of tenofovir increased with an increase in pH, thus tenofovir experienced higher instability in basic conditions (Golla et al., 2016). The degradation of tenofovir is attributed to a P – O in its structure (refer to Figure 7), which undergoes hydrolysis in basic conditions (Berger and Wittner, 1966).
Nevirapine and abacavir sulfate have conjugated double bonds (shown in Figure 7) which gives resonance to the pharmaceuticals. Thus, the pharmaceuticals can stabilize, being resistant to hydrolysis. As a result, the pharmaceuticals did not experience degradation due to the high pH. Therefore, the difference in the pharmaceutical structures lead to the difference in the degradation of the pharmaceuticals. Furthermore, the high pH degradation method conserved 96% of the urea (refer to section 4.6). Overall, the results from the current work build on the existing evidence of the pharmaceutical degradation due to high pH.