Development of Triazoles and Triazolium Salts Based on AZT and Their Anti-Viral Activity against HIV-1

We report herein a set of 3′-azido-3′-deoxythymidine (AZT) derivatives based on triazoles and triazolium salts for HIV-1 infection. The compounds were synthesized via click chemistry with Cu(I) and Ru(II) catalysts. Triazolium salts were synthesized by reaction with methyl iodide or methyl triflate in good yields. The antiviral activity of the compounds was tested using two methodologies: In method one the activity was measured on infected cells; in method two a pre-exposure prophylaxis experimental model was employed. For method one the activity of the compounds was moderate, and in general the triazolium salts showed a decreased activity in relation to their triazole precursors. With method two the antiviral activity was higher. All compounds were able to decrease the infection, with two compounds able to clear almost all the infection, while a lower antiviral activity was noted for the triazolium salts. These results suggest that these drugs could play an important role in the development of pre-exposure prophylaxis therapies.


Introduction
The extraordinary development of therapies for Human Immunodeficiency Virus (HIV) treatment over the last decades has extensively reduced the morbidity and mortality associated with the disease [1]. At present, there are over 30 drugs utilized for the treatment of HIV, divided into five main classes, which target different steps in the viral life cycle: (1) viral entry; (2) reverse transcription; (3) integration; (4) viral maturation [2]. Prophylactic treatments are also available, reducing considerably the risk of infection, either in preexposure and in post-exposure situations [3]. However, current therapies are compromised by the rapid emergence of resistant strains, side effects resulting from extended use of drugs, and their poor bioavailability [4][5][6]. The discovery that HIV-1 continues to replicate even in the patients under anti-retroviral therapy [7,8] has led to an urgent need to develop novel anti-HIV drugs with increased efficiency, minimum side effects, and higher bioavailability. Indeed, recent studies identified a specific T cell subset, the follicular cells (Tfh), as the major source of replication-competent HIV-1 in antiretroviral-treated patients [9,10].
3 -azido-3 -deoxythymidine (AZT, zidovudine) was the first approved drug for the treatment of the Human Immunodeficiency Virus (HIV) [4]. AZT is a nucleoside RT inhibitor, which lacks the 3 -OH group of the deoxyribose moiety and operates as an obligate chain terminator. AZT was initially used as a standalone drug and later as part of the highly active antiretroviral therapy (HAART) drug combination, with lamivudine and abacavir [11]. Long-term use of AZT is associated with a significant number of side obligate chain terminator. AZT was initially used as a standalone drug and later as of the highly active antiretroviral therapy (HAART) drug combination, with lamivu and abacavir [11]. Long-term use of AZT is associated with a significant number of effects, including myopathy, cardiomyopathy, and anemia, among others [5], which re from the ability of AZT to be both a substrate and inhibitor of human thymidine kin AZT can also be used to prevent HIV infection and has been used, for instance, to red mother to child transmission [12].
Appropriate modification of AZT can be used as a tool to increase the efficienc the drug and introduce additional functionalities to further expand its range of acti For example, modifications of AZT within the sugar can activate or block a spe function [13]. The presence of an azide group makes AZT a suitable substrate for fur modification via Huisgen 1,3-dipolar cycloaddition to form triazoles, known to be ac as antivirals in their own right [14]. Accordingly, the synthesis of AZT modified with and 1,5-triazoles can be easily achieved using copper [15] or ruthenium catalysts [16]. possibility has been explored previously. Earlier reports on the antiviral activity of modified with 1,4 triazoles bearing alkyl groups indicated these compounds are effective as antiviral agents for HIV-1 [17,18]. The combination of AZT with HIV inhib via click chemistry was also reported by Danel et al. [19]. Yet, the resulting dim compounds only showed moderate activity against HIV. More recently, Wang e reported the first series of AZT-derived 1,2,3-triazoles with low sub-micromolar activ against HIV-1 [20]. The active AZT derivatives present a 1,5-connectivity at the tria ring, as well as a bulky substituent. Although less effective, 1,4-triazoles with steri demanding substrates also showed anti-viral properties.
Following our work on the modification of nucleosides [21][22][23], herein we repor synthesis of 1,4-and 1,5-AZT derivatives based on triazoles, and their correspon triazolium salts, and examine their activity against HIV-1, utilizing two diffe methodologies. Specifically, we examine the different activities of 1,4-and 1,5-triaz as well as their corresponding triazolium salts, exploiting the possibility of using t compounds to prevent HIV infection.

Scheme 1. Modification of AZT via click chemistry with copper(I) and ruthenium(II) catalysts
In both cases, purification via silica gel chromatography is required. Compound 7 are obtained in good to moderate isolated yields (30-90%, Figure 1). In both cases, purification via silica gel chromatography is required. Compounds 1-7 are obtained in good to moderate isolated yields (30-90%, Figure 1).  The structure of the compounds was confirmed by NMR spectroscopy and MS spectrometry. The formation of the 1,4-and 1,5-substituted triazole is clearly indicated with the presence of a singlet, corresponding to the newly formed CH bond of the triazole ring. For 1,4-triazoles, the CHtrz singlet is located between 8.5-9.0 ppm, while for 1,5triazoles, it resonates at somewhat higher field, between 7.5 and 8.0 ppm.
Aiming to examine if the anti-viral activity is consistent with previous findings, two standard 1,4-and 1,5-phenyl derivatives, 1 and 3, were synthesized. Additionally, compound 6, previously reported to be the most active compound against HIV-1 [20], was synthesized for comparative purposes. We considered, based on the earlier reports on the relation between sterics and antiviral activity, that simultaneous substitution at positions 4 and 5 could be advantageous. Thus, compound 7, bearing two phenyl groups in positions 4 and 5, was synthesized. The synthesis requires the use of the ruthenium catalyst Ru(II)(C5Me5)(PPh3)Cl and is not efficient with copper(I). The formation of the triazole ring in 7 is clearly indicated by the presence of two quaternary carbons at 143.4 and 134.1 ppm in the 13 C{ 1 H} NMR.
Triazoles can be easily functionalized by quaternization of N3 with suitable alkylating agent [24][25][26] to yield the corresponding salts. For the synthesis of triazolium salts, we employed exclusively methylating agents, such as methyl iodide (Alfa Aeser, Kandel, Germany) and methyl triflate (Sigma-Aldrich, MO, USA), for the sake of simplicity. The generated triazolium salts present, in essence, similar properties in terms of sterics, but the solubility of the compounds may vary, due to the presence of a delocalized positive charge at the triazole ring. Additionally, the quaternization can also induce variations in the ability of the molecule to undergo hydrogen bonding, known to be of importance for antiviral activity [27]. The structure of the compounds was confirmed by NMR spectroscopy and MS spectrometry. The formation of the 1,4-and 1,5-substituted triazole is clearly indicated with the presence of a singlet, corresponding to the newly formed CH bond of the triazole ring. For 1,4-triazoles, the CH trz singlet is located between 8.5-9.0 ppm, while for 1,5-triazoles, it resonates at somewhat higher field, between 7.5 and 8.0 ppm.
Aiming to examine if the anti-viral activity is consistent with previous findings, two standard 1,4-and 1,5-phenyl derivatives, 1 and 3, were synthesized. Additionally, compound 6, previously reported to be the most active compound against HIV-1 [20], was synthesized for comparative purposes. We considered, based on the earlier reports on the relation between sterics and antiviral activity, that simultaneous substitution at positions 4 and 5 could be advantageous. Thus, compound 7, bearing two phenyl groups in positions 4 and 5, was synthesized. The synthesis requires the use of the ruthenium catalyst Ru(II)(C 5 Me 5 )(PPh 3 )Cl and is not efficient with copper(I). The formation of the triazole ring in 7 is clearly indicated by the presence of two quaternary carbons at 143.4 and 134.1 ppm in the 13 C{ 1 H} NMR.
Triazoles can be easily functionalized by quaternization of N3 with suitable alkylating agent [24][25][26] to yield the corresponding salts. For the synthesis of triazolium salts, we employed exclusively methylating agents, such as methyl iodide (Alfa Aeser, Kandel, Germany) and methyl triflate (Sigma-Aldrich, St. Louis, MO, USA), for the sake of simplicity. The generated triazolium salts present, in essence, similar properties in terms of sterics, but the solubility of the compounds may vary, due to the presence of a delocalized positive charge at the triazole ring. Additionally, the quaternization can also induce variations in the ability of the molecule to undergo hydrogen bonding, known to be of importance for antiviral activity [27].
Compounds 8-11 were thus synthesized and obtained in low to good yields (13-91%, Figure 2). Their formation was confirmed by NMR spectroscopy and MS. Thus, in the 1 H NMR, and when compared to the corresponding triazoles, a significant downfield shift of circa 1 ppm is observed for the aromatic C-H of the triazole ring for compounds 8-10. This feature is diagnostic of N3 quaternization due to the formation of the positive charge on the nitrogen that is delocalized within the triazole ring, inducing the deshielding of the H trz proton. In addition, a new singlet is observed in the region δ 4.3-4.5 ppm indicative of the presence of the methyl group at N3 for compound 8-11.
Molecules 2021, 26, x FOR PEER REVIEW 4 of 13 Compounds 8-11 were thus synthesized and obtained in low to good yields (13-91%, Figure 2). Their formation was confirmed by NMR spectroscopy and MS. Thus, in the 1 H NMR, and when compared to the corresponding triazoles, a significant downfield shift of circa 1 ppm is observed for the aromatic C-H of the triazole ring for compounds 8-10. This feature is diagnostic of N3 quaternization due to the formation of the positive charge on the nitrogen that is delocalized within the triazole ring, inducing the deshielding of the Htrz proton. In addition, a new singlet is observed in the region δ 4.3-4.5 ppm indicative of the presence of the methyl group at N3 for compound 8-11.

Antiviral Activity
Most of our current knowledge of HIV-1 pathology has been gathered from its effects on cell lines. However, primary T cells isolated from human donors can be distinctively affected by HIV-1 infection. Thus, we determined the ability of the compounds to block HIV-1 replication in human primary CD4 T cells isolated from peripheral blood [28,29]. Thus, peripheral blood mononuclear cells (PBMCs) from healthy human donors were isolated and infected ex vivo with HIV-1 NL4.3 [30]. Two main methods were tested as depicted in Scheme 2. In the first method we tested the anti-retroviral capacity of modified AZT derivatives once HIV-1 replication had already initiated (method one). In the second method we tested the ability of the compounds to act as pre-exposure prophylaxis treatment, treating CD4 T cells with the AZT derivatives for 18 h prior to HIV-1 infection (method two). As readout, the frequency of HIV-1 infected CD4 + T cells was determined by intracellular flow cytometry.
For method one, we incubated HIV-1 infected primary CD4 + T cells with the live/dead fixable Viability Dye eFluor 780 (eBioscience). The frequency of HIV-1 infected primary CD4 + T cells was determined by gating on GFP + live primary CD4 + T cells. The percentage of HIV-1 infected cells was determined in live CD4 T cells, according to the gating strategy outlined in Figure 3. With this methodology, we found that AZT derivatives have modest effects in HIV-1 replication once the primary round of infection has been established. Indeed, only compound 2 and compound 6 were able to decrease infection levels ( Figure 4). We found no significant differences between 1,4-and 1,5triazoles, but these results corroborate the high activity of 6 as previously described [20].

Antiviral Activity
Most of our current knowledge of HIV-1 pathology has been gathered from its effects on cell lines. However, primary T cells isolated from human donors can be distinctively affected by HIV-1 infection. Thus, we determined the ability of the compounds to block HIV-1 replication in human primary CD4 T cells isolated from peripheral blood [28,29]. Thus, peripheral blood mononuclear cells (PBMCs) from healthy human donors were isolated and infected ex vivo with HIV-1 NL4.3 [30]. Two main methods were tested as depicted in Scheme 2. In the first method we tested the anti-retroviral capacity of modified AZT derivatives once HIV-1 replication had already initiated (method one).
In the second method we tested the ability of the compounds to act as pre-exposure prophylaxis treatment, treating CD4 T cells with the AZT derivatives for 18 h prior to HIV-1 infection (method two). As readout, the frequency of HIV-1 infected CD4 + T cells was determined by intracellular flow cytometry.
For method one, we incubated HIV-1 infected primary CD4 + T cells with the live/dead fixable Viability Dye eFluor 780 (eBioscience). The frequency of HIV-1 infected primary CD4 + T cells was determined by gating on GFP + live primary CD4 + T cells. The percentage of HIV-1 infected cells was determined in live CD4 T cells, according to the gating strategy outlined in Figure 3. With this methodology, we found that AZT derivatives have modest effects in HIV-1 replication once the primary round of infection has been established. Indeed, only compound 2 and compound 6 were able to decrease infection levels ( Figure 4). We found no significant differences between 1,4-and 1,5-triazoles, but these results corroborate the high activity of 6 as previously described [20]. Molecules 2021, 26, x FOR PEER REVIEW 5 of 13 Scheme 2. Schematic representation of methodological approach. Human peripheral blood cells were isolated from whole blood through gradient centrifugation [28,29,31]. Primary CD4 + T cells were infected with a GFP-tagged HIV-1 NL4.3 virus for 5 days [30]. In method 1, cells were infected with HIV-1 and then treated with AZT-derivatives. In method 2, primary T cells were pre-treated with AZT-derivatives for 18      We obtained far more promising results with AZT derivatives in our pre-exposure prophylaxis experimental model (method two, Figure 5). In this experimental approach, compounds were added for 18 h prior to infection, washed out in preparation for HIV-1 infection, and then their presence restored after infection (method two, Figure 5). Under these experimental conditions, all compounds tested were able to decrease HIV-1 infection rates by at least one half. Compounds 6 and 7 were able to clear almost all infection. The results obtained for compound 6 highlight that also for pre-exposure prophylaxis this compound is highly effective, consistent with that observed with method one. The most effective compound is compound 7, substituted with phenyl groups at positions 4 and 5, which indicates that the combination of di-substitution and increased bulkiness can lead to an improvement in activity. We obtained far more promising results with AZT derivatives in our pre-exposure prophylaxis experimental model (method two, Figure 5). In this experimental approach, compounds were added for 18 h prior to infection, washed out in preparation for HIV-1 infection, and then their presence restored after infection (method two, Figure 5). Under these experimental conditions, all compounds tested were able to decrease HIV-1 infection rates by at least one half. Compounds 6 and 7 were able to clear almost all infection. The results obtained for compound 6 highlight that also for pre-exposure prophylaxis this compound is highly effective, consistent with that observed with method one. The most effective compound is compound 7, substituted with phenyl groups at positions 4 and 5, which indicates that the combination of di-substitution and increased bulkiness can lead to an improvement in activity. When examining the triazolium salts 8-11 we observed that, as was the case with their corresponding triazole precursors, compounds 8-11 show higher antiviral activity when method two is employed, that is, with pre-exposure of the cells to the compounds prior to infection. This difference in activity between the two methodologies is very evident for compound 8 ( Figure 6). For method one, it can be observed that quaternization decreases antiviral activity for complexes 8-10, when compared to the triazole precursors 4-6, with the only exception being compound 11, which shows a slightly higher activity than that found for triazole 7. For method two, the decrease of antiviral activity upon quaternization is observed for all cases, showing that the formation of the salt is detrimental and does not foster the activity. Indeed, when compared to the triazole precursors, the triazolium salts showed a decreased activity in all cases, as can been seen, for example, for complexes 6 versus 8 and 4 versus 9 (Figure 7). Triazoles are more lipophilic than their corresponding triazolium salts, which is an indication that triazoles are more prone to cross the cell membrane, which could be related to their higher activity. When examining the triazolium salts 8-11 we observed that, as was the case with their corresponding triazole precursors, compounds 8-11 show higher antiviral activity when method two is employed, that is, with pre-exposure of the cells to the compounds prior to infection. This difference in activity between the two methodologies is very evident for compound 8 (Figure 6). For method one, it can be observed that quaternization decreases antiviral activity for complexes 8-10, when compared to the triazole precursors 4-6, with the only exception being compound 11, which shows a slightly higher activity than that found for triazole 7. For method two, the decrease of antiviral activity upon quaternization is observed for all cases, showing that the formation of the salt is detrimental and does not foster the activity. Indeed, when compared to the triazole precursors, the triazolium salts showed a decreased activity in all cases, as can been seen, for example, for complexes 6 versus 8 and 4 versus 9 (Figure 7). Triazoles are more lipophilic than their corresponding triazolium salts, which is an indication that triazoles are more prone to cross the cell membrane, which could be related to their higher activity.  Thus, it can be concluded that, in general, triazolium salts were less effective than their triazole counterparts, showing, that alkylation of the triazole derivatives of AZT tends to decrease antiviral activity.  Thus, it can be concluded that, in general, triazolium salts were less effective than their triazole counterparts, showing, that alkylation of the triazole derivatives of AZT tends to decrease antiviral activity. Thus, it can be concluded that, in general, triazolium salts were less effective than their triazole counterparts, showing, that alkylation of the triazole derivatives of AZT tends to decrease antiviral activity.

Conclusions
Despite the efficacy of AZT, the search for other HIV-1 reverse transcription inhibitors has continued due to AZT poor bioavailability at the sites of HIV-1 replication (lymphoid organs), the emergence of HIV-1 resistant strains, and secondary effects. We described a series of AZT derivatives modified with triazoles that are particularly efficient if provided prior to HIV-1 infection. Under these conditions, compounds were able to decrease infection and two of them, compounds 6 and 7, were able to clear almost all the infection. The high activity found for compound 7 is indicative that substitution with aromatic groups at positions 4 and 5 of the triazole ring leads to an improvement of the antiviral activity. Lastly, we found that triazolium salts are less effective that their triazole precursors, showing, in general, a decrease in antiviral activity. Overall, these results suggest triazoles based on AZT could play an important role not only in HIV-1 treatment but also in pre-exposure prophylaxis therapies.