Original 2-(3-Alkoxy-1H-pyrazol-1-yl)azines Inhibitors of Human Dihydroorotate Dehydrogenase (DHODH)

Following our discovery of human dihydroorotate dehydrogenase (DHODH) inhibition by 2-(3-alkoxy-1H-pyrazol-1-yl)pyrimidine derivatives as well as 2-(4-benzyl-3-ethoxy-5-methyl-1H-pyrazol-1-yl)-5-methylpyridine, we describe here the syntheses and evaluation of an array of azine-bearing analogues. As in our previous report, the structure–activity study of this series of human DHODH inhibitors was based on a phenotypic assay measuring measles virus replication. Among other inhibitors, this round of syntheses and biological evaluation iteration led to the highly active 5-cyclopropyl-2-(4-(2,6-difluorophenoxy)-3-isopropoxy-5-methyl-1H-pyrazol-1-yl)-3-fluoropyridine. Inhibition of DHODH by this compound was confirmed in an array of in vitro assays, including enzymatic tests and cell-based assays for viral replication and cellular growth. This molecule was found to be more active than the known inhibitors of DHODH, brequinar and teriflunomide, thus opening perspectives for its use as a tool or for the design of an original series of immunosuppressive agent. Moreover, because other series of inhibitors of human DHODH have been found to also affect Plasmodium falciparum DHODH, all the compounds were assayed for their effect on P. falciparum growth. However, the modest in vitro inhibition solely observed for two compounds did not correlate with their inhibition of P. falciparum DHODH.


■ INTRODUCTION
As explained in more detail in our previous report, 1 in the course of a screening campaign of new chemical entities 2−11 against infectious agents, compounds 1 and 2 were found active on our whole cell measles virus replication assay ( Figure 1). 12 An initial structure−activity study led to confirmation of the potential of this original chemotype and to greatly improved antiviral compounds, including the 2-(3-isopropyloxy-1H-pyrazol-1-yl)pyrimidine derivative 3, which displayed a subnanomolar MIC 50 on this measles virus replication assay ( Figure 1). Moreover, a search for the biochemical mechanism of action of this series pointed out that, as for other recently reported compounds, 13−18 the inhibition of the cellular dihydroorotate dehydrogenase (DHODH) is at the origin of the antiviral effect. 1

■ RESULTS AND DISCUSSION
From these results, in order to increase the chemical diversity possible in this series, we initially replaced the pyrimidine ring by a pyridine moiety because its additional carbon would provide another position for structure−activity studies. Accordingly, as in our first report, 1 we are describing here a succession of synthesis campaigns followed by comments on the antiviral effect obtained. Most of these results are presented in tables in which the first result column (%) provides the yield of the reaction depicted and the second column describes the observed antiviral effect expressed as pMIC 50 values. 19 This corresponds to the negative log of the minimum compound concentration required to inhibit viral growth by 50% when using a recombinant measles virus strain expressing a luciferase as a reporter. 20 We first undertook the preparation of the 2-pyridyl derivatives 6a−t depicted in Table 1. As compound 2, these analogues were prepared from the 3ethoxypyrazole 4 10 and commercially available 2-halogenopyr-idines 5a−p. From 2-halogenopyridines featuring electronattracting substituents, the use of cesium carbonate in dry dimethylformamide or acetonitrile and heating at 150°C with a microwave oven for 1 h efficiently gave the N-pyridyl derivatives. On the other hand, from 2-fluoropyridines featuring electron donating substituents, a temperature of 180°C (under pressure) was found necessary and, because of its decomposition into dimethylamine, dimethylformamide was replaced by acetonitrile. For the preparation of compound 6a 7 or 6p, we used 2bromopyridine 5a or 5p and Taillefer and Cristau coppercatalyzed arylation conditions. 21 Moreover, as for compound 1, the regioselectivity of these reactions was unambiguously checked in a couple of instances. 7 The 5-cyclopropyl bearing derivative 6q was prepared in 48% yield via a Suzuki−Miyaura reaction between cyclopropyl boronic acid and the 5-bromo derivatives 6j. Alternatively, as described in the Experimental Section, the 2fluoropyridines 5q−s were prepared via Suzuki−Miyaura reactions between cyclopropyl boronic acid and the corresponding 5-bromo precursors. From them, the analogues 6q−s were then obtained by N-arylation of 4. The N-arylation of compound 4 with 2-(6-fluoropyridin-3-yl)propan-2-ol (5t) gave the hydroxylated analogue 6t, and a reduction of its alcohol function led to the 5-isopropyl derivative 6u. Interestingly, the palladium-catalyzed hydrogenation at room temperature of 6t was mostly inefficient. To avoid the use of high hydrogen pressure, which would probably have also resulted in the hydrogenation of the pyridine ring, 7 we used triethylsilane in the presence of trifluoromethanesulfonic acid and achieved its hydrogenation into 6u in an unoptimized 24% yield. Concerning the antiviral effects, as seen in Table 1, the "methyl scan" leading to compounds 2 and 6b−d, pointed out the importance of occupying the position 5 of the pyridine ring for a tangible antiviral effect (pMIC 50 = 6.2 for compound 2). The lack of substituents or the introduction of a small fluorine atom seen in 6a, 6e, and 6f also gave almost inactive compounds although some improvement can be observed when comparing the antiviral effect of compound 6a (pMIC 50 < 5) and the 3-fluorinated derivative 6e (pMIC 50 = 5.2). The recourse to a trifluoromethyl (compound 6g) or a chlorine atom (compound 6i) on position 5 gave slightly more active analogues (both pMIC 50 of 5.5), but the bigger bromine present in compound 6j did not improve this further (pMIC 50 = 5.3). On the other hand, the combination of the 3-fluoro and 5bromo substituents seen in compound 6k appeared to be the cause of slight synergic effect (pMIC 50 = 5.9) when considering the pMIC 50 of the parent monosubstituted compounds 6e and 6j (respectively 5.2 and 5.3). In contrast to a fluorine group, the 3methyl substituent of 5-brominated derivative 6l led only to a weak antiviral effect. A loss of effect was also seen for the three derivatives 6m, 6n, and 6o featuring respectively a cyano, an ester, or a methyl ketone on position 5. To a lesser extent, a similar phenomenon was observed with the methoxy of compound 6p (pMIC 50 = 5.5). It is only the introduction of the cyclopropyl group featured by compound 6q that led to an improved inhibition of the virus replication (pMIC 50 = 7.0 nM). Combination of this group with the fluorine of compound 6r led to an unchanged antiviral effect (pMIC 50 7.0), whereas the nitrile of compound 6s replacing this fluorine caused a loss (pMIC 50 = 6.6). Interestingly, the branched isopropyl derivative 6u turns out to be an order of magnitude less active (pMIC 50 = 5.9) than the cyclopropyl analogue 6q. Finally, the polar hydroxyl moiety of compound 6t (pMIC 50 5.3) led to an even bigger loss of effect in comparison with the hydrogen-bearing isopropyl group of compound 6u.
As for the pyrimidine-containing series, 1 by arylation of the 4aryloxypyrazoles 7a−q with the 2-fluoropyridine 5q, we prepared  the 4-aryloxy derivatives 8a−q depicted in Table 2. Later on, we also prepared the corresponding pyridazine homologues 10b−q from 3-chloro-6-cyclopropylpyridazine (9). For comparison purposes, Table 2 provides the pMIC 50 for these two series.
With few variations, the pattern of antiviral effect for analogues 8a−q featuring a 5-cyclopylpyridine is somehow mirroring the one seen for the 5-ethylpyrimidyl bearing homologues. 1 In comparison with the phenoxy-bearing compound 8a (pMIC 50 = 7.0), ortho-substitution with a fluorine, a chlorine of a bromine atom improved the antiviral effect (pMIC 50 of respectively 7.7, 7.9, and 7.8). A trifluoromethyl group instead of these halogens had a lesser effect (compound 8e, pMIC 50 = 7.3). Shifting this trifluoromethyl group on the meta position of the phenoxy ring did not alter much the antiviral effect (pMIC 50 = 7.1 for 8f), whereas placing this group on the para position led to a substantial loss (pMIC 50 = 5.3 for 8g). A similar trend is observed with the fluorine atom of compounds 8k−m, the ortho-fluoro derivative 8k being the most active (pMIC 50 = 7.4) and the para-fluorinated analogue 8m far less effective (pMIC 50 = 5.9). Trends in the polyhalogenated derivatives are somehow less clear-cut although ortho-substituted derivatives are the most active but, as in the pyrimidine series, 1 with a pMIC 50 of 9.0, the 2,6-difluorophenoxy derivative 8q is the best antiviral of this group of analogues. Very similar comments can be made on the antiviral effects of compounds 10b−q, and the differences between any given pair of analogues is always less than an order of magnitude. As depicted in Scheme 1, we also explored the preparation and use of precursors featuring a 5-cyclopropylpyridine moiety to which additional chemistry would allow the introduction of various group on the carbon 4 of the pyrazole ring. The N-arylation of the 4-iodopyrazole 11a 5 with 5-cyclopropyl-2-fluoropyridine (5q) had to be conducted at 180°C for up to 12 h and only led to a rather small amount (13%) of the 4-iodinated target compound 12a along with some (9%) of the reduced derivative 12b. This is quite in contrast with the 55% yield we reported for the N-arylation of compound 11a with 2-fluoropyridine at the same temperature for 3 h. 7 Extensive analysis of the reaction mixture pointed out the presence of a large amount of 3-ethoxy-5-methylpyrazole (11b) resulting from the reduction of the 4-iodopyrazole 11a under this long reaction time. Although an explanation for this reduction is not obvious (a carbene occurrence?), we suggest that the much decreased reactivity of the electron rich 5-cyclopropyl-2fluoropyridine (5q) toward nucleophilic reaction in comparison with 2-fluoropyridine is allowing the necessary time for this side reaction to proceed to a large extent. Somehow related to such behavior is the quick evolution of iodine, or iodochloride, upon heating 3-ethoxy-4-iodopyrazole in hydrochloric acid. 5 In any case, the iodinated target compound 12b was treated with butyllithium followed by the addition of benzoyl chloride to give the 4-benzoyl derivative 13 in 44% yield. In an attempt to improve these results, we undertook the N-arylation of the 4-bromopyr- azole 11c. Under similar conditions, we could obtain the 4brominated precursor 12c in a much improved 48% yield. The sodium borohydride reduction of ketone 13 led to the secondary alcohol 14a, whereas the addition of methyl lithium to ketone 13 provided an access to the tertiary alcohol 14b. From compound 12c, palladium-catalyzed reactions allowed the preparation of few analogues. A Suzuki−Miyaura reaction with phenylboronic acid gave 33% of compound 15, whereas Negishi reactions with benzylzinc bromide or (1-phenylethyl)zinc chloride gave compounds 6q and 16 in, respectively, 71 and 42%. Of note in this part is an optimization of the preparation of 1-phenylethylzinc chloride using the lithium chloride, dibromoethane, and trimethysilyl chloride-based protocol 22−24 as well as the use of CPhos instead of XPhos in an attempt to lessen the extent of βelimination possible with (1-phenylethyl)zinc chloride. 25 Concerning the antiviral effects of these compound, as for the pyrimidine-containing series, 1 the benzoyl derivative 13 (pMIC 50 = 6.9) was slightly less effective than the corresponding methylene analogue 6q (pMIC 50 = 7.0). On the other hand, the alcohol 14a retained a more sizable antiviral effect (pMIC 50 = 7.4) but the gain is far from what we could observe in the case of the pyrimidinecontaining series. 1 Introducing an additional methyl on this carbon led the tertiary alcohol 14b and to a much reduced antiviral activity (pMIC 50 = 6.1). A similar pattern was seen when adding a methyl to compound 6q (pMIC 50 = 7.0), leading to the much less effective analogue 16 (pMIC 50 = 5.2). Finally, the low antiviral effect of the 4-phenyl derivative 15 (pMIC 50 = 6.0) pointed out the importance of a bridge between the pyrazole and the aromatic ring. As depicted in Table 3, we then retained the 2,6difluorophenoxy component of compound 8q and prepared the 2-pyridyl derivatives 18a−s. Analogues 18a−n were obtained by the N-arylation of the pyrazole 7q by the corresponding 2halogenopyridines 5a, 5g, 5j−o, and 5q−t and the 2fluoropyridines 17l−n (prepared as described in the Experimental Section). In the case of the methoxy-bearing analogue 18p, we used again the 2-bromo-5-methoxypyridine (5p) and Taillefer− Cristau copper-catalyzed arylation conditions. 21 The volatile 2bromo-5-(1,1-difluoroethyl)pyridine (17n) was obtained from 2bromo-5-acetylpyridine (5o). The following N-arylation of compound 7q with this 2-bromopyridine 17n had to be conducted at a relatively lower temperature to avoid extensive decomposition, and the pure difluoromethylene-bearing analogue 18n was thus obtained in a modest 12% yield. A rather slow catalytic hydrogenation of the cyclopropyl ring of compound 18d enabled an access to the propyl analogue 18o in a 30% yield. The preparation of analogues 18p−r turned out to be possible as, under basic conditions, the fluorine atom on the pyridine ring of 18d could be displaced by methanol, dimethylamine, or benzylalcohol using high temperature and a microwave oven. In the last case, the catalytic hydrogenation of the resulting 3-benzyloxyderivative 18r led to the 3-hydroxypyridinyl derivative 18s. Unexpectedly, despite few trials, the direct hydrolysis of compound 18d into 18s, using sodium hydroxide in THF, was not successful. As seen in Table 3, with these compounds we could assess the effectof varying the pyridine substituents while retaining a 2,6-difluorophenoxy moiety. Going from the hydrogen of the N-pyridine derivative 18a (pMIC 50 = 4.9) to the 5-methyl homologue 18b (pMIC 50 = 7.6), a 400-fold improvement of the antiviral effect was observed. From then, the 5-cyclopropyl group of compound 8q (pMIC 50 = 9) provided another order of magnitude. On the other hand, the ethyl group of compound 18l (pMIC 50 = 7.1) caused a loss of antiviral effect. This is actually reminiscent of what was observed for the 5isopropyl bearing compound 6u (pMIC 50 = 5.9) in comparison with the 5-cyclopropyl bearing analogue 6q (pMIC 50 = 7.0) described above. Moreover, it is contrasting with what we previously reported for the 5-ethyl and 5-cyclopropyl pyrimidine analogues which had mostly the same antiviral effect(pMIC 50 =9.2 and 8.9). 1 Replacement of the methyl group of compound 18b (pMIC 50 = 7.6) by the trifluoromethyl of compound 18c (pMIC 50 = 6.5) led to more than a 10-fold loss. If adding a fluorine atom on carbon 3 of the pyridine ring has very little effect when comparing the 5-ethyl bearing analogues 18l and 18m (pMIC 50 both of 7.1), a 2-fold loss was seen in comparison with the 5-cyclopropyl pair 8q and 18d (pMIC 50 = 9 and 8.6). On the other hand, as for the pair of analogues 6j and 6k, a fluorine on the same position is improving the antiviral effect when comparing the activity of the 5-bromo derivatives 18f and 18g (pMIC 50 = 6.3 and 6.9). Attempts to introduce various polar groups on carbon 5 of the pyridine ring such as the one seen in compound 18i−k did not improve the antiviral effect, and the two fluorine atoms of compound 18n (pMIC 50 = 7.1) had no effect in comparison with the 5-ethyl analogue 18l (pMIC 50 = 7.1). Only relatively small losses were observed for the bis-substitued derivatives 18p−s featuring a cyclopropyl group. Interestingly, the influence of the methoxy moiety of compound 18p (pMIC 50 = 7.5) is comparable to the dimethylaminated derivative 18q (pMIC 50 = 7.5), whereas the hydroxyl-bearing analogue 18o (pMIC 50  relatively stronger activity. Somehow unexpectedly, the larger benzyloxy group of compound 18r (pMIC 50 = 7) only caused a relatively small loss of antiviral effect. In any case, none of the analogues described in Table 3 displayed an antiviral effect better than the nanomolar level of compound 8q. As depicted in Figure 2 and fully described in the Experimental Section, from the 2,6-difluorophenoxy pyrazole 7q, some other possible azines-bearing compounds such as 3-pyridyl and 5pyrimidyl, the 2-pyrazinyl and the 1,2,4-triazine derivatives were also prepared. The 3-pyridyl bearing compounds 19a−c were made by the Taillefer−Cristau 21 copper catalyzed N-arylation of 7q with the commercially available 5-bromo-2-methyl, 5-bromo-2-ethyl, and 5-bromo-2-methoxypyridines. The 5-pyrimidyl derivative 20a was also made with the copper catalyzed reaction between 7q and 5-bromo-2-tert-butylpyrimidine. For the preparation of the 5-cyclopropyl analogue 20b, we had recourse to the N-arylation of compound 7q with a small sample of 2cyclopropylpyrimidin-5-ylboronic acid using Lam and Chan reaction conditions. 26 The N-arylation of 2,6-difluorophenoxy pyrazole 7q with 2,5-dibromopyrazine led to the 5-bromopyrazine analogue 21a. Similarly, from 2-chloro-5-(trifluoromethyl)pyrazine, the 5-trifluoromethyl analogue 21b was obtained. The methoxy-bearing analogue 21c was then made from 21a by displacement of the bromine atom with methanol under basic conditions. As described in the Experimental Section, the preparation of 2-bromo-5-cyclopropylpyrazine allowed the synthesis of the cyclopropyl-bearing analogue 21d by N-arylation. In similar ways, the pyridazine-bearing analogues 10q and 22a−b were prepared from the corresponding chloropyridazines. The methoxy or ethoxy-bearing analogues 22c−d were made by displacement of the chlorine atom of compound 22b with the corresponding alcohols because an N-arylation trials using 3chloro-6-methoxypyridazine led to extensive side reactions, including, as seen by LC/MS, a methylation of compound 7q. The 1,2,4-triazine-bearing analogue 23 was prepared in a modest 10% yield by the N-arylation of compound 7q with 6-ethyl-3-(methylthio)-1,2,4-triazin-5-ol (preparation given) at 200°C. Finally, as fully described in the Experimental Section, we also prepared the N-ethyl imidazole-bearing analogues 24 by the copper catalyzed N-arylation of compound 7q with 1-ethyl-4iodo-1H-imidazole. In view of the antiviral effect of the azinebearing analogues 8q, 10q, 19b, and 21d, one fact clearly stands out. As for the 2-pyrimidyl analogues previously studied, an ethyl or a cyclopropyl group para to the pyrazole ring often provides a low nanomolar antiviral effect. However, this is not true for the 2cyclopropylpyrimidine derivative 20b (pMIC 50 = 6.8) or the triazine 23 (pMIC 50 = 7.6), which is in this case featuring an additional hydroxyl group. Attempt to replace such alkyl groups by side chains of similar size but featuring more polar groups mostly failed, as seen for the 2-methoxypyridine derivative 19c (pMIC 50 = 6.0), the 2-methoxypyrazine 21c (pMIC 50 = 6.1), or the 3methoxypyridazine 22c (pMIC 50 = 7.1). The same trend was observed for the halogen-bearing analogues 21a−b (pMIC 50 = 5.5 and 5.3) or 22b (pMIC 50 = 6.8). Finally, the relatively low antiviral effect of the N-ethylimidazole derivative 23 may be another example of the trend which points at a detrimental effect of polar atoms (a nitrogen in this case) in the vicinity of this alkyl side chain.
The elucidation 1 of the biochemical target of our series has led us to publish a survey of all the reported inhibitors of DHODH along with their uses. 27 This review pointed out that teriflunomide (25) depicted in Figure 3 is the only human DHODH inhibitor used in medicine against autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. Interestingly, in our cellular assay, teriflunomide (25) displayed an antiviral effect with an MIC 50 of 5 μM, which is reflected in the previously reported IC 50 of 1 μM on recombinant human DHODH. 28,29 This relatively modest effect of teriflunomide (25) on the enzyme had actually triggered the search and the discovery of some off-target inhibitions in the past. 30−32 This value can also be compared to the enzymatic IC 50 of 10 nM reported for brequinar (26), 29 a stronger inhibitor of DHODH which underwent disappointing phase II trials against solid tumors. 33−37 To assess the potential of our series in comparison with these compounds, we undertook an array of biological assays using compound 18d. In cellulo, as depicted in Figure 4, we could point out that compound 18d is affecting pyrimidine nucleoside  biosynthesis. Indeed, while adding 18d at concentration varying from 4 to 100 nM blocked the measles virus replication in cells, the addition of the pyrimidine-containing nucleoside uridine at 10 μg/ mL ( Figure 4A) restored its replication. On the other hand, the addition of the purine-containing nucleoside guanosine at 10 μg/ mL did not restore this ( Figure 4B). Moreover, a restored virus replication was seen with the addition of orotic acid at 3 mM ( Figure 4C) while, as seen in Figure 4D, dihydroorotic acid at 3 mM had no such effect. These last results thus narrowed down the biochemical target of compound 18d to DHODH. Accordingly, as reported, 1 we produced recombinant human DHODH and compound 18d was indeed found to be an inhibitor of this enzyme with an IC 50 of 25 ± 5 nM.
By using a metabolite analysis protocol, 38 the HEK-293 T cells content in adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP), and uridine triphosphate (UTP) treated for 24 h with various concentration of compound 18d could be determined. As seen in Table 4, intracellular concentrations of uridine and cytidine collapsed in cells treated with 18d, whereas purine nucleotides concentrations were slightly increased, likely as a consequence of the control loops connecting purine and pyrimidine metabolic pathways. This strongly demonstrates in cell cultures the inhibition of de novo pyrimidine biosynthesis by 18d.
We recently reported that the inhibition of pyrimidine biosynthesis amplifies cellular response to pathogen-associated molecular patterns such as exogenous RNA molecules. 17 When transfecting cells with small synthetic RNA molecules (ssRNA) that mimic viral RNA genomes or transcripts, activation of the interferon-stimulated response element (ISRE) that drives innate immunity genes was enhanced by DHODH inhibition. As depicted in Figure 5, we thus monitored that compound 18d increased the expression of an ISRE-luciferase reporter gene when transfecting cells with ssRNA molecules. This adds to the panel of cellular assays that support the inhibition of pyrimidine biosynthesis by compound 18d.
Aside from their antiviral effect, pyrimidine biosynthesis inhibitors are also well-known for blocking the proliferation of lymphocytes, and this probably accounts for their immunosuppressive property in vivo. 39 Accordingly, we also determined the effect of compound 18d on the growth of Jurkat T lymphocyte cell line, which has been demonstrated to be sensitive to DHODH inhibitors. 40,41 As shown in Figure 6, compound 18d strongly inhibited Jurkat cells proliferation (IC 50 = 0.02 μM) and its level of inhibition compared favorably with brequinar (IC 50 = 0.2 μM) or teriflunomide (IC 50 close to 60 μM, data not shown).
Also of much interest is the recent demonstration that Plasmodium falciparum DHODH is a valid target for the treatment of Malaria 42−46 and that a dual inhibition of human and P. falciparum DHODH was noticed for some series. 47−50 Accordingly, we screened all these antiviral compounds for a potential inhibition of P. falciparum growth. No growth inhibition was seen in a cellular assay at the concentration of 8.66 μg/mL (data not shown) for all the compounds aside from a modest effect for the pyridazine-bearing analogues 10q and 22a (IC 50 of 0.33 and 0.47 μM). Of course, these values led us to prepare many more pyridazine-bearing analogues, including compound 10b−p, but all of them turned out to be at least an order of magnitude less effective on the parasite growth. Just in case, compound 10q and 22a were also assayed for their eventual inhibition of recombinant P. falciparum DHODH but they turned out to be completely devoid of effect on this biochemical assay.

■ CONCLUSION
In conclusion, along with our previous report, 1 this study made good use of the alkoxypyrazole chemistry we previously reported, 2−11 which somehow cleared a sort of "chemical blind spot" existing in pyrazole chemistry. The ensuing screenings of the resulting new chemical entities led us to extensively explore the structure−activity relationship of a new series of human DHODH inhibitors. Of note is a cellular antiviral assay which greatly simplified the evaluation of the compounds prepared and probably filtered out analogues of low cell membrane permeability. Concerning the potential of this series of inhibitors against immune diseases, the current success of teriflunomide (25) 51,52 has led to renewed interest in the search for better inhibitors of   (26), are featuring a carboxylic acid function, we are currently working on the selection of compounds displaying optimal preclinical properties in order to secure a proof of effect on an animal model of autoimmune disease. This endeavor is based on the reasonable assumption that our carboxylic acid-free series of inhibitors could display a different and beneficial pharmacological profile in comparison with the inhibitors currently in preclinical or clinical studies. However, so far, our best inhibitors can be only considered as tools as a too short microsomal stability has been a recurrent feature for this series. Indeed, low half-life values were observed for compounds such as 8k (t 1/2 = 4 min) or 8q (t 1/2 = 6 min) on human microsomes and only modest improvements were seen for the "fluorine-protected" compound 18d (t 1/2 = 27−41 min), the "hydroxy-protected" compound 18s (t 1/2 = 38 min), or the less active analogues 22c (t 1/2 = 22 min) and 18g (t 1/2 = 60 min). Related to this aspect is a report mentioning similar difficulties for another type of N-arylated pyrazoles. 62 Accordingly, additional work will be required to improve this and the X-ray based determination of the binding mode of this series of inhibitors to human DHODH could be very useful in this regard. Finally, these results are probably an illustration of the interest of designing whole cell/phenotypic assays of sufficient sensitivity (a bioluminescent virus in the present case). 12 This led us to detect an unexpected biological effect and provided a simple biological mean not only to undertake further structure−activity study but also to find the mode of action of this series of compounds.

■ EXPERIMENTAL SECTION
Measles Virus Inhibition Assay. HEK-293T cells (ATCC) were maintained in Dulbecco's Modified Eagle's Medium (DMEM; Gibco-Invitrogen) containing 10% fetal calf serum (FCS), penicillin, and streptomycin at 37°C and 5% CO 2 . Antiviral activity of compounds was determined using a recombinant vaccine strain of measles virus expressing firefly luciferase (rMV2/Luc) from an additional transcription unit. 20 To determine the MIC 50 , HEK-293T cells were infected with rMV2/Luc (MOI = 0.1) and incubated in 96-well culture plates at 3 × 10 +4 cells/well with increasing concentrations of compounds or DMSO alone. After 24 h, luciferase expression was determined. The MIC 50 corresponds to the concentration of a compound inhibiting luciferase activity by 50%.
P. falciparum Growth Assay. Compounds were screened against P. falciparum 3D7 strain parasites with a starting parasitemia of 0.8% at 2% hematocrit in 96-well plates. Compound concentrations were 8.66 μg/ mL final in the assay. Parasite growth and proliferation was measured using SYBR Green I. 63 Human DHODHs. The assay was performed as described previously. 1 P. falciparum DHODHs. This enzyme was expressed as recombinant proteins in Escherichia coli and purified as previously described. 44,64 Steady-state kinetic analysis was performed using the 2,5-dichloroindophenol (DCIP)-based spectrophotometric method as described. 44 Enzyme and substrate concentrations were: DHODH (E = 5−10 nM) and substrates (0.2 mM L-dihydroorotate and 0.02 mM CoQd). The 100× compound stock solutions were made in DMSO by covering a 3-fold dilution series such that final concentrations in the assay for inhibition ranged from 0.001−100 μM.   (26). Jurkat cells were incubated with increasing doses of 18d or brequinar. As a control, cells were treated with DMSO alone. At t = 0 and after 72 h of culture, the number of living cells was determined using the CellTiter-Glo reagent. The inhibition of cellular proliferation is expressed as a percentage relative to DMSO-treated control wells. The results presented correspond to the mean ± SD of two independent experiments.
Metabolite Analyses. HEK-293T cells were plated in 6-well plates (10 6 cells per well). One day later, culture medium was supplemented with increasing doses of compound 18d or DMSO alone. After an additional 24 h of culture, cells were harvested, carefully counted, and monitored for viability by trypan blue exclusion. Cellular nucleotides were quantified as previously described. 17 ISRE-Luciferase Reporter Assay. STING-37 cell line was previously described and corresponds to HEK-293 T cells that express luciferase under control of five interferon-stimulated response elements (ISRE). 17 Cells were transfected with increasing amounts of ssRNA molecules using JetPrime PEI reagents (Polyplus transfection) and dispensed in 96-well plates at 35000 cells/well in 100 μL of Dulbecco's Modified Eagle's Medium (DMEM; Gibco-Invitrogen) containing 10% fetal calf serum (FCS), penicillin, and streptomycin. DMSO or compound 18d was added to culture medium, and after mixing, cells were cultured for 24 h at 37°C and 5% CO 2 . Finally, firefly luciferase activity was determined using the Bright-Glo reagent following manufacturer's recommendations (Promega).
Inhibition of Jurkat Cells Proliferation. Jurkat cells were cultured in at 5 × 10 4 cells per well in flat-bottom 96-well culture dishes. Cells were maintained at 37°C and 5% CO 2 in RPMI (Gibco-Invitrogen) containing 10% fetal calf serum (FCS), pyruvate sodium, nonessential amino acids, penicillin, and streptomycin. Number of living cells was determined by quantification of adenosine triphosphate (ATP) in culture wells using the CellTiter-Glo Assay (Promega) following manufacturer's recommendations. This luciferase-based assay evaluates by ATP quantification the number of metabolically active cells in culture wells.
Human Microsome Stability. The microsomal stability assessments were subcontracted to Oroxcell, Romainville, France.
Chemistry. A Biotage Initiator 2 microwave oven was used for reactions mentioning such heating method. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 spectrometer at 400 and 100 MHz, respectively. Shifts (δ)are givenin ppmwith respect to the TMS signal, and coupling constants (J) are given in hertz. Column chromatography were performed either on Merck silica gel 60 (0.035− 0.070 mm) or neutral alumina containing 1.5% of added water using a solvent pump and an automated collecting system driven by a UV detector set to 254 nm unless required otherwise. Sample deposition was carried out by adsorption of the mixture to be purified on a small amount of the solid phase followed by its deposition of the top of the column. The low resolution mass spectra were obtained on an Agilent 1100 series LC/MSD system using an atmospheric electrospray ionization system and the high resolution mass spectra (HRMS) were obtained using a Waters Micromass Q-Tof with an electrospray ion source. Unless stated otherwise, a purity of at least 95% was obtained for all the compounds by means of chromatography, recrystallization, or distillation and this level of purity was established by TLC, LC/MS and NMR spectroscopy Preparations of All the Commercially Unavailable Halogenoazines Used in This Work. 5-Cyclopropyl-2-fluoropyridine (5q). Under an inert atmosphere, 5-bromo-2-fluoropyridine (5j) (17 g, 0.096 mol) and cesium carbonate (114 g, 0.35 mol) were dispersed in a 95/5 vv mixture of toluene and water (500 mL). This was degassed by gently bubbling argon in the reaction and cyclopropyl boronic acid (10 g, 0.11 mol) and [1,1′-bis(diphenyl phosphino)ferrocene] dichloropalladium complexed with dichloromethane (1.45 g, 0.0018 mol) was added. The flask was heated to reflux for 40 min; upon cooling, the suspension was diluted in ethyl acetate, and the filtered organic layer was washed with water and brine, dried over magnesium sulfate, and concentrated to dryness. The residue was purified by a distillation at ambient pressure (E 760 mm = 206°C) to give a volatile oil (8.06 g, 62%). A lesser pure fraction (about 7%) was also collected. 1  5-Cyclopropyl-2,3-difluoropyridine (5r). By using the procedure described for the preparation of 5-cyclopropyl-2-fluoropyridine (5q), this compound was obtained from 5-bromo-2,3-difluoropyrimidine (5k) in 63% yield, as a volatile oil, after a chromatography over silica gel (cyclohexane−dichloromethane 2/1). 1  2-Chloro-5-cyclopropylnicotinonitrile (5s). By using the procedure described for the preparation of 5-cyclopropyl-2-fluoropyridine (5q), this compound was obtained from 5-bromo-2-chloronicotinonitrile in 53% yield, as a solid, after a chromatography over silica gel (cyclohexane− dichloromethane 1/1). 1  2-(6-Fluoropyridin-3-yl)propan-2-ol (5t). Under an inert atmosphere, 5-bromo-2-fluoropyridine (5j) (3.12 g, 17.7 mmol) was dissolved in dry ether (100 mL) and the solution cooled to −78°C. Butyl lithium (9.3 mL, 18.6 mmol, 2N in cyclohexane) was added. The resulting precipitate was stirred at −78°C for 15 mn, acetone (0.4 mL, 90 mmol, dried over 4 Å molecular sieves) was added, and the reaction was allowed to warm back to room temperature for 30 mn. This was diluted with a saturated solution of ammonium chloride and extracted with ethyl acetate. The organic layer was washed with a saturated solution of ammonium chloride and brine, dried over magnesium sulfate, and concentrated to dryness. The residue was further purified by a chromatography over silica gel (dichloromethane−ethanol 98.5/1.5) to yield compound 5t as an oil (0.57 g, 20%). 1  2-Fluoro-5-ethylpyridine (17l). Under an inert atmosphere, 5-bromo-2-fluoropyridine (5j) (5.35 g, 30.39 mmol) and potassium carbonate (16.8 g, 121.59 mmol) were dissolved in dry dimethylformamide (75 mL, dried over 4 Å molecular sieves). Oxygen was removed from this solution by a slow stream of argon, and [1,1′-bis(diphenylphosphino)ferrocene] dichloropalladium complexed with dichloromethane (0.62 g, 1.52 mmol) was added, followed by a 1 M solution of triethylborane (40 mL, 40.4 mmol) which was slowly added with a syringe. This darkening suspension was heatedat 85°C for 4 husing an oil bath. The resulting black suspension was diluted in diethyl ether and water, the organic layer was washed with water 5 times and brine, dried over magnesium sulfate, and cautiously concentrated to dryness to take into account the volatility of the reaction product. The residue was purified by a chromatography over silica gel (cyclohexane−dichloromethane from 2/3 to 1/4) to yield the 5-ethyl derivative 17l as a volatile oil (1.52 g, 40%). 1  2,3-Difluoro-5-ethylpyridine (17k). By using the protocol described for the synthesis of 2-fluoro-5-ethylpyridine (17l), compound 17k was obtained in 17% yield from 5-bromo-2,3-difluoropyridine (5q) as a volatile oil after a chromatography over silica gel (cyclohexane− dichloromethane from 1/1 to 1/4). 1  Another fraction of this chromatography led, after a recrystallization in cyclohexane, to 5,5′,6,6′-tetrafluoro-3,3′-bipyridine in a 25% yield as fine needles. 1  2-Bromo-5-(1,1-difluoroethyl)pyridine (17n). In a closed Teflon flask, 1-(6-bromopyridin-3-yl)ethanone (5o) (0.95 g, 4.75 mmol) and bis(2-methoxyethyl)aminosulfur trifluoride (3.6 g, 11.4 mmol) were stirred for 7 days (as only a 78% conversion was monitored by 1 H NMR, a longer reaction time may lead to an even better yield). This was diluted in water, solid calcium chloride was added, and the solution was extracted with ethyl acetate. The organic layer was washed with water and brine, dried over magnesium sulfate, and concentrated to dryness (not for too long, this compound is volatile). The residue was purified by a chromatography over silica gel (cyclohexane −dichloromethane 1/1) to yield the difluorinated pyridine 17n as a colorless and volatile oil (0.57 g, 54%). 1  2-Bromo-5-cyclopropylpyrazine. Under an inert atmosphere, 2,5dibromopyrazine (0.21 g, 0.88 mmol) and cesium carbonate (1.12 g, 3.43 mmol) were dispersed in a 95/5 v/v mixture of toluene and water (5 mL). Note: potassium carbonate works as well. This was degassed by gently bubbling argon in the reaction, and [1,1′-bis(diphenylphosphino)ferrocene] dichloropalladium complexed with dichloromethane (0.018 g, 0.022 mmol) and cyclopropyl boronic acid (0.098 g, 1. 14 mmol) was added. The flask was heated to reflux for 30 min; upon cooling, the suspension was diluted in ethyl acetate, the filtered organic layer was washed with water and brine, dried over magnesium sulfate, and concentrated to dryness. This was residue was purified by a chromatography over silica gel (cyclohexane−ethyl acetate 97/3 → 9/1 to yield the 2-bromo-5-cyclopropylpyrazine (0.08 g, 45%) as a solid. 1  3-Chloro-6-cyclopropylpyridazine (9). By using the same procedure used for the preparation of 2-bromo-5-cyclopropylpyrazine, this compound was obtained in 40% yield as a solid (on a much larger scale), from 3,6-dichloropyridazine, after two consecutive chromatography processes over silica gel (dichloromethane−ethanol 99/1) and the second using only dichloromethane to yield the 2-bromo-5-cyclopropylpyrazine. 1  6-Ethyl-3-(methylthio)-1,2,4-triazin-5-ol. First step. Preparation of 2-(2-carbamothioylhydrazono)butanoic acid: As previously described, 66 2oxobutyric acid (3.15 g, 0.0308 mol) and thiosemicarbazide (2.81 g, 0.0308 mol) were stirred in water (60 mL) at 70°C for 10 min. This was left to cool, and the precipitate was filtered, washed with water, and dried at 50°C under vacuum to give the hydrazone (4.47 g, 82%) as 4/17 mixture of isomers. 1  Preparation of 6-ethyl-3-mercapto-1,2,4-triazin-5-ol: As previously described, 66 the hydrazone (4.24 g, 0.0269 mol) and sodium carbonate (2.56 g, 0.0269 mol) were heated to reflux in water (300 mL) for 3 h. The solution was made acid with acetic acid and extracted with ethyl acetate. The organic layer was washed with brine, dried over sodium sulfate, and concentrated to dryness to yield the 1,2,4-triazin-5-ol as a white powder (3.09 g, 86%). 1  Methyl iodide (1.07 mL, 0.017 mmol), diluted in tetrahydrofuran (2 mL), was then slowly added. The solution was stirred at room temperature for 4 h; this was diluted with water, made acid with acetic acid, and extracted with ethyl acetate. The organic layer was washed with brine, dried over magnesium sulfate, and concentrated to dryness to yield the pure thioether as a white powder (2.28 g, 85%). 1  1-Ethyl-4-iodo-1H-imidazole.
Step 1: Under a moisture-protected atmosphere, a 50/50 mixture of 4,5-diiodo-1H-imidazole and 2,4,5triiodo-1H-imidazole, obtained when iodinating imidazole using the described procedure, 67 and ethyl iodide (2.14 g, 0.0137 mol) potassium carbonate (3.97 g, 0.0286 mol) were stirred in dimethylformamide (50 mL, dried over 4A molecular sieve) for 22 h. This was diluted in water, extracted with ethyl acetate, and the organic layer was washed with water and brine, dried over magnesium sulfate, and concentrated to dryness. The residue was purified by a chromatography over silica gel (dichloromethane−ethanol 99/1 to 97/3) to give the 1-ethyl-4,5diiodo-1H-imidazole (1.79 g, 37% from imidazole). 1  Step 2: Under argon, 1-ethyl-4,5-diiodo-1H-imidazole (1.8 g, 0.0051 mol) was dissolved in dry tetrahydrofuran (20 mL). The solution was cooled to 0°C, and ethyl magnesium (1.8 mL, 0.0054 mol, 3 M solution in ether) was added. The resulting suspension was stirred for 30 min, quenched with an excess of ammonium chloride in water, and extracted with ethyl acetate. The organic layer was washed with water and brine, dried over magnesium sulfate, and concentrated to dryness. The residue was purified by a chromatography over silica gel (dichloromethane−ethanol 98/2) to give the 1-ethyl-4-iodo-1H-imidazole (0.78 g, 68%) as an oil. 1  N-Arylation of 3-Alkoxypyrazoles without Copper Catalyst, General Method. In a reaction vial designed for microwave heating, the considered alkoxypyrazole (2 mmol), the considered halogenated heteroaryl (2.2 mmol), and cesium carbonate (2.8 mmol) were stirred in dimethylformamide or acetonitrile (3 mL) as specified. This was heated using a microwave at a temperature between 120 and 180°C for the specified duration. The resulting suspension was diluted in water and extracted with ethyl acetate, and the organic layer was washed with brine, dried over magnesium sulfate, and concentrated to dryness. The residue was further purified as specified below.