Discovery of a Quinoline-Based Phenyl Sulfone Derivative as an Antitrypanosomal Agent

Abstract

A series of natural products-based phenyl sulfone derivative and their property-based analogues were investigated as potential growth inhibitors of Trypanosoma brucei. Trypanosoma brucei is a kinetoplastid protozoan parasite that causes trypanosomiasis. In this work, we found that nopol- and quinoline-based phenyl sulfone derivative were the most active and selective for T. brucei, and they were not reactive towards the active thiol of T. brucei’s cysteine protease rhodesain. A thiol reactive variant of the quinoline-based phenyl sulfone was subsequently investigated and found to be a moderate inhibitor of rhodesain. The quinoline-based compound that is not reactive towards rhodesain can serve a template for phenotypic-based lead discovery while its thiol-active congener can serve as template for structure-based investigation of new antitrypanosomal agents.

Graphical Abstract

Human African Trypanosomiasis (HAT), one of the neglected tropical diseases (NTDs), caused by protozoan Trypanosoma brucei is a declining public health problem on the African continent due to a gradual decrease in the number of reported cases in the past few years. It is most prevalent, at the moment, in the Democratic Republic of the Congo. Historically, the lack of adequate and rapid diagnostic tools as well as lack of effective, safe, and accessible medicines to treat HAT resulted in the death of hundreds of thousands of people. Despite the decrease in reported case, the lack of good network of primary healthcare facilities in most rural and remote places on the continent as well as the possibility of continuous transmission of the parasite from animal reservoirs to humans, make the disease a continuous threat to millions of people.^1–4 Discovery and development of effective oral drugs remains a key objective in combating the disease. In this regard, a promising drug candidate, nitroimidazole fexinidazole, is in the approval stages for the treatment of human African trypanosomiasis. It would be the first approved oral medicine to treat human African trypanosomiasis in several decades. Fexinidazole is also being investigated as a potential treatment for Chagas Disease.^5,6 Despite these recent gains, the drug development pipeline for HAT is sparse and there is need for continued investment and investigation into new chemical entities that can be developed as treatments and/or as prophylactic agents against the disease. Many plant-derived natural products have been reported as antiprotozoal agents. See review by Schmidt and colleagues.⁷ In addition, natural products have been widely explored in anti-infective drug discovery. Most anti-infective agents are natural products-based/inspired.⁸ However, due to the complexity and scarcity of most active agents, follow-up studies are usually difficult and rarely pursued in NTDs drug discovery.

The compounds described in this work were synthesized as outlined in Schemes 1 and 2. For compounds 5 to 25, allyl phenyl sulfone (1) was reacted with bromine to obtain the 1,2-dibromide (2), in good yield (93%). This was followed by dehalogenation of the vicinal dibromide with sodium carbonate in diethyl ether to obtain (E)-((3-bromoprop-1-en-1-yl)sulfonyl)benzene (3). Compounds 4a–c were obtained via etherification reaction between the appropriate 4-hydroxyphenylacetic acids and 3 in ethanol, using potassium hydroxide and sodium iodide. Compounds 4a–c were then used to synthesize the corresponding amides (5–10) and esters (11–25) using CDI or DCC and DMAP as coupling reagents.^9–13 Detailed synthesis and compound characterization data are provided as supporting information.

Scheme 1.

Synthesis of target compounds 5–25.

Scheme 2.

Synthesis of target compound 27.

The compounds were subsequently tested for their ability to inhibit the growth of T. brucei in vitro.¹⁴ The parasites were exposed to the compounds for 48 hours. Most of the compounds displayed selective but moderate growth inhibitory activity against T. brucei when compared with mammalian cells (Hep G2).¹⁵ Compounds derived from 8-aminoquinoline (9), (1R)-nopol (15, 24), 6-bromo-2-naphthol (16), (+) fenchol (21) and 4-benzylphenol (23) were the most active (Table 1). The 8-aminoquinoline-based compound (9), being the most selective, was evaluated for in vivo antitrypanosomal activity. Two groups of T. brucei (STIB795)-infected mice were treated for 4 consecutive days intraperitoneally with 50 mg/kg/day and 100 mg/kg/day of 9, respectively.¹⁶ The infected mice were positive for parasites 24 hours posttreatment, suggesting that compound 9 lack in vivo efficacy. Several generations of aminoquinoline-based compounds have found clinical use in the treatment of malaria but not in the treatment of trypanosomiasis.¹⁷ This is perhaps due to the unique mechanism of action of aminoquinolines in plasmodium-infected cells. However, there are increasing reports of quinoline-based growth inhibitors of trypanosomes, although, the mechanism of action of the quinoline-based compounds have not been deciphered.^18–21

Table 1.

The antitrypanosomal activities of compounds 5–27.

	R1	R2	R3	T. brucei IC50	Hep G2 IC50
5	H	H		11.72 ± 0.83	>20
6	H	H		10.77 ± 0.31	>20
7	H	H		>20	>20
8	H	H		>20	>20
9	H	H		0.76 ± 0.11	>80
10	H	H		5.45 ± 0.20	>20
11	H	H		7.16 ± 0.42	>20
12	H	H		5.62 ± 0.65	>20
13	H	H		>20	>20
14	H	H		>20	>20
15	H	H		2.01 ± 0.12	>80
16	H	H		2.18 ± 0.25	>80
17	H	H		6.04 ± 0.03	>20
18	H	H		>20	>20
19	H	H		>20	>20
20	H	H		5.63 ± 0.61	>20
21	H	H		4.04 ± 0.01	11.9 ± 1.03
22	H	H		>20	>20
23	H	H		1.47 ± 0.40	>80
24	Cl	H		3.04 ± 0.07	>80
25	H	F		7.03 ± 0.17	>20
27				5.97 ± 0.12	>80
			Suramin	0.004 ± 0.001	n/a
			Podophyllotoxin	n/a	0.008 ± 0.0003

The presence of the vinyl sulfone moiety in 5–25 suggests that they are potential covalent inhibitors of trypanosoma cysteine proteases. Compounds 5–25 were then tested for inhibitory activity against the major cathepsin L protease in T. brucei, rhodesain.

Rhodesain is a validated drug target and it is known to be essential for the survival and infectivity of the parasite. Its role in the ability of the parasite to proliferate has been extensively investigated.^22–24 None of the compounds displayed noteworthy inhibition of the protease at 20 μM. The inactivity of the compounds may be because of the proximity of the vinylic Michael acceptor to the phenoxide oxygen in 5–25. It is also possible that the compounds are just unable to adopt favorable orientation at the active site of the protease. Nevertheless, a quinoline-based thiol reactive structural variant of 9 was synthesized and tested for protease inhibition, and for trypanocidal activity. Compound 27 was synthesized from boc-protected (E)-5-phenyl-1-(phenylsulfonyl)pent-1-en-3-amine (26). Compound 26 was a generous gift from Prof. J Love (University of British Columbia), and it was reported by Kiemele and co-workers in 2016.²⁵ Compound 27 was able to completely inactivate rhodesain at 20 μM after 1 hour of incubation with estimated IC₅₀ value of 800 nM, and a K_inact/K_i value of 99 M⁻¹s⁻¹ (Figure 1).²⁶

Figure 1.

Pseudo-first order inhibition plots for compound 27.

It has a moderate antitrypanosomal activity with an IC₅₀ value of 5.97 uM. Compound 27 was also tested for inhibitory activity against human cathepsin L, but it was inactive (from 0.1 μM to 125 μM).²⁶ Crystallographic investigation of rhodesain-inhibitor (27) complex has been attempted but it has not been successful. However, it is still being pursued. In order to understand the interactions responsible for the inhibitory activity of 27 on rhodesain, template docking was used.²⁷ Compound 27 was docked in the previously reported crystal structure of rhodesain using the bound ligand (D1R) as template.²⁸ Three of the top five docking poses suggests that the vinyl group is in the vicinity of active thiol (Cys25), while the homophenyl moiety occupies the P1 site, and the quinoline moiety occupies the P2 site (Figure 2a and 2b). The phenyl sulfone moiety is predicted to have steric interactions with Gln19 and His162 while the quinolyl motif have steric interactions with Met68. Quite noticeable is the empty P3 site.

Figure 2.

a. The superposition of docked compound 27 (Yellow) and D1R (Blue) at the active site of rhodesain; b. The interaction plot of compound 27 with active site residues.

In conclusion, a series of phenyl sulfone natural products-based compounds were synthesized and evaluated as potential antitrypanosomal agents. Quinoline- and nopol-based compounds, 9 and 15, were the most active and selective. The quinoline-based compound (9) can serve a template for phenotypic-based lead discovery and the thiol-active compound (27) may serve as template for structure-based investigation of new covalent antitrypanosomal agents.