淮北大白鹅养殖基地:Alkaloids of Dichroa febrifuga. I. Isolation and Degradative Studies

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★★★★ABSTRACT

Febrifugine is the active principal isolated 50 years ago from the Chinese herb chang shan (Dichroa febrifuga Lour), which has been used as an antimalarial in Chinese traditional medicine for more than 2,000 years. However, intensive study of the properties of febrifugine has been hindered for decades due to its side effects. We report new findings on the effects of febrifugine analogs compared with those of febrifugine extracted from the dry roots of D. febrifuga. The properties of the extracted febrifugine were comparable to those obtained from the standard febrifugine provided by our collaborators. A febrifugine structure-based computer search of the Walter Reed Chemical Information System identified 10 analogs that inhibited parasite growth in vitro, with 50% inhibitory concentrations ranging from 0.141 to 290 ng/ml. The host macrophages (J744 cells) were 50 to 100 times less sensitive to the febrifugine analogs than the parasites. Neuronal (NG108) cells were even more insensitive to these drugs (selectivity indices, >1,000), indicating that a feasible therapeutic index for humans could be established. The analogs, particularly halofuginone, notably reduced parasitemias to undetectable levels and displayed curative effects in Plasmodium berghei-infected mice. Recrudescence of the parasites after treatment with the febrifugine analogs was the key factor that caused the death of most of the mice in groups receiving an effective dose. Subcutaneous treatments with the analogs did not cause irritation of the gastrointestinal tract when the animals were treated with doses within the antimalarial dose range. In summary, these analogs appear to be promising lead antimalarial compounds that require intensive study for optimization for further down-selection and development.

★★★★ INTRODUCTION
Plasmodium falciparum and P. vivax are dominant pathogens of the most severe parasitic disease, malaria, which causes 300 million to 500 million clinical cases and more than 3,000 deaths each day worldwide (34). Control of the disease mostly relies on drug therapies, but most currently available antimalarials have been used for decades, with the rapid emergence of widespread drug resistance (23). The urgent need for treatments for malaria is largely dependent on the development of novel antimalarials from new and existing lead compounds. Several compounds identified decades ago, such as tryptanthrin and fosmidomycin, have been rediscovered and have been found to have remarkable antimalarial activities and other functions in clinical medicine (27). Likewise, febrifugine {(2'R,3'S)-3-[3-(3-hydroxy-2-piperidinyl) acetonyl]-4(3H) quinazolinone} is the active principle extracted from a Chinese medicinal plant, chang shan (Dichroa febrifuga Lour), that has been prescribed in traditional Chinese medicine for more than 2,000 years. The medical use of chang shan was described in an ancient Chinese pharmacopoeia (17) for the treatment of malaria and stomach cancer and as an expectorant, emetic, and febrifuge, with side effects of nausea and vomiting (10). Febrifugine was extracted (7) and was later identified as a quinazoline derivative with the molecular structure C16H21O3N3 (15). The purified febrifugine displayed potent antimalarial activity and was 100 times as active as quinine against P. lophurae in duck models and 50 times as active as quinine against P. cynomolgi infection in rhesus monkeys (12, 13). Severe gastrointestinal injury, however, was also observed in a chicken model when the chickens were overdosed (8, 29). Clinical studies of both the crude extract and isolated forms of febrifugine conducted in Yunnan Province, People's Republic of China, from the 1940s through the 1960s showed that it had excellent antipyretic and antiparasitic effects, similar to those of quinine (7, 16). Unfortunately, the emetic effects and gastrointestinal irritation caused by the compound rendered it insignificant for further investigation. During the 1960s, a series of febrifugine derivatives, including halofuginone {7-bromo-6-chloro-3-[(3-hydroxy-2-piperidinyl)-2-oxopropyl]-4(3H)-quinazolinone} (28), were synthesized by the U.S. Army Medical Research Command with the aim of identifying novel antimalarials with clinical profiles better than those of febrifugine. Some of these febrifugine derivatives were briefly screened in studies with mice right after the derivatives were submitted to the Walter Reed Army Institute of Research (WRAIR) Chemical Information System (CIS). None of the screened compounds was found to be curative for P. berghei-infected mice (2). Further study of these derivatives discontinued a few decades ago because of funding shortages and a lack of appropriate research approaches to support detailed in vivo and in vitro measurements.

Recently, state-of-the-art biomedical technology has proven that chemical modification of febrifugine decreases its toxicity, while its antiparasitic efficacy remains unchanged (11, 31). For example, it was found that the administration of febrifugine analogs did not cause changes to body or liver weights, irritation of the gastrointestinal tract, or alteration of the levels of hepatic enzyme markers in infected mice (6, 20). Febrifugine altered the production of nitric oxide and tumor necrosis factor alpha in mouse macrophages (19). These studies indicate not only that febrifugine is a plausible antimalarial lead for the search for new analogss but also that it possesses unique antimalarial mechanisms. We therefore selected a group of febrifugine analogs that have been stored in WRAIR CIS for years to reevaluate their potential antimalarial activities and possible toxicities. The synthesis and chemical characterization of the analogs were partially published before (2). This report summarizes the new data that we obtained from studies of the antimalarial properties of the febrifugine analogs.

(The compounds described in this report are all included in an invention disclosure [MRMC docket no. WRAIR 2002-42] and a utility patent application entitled Antimalarial Activities and Therapeutic Properties of Febrifugine Analogues [document no. 60/390,334] that have been filed with the U.S. Patent and Trademark Office on 20 June 2002 and 20 June 2003, respectively, by S. Jiang, T. H. Hudson, and W. K. Milhous.)

★★★★MATERIALS AND METHODS
Febrifugine isolation. The dry roots of D. febrifuga were purchased from a herbal store (Kwok Shing Trading Co., Inc., New York, N.Y.). A modified method (19) was used for the extraction of febrifugine. Briefly, 5 kg of the dry roots was ground, followed by maceration in 14 liters of methanol at room temperature for a week. After filtration, the solvent was evaporated to give approximately 153 g of crude methanol extract, which was then suspended in 130 ml of 0.1 M HCl. The HCl suspension was partitioned with 400 ml of CHCl3 three times, and the aqueous HCl portion collected was adjusted to pH 9.5 with NaOH, followed by CHCl3 extraction and evaporation to obtain 12 g of alkaloids. The alkaloidal portion was separated by chromatography on a silica gel 60 column (70 to 230 mesh; Merck), and the febrifugines were eluted with CHCl3 and methanol in proportions of 6:1 and 4:1.

Confirmation of isolated febrifugine. The isolated febrifugine was analyzed by nuclear magnetic spectrum (NMR) analysis in an Hitachi R-3000 spectrometer, and chemical shifts were recorded in units. The 1H-NMR spectrum of febrifugine indicated that the purity was >99%. The NMR signal patterns, chemical shifts, mass spectra, melting point, and optical rotation of the isolated febrifugine fully agreed with previously reported values (14, 31). Elemental analysis of C, H, and N also gave satisfactory results.

Parasite culture. The chloroquine-sensitive D6 strain and the chloroquine-resistant W2 strain of P. falciparum were cultivated in RPMI 1640 medium with 6% human erythrocytes supplemented with 10% human serum (33). The parasites were cultured in an atmosphere of 5% CO2, 5% O2, and 90% N2 at 37°C.

In vitro drug susceptibility assay. Febrifugine and its analogs were tested in a cell-based in vitro drug susceptibility assay to determine if they were capable of interrupting Plasmodium cell metabolism and growth. A semiautomated microdilution technique (3) was used to assess the sensitivities of the parasites to the compounds selected. The incorporation of [3H]hypoxanthine into the parasites was measured as a function of the compound concentration to determine 50% inhibitory concentrations (IC50s).

In vitro toxicity assay. The compounds selected were tested for their in vitro toxicities for two mammalian cell lines as described previously (9, 30).

In vivo efficacy test. In conducting the research described in this report, the investigators adhered to the Guide for the Care and Use of Laboratory Animals by the Institute of Laboratory Animal Resources, National Research Council (21A), in accordance with the stipulations mandated for a facility accredited by the Association for Assessment and Accreditation of Laboratory Animal Care. A modified Thompson test was conducted for in vivo efficacy determination. Typically, 4- to 5-week-old ICR mice were housed in plastic cages with free access to water and food. P. berghei parasite-infected erythrocytes were obtained from donor mice. On experiment day 0, the donor mice were anesthetized and exsanguinated by cardiac puncture. The pooled blood from the donor mice was then diluted with normal mouse serum so that the concentration was 106 P. berghei-infected erythrocytes per inoculum (0.1 ml). The groups of experimental and control mice were inoculated with this parasitized blood on day 0. The tested mice were treated orally or subcutaneously once a day for 8 days from day 3 to day 10 with either candidate antimalarial drugs or vehicle alone, which served as a negative control. Each experimental group received a different dose, with up to five different doses of each compound tested. Blood for blood film analysis and body weights were obtained on the third and sixth days postinfection and then at weekly intervals through day 60. Films were Giemsa stained and examined microscopically to determine the levels of parasitemia. All mice were observed twice a day to assess their clinical signs, which were recorded. All treated mice with a negative smear on day 60 were considered cured.

Reagents. Febrifugine standards were kindly provided by Yoshiteru Oshima, Tohoku University, Sendai, Japan; and halofuginone was kindly provided by Neal Farber, Collgard Biopharmaceuticals, Newton, Mass. Methanol, ethyl acetate, petroleum ether, and chloroform were obtained from Fisher Chemical Co. All other chemicals were obtained from Sigma Chemical Co., St. Louis, Mo. All cell culture media and supplement reagents were obtained from Gibco, Grand Island, N.Y. The [3H]hypoxanthine used for the in vitro drug susceptibility assay was purchased from Amersham, Piscataway, N.J.

★★★★RESULTS
A piperidinyl(acetonyl)quinazoline-based database search through WRAIR CIS identified 133 analogs, 35 of which were assayed to determine their in vitro antimalarial efficacies. Ten analogs possessed very potent antimalarial activities (Table 1) that have not been reported before. WR237645 (halofuginone), however, was the most active febrifugine analog against the parasites and had an IC50 similar to that reported previously (4). The IC50s of WR222048, WR139672, and WR092103 were less than 5 ng/ml for parasite strains W2 and D6, while the IC50s of WR221232, WR140085, WR090212, WR146115, and WR059421 ranged from 10 to 30 ng/ml (Table 1). WR088442 had the lowest potency, with an IC50 of about 200 ng/ml. Moreover, febrifugine analogs were slightly more active against chloroquine-sensitive strain D6 than chloroquine-resistance strain W2, implying that no substantial resistance to these compounds existed in the parasites.

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