Antitumor and Antiviral Activity of Colombian Medicinal Plant Extracts

Antitumor and Antiviral Activity of Colombian Medicinal Plant Extracts

Vol. 94(4): 531-535

LA Betancur-Galvis/⁺, J Saez*, H Granados*, A Salazar**, JE Ossa

Laboratorio de Virología, Departamento de Microbiología y Parasitología, Facultad de Medicina *Departamento de Química **Departamento de Biología, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia, Apartado 1226, Medellín, Colombia

Extracts of nine species of plants traditionally used in Colombia for the treatment of a variety of diseases were tested in vitro for their potential antitumor (cytotoxicity) and antiherpetic activity. MTT (Tetrazolium blue) and Neutral Red colorimetric assays were used to evaluate the reduction of viability of cell cultures in presence and absence of the extracts. MTT was also used to evaluate the effects of the extracts on the lytic activity of herpes simplex virus type 2 (HSV-2). The 50% cytotoxic concentration (CC₅₀) and the 50% inhibitory concentration of the viral effect (EC₅₀) for each extract were calculated by linear regression analysis. Extracts from Annona muricata, A. cherimolia and Rollinia membranacea, known for their cytotoxicity were used as positive controls. Likewise, acyclovir and heparin were used as positive controls of antiherpetic activity.

Methanolic extract from Annona sp. on HEp-2 cells presented a CC₅₀ value at 72 hr of 49.6x10³mg/ml. Neither of the other extracts examined showed a significant cytotoxicity. The aqueous extract from Beta vulgaris, the ethanol extract from Callisia grasilis and the methanol extract Annona sp. showed some antiherpetic activity with acceptable therapeutic indexes (the ratio of CC₅₀ to EC₅₀). These species are good candidates for further activity-monitored fractionation to identify active principles.

Key words: ethnobotany - medicinal plants - antiviral - herpes simplex virus - citotoxicity - colorimetric assay

Scientific strategies for the in vitro evaluation of natural products with biological activity have changed in the past few years. One recent development is the highly automated bioassay screening based on colorimetric methods that quantify the proliferation of cell cultures (Mosmann 1983, Denizot & Lang 1986), these techniques which are considered quick and inexpensive for the evaluation of antitumor (Carmichael et al. 1987, Rubinstein et al. 1990) and antiviral activity (Weislow et al. 1989) of a large number of natural product extracts, have also easily permitted to guide the isolation and purification of their biologically active principles (Cordell 1995).

Interest in a large number of traditional natural products has increased (Kurokawa et al. 1993, Cordell 1995, Vlietinck et al. 1995, Taylor et al. 1996). It has been suggested that aqueous and ethanolic extracts from plants used in allopathic medicine are potential sources of antiviral and antitumor agents (Chung et al. 1995, Vlietinck et al. 1995). Furthermore, the selection of crude plant extracts for screening programs has the potential of being more successful in its initial steps than the screening of pure compounds isolated from natural products (Kusumoto et al. 1995, Cordell 1995).

Finally another new research strategy for the study of the biological activity of natural products, is the search of new biological activities of natural products that have just been studied (Cordell 1995). Therefore, one of the objectives of our work was to evaluate the antiherpectic activity of some species of Annonaceae. Acetogenins isolated from the Annonaceae have been evaluated for both their cytotoxic activity in multiple ovarian cancer cell lines and their antitumor effects in a murine ovarian teratocarcinoma model in vivo (Rupprecht et al. 1990, Holschneider et al. 1994). The cytotoxic potency found for some of these compounds may be explained by the capacity that they have for inhibiting NADH-ubiquinone reductase activity in mitochondria (Degli Esposti et al.1994, Zafra-Polo et al. 1996).

Plant collection - Annona sp., was collected in the city of Montería in the department of Córdoba, Colombia, at a mean altitude of 20 m; voucher specimen was deposited under the number 12650 at the herbarium of the Joaquin Antonio Uribe Botanical Garden in the city of Medellín. R. membranacea, and Aristolochia cordiflora, were collected in the Rio Claro canyon, in the town of San Luis, department of Antioquia, Colombia, at a mean altitude of 700 m; voucher of the R. membranacea and A. cordiflora were deposited under the numbers 1106 and 3712, respectively, in the Gabriel Gutierrez Villegas Herbarium of the National University at Medellín (HNUM). A. cherimolia was collected in the town of San Cristobal, department of Antioquia at a mean altitude of 1700 m. C. gracilis was collected in the Department of Valle, Colombia, at a mean altitude of 700 m. A. cherimolia and and C. gracilis were deposited under the numbers 4561 and 17C35 in the HNUM. Crescentia cujete and Chenopodium ambrosioides were collected in the city of Medellín and were deposited under the numbers 121 and 33 in the HNUM. The species collected are listed in Table I, with their common names and folk medicinal uses.

Extract preparation - The material (100 g) was dried in an oven at a temperature of 40°C. The plant material of the A. muricata, the A. cherimolia and the C. grasilis were percolated with ethanol. The seeds of the Annona sp., and leaf/stem of the A. cordiflora were percolated with methanol. Then, the different extracts were evaporated near dryness under reduced pressure and were stored at a temperature of 4°C protected from the light, until there were used in the different cytotoxic assays, no longer than two months. The plant parts used for the extraction process are described in Table II.

Cell culture and virus - Bovine kidney cells (MDBK cell line no. ATCC CCL22) and human larynx epidermoid carcinoma cells (HEp-2 cell line no. ATCC CCL23) were grown as a monolayer culture in Eagle minimum essential medium (MEM) supplemented with 10% fetal bovine serum (FBS), 100 units/ml penicillin, 100 mg/ml streptomycin, 20 mg/ml glutamine, 0.14% NaHCO₃ and, MEM non-essential aminoacid and vitamins solution. The culture were maintained at 37°C in a humidified 5% CO₂ atmosphere.

HSV-2 was obtained from Center for Diseases Control and Prevention, Atlanta, Georgia. USA. The virus stock was prepared from HSV-2-infected HEp-2 cell cultures. The infected cultures were then subjected to three cycles of freezing-thawing, and centrifuged at 2000 rpm for 10 min. The supernatant was collected, titrated and stored at -170^oC in 1 ml alliquots.

Cytotoxicity assay- The dry crude extracts were redissolved in dimethyl sulfoxide (DMSO, Sigma), then diluted 1:100 in cell culture medium at 2.5% FBS before preparing serial half-log 10 dilution in a 100 mg/ml to 10^-6 mg/ml range. Both for tetrazolium-dye (MTT) (Mosmann 1983, Niks & Otto 1990) and Neutral Red (NR) cytotoxicity assays (Flick & Gifford 1984), adherent cell monolayers (MDBK, HEp-2) in culture were trypsinized and washed with culture medium. The cells were plated at 15,000 cells/well in 96-well flat-bottomed plate. After a 24 hr preincubation period, extract dilutions were added to the appropriate wells and the plates were incubated for 24, 48 or 72 hr at 37^oC in a humidified incubator with 5% CO₂. Untreated cells were used as controls. By MTT technique, the supernatants were removed from all wells and 25 ml of MTT (Sigma) (2 mg/ml) solution in phosphate buffered saline (PBS) was added to each well and the plates were incubated for 2 hr at 37°C. Then 125 ml of DMSO was added to the wells to solubilize the MTT crystals. The plates were placed on a shaker for 15 min and absorbency was read at 492 nm on multiwell spectrophotometer (Titertek Uniskan). Similarly by NR technique, 50 ml/well solution containing 0.033% NR (w/v) in PBS was added. After incubation 1 hr at 37^oC, dye containing medium was removed and the wells were washed twice with 150 ml/well warmed PBS. The cells were then lysed with 125 ml of a 50% v/v mixture of ethanol and 0.1M Monobasic Sodium Phosphate to solubilize the NR. The plates were gently rocked for 15 min and the absorbency in each wells was read 550 nm. Control cells lysed with DMSO 2 hr previous to evaluation of cellular viability by the MTT technique were used to blank the spectrophotometer.

The percentage of cytotoxicity was calculated as (A-B)/A x 100, where A is the mean optical density of untreated wells and B is the optical density of wells with plant extracts.

Antiviral assays - To screen for antiviral activity, MDBK or HEp-2 cell monolayer were grown in 96 well microtiter plates. Dilutions of the extracts were added 2 hr before the viral infection. Ten infectious dosis were added to each well and incubated at 37°C in a humidified 5% CO₂ atmosphere for a period of 72 hr. Controls consisted of untreated infected, treated noninfected and untreated noninfected cells. Furthermore all tests were compared with positive controls (acyclovir and heparin) tested simultaneously under identical conditions. The cellular viability was evaluated by MTT technique.

Data analysis - The 50% cytotoxic concentration (CC₅₀) and the 50% inhibitor concentration of the viral effect (EC₅₀) for each extract were calculated from concentration-effect-curves after linear regression analysis. The therapeutic index or selective index is defined as CC₅₀ over EC₅₀ (Piñeros et al. 1992).

New scientific strategies for the evaluation of natural products with biological activity require the implementation of large-scale screening programs. Our laboratory has adopted a microculture assay based on metabolic reduction of MTT to evaluate the cytotoxic effect as well as the antiviral activity of plant extracts on different cells. Tetrazolium salt is metabolically reduced by viable cells to yield a blue Formosan product measurable in a multiwell scanning spectrophotometer. This technique permitted to evaluate dose-dependent-effect, by linear regression analysis showing acceptable R² values and correlation coefficients.

Tables II and ^III, shows concentration of extract to produce a reduction of viability of cell cultures of a 50% (50 cytotoxic concentration values CC₅₀), as determined by NR and MTT asssays. The ethanol extract from Annona sp. on MDBK and HEp-2 cells presented CC₅₀ values of 34.5 and 55 mg/ml at 24 hr respectively, furthermore the value for the same extract on HEp-2 cells at 72 hr was 49.6x10^-3 mg/ml. The cytotoxic activity of the A. muricata, A. cherimolia and R. membranacea species has been extensively proven. From these species, Bis-tetrahydrofuran Acetogenins such as rolliniastatin-1 and rolliniastatin-2 have been isolated and identified, both are potent inhibitors of the NADH-ubiquinone reductase (Complex I) activity of mammalian mitochondria (Degli Esposti et al. 1994). The comparison of the CC₅₀ values of the extracts of these species with the CC₅₀ values of the methanolic extract from the Annona sp. allows us to conclude that this extract is a good candidate for further studies of activity-monitored fractionation to identify the active principles, which according to chemical structure studies performed (Degli Esposti et al. 1994, Zafra-Polo et al. 1996) may be similar to a Mono or Bis-tetrahydrofuran Acetogenin. Neither of the other extracts examined showed an important cytotoxicity.

Annona sp. extract also inhibited HSV-2 replication in HEp-2 cells. Comparing the CE₅₀ and therapeutic indexes values (the ratios of CC₅₀ to CE₅₀) for Acyclovir and Heparin (Wiltink & Janknegt 1991, Bean 1992, Clercq 1995, Herold et al. 1995), Table IV, compounds whose antiherpetic activity has been proven, with the values of the Annona sp. extract, we conclude that they have an acceptable antiviral activity. Similarly the aqueous extract from Beta vulgaris showed a good therapeutic index. These species are good candidates for further activity-monitored fractionation to identify active principles.

To the personnel of the Virology Laboratory, specially to Gloria Mercedes Mejía-Londoño, for collaboration.

Table I | Table II | ^{Table III} | Table IV

This work received financial support from the University of Antioquia.

⁺Corresponding author. Fax: [phone]. E-mail: [e-mail]

Received 29 July 1998

Accepted 10 February 1999

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Publication Dates

• Publication in this collection
  12 Aug 1999
• Date of issue
  July 1999

History

• Received
  29 July 1998
• Accepted
  10 Feb 1999