Antitrypanosomal Activity of Novel Benzaldehyde-Thiosemicarbazone Derivatives from Kaurenoic Acid †

A series of new thiosemicarbazones derived from natural diterpene kaurenoic acid were synthesized and tested against the epimastigote forms of Trypanosoma cruzi to evaluate their antitrypanosomal potential. Seven of the synthesized thiosemicarbazones were more active than kaurenoic acid with IC50 values between 2-24.0 μM. The o-nitro-benzaldehyde-thiosemicarbazone derivative was the most active compound with IC50 of 2.0 μM. The results show that the structural modifications accomplished enhanced the antitrypanosomal activity of these compounds. Besides, the thiocyanate, thiosemicarbazide and the p- methyl, p-methoxy, p-dimethylamine, m-nitro and o-chlorobenzaldehyde-thiosemicarbazone derivatives displayed lower toxicity for LLMCK2 cells than kaurenoic acid, exhibing an IC50 of 59.5 μM.

Considering the wide range of anti-cancer and antiparasitic activities derived from natural products, and considering that the vast majority of the thiosemicarbazones described in the literature only present structural variations on the imine carbon, our research group began the synthesis of new thiosemicarbazones containing mono-and sesquiterpenic units linked to the terminal nitrogen.
In previous research, several thiosemicarbazone derivatives of (-)-α-bisabolol exhibited inhibitory effects on the growth of eight cancer cell lines outlining myeloid leukemia (K-562) as especially sensitive to all of the tested compounds. Among the analogues, the methyl-phenyl-ketone derivative was the most active, exhibiting potent antitumoral activity (GI 50 0.01 µM) and high selectivity for K-562 (δTGI 505). This derivative also demonstrated high cytotoxicity (IC 50 1.55 µM) for K-562 with moderate selectivity (δLC 50 38.5 µM) [32]. Additionally, a thiosemicarbazide derivative of camphene displayed activity against Trichophyton mentagrophytes, a dermatophyte fungus [33]. In a continuation of our research efforts, we selected kaurenoic acid as a starting point to find new drugs with reduced side effects and greater efficacy in the chemoprophylaxis and chemotherapy of Chagas' disease, as kaurenoic acid has been previously described as a valuable asset against T. cruzi.

Synthesis
The synthetic route for the preparation of benzaldehyde-thiosemicarbazone derivatives of kaurenoic acid is presented in Scheme 1. The kaurenoic acid isothiocyanate and thiocyanate derivatives 2 and 3 were obtained as a mixture from a reaction between kaurenoic acid (1) and HSCN performed according to a previously reported procedure [34,35]. The reaction yield was of 58% of 2 and 33% of 3. The mainly difference between this compounds was observed by the signals moieties in 13 C-NMR spectrum at δ c 128.6 (NCS) and δ c 113.5 (SCN). In product 2, the presence of the NCS moiety was also demonstrated by the IR absorption band at 2,125 cm -1 and the diterpenic substituent was further characterized by the signals at δ H /δ C 1.52 (s, 3H, H-17)/23.6 (C-17), 1 The kaurenoic acid thiosemicarbazide derivative 4 was obtained in 82% yield from the addition reaction of 2 with hydrazine according to a previously reported procedure [32,33]. In product 4, the presence of the NH moieties were determined by the IR absorption bands at 3,311, 3,198 and 1,620 cm -1 (NH 2 ) as well as at 2,941 cm -1 (NH). This IR information was combined with the signals at δ H 4.48 (brs, 2H, N-1), 8.40 (brs, 1H, N-2), and 7.45 (s, 1H, N-4) in the 1 H-NMR spectrum and the signal at δ C 179.3 (C-3) in the 13 C-NMR spectrum.
The kaurenoic acid benzaldehyde-thiosemicarbazone derivatives 5-15 were obtained with yields between 60 and 96%, through the condensation of thiosemicarbazide 4 with benzaldehyde and its derivatives, which were substituted at the para position with methyl, methoxyl, hydroxyl, and dimethylamine groups and at the ortho, meta, and para positions with nitro and chloro groups according to a previously reported procedure [32]. The benzaldehyde-thiosemicarbazone moiety was characterized by NH signals, in which one was linked to the imine portion between δ H 11.11-11.67 (s, 1H, N-2), another linked to the terpenic portion between δ H 7.41-7.63 (s, 1H, N-4), and the last to the imine hydrogen between δ H 7.93-8.44 (s, 1H, H-1″)/ δ C 121.4-142.5 (C-1''). In addition, the signals for the hydrogens and carbons of the aromatic system (H-2″ -H-7″ and C-2″ -C-7″) between δ H 6.72-8.43/δ C 111.9-160.7 in the 1 H/ 13 C-NMR spectrum were also used for characterization. The remaining NMR data were consistent for the various substituents on the phenyl moiety discussed in the Experimental Section.

Biological assays
The results of the antitrypanosomal (IC 50 ) and cytotoxic (CC 50 ) assays, as well as their respective selectivity indexes (SI), are shown in Table 1. Through the antitrypanosomal assay evaluation of kaurenoic acid (1) and its derivatives 2-15, it was verified that excluding the thiosemicarbazide 4 and p-nitrobenzaldehyde-thiosemicarbazone 12, all of the derivatives synthesized were more active than kaurenoic acid (1). The o-nitrobenzaldehyde-thiosemicarbazone derivative 10 was of particular interest, as it was the most active compound, with an IC 50 of 2.0 µM. These results indicated that these structural modifications enhanced the antitrypanosomal activity of these compounds when compared to kaurenoic acid in this assay. The incorporation of HSCN into kaurenoic acid increased the activity of the isothiocyanate and thiocyanatederivatives 2 and 3 by approximately two-fold (58.4 and 43.7 µM, respectively). With the incorporation of the hydrazine on the isothiocyanate 2, there was a decrease in the activity of the thiosemicarbazide 4 to the same order of activity displayed by kaurenoic acid (1) (101.7 µM). Hence, the incorporation of the benzaldehyde increased the activity of the benzaldehyde-thiosemicarbazone derivative 5 to the same order of magnitude as the isothiocyanate derivative 2 (68.2 µM).
The methyl, methoxyl and hydroxyl substituents, which are electron density donors, at the para position of the aromatic ring increased the activity of the thiosemicarbazones 6, 7, and 8 (23.4, 16.0, and 18.3 µM) approximately three-fold when compared to compound 5. Conversely, the incorporation of a dimethylamine substituent, also an electron density donor, at the same position decreased the activity of compound 9 to 79.5 µM.
These results indicated that electronic effects were not the only factor governing biological activity. However, further investigations are necessary to elucidate the mode of action, and structure-activity correlations must be done to understand the effects involved in activity. With this information in hand, these compounds could then be used as new medicines for combating Chagas' disease.

General
All melting points were determined using a Microquímica model MQAPF-301 apparatus and are uncorrected. IR spectra were obtained using KBr pellets on an FT-IR BOMEM spectrophotometer. Low resolution mass spectra were recorded by means of a SHIMADZU-CG/MS model QP 2000A spectrometer at 70 eV with a prob for solids. The optical rotations were determined in CHCl 3 or DMSO as a solvent with a Perkin Elmer polarimeter 343 model at 20 °C. Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded using CDCl 3 or DMSO-d 6 as a solvent, at ambient temperature, on a Varian Mercury plus BB 300 spectrometer (300 MHz) with TMS as an internal standard. The chemical shifts (δ) are given in parts per million relative to TMS. Carbon-13 nuclear magnetic resonance ( 13 C NMR) spectra were recorded at 75.5 MHz with the same internal standard.
Chromatography was performed on silica gel Merck 230-400 mesh ASTM.
The solvents were previously treated under reduced pressure and the reagents (Sigma-Aldrich) were used as received.

Plant material
Dried bark of Croton floribundus (Euphorbiaceae) was collected in the 2-km area of the PR 323 highway in Maringá, Paraná, Brazil, during 2006 and was identified by comparison with a voucher specimen deposited at the Herbarium of the Universidade Estadual de Maringá under registration code HUEM 8406. (1) Dried, powdered material (5.5 kg) was extracted with hexane (1 L) for 12 h using a Soxhlet apparatus. The extract was then concentrated using a rotary evaporator under reduced pressure below 40 °C and then concentrated under vacuum at room temperature. The dried, crude hexane extract was chromatographed on a silica gel column using a hexane-dichloromethane gradient solvent system. The fractions that contained kaurenoic acid were gathered and recrystallized from cold methanol. This procedure resulted in 27.0 g of pure kaurenoic acid (1, ent-kaur-16-en-19-oic acid) as a white powder. m.p. 177 °C, [α] 20 D : -105.20 deg dm -1 g -1 cm 3 (c 0.102, CHCl 3 ). The spectroscopic data were consistent with the literature [13,16].

General procedure for the preparation of isothiocyanate 2
Isothiocyanic acid [34,35]: a slurry of powdered KSCN (7.92 mmol) in CHCl 3 (20 mL) was triturated with KHSO 4 (7.92 mmol) in a mortar for 5 min. The HSCN chloroform solution was decanted, an additional 10 mL of CHCl 3 was added to the solid mixture, and the solution was again decanted. The combined organic solutions totaled 30 mL in volume. The isolated kaurenoic acid (1) (0.66 mmol) and the HSCN/CHCl 3 solution (30 mL) were stirred at room temperature for 60 h. The reaction was monitored by TLC using kaurenoic acid (1) as the reference standard. After filtration, the solvent was removed under reduced pressure. The residue was purified by chromatography on a silica gel column eluted with hexane/ethyl acetate 5-15%, giving isothiocyanate (2) (58% yield) and the more polar thiocyanate (3) (33% yield).

General procedure for the preparation of thiosemicarbazide 4
Isothiocyanate 2 (0.55 mmol), dissolved in n-butanol (50 mL) and hydrazine (1.10 mmol), was stirred at 90 °C for 6.5 h. The reaction was monitored by TLC using isothiocyanate 2 as the reference standard. The mixture was then partitioned in n-butanol/distilled water, and the butanol phase was washed with CHCl 3 to afford the thiosemicarbazide 4 in 82% yield.

General procedure for the synthesis of benzaldehyde-thiosemicarbazones 5-15
Thiosemicarbazide 4 (0.50 mmol) was dissolved in n-butanol (50 mL) and then treated with 0.2% hydrochloric acid (50 µL) and benzaldehyde derivatives (0.50 mmol); it was then stirred at 90 °C for a period of time specific to each synthesized compound. The reactions were monitored by TLC using thiosemicarbazide 4 as the reference standard. This mixture was partitioned between n-butanol and distilled water, and the solvent was removed from the butanol phase under reduced pressure. The benzaldehyde-thiosemicarbazones 5-15 were then recrystallized from acetone.    (8) (11) (12) (15)

Antitrypanosomal assay
Stock solutions of the synthetic compounds and kaurenoic acid (5, 10, 50, and 100 µg/mL) were prepared in dimethylsulfoxide with their final concentrations not exceeding 1.0%. For the assay, epimastigote forms of T. cruzi (Y strain) were harvested during the exponential phase of growth, resuspended in liver infusion tryptose broth supplemented with 10% inactivated fetal bovine serum (Gibco Invitrogen Corporation, New York, NY, USA) and plated on 24-well plates at a concentration of 1 × 10 6 cells/mL. One milliliter of diluted compounds were included in each well and incubated for 96 h at 28 °C. Cell density was determined by counting the parasites in a hemocytometer chamber (Improved Double Neubauer) under a light microscope. All assays were carried out twice, in duplicate, on separate occasions.

Cytotoxicity assay
The cytotoxic effect was evaluated against LLMCK 2 cells in 96-well plates. A suspension of 2.5 × 10 4 cells was added to each well and left to grow as a monolayer for 24 h at 37 °C in a 5% CO 2 /air mixture. After this period, different concentrations of the synthetic compounds and kaurenoic acid (10 to 100 µg/mL) were added to the wells, and the plate was incubated for 96 h under the same conditions described above. The cells were fixed in 10% trichloroacetic acid at 4 °C for 1 h, washed five times with distilled water, and allowed to dry at room temperature. A solution of 4% sulforhodamine B (in 1% acetic acid) was added to each well, and the plate was kept protected from light for 30 min at 4 °C. The wells were then washed four times with 1% acetic acid, an aliquot (150 µL) of 10 mM Tris-base was added, and the aliquot was homogenized for 15 min. The absorbance was read at 530 nm in a microplate spectrophotometer, and data were calculated as the percentage of inhibition of growth. A concentration for 50% cellular toxicity (CC 50 ) was defined as the concentration that reduced the optic density of treated cells by 50% relative to untreated cells.
We have synthesized a series of new thiosemicarbazones derived from the natural diterpene kaurenoic acid. These compounds were found to be selective for protozoa and displayed an enhancement of their antitrypanosomal activity when compared to kaurenoic acid (1) without significant decreases in cytotoxicity for the tested cells. Nevertheless, the high cytotoxicities expressed by these derivatives could be an indication of their potential as anticancer agents. Further investigations are currently underway to confirm these hypotheses and to elucidate the mode of action and the structure-activity correlations that are involved in antitrypanosomal activity.