Determinação do mecanismo de ação citototóxica de alguns compostos extraídos de plantas do Nordeste brasileiro

Abstract

Eleven phytochemicals derived from plants to Northeast Brazil were studied for their antiproliferative effects in the human tumor cell lines CEM (leukemia) and SW-1573 (lung tumor), and in the normal human fibroblast cell line CCD-922. Using the MTT microtiter assay, five compounds were found to have IC50 values less than 10 gg/mL in the tumor cell lines. Jatrophone showed non-specifíc cytotoxicity, with IC50 values less than 1 gg/mL for all three cell lines. Meanwhile, a diterpenoid and a flavanoid, both from Egletes viscosa (Compositae family) had moderate activity. The 12-acetoxy hautriwaic acid lactone (clerodane A) was cytotoxic, with IC50 values of 6, 6 and 10 gg/mL, while 4’5-dihydroxy- 3,3’,7,8-tetramethoxy-flavone (tematin) was growth-inhibitory, with IC50 values of 2, 2 and 10 gg/mL in CEM, SW-1573 and CCD-922, respectively. Those compounds showing more differential cytotoxicity were oncocalyxones A (re/-8a-hydroxy-5-hydroxymethyl-2- methoxy-8al3-mehtyl-7,8,8a,9-tetrahydro-l,4-anthracenodione) and C (re/-10a,HJ3-epoxy- 1 la.-ethoxy-8a-hydroxy-2-methoxy-8aP-methyl-5,6,7,8,8a,9,10,10aP-octahydro-l,4- anthracenodione), both derived from Auxemma oncocalyx (Boraginaceae family), with IC50 values of 0.8-2, 7-8 and 12-13 gg/mL in CEM, SW-1573 and CCD-922, respectively. These four latter compounds were examined for their DNA reactivity, with regard to inhibition of DNA synthesis (5-BrdU incorporation) and induction of apoptosis (TUNEL assay and annexin V binding). Except for tematin, these compounds caused DNA damage, marked inhibition of 5-BrdU incorporation, DNA strand breaks and strong induction of apoptosis. Derivatives of oncocalyxone A obtained by chlorination and acetylation were less cytotoxic relative to the parent compound when tested against human leukemia cells. The cytotoxicity of oncocalyxone A when compared with other anticancer drugs belonging to the anthroquinone group, showed a low potency. Nonetheless, in multidrug resistant tumor cells with 10 to 500 fold lower sensitivity to doxorubicin, oncocalyxone A cytotoxicity was only moderately reduced twofold. In vivo, studies showed substantial inhibition of tumor growth of approximately 42% and 50% in L1210 leukemia and Ehrlich carcinoma, respectively. Oncocalyxone A (1,4-anthracenedione) could be considered as a new benzoquinone, with therapeutic potential in câncer chemotherapy, that possibly involving DNA reactivity leading to apoptosis in the câncer cells.