McCluskey Adam, Ackland Stephen P, Bowyer Michael C, Baldwin Monique L, Garner James, Walkom Cecilia C, Sakoff Jennette A
Medicinal Chemistry Group, Chemistry, The University of Newcastle, University Drive, Callaghan, NSW, 2308, Australia.
Bioorg Chem. 2003 Feb;31(1):68-79. doi: 10.1016/s0045-2068(02)00524-2.
Diels-Alder addition of furans (furan, furfuryl alcohol, and 3-bromofuran) to maelic anhydride yields three distinct 5,6-dehydronorcantharidins. Hydrogenation of (4,10-dioxatricyclo[5.2.1.0]decane-3,5-dione) (4a), in dry ethanol affords the monoester (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic aid monoethyl ester) (6). Subsequent transesterification affords a series of monoesters (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monomethyl ester (7)), 7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monopropyl ester (8), (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monohexyl ester (9)) and differentially substituted diesters (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid 2-ethyl ester 3-isopropyl ester) (10), and (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid 2-ethyl ester 3-phenyl ester) (11). Analogues were firstly screened for their ability to inhibit protein phosphatases 1 (PP1) and 2A (PP2A) as the lead compounds cantharidin (1) and norcantharidin (2) are known PP1 and PP2A inhibitors. Only analogues 4a, 6-8 displayed good PP1 and PP2A inhibition (PP1 IC(50)'s=2.0, 2.96, 4.71, and 4.82 microM, respectively; PP2A IC(50)'s=0.2, 0.45, 0.41, and 0.47 microM, respectively). All analogues were also screened for their anti-cancer potential against a panel of tumour cell lines, HL60, L1210, SW480, WiDr, HT29, HCT116, A2780, ADDP, and 143B, producing GI(50) values ranging from 6 microM to >1000 microM. Analogues possessing good PP1 and/or PP2A inhibition also returned moderate to good anti-cancer activity. Analogues with substituents directly attached to the intact bicyclo[2.2.1]heptane skeleton were poor to moderate anti-cancer agents. This correlates well with their lack of PP1 or PP2A activity. Analogues capable of undergoing a facile ring opening of the anhydride or with a single carboxylate were good PP1 and PP2A inhibitors, largely correlating to the observed anti-cancer activity in all cases, except 11. Analogue 11, whist neither a PP1 nor a PP2A inhibitor shows anti-cancer activity comparable to 1 and 2. We believe that intracellular esterases generate the corresponding dicarboxylate, which is a potent PP1 and PP2A inhibitor, and that it is this species which is responsible for the observed anti-cancer activity.
呋喃（呋喃、糠醇和3-溴呋喃）与苹果酸酐发生狄尔斯-阿尔德加成反应，生成三种不同的5,6-脱氢去甲斑蝥素。在干燥乙醇中，（4,10-二氧杂三环[5.2.1.0]癸烷-3,5-二酮）（4a）进行氢化反应，得到单酯（7-氧杂双环[2.2.1]庚烷-2,3-二羧酸单乙酯）（6）。随后进行酯交换反应，得到一系列单酯（7-氧杂双环[2.2.1]庚烷-2,3-二羧酸单甲酯（7））、7-氧杂双环[2.2.1]庚烷-2,3-二羧酸单丙酯（8）、（7-氧杂双环[2.2.1]庚烷-2,3-二羧酸单己酯（9））以及不同取代的二酯（7-氧杂双环[2.2.1]庚烷-2,3-二羧酸2-乙酯3-异丙酯）（10）和（7-氧杂双环[2.2.1]庚烷-2,3-二羧酸2-乙酯3-苯酯）（11）。首先筛选类似物抑制蛋白磷酸酶1（PP1）和2A（PP2A）的能力，因为先导化合物斑蝥素（1）和去甲斑蝥素（2）是已知的PP1和PP2A抑制剂。只有类似物4a、6 - 8表现出良好的PP1和PP2A抑制作用（PP1的IC(50)值分别为2.0、2.96、4.71和4.82微摩尔；PP2A的IC(50)值分别为0.2、0.45、0.41和0.47微摩尔）。还针对一组肿瘤细胞系HL60、L1210、SW480、WiDr、HT29、HCT116、A2780、ADDP和143B筛选了所有类似物的抗癌潜力，得出的GI(50)值范围为6微摩尔至>1000微摩尔。具有良好PP1和/或PP2A抑制作用的类似物也表现出中等至良好的抗癌活性。带有直接连接到完整双环[2.2.1]庚烷骨架上的取代基的类似物是较差至中等的抗癌剂。这与它们缺乏PP1或PP2A活性密切相关。能够使酸酐容易开环或带有单个羧酸盐的类似物是良好的PP1和PP2A抑制剂，在所有情况下，除了11，这在很大程度上与观察到的抗癌活性相关。类似物11，虽然既不是PP1也不是PP2A抑制剂，但显示出与1和2相当的抗癌活性。我们认为细胞内酯酶产生相应的二羧酸盐，它是一种有效的PP1和PP2A抑制剂，正是这个物种导致了观察到的抗癌活性。
