Synthesis and Antiproliferative Activity of Triphenylphosphonium Derivatives of Natural Allylpolyalkoxybenzenes.

Derivatives of natural allylpolyalkoxybenzenes conjugated to triphenylphosphonium (TPP) cations by aliphatic linkers of three, six, seven, and eight atoms were synthesized to examine the role of the polyalkoxybenzene pharmacophore, TPP fragment, and linker length in antiproliferative activities. The key synthetic procedures included (i) hydroboration-oxidation of apiol, dillapiol, myristicin, and allyltetramethoxybenzene; (ii) acylation of polyalkoxybenzyl alcohols or amines; and (iii) condensation of polyalkoxybenzaldehydes followed by hydrogenation and cyclopropyl-homoallyl rearrangement. The targeted TPP conjugates as well as the starting allylbenzenes, the corresponding alkylpolyalkoxybenzenes, and the respective alkyl-TPP salts were evaluated for cytotoxicity in a panel of human cancer cell lines using MTT and Click-iT-EdU assays and in a sea urchin embryo model. The linker of three carbon atoms was identified as favorable for selective cancer cell growth inhibition. Although the propyl-TPP salt was cytotoxic at low micromolar concentrations, the introduction of a polyalkoxybenzene moiety significantly potentiated inhibition of both cell growth and de novo DNA synthesis in several human cancer cell lines, HST-116 colon cancer, A375 melanoma, PC-3 prostate cancer, and T-47D breast carcinoma cells, while it failed to produce any developmental abnormalities in the sea urchin embryos.

Sea Urchin Embryo Model As a Reliable in Vivo Phenotypic Screen to Characterize Selective Antimitotic Molecules. Comparative evaluation of Combretapyrazoles, -isoxazoles, -1,2,3-triazoles, and -pyrroles as Tubulin-Binding Agents.

A series of both novel and reported combretastatin analogues, including diarylpyrazoles, -isoxazoles, -1,2,3-triazoles, and -pyrroles, were synthesized via improved protocols to evaluate their antimitotic antitubulin activity using in vivo sea urchin embryo assay and a panel of human cancer cells. A systematic comparative structure-activity relationship studies of these compounds were conducted. Pyrazoles 1i and 1p, isoxazole 3a, and triazole 7b were found to be the most potent antimitotics across all tested compounds causing cleavage alteration of the sea urchin embryo at 1, 0.25, 1, and 0.5 nM, respectively. These agents exhibited comparable cytotoxicity against human cancer cells. Structure-activity relationship studies revealed that compounds substituted with 3,4,5-trimethoxyphenyl ring A and 4-methoxyphenyl ring B displayed the highest activity. 3-Hydroxy group in the ring B was essential for the antiproliferative activity in the diarylisoxazole series, whereas it was not required for potency of diarylpyrazoles. Isoxazoles 3 with 3,4,5-trimethoxy-substituted ring A and 3-hydroxy-4-methoxy-substituted ring B were more active than the respective pyrazoles 1. Of the azoles substituted with the same set of other aryl pharmacophores, diarylpyrazoles 1, 4,5-diarylisoxazoles 3, and 4,5-diaryl-1,2,3-triazoles 7 displayed similar strongest antimitotic antitubulin effect followed by 3,4-diarylisoxazoles 5, 1,5-diaryl-1,2,3-triazoles 8, and pyrroles 10 that showed the lowest activity. Introduction of the amino group into the heterocyclic core decreased the antimitotic antitubulin effect of pyrazoles, triazoles, and to a lesser degree of 4,5-diarylisoxazoles, whereas potency of the respective 3,4-diarylisoxazoles was increased.

Regioselective synthesis of 3,4-diaryl-5-unsubstituted isoxazoles, analogues of natural cytostatic combretastatin A4.

4,5-Diarylisoxazoles are potent antiproliferative tubulin-targeting agents. Their isomeric 3,4-diaryl-5-unsubstituted isoxazoles are hardly accessible. The synthesis of 3,4-diaryl-5-unsubstituted isoxazoles 13 was designed based on a condensation of arylbenzaldehydes, arylnitromethanes, and ethoxycarbonylmethylpyridinium bromide followed by a selective one-step transformation of intermediate 3,4-diaryl-5-ethoxycarbonyl-4,5-dihydroisoxazole 2-oxides 8. The orientation of aryl rings in relation to isoxazole heterocycle was confirmed by X-ray crystallography. Targeted compounds were evaluated for antimitotic microtubule destabilizing activity using a phenotypic sea urchin embryo assay. 3-(4-Methoxyphenyl)-4-(3,4,5-trimethoxyphenyl)isoxazole 13e and 13h with a single methoxy substituent were the most potent. Compound 13e showed strong cytotoxicity in NCI60 screen with GI50 for NCI-H522 human lung cancer cell line of 0.023 µM.

Synthesis and antiproliferative activity of conformationally restricted 1,2,3-triazole analogues of combretastatins in the sea urchin embryo model and against human cancer cell lines.

A series of 1,5-diaryl- and 4,5-diaryl-1,2,3-triazole derivatives of combretastatin A4 were synthesized and evaluated as antimitotic microtubule destabilizing agents using the sea urchin embryo model. Structure-activity relationship studies identified compounds substituted with 3,4,5-trimethoxyphenyl and 3,4-methylenedioxy-5-methoxyphenyl ring A and 4-methoxyphenyl ring B as potent antiproliferative agents with high cytotoxicity against a panel of human cancer cell lines including multi-drug resistant cells. 4,5-Diaryl-1,2,3-triazoles (C-C geometry) were found to be considerably more active than the respective 1,5-diaryl-1,2,3-triazoles (N-C geometry). Compound 10ad' induced G2/M cell cycle arrest and apoptosis in human T-leukemia Jurkat cells via caspase 2/3/9 activation and downregulation of the antiapoptotic protein XIAP. A mitotic catastrophe has been evaluated as another possible cell death mode.

Synthesis and comparative evaluation of 4-oxa- and 4-aza-podophyllotoxins as antiproliferative microtubule destabilizing agents.

A series of novel 4-oxa-podophyllotoxin derivatives 7 featuring the intact lactone ring D and various substituents in rings B and E has been synthesized and evaluated in a phenotypic sea urchin embryo assay along with the representative 4-aza-analogs 5 for their antimitotic and microtubule destabilizing activity. The most active compounds exhibited myristicin-derived or a 3',5'-dimethoxy substitution pattern in the ring E and a 6-methoxy moiety replacing the methylenedioxy ring A. Compounds 5xb, 5xe, 5yb, 7xa, 7xb, and 7xc showed potent antiproliferative effects in the NCI60 cytotoxicity screen. Notably, growth of the multi-drug resistant NCI/ADR-RES cells was more affected by these agents than the parent OVCAR-8 cell line. Although generally 4-oxa-podophyllotoxins were less potent than the respective 4-aza-derivatives in these assays, stability of the former series towards oxidation may prove to be of interest for the development of anticancer agents with in vivo activity.