Cytotoxic Dehydroabietylamine Derived Compounds.

BACKGROUND AND METHODS: Chemotherapy remains one of the most important methods for the treatment of cancer. More recently in this context, some products derived from natural products have raised scientific interest which especially include many terpenes. Thereby, diterpenoids represent a special class, and within this class of important secondary natural products, especially compounds derived from Dehydroabietylamine (DA), are of particular interest. RESULTS: This review not only gives a summary of the most important findings on the cytotoxic behavior of DAderived compounds but also shows some drawbacks of these compounds, such low bioavailability and/or poor solubility of several derivatives of DA. It focusses on the chemical aspects and summarizes the DA related biological effects without deep discussion of underlying biochemical pathways. CONCLUSION: Dehydroabietylamine-derived cytotoxic compounds hold a high potential to be developed into efficient antitumor active drugs.

Novel 12-hydroxydehydroabietylamine derivatives act as potent and selective butyrylcholinesterase inhibitors.

The skeleton of the diterpene dehydroabietylamine was modified, and a set of 12-hydroxy-dehydroabietylamine derivatives was obtained. The compounds were screened in colorimetric Ellman's assays to determine their ability to act as inhibitors for the enzymes acetylcholinesterase (AChE, from electric eel) and butyrylcholinesterase (BChE, from equine serum). Additional investigations concerning the enzyme kinetics were performed and showed 12-hydroxy-N-(4-nitro-benzoyl)dehydroabietylamine (13) and 12-hydroxy-N-(isonicotinoyl)dehydroabietylamine (17) as selective BChE inhibitors holding good inhibition constants Ki = 0.72 ±â€¯0.06 µM and Ki = 0.86 ±â€¯0.19 µM, respectively.

Transformation of asiatic acid into a mitocanic, bimodal-acting rhodamine B conjugate of nanomolar cytotoxicity.

Based on their biological activity natural products continue to represent optimal lead structures for the development of novel drug candidates. We focused on the syntheses of several derivatives of the triterpene asiatic acid and on the evaluation of their cytotoxic activity in a photometric sulforhodamin B assay. Especially, benzamide 2 and rhodamine B conjugate 11 show a distinct cytotoxicity for several human tumor cell lines, e.g. EC50 (A2780) = 110 ±â€¯1 nM and EC50 (A2780) = 8 ±â€¯2 nM, respectively. Interestingly, compound 11 showed for two human tumor cell lines (HT29 and 518A2) non-linear, bimodal dose-response relationships.

Ugi multicomponent-reaction: Syntheses of cytotoxic dehydroabietylamine derivatives.

Isocyanide-based multicomponent reactions - especially the standard four component Ugi reaction - provide an easy and powerful access to compounds with an auspicious pharmacological potential. Therefore, a set of 16 novel derivatives of the diterpene dehydroabietylamine was synthesized by the Ugi-4CR. The subsequent screening of the synthesized α-acylamino carboxamides in colorimetric sulforhodamine B assays revealed an in vitro cytotoxicity towards several human tumor cell lines. Particularly, the rhodamine B conjugates 14-16 showed a remarkable cytotoxic activity, characterized by EC50 values in a low three-digit nanomolar range. The screening of rhodamine B amide 17 that was obtained for comparison by a Schotten-Baumann reaction showed that the linkage of the rhodamine B moiety and the diterpene influences significantly its cytotoxic potency. While 14 was highly cytotoxic and acted as a mitocan, compound 17 was not cytotoxic at all. This observation underlines the importance of the type of coupling between the diterpene and the rhodamine part. The presence of a rhodamine B moiety in the molecules doesn't necessarily guarantee that the compound is cytotoxic.

Unexpected AChE inhibitory activity of (2E)α,ß-unsaturated fatty acids.

A small library of (E) α,ß-unsaturated fatty acids was prepared, and 20 different saturated and mono-unsaturated fatty acids differing in chain length were subjected to Ellman's assays to determine their ability to act as inhibitors for AChE or BChE. While the compounds were only very weak inhibitors of BChE, seven molecules were inhibitors of AChE holding IC50 = 4.3-12.8 M with three of them as significant inhibitors of this enzyme. The results have shown trans 2-mono-unsaturated fatty acids are better inhibitors for AChE than their saturated analogs. Furthermore, the screening results indicate that the chain length is crucial for obtaining an inhibitory efficacy. The best results were obtained for (2E) eicosenoic acid (14) showing inhibition constants Ki = 1.51 ±â€¯0.09 M and Ki' = 7.15 ±â€¯0.55 M. All tested compounds were mixed-type inhibitors with a dominating competitive part. Molecular modelling calculations indicate a different binding mode of active/inactive compounds for the enzymes AChE and BChE.

Syntheses of C-ring modified dehydroabietylamides and their cytotoxic activity.

Due to their auspicious pharmacological efficacy as future drug candidates, natural products have been attracting scientific interest for centuries. An interesting field of research concerns the natural product class of terpenes. In this regard, a multitude of studies have already shown their promising biological potential. Therefore, a set of 27 derivatives of the diterpene dehydroabietylamine was synthesized, focusing on C-ring modifications and the derivatization of the amino moiety at C-18. Subsequent screening of the compounds in colorimetric sulforhodamine B-assays revealed an in vitro cytotoxicity especially towards malignant cell line MCF7. Particularly, 12-hydroxy-N-(isonicotinoyl)dehydroabietylamine and N-(4-methoxybenzoyl)dehydroabietylamine showed good cytotoxic activities (EC50 (MCF7) = 4.3 ±â€¯0.2 µM and EC50 (MCF7) = 4.5 ±â€¯1.5 µM, respectively) and significant selectivities (SI = 6.2 and SI = 8.8, respectively) towards malignant cell lines.

An access to a library of novel triterpene derivatives with a promising pharmacological potential by Ugi and Passerini multicomponent reactions.

The promising combination of natural product leads and their derivatization by isocyanide-based multicomponent reactions (IMCRs) has gained interest in accessing diversity-oriented libraries with auspicious pharmacological potential. Therefore, a set of 34 Ugi and 3 Passerini products was successfully synthesized starting from naturally occurring triterpenoids, i.e. oleanolic acid (OA) and maslinic acid (MA), followed by a biological evaluation of the novel α-acylamino carboxamides and the α-acyloxy carboxamides in colorimetric SRB assays to determine their cytotoxic potential. Especially, the MA-Ugi products 6a, 6b and 7b showed a remarkable cytotoxicity for A2780 ovarian carcinoma cells in a low µM range. Compounds 6a and 7b induced programmed cell death in part through the apoptosis pathway.

Piperlongumine B and analogs are promising and selective inhibitors for acetylcholinesterase.

Piperlongumine B (19), an alkaloid previously isolated from long pepper (Piper longum) has been synthesized for the first time in a short sequence and in good yield together with 19 analogs. Screening of these compounds in Ellman's assays showed several of them to be good inhibitors of acetylcholinesterase while being less active for butyrylcholinesterase. Activity of the compounds increased with the ring size of the heterocycle, and a maximum of activity was observed for an analog holding 12 methylene groups in the aliphatic side chain. These compounds may be regarded as promising candidates for the development of efficient inhibitors of acetylcholinesterase being useful for the treatment of Alzheimer's disease.

Novel dehydroabietylamine derivatives as potent inhibitors of acetylcholinesterase.

Nowadays, the inhibition of acetylcholinesterase is one of the main pharmacological strategies for the treatment of Alzheimer's disease. Therefore, a set of thirty-four derivatives of the diterpenoid dehydroabietylamine has been synthesized and screened in colorimetric Ellman's assays to determine their ability to inhibit the enzymes acetylcholinesterase (AChE, from electric eel) and butyrylcholinesterase (BChE, from equine serum). A systematic variation of the substitution of dehydroabietylamides enabled an approach to analogs showing a remarkable inhibition potency for AChE. Particularly N-benzoyldehydroabietylamines 11, 12 and 13 were excellent inhibitors for AChE, showing inhibition rates comparable to standard galantamine hydrobromide.

Synthesis and proapoptotic activity of oleanolic acid derived amides.

Thirty-one different 3-O-acetyl-OA derived amides have been prepared and screened for their cytotoxic activity. In the SRB assays nearly all the carboxamides displayed good cytotoxicity in the low µM range for several human tumor cell lines. Low EC50 values were obtained especially for the picolinylamides 14-16, for a N-[2-(dimethylamino)-ethyl] derivative 27 and a N-[2-(pyrrolinyl)-ethyl] carboxamide 28. These compounds were submitted to an extensive biological testing and proved compound 15 to act mainly by an arrest of the tumor cells in the S phase of the cell cycle. Cell death occurred by autophagy while compounds 27 and 28 triggered apoptosis.

Amino(oxo)acetate moiety: A new functional group to improve the cytotoxicity of betulin derived carbamates.

While 3-O-acetylated betulin derivatives carrying a carbamate moiety at position C-28 are of rather low cytotoxicity for human tumor cell lines, the corresponding C-3 amino(oxo) acetates show good cytotoxicity. For example, an EC50 as low as 2.0µM was found for (3ß) 28-{[(hexylamino)carbonyl]oxy}lup-20(29)-en-3-yl amino(oxo)acetate (16) employing the ovarian cancer cell line A2780.

Targeting cancer cells with oleanolic and ursolic acid derived hydroxamates.

Oleanolic and ursolic acid derived hydroxamates were easily obtained from their parent compounds; they were screened for their cytotoxicity applying SRB assays employing several human tumor cell lines. Low EC50 values were determined for compounds in which the nitrogen as well as the oxygen in the hydroxamic acid part still holds acidic hydrogens. Thus, ursolic acid derived compounds having at least an OH and/or NH moiety in the hydroxamate part of the molecule showed good cytotoxicity but they are significantly less selective for the tumor cells than oleanolic acid derived compounds. Good results were determined for oleanolic acid derived 7 for tumor cell lines 518A2 (melanoma, EC50=3.3 µM), A2780 (ovarian carcinoma, EC50=3.4 µM) and HT29 (colon adenocarcinoma, EC50=5.6 µM) while being significantly less cytotoxic for fibroblasts (EC50=20.4 µM).

First Occurrence of a Furano-glycyrrhetinoate and Its Cytotoxicity.

(18α)-Glycyrrhetinic acid (4) was prepared from (18ß)-glycyrrhetinic acid (1), and the cytotoxicity of some derivatives was investigated by photometric SRB assays employing several human tumor cell lines. In summary, (18ß)-1 is slightly more cytotoxic than its (18α) epimer 4, but its cytotoxicity is negligible. Higher cytotoxicity was observed for the esters 2 and 5 and for the 3-O-acetylated esters 3 and 6. Cytotoxicity was improved dramatically when the hydroxyl group at position C-3 was replaced by an amino moiety. SeO2 oxidations gave access to a novel furano-glycyrrhetinoate 15. Interestingly, its seleno analog 16 is approximately five to six times less cytotoxic for the tumor cell lines tested, and tumor/non-tumor selectivity is lost upon replacement of the oxygen by a selenium substituent.

Betulinic acid derived hydroxamates and betulin derived carbamates are interesting scaffolds for the synthesis of novel cytotoxic compounds.

The betulinic acid-derived hydroxamates 5-18, the amides 19-24, and betulin-derived bis-carbamates 25-28 as well as the carbamates 31-40 and 44-48 were prepared and evaluated for their antiproliferative activity in a photometric sulforhodamine B (SRB) assay against several human cancer cell lines and nonmalignant mouse fibroblasts (NIH 3T3). While for 3-O-acetyl hydroxamic acid 5 EC50 values as low as EC50 = 1.3 µM were found, N,O-bis-alkyl substituted hydroxamates showed lowered cytotoxicity (EC50 = 16-20 µM). In general, hydroxamic acid derivatives showed only reduced selectivity for tumor cells, except for allyl substituted compound 13 (EC50 = 5.9 µM for A2780 human ovarian carcinoma cells and EC50 > 30 µM for nonmalignant mouse fibroblasts). The cytotoxicity of betulinic acid derived amides 19-24 and of betulin derived bis-carbamates 25-28 was low, except for N-ethyl substituted 25. Hexyl substituted 39 showed EC50 = 5.6 µM (518A2 cells) while for mouse fibroblasts EC50 > 30 was determined.

The chemical and biological potential of C ring modified triterpenoids.

A convenient and elegant route has been developed to separate the natural regioisomers triterpenoids ursolic acid (UA) and oleanolic acid (OA) as well as derivatives thereof. Eleven unknown derivatives of OA were designed, synthesized, and their cytotoxicity was investigated. Further sixteen compounds were prepared to correlate all compounds in a SAR study. It could be shown that C-ring modifications of OA and UA have only a moderate influence onto the cytotoxic activity of the compounds but a significant impact onto the ability to trigger apoptosis in ovarian cancer cells (cell line A2780).

A bioassay-driven discovery of an unexpected selenophene and its cytotoxicity.

During the reaction of methyl 3ß-acetoxy-glycyrrhetinate (1) with SeO2 significant amounts of a cytotoxic hitherto unprecedented triterpenoic selenophene 3 are formed. This compound stops cell proliferation and acts by apoptosis.