Antitumor agents. 148. Synthesis and biological evaluation of novel 4 beta-amino derivatives of etoposide with better pharmacological profiles

Recent progress on C-4-modified podophyllotoxin analogs as potent antitumor agents

Podophyllotoxin (PPT), as well as its congeners and derivatives, exhibits pronounced biological activities, especially antineoplastic effects. Its strong inhibitory effect on tumor cell growth led to the development of three of the most highly prescribed anticancer drugs in the world, etoposide, teniposide, and the water-soluble prodrug etoposide phosphate. Their clinical success as well as intriguing mechanism of action stimulated great interest in further modification of PPT for better antitumor activity. The C-4 position has been a major target for structural derivatization aimed at either producing more potent compounds or overcoming drug resistance. Accordingly, numerous PPT derivatives have been prepared via hemisynthesis and important structure-activity relationship (SAR) correlations have been identified. Several resulting compounds, including GL-331, TOP-53, and NK611, reached clinical trials. Some excellent reviews on the distribution, sources, applications, synthesis, and SAR of PPT have been published. This review focuses on a second generation of new etoposide-related drugs and provides detailed coverage of the current status and recent development of C-4-modified PPT analogs as anticancer clinical trial candidates.

Design, synthesis and cytotoxic activity of novel sulfonylurea derivatives of podophyllotoxin

Three series of novel sulfonylurea podophyllotoxin derivatives were designed, synthesized, and evaluated for in vitro cytotoxicity against four tumor cell lines (A-549, DU-145, KB and KBvin). Compounds 14c (IC₅₀: 1.41-1.76 μM) and 14e (IC₅₀: 1.72-2.01 μM) showed superior cytotoxic activity compared with etoposide (IC₅₀: 2.03 to >20 μM), a clinically available anticancer drug. Significantly, most of the compounds exhibited comparable cytotoxicity against the drug-resistant tumor cell line KBvin, while etoposide lost activity completely. Preliminary structure-activity relationship (SAR) correlations indicated that the 4'-O-methyl functionality in podophyllotoxin analogues may be essential to maintain cytotoxic activity, while an arylsulfonylurea side chain at podophyllotoxin's 4β position can significantly improve cytotoxic activity.

Novel Nitrobenzazolo[3,2-a]quinolinium Salts Induce Cell Death through a Mechanism Involving DNA Damage, Cell Cycle Changes, and Mitochondrial Permeabilization

This study reports the capacity of three nitro substituted benzazolo[3,2-a]quinolinium salts NBQs: NBQ 95 (NSC-763304), NBQ 38 (NSC 763305), and NBQ 97 (NSC-763306) as potential antitumor agents. NBQ's are unnatural alkaloids possessing a positive charge that could facilitate interaction with cell organelles. The anticancer activities of these compounds were evaluated through the National Cancer Institute (NCI) 60 cell line screening which represents diverse histologies. The screening was performed at 10 µM on all cell lines. Results from the NCI screening indicated cytotoxicity activity on six cell lines. In order to explore a possible mechanism of action, a detailed biological activity study of NBQ 95 and NBQ 38 was performed on A431 human epidermoid carcinoma cells to determine an apoptotic pathway involving, cell cycle changes, DNA fragmentation, mutations, mitochondrial membrane permeabilization and caspases activation. DNA fragmentation, cell cycle effects, mutagenesis, mitochondrial permeabilization and activation of caspases were determined by fluorimetry and differential imaging. Our data showed that A431 growth was inhibited with an average IC50 of 30 µM. In terms of the mechanism, these compounds interacted with DNA causing fragmentation and cell cycle arrest at sub G0/G1 stage. Mutagenesis was higher for NBQ 38 and moderate for NBQ 95 Mitochon-drial permeabilization was observed with NBQ 38 and slightly for NBQ 95. Both compounds caused activation of Caspases 3 and 7 suggesting an apoptotic cell death pathway through an intrinsic mechanism. This study reports evidence of the toxicity of these novel compounds with overlapping structural and mechanistic similarities to ellipticine, a known anti-tumor compound.

N-hydroxyethyl-4-aza-didehydropodophyllotoxin derivatives as potential antitumor agents

Three different series of N-hydroxyethyl aza-podophyllotoxin derivatives containing (i) a five-membered methylenedioxy ring (7a-f), (ii) a five-membered ring with no heteroatom (8a-f) or (iii) a six membered ethylenedioxy ring (9a-f) as ring A were synthesized using a convenient one-pot multi-component reaction. Further variation on ring E was done by decorating it with methoxy and hydroxy groups at different positions. Calculation of logP values of these compounds indicates them to be better soluble than corresponding non-hydroxy derivatives. These novel aza-podophyllotoxin derivatives were screened for their cytostatic and cytotoxic activities on National Cancer Institute's 60 human tumor cell lines to study the structure activity relationship. The overall anticancer activity of these compounds was in the order of 8a-f>9a-f>7a-f. Furthermore, the compounds having 3'-methoxy and 3',4',5'-trimethoxy substitution at ring E were the most active within the series. The cytotoxicity of all the active compounds was low, while their antiproliferative (or cytostatic) activity was high, providing a wide therapeutic window for their potential application as anticancer drugs.

Plant-based anticancer molecules: a chemical and biological profile of some important leads

A number of natural products, with diverse chemical structures, have been isolated as anticancer agents. Several potential lead molecules such as camptothecin, vincristine, vinblastine, taxol, podophyllotoxin, combretastatins, etc. have been isolated from plants and many of them have been modified to yield better analogues for activity, toxicity or solubility. Several successful molecules like topotecan, irinotecan, taxotere, etoposide, teniposide, etc. also have emerged as drugs upon modification of these natural leads and many more are yet to come. In this review, the authors have focused on four important anticancer leads, that is, camptothecin, taxol, combretastatin A-4 and podophyllotoxin. Their chemistry, structure and activity relationships, biological activities, modes of action, analogue synthesis and future prospects have been discussed.

Hydrolytically activated etoposide prodrugs inhibit MDR-1 function and eradicate established MDR-1 multidrug-resistant T-cell leukemia

Effective therapy of high-risk leukemia with established cytotoxic drugs may be limited by poor antitumor efficacy, systemic toxicity, and the induction of drug resistance. Here, we provide the first evidence that hydrolytically activated prodrugs may overcome these problems. For this purpose, VP16 was functionally blocked by hydrolytically cleavable carbonate linkers with unique characteristics to generate 2 novel prodrugs of VP16. First, we established a more than 3-log higher efficacy of the 2 prodrugs compared with VP16 on a panel of naturally drug-resistant tumor cell lines. Second, the prodrugs did overcome VP16-induced multidrug resistance-1 gene (MDR-1)-mediated multidrug resistance in vitro in a newly established VP16-resistant T-cell leukemia cell line MOVP-3 by functionally blocking MDR-1-mediated efflux. Third, in vivo studies showed a maximum tolerated dose of ProVP16-II (> 45mg/kg), which was at least 3-fold higher than that of VP16 (15 mg/kg). Finally, tests of ProVP16-II in a multidrug-resistant xenograft model of T-cell leukemia expressing MDR-1 indicated that only the mice treated with this prodrug revealed a complete and long-lasting regression of established, drug-resistant leukemia. In summary, the hydrolytically activated etoposide prodrugs proved effective against multidrug-resistant T-cell leukemia in vitro and in vivo and provide proof of concept for a highly promising new strategy for the treatment of MDR-1 drug-resistant malignancies.

Synthesis, hydrolytic activation and cytotoxicity of etoposide prodrugs

Two 4'-propylcarbonoxy derivatives (2,3) of etoposide (1), a topoisomerase II inhibitor, were synthesized and evaluated as potential prodrugs for anticancer therapy. Their activation via hydrolysis mechanisms was determined as a function of pH in buffer solutions, in human serum and in the presence of carboxyl ester hydrolase. Cytotoxicity was determined on various tumor cell lines and compared to the parent compound. On cell lines exhibiting resistance to etoposide we observed an enhanced cytotoxicity of the prodrugs of up to three orders of magnitude.

Facile and improved synthesis of 4 beta-aminopodophyllotoxin congeners

An efficient synthesis of 4 beta-aminopodophyllotoxin from 4 beta-bromopodophyllotoxin using ammonia, and also a facile synthesis of 4 beta-amino-4'-O-demethylpodophyllotoxin from 4 beta-azidopodophyllotoxin by simultaneous azido reduction and selective demethylation at 4'-position employing chlorotrimethylsilane and sodium iodide, has been described. These are potential precursors for the various 4 beta-amino analogs of podophyllotoxin possessing DNA topoisomerase II inhibition activity.

Discovery of podophyllotoxins

This review deals with the historical discovery of particularly important lignan derivatives used in cancer chemotherapy. From isolation of the naturally occurring podophyllotoxin, an inhibitor of microtubule assembly, to hemisynthesis of the clinically important anticancer drugs etoposide and teniposide, it will be demonstrated how the activities and the ability of this class of compounds to inhibit topoisomerase II were discovered by different research teams. By virtue of these discoveries, new hemisynthetic derivatives, with different mechanisms of action, are bringing improvements in the ability to treat cancer.