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A main problem of common cancer chemotherapy is the occurrence of severe side effects caused by insufficient selectivity of the applied drugs. A possible concept to overcome this limitation is light-driven prodrug monotherapy. The synthesis as well as photochemical and biological evaluation of new photoactivatable prodrugs is described. Best results were obtained with prodrug (S,S)-7a. The photochemical labile protecting groups in (S,S)-7a can easily be removed by irradiation with UV-A light in 30 min with a power of only 2 J cm(-2). The determination of the in vitro cytotoxicity by using an HTCFA-test reveals a QIC(50) value of 8200 and the prodrug is more than two million times less cytotoxic than the corresponding seco-drug (-)-(S,S)-5 with an IC(50) value of about 110 fM. The big therapeutic window makes (S,S)-7a very suitable for its use in selective cancer therapy., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

For a better understanding of the mode of action of duocarmycin and its analogs, the novel fluorescent duocarmycin derivatives 13-15 and 17b-19b were synthesized, and their bioactivity as well as their cellular uptake investigated using confocal laser scanning microscopy (CLSM) in live-cell imaging experiments., (Copyright © 2012 Verlag Helvetica Chimica Acta AG, Zürich.)

Chemotherapy of malign tumors is usually associated with serious side effects as common anticancer drugs lack selectivity. An approach to deal with this problem is the antibody-directed enzyme prodrug therapy (ADEPT) and the prodrug monotherapy (PMT). Herein, the synthesis and biological evaluation of new glycosidic prodrugs suitable for both concepts are described. All prodrugs but one are stable in human serum and show QIC(50) values (IC(50) of prodrug/IC(50) of prodrug in the presence of the appropriate glycohydrolase) of up to 6500. This is the best value found so far for compounds interacting with DNA., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis of the first spacer containing, duocarmycin analogue prodrug was realised, its biological properties evaluated and compared to its counterpart prodrug without a spacer unit. The synthesis comprises the manufacture of the new acetylated derivatives and of two double spacer systems, their activation and coupling to the pharmacophoric seco-drug (+)-. Unprecedented biological results were found as the new prodrug showed a fairly low QIC(50) value of 20, but on the other hand a high stability and very low DNA alkylation efficiency. These findings indicate a changed cytostatic mode of action induced by the self-immolative spacer moiety which was employed.

The natural antibiotics CC‑1065 and the duocarmycins are highly cytotoxic compounds which however are not suitable for cancer therapy due to their general toxicity. We have developed glycosidic prodrugs of seco-analogues of these antibiotics for a selective cancer therapy using conjugates of glycohydrolases and tumour-selective monoclonal antibodies for the liberation of the drugs from the prodrugs predominantly at the tumour site. For the determination of structure activity relationships of the different seco-drugs, experiments addressing their interaction with synthetic DNA were performed. Using electro-spray mass spectrometry and high performance liquid chromatography, the experiments revealed a correlation of the stability of these drugs with their cytotoxicity in cell culture investigations. Furthermore, it was shown that the drugs bind to AT-rich regions of double-stranded DNA and the more cytotoxic drugs induce DNA fragmentation at room temperature in several of the selected DNA double-strands. Finally, an explanation for the very high cytotoxicity of CC-1065, the duocarmycins and analogous drugs is given.

Circular dichroism (CD) spectroscopy is a well-known method for the analysis of chiral chemical compounds and is often used for studying the structure and interaction of proteins, DNA and bioactive compounds in solution. Here we demonstrate that CD spectroscopy is also a powerful tool for investigating the cellular uptake and mode of action of drugs in live cells. By means of CD spectroscopy, we identified DNA as the cellular target of several novel anticancer agents based on the highly cytotoxic natural antibiotic CC-1065. Furthermore, time-dependent changes in the CD spectra of drug-treated cells enabled us to rationalize differences in drug cytotoxicity. The anticancer agents rapidly penetrate the cell membrane and bind to cellular DNA as their intracellular target. Thereby, the formation of a reversible noncovalent complex with the DNA is followed by a covalent binding of the drugs to the DNA and the more toxic compounds show a higher stability and a lower alkylation rate. Since no drug manipulation is necessary for this kind of investigation and achiral compounds bound to chiral biomolecules may also show induced CD signals, CD spectroscopy of live cells is not limited to the study of analogues of CC-1065. Thus, it constitutes a general approach for studying the mode of action of bioactive compounds on the cellular and molecular level.

In the course of our chemical and biological screening program for yet unidentified microbial metabolites, we selected plants of Traditional Chinese Medicine (TCM) as habitats for talented Streptomycetes producer strains for the first time. Liquid pure cultures of strain Streptomyces sp. GS DV232 were found to contain 4-methyl-2-quinazolinamine (1), a potent alkaloid yet unknown from nature. In this study, we investigated the chemical and crystal structure of 1, as well as its antiproliferative bioactivity, and addressed the unusual biosynthesis using feeding experiments.

The synthesis and biological evaluation of novel prodrugs for use in the antibody directed enzyme prodrug therapy (ADEPT) of cancer based on the cytotoxic antibiotic duocarmycin SA (1) are described. In this approach, we investigated the influence of the sugar moiety of the glycosidic prodrug on the QIC(50) values as well as on the stability and the water solubility. The best result was found for prodrug 22 containing an alpha-mannoside moiety with a QIC(50) value of 4500.

A severe limitation in cancer therapy is the often insufficient differentiation between malign and benign tissue using known chemotherapeutics. One approach to decrease side effects is antibody-directed enzyme prodrug therapy (ADEPT). We have developed new glycosidic prodrugs such as (-)-(1S)-26 b based on the antibiotic (+)-duocarmycin SA ((+)-1) with a QIC(50) value of 3500 (QIC(50)=IC(50) of prodrug/IC(50) of prodrug+enzyme) and an IC(50) value for the corresponding drug (prodrug+enzyme) of 16 pM. The asymmetric synthesis of the precursor (-)-(1S)-19 was performed by arylation of the enantiomerically pure epoxide (+)-(S)-29 (> or = 98 % ee).

The synthesis and biological evaluation of novel prodrugs based on the cytotoxic antibiotic duocarmycin SA (1) for a selective treatment of cancer using a prodrug monotherapy (PMT) are described. Transformation of the phenol 8 with the glucuronic acid benzyl ester trichloroacetimidate 9b followed by reaction with DMAI x HCl (10) gives the glucuronide 11b, which is deprotected to afford the desired prodrug 4a containing a glucuronic acid moiety. In addition, the prodrug 4b with a glucuronic methyl ester unit is prepared. The cytotoxicity of the glucuronides is determined using a HTCFA-assay with IC(50) values of 610 nM for 4a and 3300 nM for 4b. In the presence of beta-glucuronidase, 4a expresses an IC(50) value of 0.9 nM and 4b of 2.1 nM resulting in QIC(50) values of about 700 for 4a and 1600 for 4b.

A novel carbamate prodrug 2 containing a pentagastrin moiety was synthesized. 2 was designed as a detoxified analogue of the highly cytotoxic natural antibiotic duocarmycin SA (1) for the use in a targeted prodrug monotherapy of cancers expressing cholecystokinin (CCK-B)/gastrin receptors. The synthesis of prodrug 2 was performed using a palladium-catalyzed carbonylation of bromide 6, followed by a radical cyclisation to give the pharmacophoric unit 10, coupling of 10 to the DNA-binding subunit 15 and transformation of the resulting seco-drug 3b into the carbamate 2 via addition of a pentagastrin moiety.

Two high-yielding strategies for the synthesis of 4H-anthra[1,2-b]pyran antibiotics have been developed giving access to novel antitumor agent (ED(50) 1.5 microm) and to (S)-espicufolin (3). A key step for the assembly of the tetracyclic 4H-anthra[1,2-b]pyran-4,7,12-trione skeleton is the nucleophilic addition of an aryl lithium species onto an aldehyde which allows the introduction of either an ynone or 1,3-diketo side chain, serving as precursors for an acid-catalysed cyclisation.

Novel diastereomerically pure beta-D-galactosidic prodrugs (+)-12 a-e of the cytotoxic antibiotics CC-1065 and the duocarmycins were prepared for an antibody directed enzyme prodrug therapy (ADEPT) using 4 as a substrate via a radical cyclization to give rac-5 and rac-6 followed by a chromatographic resolution of the enantiomers of rac-5, glycosidation and linkage to the DNA-binding units 10 a-e. These only slightly toxic compounds can be toxified enzymatically by an antibody-beta-D-galactosidase conjugate at the surface of malignant cells to give the cytotoxic drugs, which then alkylate DNA. The new prodrugs were tested in in vitro cytotoxicity assays showing excellent QIC(50) values of 4800 and 4300 for (+)-12 a and (+)-12 b, respectively. The absolute configuration of precursor (+)-5 was determined by comparison of the experimental CD spectrum with the theoretically predicted CD spectra and by X-ray structure analysis.

The synthesis of the novel unprotected carboranyl C-glycosides 2 and 20-24 starting from ethynyl C-glycosides 1, 5-8, 10, and 13 is described. The new compounds are highly water-soluble and display only a very low cytotoxicity, which makes them promising candidates for use in boron neutron capture therapy for the treatment of cancer.

Novel prodrugs of the cytotoxic antibiotic CC-1065 for an antibody-directed enzyme prodrug therapy (ADEPT) were prepared that show an excellent selectivity with a high toxicity of the corresponding drug. In particular, the seco-CBI analogue of CC-1065, 1-chloromethyl-5-hydroxy-1,2-dihydro-3H-benz[e]indole, as well as the novel methyl-seco-CBI analogue 1-(1'-chloroethyl)-5-hydroxy-1,2-dihydro-3H-benz[e]indole, were synthesized and transformed into their galactosides 10 a and 10 b, respectively. These galactosides can be cleaved with beta-D-galactosidase to give the free cytotoxic compounds. They were tested in in vitro cytotoxicity assays by using human bronchial carcinoma cells of line A549 in the presence and in the absence of beta-D-galactosidase. While the seco-CBI prodrugs revealed only modest selectivity, prodrugs of the methyl-seco-CBI analogue bearing an anti orientation of the substituents at the two stereogenic centers of the N-heterocycle displayed an excellent selectivity with an ED(50) quotient of about 750. The cytotoxicity of the corresponding phenol was rather high, with an ED(50) of 1.3 nM. The diastereomer with a syn orientation at the stereogenic centers was much less toxic.

Hetero-Diels-Alder reaction of the steroidal exocyclic enol ethers 14 and 15, obtained from the secoestrones 8 and 9 by reduction, iodoetherification, and elimination, with ethyl O-benzoyldiformylacetate (16) leads to the spiroacetals 17 and 18 as a mixture of four diastereomers. Reduction of the major diastereomers 17a and 18a with DIBAH and subsequent hydrogenation yields the novel natural product hybrids 21, 23, 24, and 25, which possess the structural features of the steroid estrone (7) and the mycotoxin talaromycin 6.

The synthesis of new ortho-carboranyl lactosides 8, 17, 19 and glucosides 22 and 23 for the use in boron neutron capture therapy is reported. Carboranyl lactosides 17 and 19 as well as the glucosides 22 and 23 contain a fluorine atom to allow a noninvasive determination of these compounds in tumor cells by 19F-NMR spectroscopy. In cloning efficiency tests on human bronchial carcinoma cells the carboranyl lactosides 17 and 19 displayed almost no cytotoxicity. Thus, the considerably cytotoxic carboranyl alcohol 11 is detoxified when linked to a sugar moiety such as in carboranyl glucoside 22.