Pyrindamycin A

The duocarmycins and (+)-CC-1065 are amongst the most potent antitumour antibiotics discovered to date and yet have not progressed into the clinic. The natural products are extremely stable to nucleophilic attack until bound to their DNA target and are not substrates for any other biological nucleophile. The mechanism for this target activation of the duocarmycins is discussed with relation to both an acid-catalyzed activation and a binding-induced conformational change leading to ground state destabilization. It is suggested that targeting of the duocarmycins to their site of action in a tumour may be more important than introducing systemically-activated prodrugs as the natural product itself can be considered to be a type of prodrug, activated only on binding to its targets. Methods that have been used to target CC-1065 and the duocarmycins are reviewed as well as efforts towards systemically activated prodrugs. A simple analysis of the approaches that could be taken to vary the structure for targeting is suggested.

The design, synthesis and biological evaluation of novel seco-iso-cyclopropylfurano[2,3-e]indoline (seco-iso-CFI) and the seco-cyclopropyltetrahydrofurano[2,3-f]quinoline (seco-CFQ) analogues of the duocarmycins are described. These novel analogues (4-7) were designed on the premise that the lone pair of electrons on the furano-oxygen atom could enter into conjugation with the isocyclopropylfurano[e]indolone (iso-CFI) alkylating moiety, formed from the loss of HCl in compounds 4-7. The seco-iso-CFI DNA alkylating pharmacophore was synthesized through a well precedented approach of 5-exo-trig aryl radical cyclization with a vinyl chloride. In our studies, in addition to the formation of the seco-iso-CFI product, an equal amount of an unexpected seco-CFQ product was also generated during the radical cyclization reaction. Like CC-1065 and adozelesin, using Taq DNA polymerase stop and thermal cleavage assays, the seco-iso-CFI compounds (4 and 6) and the seco-CFQ compounds (5 and 7) were shown to preferentially alkylate the adenine-N3 position within the minor groove of long stretches of A residues. A MM2 energy optimized molecular model of a 1:1 complex of compound 6 with DNA reveals that the iso-CFI compound fits snugly within the minor groove. Using a MTT based experiment, the cytotoxicity of compounds 4-7 were determined against the growth of murine leukemia (L1210), mastocytoma (P815) and melanoma (B16) cell lines. The concentrations of compounds required to inhibit the growth of these tumor cells by 50% is in the range of 10(-8)M. These compounds were also tested against a panel of human cancer cells by the National Cancer Institute, demonstrating that the compounds exhibited a high level of activity against selected solid tumors. At a concentration of 0.0084 microM (based on the IC(50) of compound 17 (seco-CBI-TMI) against the growth L1210 cells), while compounds 4 and 17 were toxic against murine bone marrow cells as judged by a colony forming study of freshly isolated murine progenitor hematopoeitic cells, compound 5, a seco-CFQ compound, was significantly less toxic. Flow cytometric analysis of P815 cells that had been incubated for 24h with compounds 4 and 5 at their cytotoxic IC(50) concentrations indicated the induction of apoptosis in a large percentage of cells, thereby suggesting that this might be the mechanism by which the iso-CFI compounds kill cells.

Racemic seco-iso-CFI (cyclopropylfurano[e]indoline) analogs of the duocarmycins and CC-1065 have recently been reported by our group. These compounds covalently react with AT-rich sequences of DNA, and they exhibit potent cytotoxicity against cancer cells but are less toxic to normal bone marrow cells. This article details the synthesis of enantiomerically pure (S)-(-)- and R-(+)-seco-iso-CFI (cyclopropylfurano[e]indoline)-5,6,7-trimethoxyindole-2-carboxamide analogs, (S)-(-)-1 and (R)-(+)-1, respectively. The covalent DNA binding properties and cytotoxicity of both enantiomers against L1210 murine leukemia and B16 murine melanoma cells grown in culture are reported and compared to racemate (+/-)-1. The natural (S)-(-)-enantiomer of 1 is more reactive with DNA and more cytotoxic than its unnatural mirror image and the racemic mixture.