A Proposal for the Mechanism-of-Action of Diazoparaquinone Natural Products

Abstract

The diazoparaquinone family of antibiotics, exemplified by the naturally occurring species prekinamycin (1a), 1 kinamycin F (2), 2 and lomaiviticin A (3), 3 has had a long and storied history, featuring two major structural revisions (N-cyanocarbazole → diazofluorene for the kinamycins, 4 and diazobenzo [b]fluorene → diazobenz[a]-fluorene for isoprekinamycin (not shown) 5 ). Following these corrections, two mechanism-of-action hypotheses have been proposed. Jebaratnam, using diazofluorene as a model system, suggested that oxidative activation of the diazo function might lead to DNA damaging reactive oxygen species via a putative C(11) radical. 6 Dmitrienko, employing the isoprekinamycin structure as a probe molecule, favored an alternative mechanism whereby enhanced diazonium character of the N 2 group as a consequence of an internal hydrogen bond network provided a site for nucleophilic attack by DNA amino groups, possibly leading to DNA damaging C(11) radicals as well. 7 A disclosure by He et al. on the structure and biological activity of lomaiviticin A (3) attributed its profound cytotoxicity (IC 50 's of 0.7-6.0 nM for a variety of tumor cell lines) 3 to double-stranded DNA cleavage under reducing conditions.Correspondence to: Ken S. Feldman, ksf@chem.psu.edu. Supporting Information Available: Experimental procedures and characterization data for 10a-e and 11a-g. This material is available free of charge via the Internet at http://pubs.acs.org. Neither Jebaratnam nor Dmitrienko utilized a diazoparaquinone-containing species for their studies, and an alternative mechanism-of-action that directly incorporates both this specific functionality and the reductive activation observation of He can be envisioned (Scheme 1). This proposal does not stray far from orthodox and precedented mechanisms of known paraquinoid antitumor agents, such as mitomycin and the anthracyclins. 8 The observation that lomaiviticin A operates through bioreductive activation is suggestive of one-electron addition to the quinone of generic diazoparaquinone 4 to give a reactive semiquinone 5 following protonation (not obligatory). The radical 5 so generated can be represented by the resonance form 6. This pivotal radical (or radical anion) 5/6 can furnish the second key intermediate of this hypothesis, a C(11) radical 7, provided that C(11) is pyramidalized sufficiently to permit adequate overlap between the enol (enolate) orbitals and σ* C-N for rapid loss of N 2 . If this sequence occurs in proximity to DNA, then H-atom abstraction seems plausible by analogy with the similar reaction of a related indenyl monoradical derived from neocarzinostatin and p-hydroxythiophenol, 9 and the hydroxymethylacylfulvene-containing intermediate 8 along with a DNA radical will be formed. At this point in the mechanistic speculation, there are two distinct avenues by which these two species, 8 and DNA • , might react further to lead to the observed result with lomaiviticin A. Generation of a DNA-based radical (C(4′), C(5′), or C(1′)) itself may be sufficien...

Mechanism of the cytotoxicity of the diazoparaquinone antitumor antibiotic kinamycin F

“…While it has been shown that the kinamycin analog prekinamycin forms radical-trapping arene products that involved the loss of dinitrogen in organic aromatic solvents containing a radical initiator and an organic tin reducing agent [8,9], the non-physiological conditions of these studies make direct comparisons with our study problematic.…”

“…Recent studies on prekinamycin, a precursor analog of kinamycin, showed that in aromatic solvents containing a radical initiator and an organic tin reducing agent, radical-trapping arene products were formed that involved the loss of dinitrogen to form a postulated reactive radical intermediate and an orthoquinonemethide electrophile [8,9]. A role for reductive activation of kinamycin D by DTT in its DNA-cleaving ability has also recently been suggested [10].…”

Kinamycins A and C, bacterial metabolites that contain an unusual diazo group, as potential new anticancer agents: antiproliferative and cell cycle effects

“…While kinamycin C has been subjected to the 60-cell National Cancer Institute panel, kinamycin A has not. More recently prekinamycin, a precursor analog of kinamycin, was shown to form a phenyl adduct in the presence of a mild organic reductant and a radical initiator in benzene [6]. The kinamycins were initially assigned an N-cyanocarbazole structure and it was initially thought that, due to similarities of the indoloquinone system to that of the clinical antitumor agent mitomycin C, the kinamycins likely functioned as reductively activated DNA alkylating agents by a mechanism related to that for the mitomycins [3].…”

“…In the 60-cell panel, kinamycin C generally displayed submicromolar GI 50 values (http://dtp.nci.nih.gov). Prekinamycin was proposed to undergo a two-step mechanism involving loss of dinitrogen to form a reactive radical intermediate that could damage biological components [6]. We [4] and another group [5], however, independently determined that the structures assigned to the kinamycins were incorrect, and that the kinamycins are in fact derivatives of diazobenzo [b]fluorene, rather than of Ncyanobenzo[b]carbazole.…”

Synthesis of the Benzo[b]fluorene Core of the Kinamycins by Arylalkyne–Allene and Arylalkyne–Alkyne Cycloadditions

“…[10,11] This loss of dinitrogen is probably initiated by a one-electron addition to the p-quinone. [12] Lomaiviticins A (9) and B (10) are particularly interesting dimeric compounds, as they show very potent antitumor activity, in addition to antibacterial activity against Staphylococcus aureus. [13] The novel structures and antitumor activities of the kinamycins have stimulated the interest for the development of their total synthesis.…”