chemistry and medicine since this group of compounds contains some of the most potent anti-tumour
antibiotics known. They are about a thousand times more toxic than the clinically used adriamycinand
anthracycline antibiotics and generally induce apoptosisthrough a caspase-mediated mitochondrial
amplification loop.^54 In particular, the high cytotoxicity of calicheamicin has been harnessed by means of
antibody directed drug delivery^55 to target human myeloid leukaemia tumour cells in a Food and Drug
Administration (FDA) approved product, Mylotarg™.
The mode of action of the enediyne antibiotics involves single or double strand scission of DNA and
depends on the formation of carbon radicals followed by hydrogen atom abstraction from specific
nucleotides. The key enediyne reaction is an electrocyclisation, as reported by Bergman in 1972, which
generates a 1,4-benzenoid diradical (Figure 8.26b). In the case of the very unstable antibiotic C-1027, this
process takes place merely on warming the antibiotic to 50°C in solution in ethanol. For the other com-
pounds, a distortion of the enediyne ring inhibits the Bergman cyclisation.^56 The rearrangement is ‘triggered’
by a chemical process that relaxes the ring and so facilitates a change in conformation of the enediyne to
allow the electrocyclisation to take place spontaneously. In the cases of NCS, calicheamicin and esperam-
icin, this trigger is the attack of a thiol, possibly glutathione, at the site indicated (arrows in Figure 8.26a).
In the case of dynemicin, opening of the epoxide follows initial biological reduction to a quinol by nucleo-
philic attack at the position indicated.
While the enediyne provides the warhead for strand breaking, sequence selectivity and orientation of
the enediyne to the DNA target is delivered by minor groove binding and/or intercalation of peripheral
components of these antibiotics.
The neocarzinostatinchromophore acts primarily by single-strand cleavage of DNA, and this requires
oxygen. NCS first intercalates its naphthoate residue into the DNA duplex, which positions the remainder
of the molecule in the minor groove (Section 8.7.1). Following activation of the molecule by thiol addition
at C-12, Bergman cyclisation generates a benzenoid diradical, which abstracts a 5-hydrogen atom from a
residue in the DNA recognition sequence.^57 Such action takes place preferentially at dA and dT sites with
at least 80% of the DNA cleavage resulting in the formation of 5-aldehydes of A and T residues. Less than
20% of strand breaks result from pathways initiated by a second hydrogen abstractionin the alternate
strand from a deoxyribose at C-4or C-1.
The NMR structure in solution of a complex formed between calicheamicinand a DNA hairpin con-
taining the preferred recognition sequence d(T4-C5-C6-T7)(A17-G18-G19-A20) has been determined by
Patel (Figure 8.27).^58 Sequence-specific binding of calicheamicin 1 Ito the (T-C-C-T) containing DNA
hairpin duplex is favoured by the complementarity of fit through hydrophobic and hydrogen-bonding
interactions between the antibiotic and the floor and walls of the minor groove of a minimally perturbed
DNA helix (Section 9.7). Calicheamicin 1 Ibinds with its arene-tetrasaccharide segment in an extended
conformation spanning the (T-C-C-T)(A-G-G-A) segment of the duplex minor groove. Its thiol-sugar and
thiobenzoate rings are inserted in an edgewise manner deep into the minor groove with their faces sand-
wiched between its walls, where hydrophobic and hydrogen-bonding interactions account for the (TCCT)
sequence recognition in the complex (Figure 8.27a).
This positioning of the arene-tetrasaccharide moiety orientates the enediyne ring deep in the minor
groove, spanning both strands, such that its pro-radical carbon centers, C-3 and C-6, are proximal to the
anticipated H-5and H-4sites for hydrogen atom abstraction. When a thiolate nucleophile adds in
Michael fashion to the proximate ,-unsaturated ketone, the resulting change in geometry of the
enediyne triggers the Bergman cyclisation to generate a benzenoid 3,6-diradical. This is suitably orien-
tated to abstract one 5-hydrogen atom from the first deoxycytidine residue in the recognition sequence
d(TCCT) and a second 4-hydrogen atom from the opposite strand and this leads oxidatively to a double
strand cleavageprocess (Figure 8.28). It is worth noting that the affinity of calicheamicin for DNA has
been increased 1000-fold through synthesis of head-to-head and head-to-tail dimers with significant bene-
fit to sequence-selectivity for TCCT and ACCT sequences.^59
Esperamicin A 1 works in a similar fashion but with low sequence selectivity and favours cleavage
at TCAG. Its binding to DNA involves a combination of upstream intercalation of an
314 Chapter 8