E Speramicin- a Cytotoxic Antitumor Antibiotic

Home , Calicheamicin, Esperamicin, Neocarzinostatin

E E speramicin- a cytotoxic antitumor antibiotic

Discovery1

The antitumor antibiotic esperamicin A1 is produced by the bacterial culture actinomadura verrucosospora and was discovered by the Bristol- Meyers Squibb Company, located in Wallingford. The company concentrates on preclinical antitumor research and isolated the culture from a soil sample that was collected at Esperanza, Missiones in Argentinia. Consequently, the antibiotic is called esperamicin. Furthermore, there are also other types of esperamicin, for instance A1b, A2, A1c, A2b, A2c and P. Scientists also found out, that the different types of esperamicin A1 can be synthesized by hydrolysis. It follows, that esperamicin C and esperamicin D are less potent than esperamicin A. In face of structural analysis and biological evaluation, esperamicin A1 played an active role in experimental tumor models, such as the Lewis lung carcinoma. Apart from this, esperamicin's complex chemical structure is characterized by a bicyclo (7.3.1) ring system, an allylic trisulfide attached to the bridging atom, a 1,5-diyn-3-ene as a part of the ring system and an α,β- unsaturated ketone. Those functional groups form the central core of esperamicin. For example, esperamicin A1 also has “four sugars attached to the bicyclic core, as well as an aromatic chromophore attached to one of the sugars.2” Moreover, scientists also used different experiments to analyse the function of esperamicin A1 and its analogs. After a DNA cleavage reaction, that was first initiated only by the addition of dithiothreitol and in the second experiment by the addition of esperamicin and diothiothreitol, a nucleotid sequence analysis followed. In the experiments, materials such as different types of esperamicins, distymicin A, plasmid fragments, Eco RI, netropsin, actinomycin D and others, were used. As a result, scientists could analyze, that even after a reaction time of 1 min. esperamicin A1 was able to stimulate DNA breakage, forming open-circular and linear DNA, in presence of dithiothreitol. After up to 120 min reaction time, “extensive fragmentation of DNA was observed.3” Furthermore, reducing agents, such as NADPH and L-absorbic acid had no impact on the cleavage of esperamicin on the DNA. Only “sulfur-containing reducing agents4” accelerated the cleavage process. As another result, oxygen and oxygen-radical species did not effect the potency of esperamicin A1 on the DNA. In face of cleavage, esperamicin A1 and its analogs C and D showed similar nucleotide sequence-specific cleavage patterns, even though esperamicin A1 is more active than its analog esperamicin D and esperamicin C showing an effective activity according to DNA breakage.

1 Other Source: Cytotoxic Anticancer Drugs: Models and Concepts for Drug Discovery and Development, edited by Frederick A.Valeriote, Thomas H.Corbett and Laurence H.Baker 1992, Springer.com 2 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC286391/ 3 http://www.pnas.org/content/86/20/7672.full.pdf 4 http://www.pnas.org/content/86/20/7672.full.pdf

1 Medical impact of esperamicin A1

Esperamicin A1 main function is DNA splicing. Not only the thiol compounds of esperamicin A1 affect the breakage of the DNA but also thymidylate residues belong to the preferred cutting sides of esperamicin A1 (T>C>A>G). Consequently, this preferred “frequency of bases attacked5” differs from those of calicheamicin, neocarzinostatin and bleomycin. Whereas calicheamicin mainly cuts double strands, esperamicin cuts single strands. Furthermore, esperamicin wraps itself around the bases of the DNA helice. As a consequence of esperamicin's high activity, it is very toxic, carcinogen, mutagen, teratogenic and immunodeficient. Eventhough esperamicin can be used against tumor cells and therfore for chemotherapy, it can not differ between healthy and tumor cells. Consequently, esperamicin leads to both, the splicing of tumor DNA and healthy DNA that is followed by apoptosis of tumor and healthy cells. Nevertheless, research tries to minimize the mischievousness of esperamicin.

Structural analysis and interaction with the DNA

The structural analysis of esperamicin is known since 1987/1989 and is characterized by an oligosaccharide chain, a trisulfide group and an enediyne core, with each of them having a specific function in face of the DNA cleavage reaction. In comparison with the calicheamicins, both, esperamicin and calicheamicin share similar structures. Only esperamicin has an oligosaccharid chain that calicheamicin does not have.

The oligosaccharide chain targets where and at which base pair sequence the molecule binds to the minor groove of the DNA. In the following step, the molecule binds to the minor groove through hydrophobic interactions and hydrogen bonding of the sugar side chain with the DNA. Thus, the trisulfide group is reductive activated and produces thiolate, that performs an intramolecular Michael addition. The Michael addition leads to a change of the molecule's geometry, which changes from a trigonal bridgehead to a tetragonal centre. Last but not least, the Michael addition is followed by a Bergman cycloaromatization of the enediyn group, which leads to a p-benzyne diradical as a product. The newly formed diradical abstracts hydrogen from the DNA backbone, which forms a DNA backbone radical. Those radicals react with molecular oxygen, with a double strand cleavage as a result. Nevertheless, this double strand cleavage is the reason why esperamicin leads to permanent damage of the genetic material.

5 http://www.pnas.org/content/86/20/7672.full.pdf