B. A. Baviskar1, S. S. Khadabadi2, S. L. Deore1*, R. P.Marathe1
1Government College of Pharmacy, Kathora Naka,
Amravati – 444604, MS, INDIA.
2Government College of Pharmacy, Aurangabad Opp. Govt. Polytechnic, Osmanpura, Aurangabad-431005,
Abstract: Topoisomerase is an enzyme that alters/regulates the super coiling of double-stranded DNA by transiently cutting one or both strands of the DNA and hence important target for anticancer activity. The present review articles is focusing a light on points like What is topoisomerase, its types, mode of action of cancer inhibition, Top-I targeted anticancer drugs, Top-II targeted anticancer drugs and Topoisomerase Cellular resistance.
Cancer is one of the most dreaded diseases of the 20th century and spreading further with continuance and increasing incidence in 21st century. Cancer, due to anomaly in mitotic cell cycle, is the second common cause of death in developed countries. In the United States, as the leading causes of death, it accounts for 25% of all the deaths in humans presently. In industrialized countries, approximately one in five die of cancer. According to WHO, out of an estimated total of 50 million deaths annually in the world, more than 5 million are attributed to Cancer and the number of deaths from cancer throughout the world is increasing. Approximately 500,000 new cases of cancer occur every year in India. As the human life span is increasing in India, more cases of cancer are observed here also. Cancer is thus regarded as an emerging health problem in India.
There are three main types of topology: supercoiling, knotting and catenation to keep DNA as compact as possible essential for the survival of eukaryotic and prokaryotic cells. However, when transcription or replication occurs, DNA must be free, and these states seriously hinder the processes. (1) During replication, the newly replicated duplex of DNA and the original duplex of DNA become intertwined and must be completely separated in order to ensure genomic integrity as a cell divides. One of the most essential topological problem occurs at the very end of replication, when daughter chromosomes must be fully disentangled before mitosis occurs. Topoisomerase IIA plays an essential role in resolving these topological problems. Topoisomerases are enzymes that interconvert different topological states of DNA. Topoisomerases are isomerase enzymes that act on the topology of DNA. Topoisomerases catalyze and guide the unknotting of DNA by creating transient breaks in the DNA using a conserved Tyrosine as the catalytic residue.
Topoisomerase enzyme was originally termed as gyrase. It is an enzyme that alters/regulates the supercoiling of double-stranded DNA by transiently cutting one or both strands of the DNA.  This is called as transient breaking and rejoining of DNA strands. DNA Topoisomerase aids the transcription and replication of DNA. As well as DNA topoisomerase I and DNA topoisomerase II, there is DNA topoisomerase III and DNA topoisomerase IV. DNA Topoisomerase III may regulate recombination. DNA Topoisomerase IV regulates the segregation of newly replicated chromosomes. These enzymes are essential for DNA replication and are important targets for anti viral, anti bacterial and anti tumor drugs. These breaks in DNA accumulate, ultimately leading to programmed cell death, or apoptosis by hijacking the natural ability of topoisomerase to create breaks in chromosomal DNA. However these enzymes are structurally and mechanistically different. 
What is topoisomerase-I:
It is nomenclatures as type I: EC 18.104.22.168. Topoisomerase type I cuts one strand of DNA. Type I topoisomerases are additionally divided into two subtypes: A and B. Enzymes belonging to the subtype A have a complex mechanism of action that involves passage of the uncut strand through the enzyme-bridged cleavage of the other strand. Interestingly, while acting on DNA with nicks or with single-stranded regions, type IA topoisomerases can cleave the continuous strand and allow the passage of a segment of duplex DNA of the same or another DNA molecule through the cut strand. Topoisomerases of the subtype IB act by a simpler mechanism that involves free rotation of DNA at the transient nick site. 
· Type IA topoisomerases, which share many structural and mechanistic features with the type II topoisomerases, type IA topoisomerases form a covalent intermediate with the 5' end of DNA,
· Type IB topoisomerases, which utilize a controlled rotary mechanism. IB topoisomerases form a covalent intermediate with the 3' end of DNA.
· Type IC topoisomerase has been identified, called topo V. While it is structurally unique from type IA and IB topoisomerases, it shares a similar mechanism with type IB topoisomerase.
Topoisomerase I has several unusual features. Unlike type II topoisomerases, topoisomerase I does not require ATP hydrolysis to catalyze the complex topological rearrangements of DNA for which it is responsible. Whereas most enzymes involved in complex rearrangements of DNA are oligomeric, topoisomerase I appears to be a fully functional monomer. 
Top-I targeted anticancer drugs:
Top1 inhibitors can be grouped into two main classes: top1 poisons and top1 suppressors. Top1 poisons kill cells by trapping cleavage complexes rather than inhibiting top1 catalytic activity. 
Anticancer Mode of Action of topoisomerase-I
Topoisomerase-I is active during S phase, forms and rejoins single strand breaks in DNA which reduce torsion strain ahead of the DNA replication fork. Camptothecin and topotecan inhibit Topoisomerase I and induces DNA strand breaks localized near replication forks. Toisomerase I inhibitors are camptothecin and topotecan The mechanism of action of topoisomerase I inhibitors is Induce DNA strand breaks localized near replication forks by inhibiting religation of the broken DNA strand. 
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