Major Targets For Antineoplastic Agents Biology Essay


The nuclear enzyme DNA topoisomerase II is a major target for antineoplastic agents. A voir To clearly understand why, a closer look at the function and the mechanism of the enzyme is important. Toposisomerases are molecules which are able to wind and wind the molecule of DNA. Two major types of topoisomerases exist: type I and type II. Parler des autres. They both involve different mechanisms. During DNA transcription and replication, DNA helix needs to be unwond to allow larger enzyme such as RNA and DNA polymerases to operate on its strand in order for DNA to control the synthesis of proteins and to facilitate DNA replication. Reference a trouver, sur toposisomerase de ta daronne.

Topoisomerases II inhibitors are of two kinds. Each of them interferes with one of the step of the catalytic cycle of the enzyme. A distinction is made between topoisomerase II catalytic inhibitors and topoisomerase II poisons. Topoisomerase II catalytic inhibitors are compounds that will disrupt catalytic turnover, whereas topoisomerase II poisons will interact with the complex formed of DNA and topoisomerase II, stabilizing it.

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Topoisomerase II poisons are the most frequently used class of topoisomerase inhibitors in clinical use; they are used for their antitumor activity whereas catalytic inhibitors are used for several reasons. Depending on the compound, catalytic inhibitors can have antineoplastic activity, cardioprotectors or modulators in order to increase the efficacy of other agents. Avoir si important

Poisons are called this way because by stabilizing the covalent DNA complex topoisomerase complex, permanent lesions to the DNA will be observed. Having for finalité, the destruction of the whole molecule of DNA, hence the destruction of the cell.

Catalytic inihibitors kills cells by eliminating the essential activity of the topoisomerase activity.

Topoisomerase II is an important enzyme which is involved in many molecular processes such as replication and transcription. Toposisomerase has the ability to uncoil DNA's molecule by opening the double helix which is the first essential step that leads to both transcription and replication. Accessing genetic data, represent a tough topological challenge because the genetic material is anchored to the chromosome scaffold, and the two strands of the double helix are coiled. It helps to regulate DNA under and overwinding in order to remove knots and tangles. It is also involved in the fragmentation of the genome when it is necessary for cell survival.

The enzyme is a homodimer that creates double stranded breaks in DNA, passes another unbroken DNA helix through it, and then reanneals the cut strand.

Unlike lower eukaryotes, vertebrate species possess two isoforms of the enzyme, called topoisomerase IIα and topoisomerase IIβ. Their structures are really similar and only differ in two ways. Their protomer molecular weight is slightly different (170 kDa for topoisomerase IIα and 180 kDa for the second one) are encoded by separate genes and most important they have separate cellular functions. Topoisomerase IIα level has been observed high during period of cell growth in the work of reference, it is believed that it is essential for the survival of proliferating cells. Indeed this isoform plays a role in both replication and chromosome segregation processes. Action of topoisomerase IIβ from what is known seems to be independent of proliferative status and cell cycle although its function IIβ has yet to be defined clearly.

All type of topoisomerase has the ability to cleave and ligate DNA by utilizing its active site tyrosyl residues. Each protomer subunit of both topoisomerase IIα and β contain one (Tyr 805 and Tyr 821 respectively). DNA cleavage is initiated by the nucleophilic attack of the enzyme's active site tyrosin on the phosphate of the nucleic acid backbone. It generates a phosphotyrosyl bond that links together enzyme and DNA. Therefore a break in sequence is created and 5'-terminus and 3'-hydroxyl moieties are generated on each side of it. Topoisomerase is linked to the newly generated 5'terminus side of both strand of the DNA leaving each strand separated in two.

Phosphotyrosyl bond plays two important roles in the mechanism in the way that it maintains the integrity of the sequence and conserves the bond energy of the sugar-phosphate DNA backbone. The intermediate creating during this reaction is called cleavage complex and is the key feature for the pharmaceutical activities of the enzyme.

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The enzyme cleaves at preferred sites. It is impossible to predict at the moment where will the enzyme perform its task, the consensus sequence for it being weak. Contrary what one might think, topoisomerase II do not directly recognize the bases that comprise the sequence and its specificity to bind DNA is believed to depend on the local structure, flexibility or malleability of the molecule. reference

The enzyme can be potentially dangerous; it creates breaks in DNA's molecule and can trigger the fragmentation of the molecule. The key factor is the complex cleavage level and depending on it, different situations might be encountered. When level drops below threshold concentrations, daughter chromosome will not be able to segregate properly during mitosis as they will remain entangled after replication. Cell will consequently die. When levels of cleavage complex are too high, permanent DNA breaks will appear in high numbers. Recognition/repair system of DNA will be overwhelmed and won't be able to fill in DNA's breaks if they are too numerous and cell will initiate death pathway. Enzymes from DNA tracking system by their action will encounter cleavage complexes on DNA's molecule if their concentration is high. Collisions will occur (helicase will encounter cleavage complexes on DNA strands) and will disrupt cleavage complexes provoking permanent double strand breaks. (Parler de ça plus tard) As a result recombination/repair pathways will be initiated and DNA aberrations might be generated, leading to possible mutations that might provoke cancer. Cell might even get overwhelmed if too many strand breaks are occurring and apoptosis will be triggered. Having a high concentration of cleavage complex can provoke severe harms to the molecule of DNA, thus several anticancer drugs work on this basis.

Molecules which modify activity of topoisomerase II are divided in two classes. A distinction is made between chemicals decreasing the overall activity of the enzyme and those increasing levels of topoisomerase II-DNA cleavage complexes. The first class of products is known as catalytic inhibitors whereas the second class of compound are called topoisomerase II poisons.

Nowadays, of all type of anticancer drug, topisomerase II poisons are the most widely prescribed in clinical use. Six toposiomerase II targeted anticancer agents are approved in the United State and additional drugs are prescribed elsewhere in the world. reference

Many topoisomerase II poisons have shown to have anticancer activity. Toposiomerase II poisons convert the enzyme into a lethal agent that will provoke apoptosis (more information will be provided further). They act on the concentration of cleavage complexes making it raise until a certain point that will triggers death pathway. Poisons are believed to bind to DNA, the topoisomerase, or either molecule at or near the region of the enzyme involved in the formation of the DNA-protein covalent linkage. Anthracyclines and DNA Intercalators/Epipodophyllotoxins/DNA Topoisomerases

Eric H Rubin, MD and William N Hait, MD, PhD

Two different pathways can lead to high concentration of cleavage complex. Some chemicals act without inhibiting overall catalytic activity, they work mainly by enhancing the forward rate of scission (clarifier) while other compounds perform their work by inhibiting the ability of the enzyme to ligate cleaved DNA molecules.

Most currently used topoisomerase targeted agents act this way, blocking the catalytic cycle between the cleavage and the re-ligation steps. Anthracyclines and epipodophyllotoxins (etoposide) are one of these compounds. Generation of high level of topoisomerase II-DNA cleavage complexes is the main effect that these compound cause but they also prevent enzyme turnover and are considered as strong inhibitors of catalytic activity. Voir si ce n'est pas la meme chose que je viens de dire dans la dreniere phrase.

They can be furthermore divided into intercalating and non-intercalating agents. Intercalating agents are compound that presents various and different structural features. Their ability to intercalate DNA is the only characteristic that compounds like anthracyclines, mitoxanthrone or mAMSA are sharing (compounds that are already in clinical use). The geometry required for re-ligation is disrupted when an intercalating topoisomerase II poisons interact with DNA. Some experiments performed with the use of small intercalating agents such as ethidium bromide where unsuccessful, leading to the following conclusion: intercalating agents have to be voluminous enough for topoisomerase II to be trapped as a covalent complex on DNA. reference

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Anthracycline like certain other intercalating agents acting on topoisomerase II present a range of effects on cell that are independent of their action on topoisomerase II. Doxorubicin is known to produce free radicals, cause protein-DNA crosslinks and membrane damage. By causing protein-DNA crosslinks, these molecules prevent the attachment of the enzyme to DNA.Voir si plus tot j'ai déjà introduit cette famille de compose sinon le faire et dire que doxorubicin ne fait partie.

Non-intercalating agents acting on topoisomerase II do not interact strongly with DNA, and it is believed that their mechanism of action is linked to protein-drug interactions. Nevertheless consequence for the enzyme is the same, it will be trapped on DNA and cascade of events will be triggered which might lead to apoptosis. Etoposide is one of the most used non-intercalating topoidomerase II poison.

Double strand cleavage is not necessary for the poison to be effective. Single strand cleavage by topoisomerase II can also result in cytotoxicity. Therefore topoisomerase II poisons can only interact with one of the dimer of the enzyme.

Beyond Topoisomerase II poisons (exception of DNA lesions) act by two different mechanisms. One the mechanism requires redox activity to facilitate their actions against topoisomerase II whereas the other is redox-independent. Their actions against the enzyme are not affected by reducing agents such as dithithreitol. Unlike the other mechanism, which require redox chemistry for activation which will be abrogated by reducing agents.

Catalytic inhibitors are a heterogenous group of compounds that might interfere with the binding between DNA and toposiomerase ( aclarubicin and suramin), stabilize noncovalent DNA topoisomerase II complexes (merbarone,ICRF-187 and structurally realted bisdioxopiperazine derivatives) ot inhibit ATP binding (Novobiocin).

One of the big challenges in finding or designing inhibitors of topisomerase II is their specificity. Most of them are not specific. An exception is bisdioxopiperazines but it generates DNA damage responses only after a long time exposure. Catalytic inhibitors of toposiomerase II antagonize the toxicity of topoisomerase II poisons. These are conclusions made after performing experiments in vitro. Topoisomerase II inhibitors that are not poisons still need to be studied to answer the question of whether or not they can be active anticancer agents. Voir si place dans la conclusion ou pas.

Inhibitors that act on topoisomerase II can act at different moment in its reaction cycle. Each of these actions will have consequently different effects and consequences on the cell. Toposisomerase II operates by a strand passage mechanism in which one segment of DNA is passed through a transient break in another. The DNA strand passage reaction of topoisomerase II is coupled to the hydrolysis of ATP. CouplingATPhydrolysistoDNAstrandpassageintypeIIADNAtopoisomerasesA.Maxwell*1,L.Costenaro*,S.Mitelheiser*andA.D.Bates†*DepartmentofBiologicalChemistry,JohnInnesCentre,Colney,NorwichNR47UH,U.K.,and†SchoolofBiologicalSciences,UniversityofLiverpool,CrownStreet,LiverpoolL697ZB,U. Competitive inhibitors of ATP binding prevent strand passage and do not generate enzyme-mediated DNA damage.

Novobiocin and coumerycin are able to inhibit both prokaryotic and eukaryotic type II topoisomerase enzyme but coumermycin is poorly taken up by mammalian cells and novobiocin is less potent and non specific.

Aclarubicin acts by preventing the attachment of the enzyme to DNA. Merbarone is an agent that prevents DNA cleavage by the enzyme and so acts as a catalytic inhibitor. Most currently used topoisomerase targeted agents block the catalytic cycle between the cleavage and the re-ligation steps. Anthracyclines and epipodophyllotoxins (etoposide) are one of these compounds. Generation of high level of topoisomerase II-DNA cleavage complexes is the main effect that these compound cause but they also prevent enzyme turnover and are considered as strong inhibitors of catalytic activity. Voir si ce n'est pas la meme chose que je viens de dire dans la dreniere phrase. A relier avec le paragraphe plus haut

The enzyme can be inhibited after strand passage but before ATP hydrolysis and dissociation and dissociation of amino-terminal dimerization. Dexrazoxane is able to inhibit ATP hydrolysis and maintain the enzyme structure as a closed clamp. Like anthracyclines and epipodophyllotoxin, bisdioxopiperazines (dexrazoxane) inhibit the enzyme catalytic activity mainly by blocking enzyme turnover. These agents leave toposiomerase II stuck on DNA and this might lead to interferences with DNA metabolism. Even though based on this particularity they might provoke the same consequences than topoisomerase II poisons, they are classified as catalytic inhibitors. As they are specific to topoisomerase II, they are the most commonly used catalytic inhibitors of topoisomease II in mammalian cells.

Calatyic inhibitors such as bisdioxopiperazines have modest tumour activity and they are principally used for reducing the cardiotoxicity of anthracyclines. Nonetheless being the most specific type of catalytic inhibitors they are good tools for studying the effect of topoisomerase II inhibition. Parler des different ICRF et MST

The first topoisomerase II targeted agent to be discovered was Etoposide which is derived from podophyllotoxin. Podophyllotoxin presented a high toxicity but etoposide and teniposide discovered later on based on the pharmacophore of the natural product shown increased antineoplastic activity and decreased toxicity. mitoxantrone in anticancer regimens, it is used as a treatment for autoimmune diseases, such as multiple sclerosis

Topoisomerase II target drugs constitute essential compounds that will be used to treat a cancer curable by chemotherapy. They will be prescribed to each patient requiring chemotherapy.

Mitoxanthrone is used to treat breast cancer, while Etopoxide, doxorubicin and other drugs are the first therapy envisaged toward solid tumors and cancer such as leukemia, lymphomas, lung and breast cancer.

Amsacrine along mitoxanthrone are used at the same time to treat acute myeloid leukemia.


Topoisomerase II poisons are also found in the alimentation, and many food products. Bioflavonoids are the most important class of natural product causing this effect. Bioflavonoids are the component of many fruits, vegetables and leaves (donner lesquelles). They have a broad range of effects on the human body (mammalian?) but certain of them are potent topoisomerase II poisons. Geinistein is one of them.

Genistein's ring structure presents lots of similarities compared to the quinolone. They both increase the level of toposisomerase II-DNA cleavage complexes as

Metabolites from some drugs and chemical used in the industry are topoisomerase II poison. In all cases know until now, they all present the same characteristic, The one of possessing a quinone ring (aromatic ring featuring a ketone group). (Mettre petite image de structure)

Acetaminophen is an often used analgesic. N-acetyl p-benzoquinone imine (NAPQI), its metabolite is a potent topoisomerase II poison.

Benzene present as well metabolite which act as toposiomerase II poison: 1,4-benzoquinone. This solvent is believed to cause human leukemias.

Polychlorinated biphenyls (PCBs) to finish with this part, have quinone metabolites which binds the enzyme covalently. They present a high activity against human type II topoisomerase.