Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

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prof. aza prof. aza Drugs that Target Nucleic Drugs that Target Nucleic Acids Acids Reference: Reference: Gareth Gareth Thomas Thomas Week 15 Week 15

Transcript of Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

Page 1: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

Drugs that Target Nucleic Drugs that Target Nucleic AcidsAcids

Reference: Reference: Gareth Gareth ThomasThomas

Week 15Week 15

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1313.. Drugs that Target Nucleic Acids Drugs that Target Nucleic Acids

•Drugs that target DNA and RNA Drugs that target DNA and RNA either either inhibit their synthesis inhibit their synthesis or or act on existing nucleic acid act on existing nucleic acid molecules. Those that inhibit the molecules. Those that inhibit the synthesis of nucleic acids usually synthesis of nucleic acids usually act as either act as either antimetabolites or antimetabolites or enzyme inhibitorsenzyme inhibitors. .

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•The drugs The drugs that target existing that target existing nucleic acid molecules nucleic acid molecules can, can, for convenience be broadly for convenience be broadly classified into classified into intercalating intercalating agents, alkylating agents and agents, alkylating agents and chain-cleaving agentschain-cleaving agents..

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• However, it should be realised that However, it should be realised that these classifications are not rigid: these classifications are not rigid: drugs may act by more than one drugs may act by more than one mechanismmechanism. . tthose drugs acting on hose drugs acting on existing DNA usually existing DNA usually inhibit inhibit transcription whereas those acting transcription whereas those acting on RNA normally inhibit translation. on RNA normally inhibit translation.

• In both cases the net result is the In both cases the net result is the prevention or slowing down of cell prevention or slowing down of cell growth and division. growth and division.

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• Consequently, the discovery of new Consequently, the discovery of new drugs that target existing DNA and drugs that target existing DNA and RNA is a major consideration when RNA is a major consideration when dedevveloping new drugs for the eloping new drugs for the treatment of cancer (see Appendix 4) treatment of cancer (see Appendix 4) and bacterial and other infections and bacterial and other infections due to microorganisms.due to microorganisms.

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13.1 Antimetabolites13.1 Antimetabolites

• Antimetabolites are compounds that Antimetabolites are compounds that block the normal metabolic pathways block the normal metabolic pathways operating inoperating in cells. cells.

• They act by either They act by either replacing an replacing an endogenous compound endogenous compound in the in the pathway by apathway by a compound whose compound whose incorporation into the system results incorporation into the system results in a product that can no longer playin a product that can no longer play any further part in the pathway, any further part in the pathway, or or inhibiting an enzyme in the metabolic inhibiting an enzyme in the metabolic pathway in thepathway in the cell. cell.

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•Both these types of Both these types of intervention intervention inhibit the inhibit the targeted metabolic pathway targeted metabolic pathway to a level to a level that hopefully has it that hopefully has it significant effect on the significant effect on the health of the patient.health of the patient.

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• The structures of antimetabolites are The structures of antimetabolites are usually very similar to those of the usually very similar to those of the normal metabolites used by the cell. normal metabolites used by the cell. Those used to prevent the formation Those used to prevent the formation of DNA may of DNA may bbe classified as e classified as antifolates, pyrimidine antifolates, pyrimidine antimetabolites and purine antimetabolites and purine antimetabolitesantimetabolites. .

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• However, because of the difficult of However, because of the difficult of classifying biologically active classifying biologically active substances (see section 1.6), substances (see section 1.6), antimetabolites that inhibit enzyme antimetabolites that inhibit enzyme action are also classified as enzyme action are also classified as enzyme inhibitors.inhibitors.

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Figure 10.23. (a) The structure of folic acid. In blood, folic acids usually have one glutamate residue. However, in the cell they are converted to polyglutamates. (b) A Fragment of a polyglutamate chain.

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13.1.113.1.1.. Antifolates Antifolates

• Folic acid (Figure 10.23) is usually Folic acid (Figure 10.23) is usually regarded as the parent of a family of regarded as the parent of a family of naturallv occurring compounds naturallv occurring compounds known as folates. known as folates.

• These folates are widely distributed These folates are widely distributed in food. They differ from folic acid in in food. They differ from folic acid in such ways as the state of reduction such ways as the state of reduction of the pteridine ring and having of the pteridine ring and having carbon units attached to either or carbon units attached to either or both of the N5 and N 10 atoms.both of the N5 and N 10 atoms.

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• In the body folates are converted by a two-In the body folates are converted by a two-step process into tetrahvdrofolates (FH4) by step process into tetrahvdrofolates (FH4) by the action of the enzyme dihydrofolate the action of the enzyme dihydrofolate reductase (DHFR). reductase (DHFR). Tetrahydrofolic acid is an Tetrahydrofolic acid is an essential cofactor in the biosynthesis of essential cofactor in the biosynthesis of purines and thymine. which are required for purines and thymine. which are required for DNA synthesis.DNA synthesis.

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• Folic acid antimetabolites have Folic acid antimetabolites have structures that resemble folic acid structures that resemble folic acid (Figure 10.24). (Figure 10.24). They have a stronger They have a stronger affinity for DHFR than folic acid affinity for DHFR than folic acid and act and act by inhibiting this enzyme at both stages by inhibiting this enzyme at both stages in the conversion of folic acid to FH4. in the conversion of folic acid to FH4.

• This has the effect of This has the effect of inhibiting the inhibiting the formation of purines and thymine formation of purines and thymine required for DNA synthesis. required for DNA synthesis.

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MethotrexateMethotrexate

• This inhibits cell growth, which This inhibits cell growth, which prevents replication and ultimately prevents replication and ultimately leads to cell death.leads to cell death.

• MethotrexateMethotrexate is the only folate is the only folate antimetabolite in clinical use. It is antimetabolite in clinical use. It is distributed to most body fluids but distributed to most body fluids but has a low lipid solubility, which has a low lipid solubility, which means, that means, that does not readilv cross does not readilv cross the blood-brain barrier.the blood-brain barrier.

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Figure 10.24. A comparison of the structures of folic acid antimetabolites with folic acid.

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• It is transported into cells by the It is transported into cells by the folate transport system and at high folate transport system and at high blood levels an additional second blood levels an additional second transport mechanism comes into transport mechanism comes into operationoperation. .

• Once in the cell it is metabolised to Once in the cell it is metabolised to the polyglutamate, which is retained the polyglutamate, which is retained in the cell for considerable periods of in the cell for considerable periods of time. This is probably due to the time. This is probably due to the polar nature of the polymer. polar nature of the polymer.

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• MethotrexateMethotrexate is used to treat a is used to treat a variety of cancers, including head variety of cancers, including head and neck tumours, and, in low doses, and neck tumours, and, in low doses, rheumatoid arthritis. rheumatoid arthritis.

• It can cause vomiting, nausea, oral It can cause vomiting, nausea, oral and gastric ulceration and depression and gastric ulceration and depression of bone marrow, a well as other of bone marrow, a well as other unwanted side effects.unwanted side effects.

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13.1.2 Purine Antimetabolites13.1.2 Purine Antimetabolites• Purine antimetaholites are Purine antimetaholites are

exogenous compoundsexogenous compounds,, such as 6- such as 6-mercaptopurine and 6-thioguanine, mercaptopurine and 6-thioguanine, with structures based on the purine with structures based on the purine nucleus (Figure 1O.25).nucleus (Figure 1O.25).

• They inhibit the synthesis of DNA and They inhibit the synthesis of DNA and in some cases RNA by a number oin some cases RNA by a number off different mechanisms. For example, different mechanisms. For example, 6-mercaptopurine is metabolised to 6-mercaptopurine is metabolised to the ribonucleotide 6-thioguanosine-the ribonucleotide 6-thioguanosine-5’-pltosphate. 5’-pltosphate.

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Figure 10.25. Examples of purine antimetabolites. The purine nucleus, on which the structures of the antimetabolites and the endogenous compounds they replace are based, is shown in square brackets.

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• This exogenous nucleotide inhibits This exogenous nucleotide inhibits several pathways several pathways fofor the r the biosynthesis of endogenous purine biosynthesis of endogenous purine nucleotides. nucleotides.

• In contrastIn contrast,, 6-thioguanine is 6-thioguanine is converted in the cell to the converted in the cell to the ribonucleotide 6-thioinosine-5’-ribonucleotide 6-thioinosine-5’-phphoosphate. sphate.

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• This ribonucleotide disrupts DNA This ribonucleotide disrupts DNA synthesis synthesis by being incorporated into by being incorporated into the structure of DNA as a false nucleic the structure of DNA as a false nucleic acid. acid.

• Resistance to these two drugs arises Resistance to these two drugs arises because of a loss of the posphorybosil because of a loss of the posphorybosil transferase required for the formation transferase required for the formation of their ribonucleotides.of their ribonucleotides.

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13.1.3 Pyrimidine Antimetabolltes13.1.3 Pyrimidine Antimetabolltes

•These are antiThese are antimmetaetabbolites whose olites whose structures closely structures closely to to those of the those of the endoendoggenous penous pyyrimidine bases rimidine bases (Figure l0.26a).(Figure l0.26a).

•They usually act by inhibiting They usually act by inhibiting one or more of the enzymes that one or more of the enzymes that areare required for DNA synthesis. required for DNA synthesis.

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•The presence of the The presence of the unreactive unreactive C5 F bond in FUdRP blocks this C5 F bond in FUdRP blocks this methylationmethylation, which prevents the , which prevents the formation of deoxythymidylic formation of deoxythymidylic acid (TdRP) and its subsecacid (TdRP) and its subsecqquent uent incorporation into DNA (Figure 1 incorporation into DNA (Figure 1 0.0. 261).261).

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• Fluorine was chosen to replace Fluorine was chosen to replace hydrogen at the C5 position of uracil hydrogen at the C5 position of uracil because because it is of a similar size to it is of a similar size to hydrogenhydrogen (atomic radii: F. 0.13 nm: (atomic radii: F. 0.13 nm: 1-1. 0.l2nrn). 1-1. 0.l2nrn).

• IIt was thought that this similarity in t was thought that this similarity in size would give a drug that ould size would give a drug that ould cause little steric disturbance to the cause little steric disturbance to the biosynthetic pathwaybiosynthetic pathway as well as as well as being chemically inert. being chemically inert.

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•Analogues containing larger Analogues containing larger halogen atoms do not have any halogen atoms do not have any appreciable activity.appreciable activity.

•For example, fluorouracil is For example, fluorouracil is metabolised by the same metabolic metabolised by the same metabolic pathway as uracil to 5-fluoro-2’-pathway as uracil to 5-fluoro-2’-deoxyuridylic acid (FUdRP). deoxyuridylic acid (FUdRP).

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• FUdRP inhibits the enzyme FUdRP inhibits the enzyme thythymmidylate synthetaseidylate synthetase,, which in its which in its normal role is responsible for the normal role is responsible for the transfer (transfer (ofof a meth a methylyl group from the group from the coenzyme coenzyme mmelhylenetetrahydrofolic elhylenetetrahydrofolic acid (MeFI 14) to the C5 atom of acid (MeFI 14) to the C5 atom of deoxyuridylic acid (UdRP). deoxyuridylic acid (UdRP).

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Figure 10.26. (a) Examples of pyrimidines that act as antimetabolites. It should be noted that cytarabine only differs from cytidine by the stereochemistry of the 2’ carbon. (b) The intervention of fluorouracil in pyrimidine biosynthesis.

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Figure 10.27. Examples of topoisomerase inhibitors. Ellipticene acts h intercalation and inhibition of topoisomerase II enzymes. It is active against nasophar ngeal carcinomas. Amsacrinc is used to treat oarian carcinomas. lymphoinas and myelogenous leukaemias. (camptotheci n is an antitumour.

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13.213.2.. Enzyme Inhibitors Enzyme Inhibitors

• Enzyme inhibitors may Enzyme inhibitors may bbe classified e classified for convenience as those for convenience as those that inhibit that inhibit the enzymes directly responsible for the enzymes directly responsible for the formation of nucleic acids the formation of nucleic acids or the or the variety of variety of enzymes that catalyse the enzymes that catalyse the various stages in the formation of the various stages in the formation of the pirimidine and purine bases pirimidine and purine bases required required for the formation of nucleic acids.for the formation of nucleic acids.

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13.2.1 Topoisomerases13.2.1 Topoisomerases

• Topoisomerases are a group of Topoisomerases are a group of enzymes that are responsible enzymes that are responsible for the for the supercoiling, the cleavage and supercoiling, the cleavage and rejoining of DNA. rejoining of DNA.

• Their inhibition has the effect of Their inhibition has the effect of preventing transcriptionpreventing transcription. A number . A number of compounds (Figure 10.27) are of compounds (Figure 10.27) are believed to act by inhibiting these believed to act by inhibiting these enzymes. enzymes.

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• It is thought that some It is thought that some intercalators act in this manner intercalators act in this manner although it is not clear whether although it is not clear whether the drug the drug bbinds to the inds to the topoisotopoisommerase prior to or after erase prior to or after the enzyme has formed a DNA—the enzyme has formed a DNA—enzyme complex.enzyme complex.

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113.2.2 Enzyme Inhibitors for Purine 3.2.2 Enzyme Inhibitors for Purine and Primidine Precursor Systemsand Primidine Precursor Systems

• A wide range of compounds are A wide range of compounds are active against a number of the active against a number of the enzyme s stems that arenzyme s stems that are e involved in involved in the biosynthesis of purines and the biosynthesis of purines and pyrimidines in bacteria. pyrimidines in bacteria.

• In both of these examples the overall In both of these examples the overall effect is the inhibit ion of purine and effect is the inhibit ion of purine and pyrirpyrirmmidine synthesis, which results idine synthesis, which results in thein the inhibition of the synthesis of inhibition of the synthesis of DNA. DNA.

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• This restricts the groThis restricts the growwth of the th of the bacteria and ultimately prevents it bacteria and ultimately prevents it from replicating, which gives the from replicating, which gives the bodys natural defences time to bodys natural defences time to destroy the bacteria. destroy the bacteria.

• Because suBecause sullphonamides and phonamides and trimethoprim inhibit different stages trimethoprim inhibit different stages in the same metabolic pathway, they in the same metabolic pathway, they are often used in conjunction (Figure are often used in conjunction (Figure 1 0.28).This allows the clinician to 1 0.28).This allows the clinician to use louse lowerwer and there fore safer doses. and there fore safer doses.

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• For exampleFor example, , sulphonamides inhibit sulphonamides inhibit dihydropteroate sdihydropteroate syynthetase (see nthetase (see section 6.12.1), which prevents the section 6.12.1), which prevents the formation of folic acid, whereas formation of folic acid, whereas trimethoprim inhibits dihydrofolate trimethoprim inhibits dihydrofolate reductase, which prevents the reductase, which prevents the conversion of folic acid to conversion of folic acid to tetrahydrofolate (see section tetrahydrofolate (see section 10.13.1.1).10.13.1.1).

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• In both of these examples the overall In both of these examples the overall effect is the effect is the inhibit ion of purine and inhibit ion of purine and pyripyrimimidine synthesisdine synthesis, which results in , which results in thethe inhibition of the synthesis of DNA. inhibition of the synthesis of DNA. This restricts the groThis restricts the growwth of the bacteria th of the bacteria and ultimately and ultimately prevents it from prevents it from replicating,replicating, which gives the bodys which gives the bodys natural defences time to destroy the natural defences time to destroy the bacteria. bacteria.

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• Because suBecause sullphonamides and phonamides and trimethoprim trimethoprim inhibit different stages inhibit different stages in the same metabolic pathwayin the same metabolic pathway, they , they are often used in conjunction (Figure are often used in conjunction (Figure 1 0.28).This allows the clinician to 1 0.28).This allows the clinician to use louse lowerwer and therefore safer doses. and therefore safer doses.

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Figure 10.28. Sequential blocking using sulphamethoxazole and Trimethoprim.

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13.313.3.. Intercalating Agents Intercalating Agents

• Intercalating agents are compounds Intercalating agents are compounds that insert themselves between the that insert themselves between the bases of the DNA helix (Figure bases of the DNA helix (Figure 10.29). 10.29). This insertion causes the DNA This insertion causes the DNA helix to partially unwind at the site of helix to partially unwind at the site of the intercalated moleculethe intercalated molecule. .

• This inhibits transcription, which This inhibits transcription, which blocks the replication process of the blocks the replication process of the cell containing the DNA. cell containing the DNA.

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• However, it is not known how the However, it is not known how the partial unwinding presents partial unwinding presents transcription transcription bbut some workers think ut some workers think that it inhibits topoisomerasesthat it inhibits topoisomerases (see (see section 10.12.2.1). Inhibition of cell section 10.12.2.1). Inhibition of cell replication can lead to cell death, replication can lead to cell death, which reduces the swhich reduces the sizeize of a tumour, of a tumour, the number of ‘free’ cancer cells or the number of ‘free’ cancer cells or the degree of infection, all of which the degree of infection, all of which will will contribute to improving the health contribute to improving the health of the patient.of the patient.

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Figure 10.29. A schematic representation of the distortion of the DNA helix by intercalating agents. The horizontal lines represent the hydrogen-bonded bases. The rings of these bases and intercalating agent are edgeon to the reader.

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• The insertion of an intercalation The insertion of an intercalation agent appears to occur agent appears to occur via either the via either the minor or major grooves of DNAminor or major grooves of DNA. .

• Compounds that act as intercalating Compounds that act as intercalating agents must have structures that agents must have structures that contain contain a flat fused aromatic or a flat fused aromatic or heteroarornatic ring section that can heteroarornatic ring section that can fit between the flat structures of the fit between the flat structures of the bases of the DNAbases of the DNA. .

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• It is believed that these aromatic It is believed that these aromatic structures are structures are held in place by held in place by hydrogen bonds, van der Waals’ hydrogen bonds, van der Waals’ forces and charge-transfer bonds forces and charge-transfer bonds (see section 5.2).(see section 5.2).

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Figure 10.30. Examples of intercalating agents. Trade name.

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• Drugs whose mode of action includes Drugs whose mode of action includes intercalation arintercalation aree the antimalarials the antimalarials quinine and chquinine and chlloroquine, the oroquine, the anticancer agents mitoxantrone and anticancer agents mitoxantrone and doxorudoxorubbicin, and the antibiotic icin, and the antibiotic proflavine (Figure 10.30). proflavine (Figure 10.30).

• In each of these compounds it is the In each of these compounds it is the flat aromatic ring system flat aromatic ring system that is that is responsible for the intercalation. responsible for the intercalation.

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• However, other groups in the However, other groups in the structures may also contribute to the structures may also contribute to the binding of a drug to the DNA. binding of a drug to the DNA.

• For example, the For example, the amino group of the amino group of the sugar residue of doxorubicin forms sugar residue of doxorubicin forms an ionic bond with the negatively an ionic bond with the negatively charged oxygens of the phosphate charged oxygens of the phosphate groups groups of the DNA chain, which of the DNA chain, which effectively locks the drug into place. effectively locks the drug into place.

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• A number of other drugs appear to have A number of other drugs appear to have groups that act in a similar manner.groups that act in a similar manner.

• Some intercalating agents Some intercalating agents eexhibit a xhibit a preference preference toto certain combinations of certain combinations of bases in DNA. bases in DNA.

• For example, mitoxantrone appears to For example, mitoxantrone appears to prefer to intercalate with prefer to intercalate with cytosine—cytosine—guanosine-rich sequences. guanosine-rich sequences. This type of This type of behaviour does open out the possibility behaviour does open out the possibility of selective action in some cases.of selective action in some cases.

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13.4 Alkylating Agents13.4 Alkylating Agents

• Alkylating agents are Alkylating agents are believed to bond believed to bond to the nucleic acid chainsto the nucleic acid chains in either the in either the major or minor grooves. major or minor grooves.

• In DNA the alkylating agent frequently In DNA the alkylating agent frequently forms either intrastrand or informs either intrastrand or interterstrand strand crosslinks. crosslinks.

• Intrastrand cross-linking agents form a Intrastrand cross-linking agents form a bridge between two parts of the same bridge between two parts of the same chain (Figure 10.31). This has the echain (Figure 10.31). This has the efffect fect of distorting the strand, which of distorting the strand, which inhibits inhibits transcription.transcription.

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Figure 10.31. A schematic representation of the intrastrand cross-linking.

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• lnterstrand cross—links are formed lnterstrand cross—links are formed between the two separate chains of between the two separate chains of the DNAthe DNA,which,which has the has the effect of effect of bblocking them together (Figure locking them together (Figure 10.32). 10.32). This also inhibits This also inhibits transcription. transcription.

• In RNA only intrastrand cross-links In RNA only intrastrand cross-links are possible. are possible. However, irrespective of However, irrespective of whether or not it forms a bridge.whether or not it forms a bridge. the the bonding of an alkylating agent to a bonding of an alkylating agent to a nucleic acid inhibits replication of nucleic acid inhibits replication of that nucleic acidthat nucleic acid

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• InIn the case of bacteria the case of bacteria this prevents this prevents an increase in the sian increase in the sizeze of the of the infection infection and so buys theand so buys the time for its time for its immune system to destroy the immune system to destroy the existing bacteria. However, in the existing bacteria. However, in the case of cancer itcase of cancer it may lead to cell may lead to cell death and a benefdeath and a benefiicial reduction in cial reduction in tumour stumour sizeize..

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Figure 10.32. (a) The general structure of nitrogen mustards (h) The proposee mechanism for tormimu’ interstrandcross-links by the action of aliphatic nitrogen mustards.

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• The nucleophilic nature of the nucleic The nucleophilic nature of the nucleic acids means that acids means that alkylating agents alkylating agents are usuallyare usually electrophiles or give rise electrophiles or give rise to electrophiles. to electrophiles.

• For example, it is believed that For example, it is believed that a a weaklyweakly electrophilic β-carbon atom of electrophilic β-carbon atom of an aliphatic nitrogenan aliphatic nitrogen mustard mustard alkylating agent, such asalkylating agent, such as mechlorethamine (Mustine), mechlorethamine (Mustine),

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• is converted to the more highly is converted to the more highly electrophilic aziridine electrophilic aziridine ion by an ion by an internal nucleophilic substitution of a internal nucleophilic substitution of a b-chlorine atom. b-chlorine atom.

• This is thought to be followedThis is thought to be followed by the by the nucleophilic attack of the N7 of a nucleophilic attack of the N7 of a guanine residue guanine residue on this ion by what on this ion by what appears to bean SN2 type of appears to bean SN2 type of mechanism. mechanism.

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• Since these drugs have two Since these drugs have two hydrocarbon chains with hydrocarbon chains with b-b-chlorogroups,chlorogroups, each of these each of these chlorogroups is believed to react with chlorogroups is believed to react with a guanine residue in aa guanine residue in a different chain different chain of the DNA strand to of the DNA strand to form a cross-form a cross-link between the two nucleic acidlink between the two nucleic acid chains chains (Fig. 10.38).(Fig. 10.38).

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Figure 10.33. (a) The structure of chlorambucil and (b) a proposed mode of action for some aromatic nitrogen mustards.

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• The electrophilic nature of alkylating The electrophilic nature of alkylating agents means that they can also agents means that they can also react with a wide variety of other react with a wide variety of other nucleophilic biomanucleophilic biomaccromolecules. romolecules.

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• This accounts for many of the unwanted This accounts for many of the unwanted toxic effects that are frequently toxic effects that are frequently observed with the use of these drugs. In observed with the use of these drugs. In the case of the nitrogen mustards. the case of the nitrogen mustards. attempts to reduce these side effects attempts to reduce these side effects have centred on reducing their reactivity have centred on reducing their reactivity by discouraging the formation of the by discouraging the formation of the aiaizziridine ion before the drug reaches its iridine ion before the drug reaches its site of actionsite of action. .

prof. azaprof. aza

Page 58: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• The approach adopted has been to The approach adopted has been to reduce the nucleophilic character of reduce the nucleophilic character of the nitrogen atom the nitrogen atom by attaching iby attaching itt to to an electron-withdrawing aromatic an electron-withdrawing aromatic ringring. This produced analogues that . This produced analogues that would only react with strong would only react with strong nucleophiles and resunucleophiles and resullted in ted in the the development of chlorambucil. development of chlorambucil.

prof. azaprof. aza

Page 59: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• This drug is one of the This drug is one of the least toxic least toxic nitrogen mustardsnitrogen mustards, being active , being active against malignant lymphomasagainst malignant lymphomas,, carcinomas of the breast and ovary carcinomas of the breast and ovary and lymphocytic and lymphocytic lleueuccaemia. aemia.

prof. azaprof. aza

Page 60: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• It has been suggested that because It has been suggested that because of the reduction in the nucleophilicity of the reduction in the nucleophilicity of the nitrogen atom these aromatic of the nitrogen atom these aromatic nitrogen mustards nitrogen mustards do not form an do not form an aziriaziriddine ion. ine ion.

• Instead they react by direct Instead they react by direct substitution of the substitution of the 13-chlorine atoms 13-chlorine atoms by guanineby guanine,, which is a strong which is a strong nucleophile, by an S.I type of nucleophile, by an S.I type of mechanism (Figure 10.33).mechanism (Figure 10.33).

prof. azaprof. aza

Page 61: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

Figure 10.34. Cyclophosphamide and the formation of phosphoramide mustard, the active of the drugs

Page 62: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

• Further attempts to reduce the Further attempts to reduce the toxicity otoxicity off nitrogennitrogen mustards were mustards were based on making the drug more based on making the drug more selective. selective. Those Those approaches have approaches have yielded useful drugs. The first was yielded useful drugs. The first was based on the fact that the rapid based on the fact that the rapid synthesis of proteins that occurs in synthesis of proteins that occurs in tumour cells tumour cells requires a large supply requires a large supply of amino acid raw material from of amino acid raw material from outside the cell. outside the cell.

Page 63: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• ConsequentlyConsequently,, it was thought that it was thought that the presence of an athe presence of an amimino acid no acid residue in the structure of a nitrogen residue in the structure of a nitrogen mustard might lead to an increased mustard might lead to an increased uptake of that compound. uptake of that compound.

• This approach resulted in the This approach resulted in the synthesis of the phensynthesis of the phenyylalanine lalanine mustard meiphalan (mustard meiphalan (Table 10.6)Table 10.6)

prof. azaprof. aza

Page 64: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• The The L-L-form of this drug is more active form of this drug is more active than the than the D-D-form and so it has been form and so it has been suggested that the Lsuggested that the L--form may be form may be transported into the cell by means of transported into the cell by means of a a LL -phen -phenyylalanine activelalanine active transport transport system.system.

prof. azaprof. aza

Page 65: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

• The second approach was based on The second approach was based on the fact that some tumours the fact that some tumours wwere ere thought to contain a highthought to contain a high concentration of phosphoramidases. concentration of phosphoramidases.

• This resulted in the synthesis of This resulted in the synthesis of nitrogen mustard ananitrogen mustard analogues logues mechanismmechanism, , whose structures whose structures contained phosphorus functional contained phosphorus functional groups that could groups that could bbe attacked bye attacked by this this enzyme. enzyme.

Page 66: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• It leIt leaad to the development of the d to the development of the cyclophosphamide (Figure 10.34). cyclophosphamide (Figure 10.34). which haswhich has a wide spectrum of a wide spectrum of activity. activity.

• However, the action of this prodrug However, the action of this prodrug has now been shown tohas now been shown to b be due to e due to phosphoramide mustard formed phosphoramide mustard formed by by oxidation by microsomal enzymes in oxidation by microsomal enzymes in the liverthe liver rather than hydrolysis by rather than hydrolysis by tumour tumour p phosphoramidases.hosphoramidases.

prof. azaprof. aza

Page 67: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• The acrolein produced in this proceThe acrolein produced in this process bbe-lieved to e-lieved to bbe the source of me the source of myyeloelo- - suppression and haemorrhagic suppression and haemorrhagic ccyystitis associatedstitis associated with the use of with the use of ccyyclophosphamide. clophosphamide.

• However, coadministration of the However, coadministration of the drug with sodiumdrug with sodium 2-2-mmercaptoethaneercaptoethane sulphonate (MESNA) can relieve sulphonate (MESNA) can relieve some of these symptoms. some of these symptoms.

• MESNA MESNA formsforms a water-soluble adduct a water-soluble adduct with the acrolein,with the acrolein, which is then which is then excreted in the urine.excreted in the urine.

prof. azaprof. aza

Page 68: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

• Some alkylating agents act by Some alkylating agents act by decomposing to produce an decomposing to produce an electrophile that bonds to a electrophile that bonds to a nunuclcleophilic group of a base in the eophilic group of a base in the nucleic acid. nucleic acid.

• For example. temozolomide (Table For example. temozolomide (Table 10.10.66) enters) enters the major groove of DNA the major groove of DNA where it reacts with water to from where it reacts with water to from nitrogen. carbon dioxide, an nitrogen. carbon dioxide, an aamminominomiidazole and a methyl dazole and a methyl carbonium ion (CH3). carbonium ion (CH3).

Page 69: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• This methyl carThis methyl carbbonium ion then onium ion then methmethyylates the strongly nucleophilic lates the strongly nucleophilic N7 of the guanine bases in the major N7 of the guanine bases in the major groo e. groo e.

prof. azaprof. aza

Page 70: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

• A range of different classes of A range of different classes of compound can act as nucleic acid compound can act as nucleic acid alkylating agents alkylating agents (T(Table 10.6). able 10.6).

• Within these classes a Within these classes a nnumber of umber of compounds have been fcompounds have been founound to d to bbe e ususefuleful d drrugs. ugs.

• IInn ma manyny, cases their effectiveness is , cases their effectiveness is improved by the use of combinations improved by the use of combinations of drugs. of drugs.

prof. azaprof. aza

Page 71: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

•Their modes of action are usually Their modes of action are usually not fully understood but a large not fully understood but a large amount of information is amount of information is available concerning their available concerning their structure-action relationships.structure-action relationships.

prof. azaprof. aza

Page 72: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

Table 10.6. Some examples of the classes and compounds of anticancer agents that act by aik alkylation of nucleic acids. it is emphasised that this table only lists some of the classes of alkylating compound that are active against cancers.

Page 73: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

Page 74: Prof. aza Drugs that Target Nucleic Acids Reference: Gareth Thomas Week 15.

prof. azaprof. aza

Figure 10.35. Development routes for antisense drugs. Examples of: (a) a section of the backbone of a deoxy ribonuCWICIId cleic chain; (b) backbone modifications; (c) sugar residue modifications; and (d) base modifications,