ANTICANCER AGENTS FARNESYL TRANSFERASE INHIBITORS Chapter 21

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  • Slide 1
  • ANTICANCER AGENTS FARNESYL TRANSFERASE INHIBITORS Chapter 21
  • Slide 2
  • Ras Protein Ras Protein Notes Signalling protein that is crucial to cell growth and division Abnormal form is present in 30% of cancers Prevalent in colonic and pancreatic cancers Abnormal Ras is coded by a mutated ras gene Small G-protein Binds GDP in resting state and GTP in active state Active Ras normally autocatalyses hydrolysis of GTP back to GDP Abnormal Ras fails to hydrolyse GTP Abnormal Ras remains permanently active Three human Ras proteins (H-Ras, N-Ras and K-Ras)
  • Slide 3
  • Farnesyl transferase Farnesyl transferase Notes Zinc metalloproteinase Catalyses attachment of a farnesyl group to Ras Hydrophobic farnesyl group anchors Ras to the inner part of the cell membrane Farnesylation is necessary for Ras to become activated during signal transduction Inhibition of farnesyl transferase should inhibit this process
  • Slide 4
  • FTase Furtherprocessing Methyl ester Farnesyl transferase Farnesyl transferase Enzyme mechanism
  • Slide 5
  • Farnesyl transferase Notes Farnesyl diphosphate (FPP) binds first to the active site FPP aids binding of Ras protein to the active site Magnesium and iron ions are present as cofactors Magnesium ion interacts with the pyrophosphate group Results in a better leaving group Iron ion interacts with the thiol group of cysteine Results in a better nucleophile
  • Slide 6
  • FT Substrates C-a-a-XSubstrate C = cysteine a = valine, isoleucine or leucine X = methionine, glutamine or serine Substrates share a terminal tetrapeptide moiety called the CaaX peptide
  • Slide 7
  • FT Inhibitors Aims Good inhibitory activity vs enzyme Ability to cross the cell membrane to reach the enzyme Metabolic stability Aqueous solubility Oral absorption Favourable pharmacokinetic properties
  • Slide 8
  • FT Inhibitors Notes Inhibitors were developed to mimic the terminal tetrapeptide moiety - CaaX peptideInhibitors were developed to mimic the terminal tetrapeptide moiety - CaaX peptide Tetrapeptides having Phe next to X act as inhibitors Serve as lead compoundsC-a-a-XSubstrateC-a-Phe-XInhibitor C = cysteine a = valine, isoleucine or leucine X = methionine, glutamine or serine
  • Slide 9
  • Lead compound CysValPheMetDisadvantages Terminal carboxylic acid likely to be ionised - bad for absorption Peptide bonds are susceptible to enzyme-catalysed hydrolysis Poor stability to digestive or metabolic enzymes (e.g. aminopeptidases)
  • Slide 10
  • Lead compound Cys Val Phe Met Drug design Notes Modifications carried out to remove peptide nature - peptidomimeticsModifications carried out to remove peptide nature - peptidomimetics Ester masks polar carboxylic acid or carboxylate ion - acts as prodrugEster masks polar carboxylic acid or carboxylate ion - acts as prodrug Methyleneamino link replaces N-terminal peptide bondMethyleneamino link replaces N-terminal peptide bond Methyleneamino link introduces a resistance to aminopeptidasesMethyleneamino link introduces a resistance to aminopeptidases Peptide bond isostere introduced to mimic central peptide bondPeptide bond isostere introduced to mimic central peptide bond Isostere should be capable of mimicing any binding interactionsIsostere should be capable of mimicing any binding interactions Isostere should be stable to enzyme-catalysed hydrolysisIsostere should be stable to enzyme-catalysed hydrolysis Peptidomimetic Methylene- amino link Peptidomimetic Ester Methylene- Peptidomimetic Peptide bond isostere Ester Methylene- amino link Peptidomimetic
  • Slide 11
  • Examples of FT Inhibitors R=H FTI 276 R= i Pr FTI 277 Terminal amino group Thiol Aromaticsubstituent Notes Thiol group forms important interactions with the zinc ion cofactorThiol group forms important interactions with the zinc ion cofactor Methyleneamino link is stable to aminopeptidasesMethyleneamino link is stable to aminopeptidases Aromatic substituent is important for inhibitory activityAromatic substituent is important for inhibitory activity Aromatic ring acts as a peptide bond isostereAromatic ring acts as a peptide bond isostere Terminal amino group is ionisedTerminal amino group is ionised Terminal amino group forms an ionic bond to the phosphate group of FPPTerminal amino group forms an ionic bond to the phosphate group of FPP Terminal carboxylate group is important to bindingTerminal carboxylate group is important to binding Stable methylene- amino link Peptide bond isostere Stable methylene- amino link
  • Slide 12
  • Examples of FT Inhibitors SulfoneAromaticsubstituent Terminal amino group Thiol R=H L739750 R= i Pr L744832Notes Thiol group forms important interactions with the zinc ion cofactorThiol group forms important interactions with the zinc ion cofactor Methyleneamino link is stable to aminopeptidasesMethyleneamino link is stable to aminopeptidases Aromatic substituent is important for inhibitory activityAromatic substituent is important for inhibitory activity Methyleneoxy group acts as the peptide bond isostereMethyleneoxy group acts as the peptide bond isostere Terminal amino group is ionisedTerminal amino group is ionised Terminal amino group forms an ionic bond to the phosphate group of FPPTerminal amino group forms an ionic bond to the phosphate group of FPP Terminal carboxylate group is important to bindingTerminal carboxylate group is important to binding Sulfone increases activity over a methylthio groupSulfone increases activity over a methylthio group Peptide bond isostere Stable methylene- amino link
  • Slide 13
  • AZD-3409 Pyrrolidine Aromatic substituent Examples of FT Inhibitors Notes Thiol and carboxylic acid groups are both masked in the prodrugThiol and carboxylic acid groups are both masked in the prodrug Lowers the toxicity risk of the thiol groupLowers the toxicity risk of the thiol group Protects the thiol from possible metabolismProtects the thiol from possible metabolism Pyrrolidine ring introduces conformational rigidityPyrrolidine ring introduces conformational rigidity Potent inhibitor (K i < 1 nM)Potent inhibitor (K i < 1 nM) Also inhibits geranylgeranyltransferase - catalyses prenylation with geranylgeranyl diphosphateAlso inhibits geranylgeranyltransferase - catalyses prenylation with geranylgeranyl diphosphate Agents inhibiting both enzymes are potentially advantageousAgents inhibiting both enzymes are potentially advantageous Masking group AZD-3409 Peptide bond isostere Masking group AZD-3409
  • Slide 14
  • Structure I IC 50 1.4 nM Examples of FT Inhibitors Notes Non-peptide inhibitorNon-peptide inhibitor Imidazole ring acts as the zinc ligandImidazole ring acts as the zinc ligand Decreases the risk of toxicity due to a free thiol groupDecreases the risk of toxicity due to a free thiol group Imidazole ring Structure I IC 50 1.4 nM
  • Slide 15
  • Lonafarnib IC 50 1.9 nM Examples of FT Inhibitors Notes Non-peptide inhibitorNon-peptide inhibitor Developed from lead compound discovered by screening compound librariesDeveloped from lead compound discovered by screening compound libraries 10,000 times more active than the lead compound10,000 times more active than the lead compound No ligand for the zinc cofactor is present!No ligand for the zinc cofactor is present!
  • Slide 16
  • Examples of FT Inhibitors Non-peptide inhibitor Developed from lonafarnib by structure- based drug design Imidazole ring introduced as zinc ligand Aromatic ring introduced as a steric shield vs metabolism Sch 226374 IC 50 0.36 nM Lonafarnib IC 50 1.9 nM Stericshield Sch 226374 IC 50 0.36 nM Imidazolering Stericshield Sch 226374 IC 50 0.36 nM
  • Slide 17
  • Development of Tipifarnib Lead compound Identified from screening compound libraries Imidazole ring present - zinc ligand Both aromatic rings are important to activity I; IC 50 180 nM Imidazole Quinolone
  • Slide 18
  • Strategy - variation of substituents Activity increases with introduction of meta-chloro substituent II; IC 50 35 nM Development of Tipifarnib Imidazole Quinolone I; IC 50 180 nM
  • Slide 19
  • Strategy - variation of substituents Activity increases with addition of N-methyl substituent II; IC 50 35 nM III; IC 50 15 nM Development of Tipifarnib Development of Tipifarnib Imidazole Quinolone I; IC 50 180 nM
  • Slide 20
  • II; IC 50 35 nM III; IC 50 15 nM IV; IC 50 2.5 nM Strategy - variation of ring substitution Activity increases Development of Tipifarnib Development of Tipifarnib Imidazole Quinolone I; IC 50 180 nM
  • Slide 21
  • III; IC 50 15 nM IV; IC 50 2.5 nM Tipifarnib; IC 50 0.6 nM Extension strategy Extra functional group Extra binding interactions Activity increases Development of Tipifarnib Imidazole Quinolone I; IC 50 180 nM II; IC 50 35 nM
  • Slide 22
  • Other Factors Notes FT-Inhibitors show potential as anticancer agentsFT-Inhibitors show potential as anticancer agents Anticancer activity may not necessarily be due solely to FT-inhibitionAnticancer activity may not necessarily be due solely to FT-inhibition FTIs inhibit farnesylation of H-Ras, N-Ras and K-RasFTIs inhibit farnesylation of H-Ras, N-Ras and K-Ras But N-Ras and K-Ras can by prenylated by GGTaseBut N-Ras and K-Ras can by prenylated by GGTase GGTase provides alternative mechanism of attaching Ras to cell membranesGGTase provides alternative mechanism of attaching Ras to cell membranes FTIs still have anticancer activi