Chemistry in Cancer Chemotherapy lecture Flashcards
(32 cards)
The nitrogen mustards contain
The nitrogen mustards contain highly electrophilic groups and form covalent bonds to nucleophilic groups in DNA, e.g. alkylation of 7‐N of guanine, causing interstrand and intrastrand crosslinking due to the two electrophilic groups present.
Cyclophosphamide
Cyclophosphamide is the most commonly used alkylating agent. It is a non‐toxic prodrug which is orally active.
Mitomycin
Mitomycin is also a prodrug activated in vivo to form an alkylating agent. However, it is one of the most toxic anticancer drugs in clinical use.
Platinum compounds and their mechanism of action
Cisplatin is a prodrug. Platinum covalently bonded to the chloro substituents while ammonia molecules act as ligands. The drug is activated in cells with low chloride ion concentration. The chloro substituents are replaced with neutral water ligands which produces a positively charged species that reacts with DNA. It binds to DNA in regions rich in guanine units. Intrastrand links are formed rather than interstrand; this causes localised unwinding of the DNA double helix and inhibits transcription .
intercalating agents
The anthracyclines are intercalating agents. The planar anthraquinone moiety intercalates between the DNA base pairs and the carbohydrate moiety binds into the
minor groove. The carbohydrate also interacts with the DNA topoisomerase and binds to the amino acid residues highlighted in the diagram.
inhibitor of thymidylate synthase.
5‐Fluorouracil (5‐FU) is a prodrug that is metabolised to give 5‐F‐dUMP which then acts an inhibitor of thymidylate synthase. The metabolic pathway by which 5‐FU is activated is shown.
The mechanism by which 5‐F‐uDMP inhibits thymidylate synthase is shown: the carbon‐fluorine bond is much stronger than the carbon‐hydrogen bond which is normally cleaved by thymidylate synthase. This causes the enzyme to be inhibited irreversibly
Capecitabine is a produg of 5‐fluorouracil which is itself a prodrug
inhibitors of DNA polymerases
Pancreatic ductal adenocarcinoma (PDAC) is an epithelial, exocrine pancreatic malignancy, and it account for about 85% of the malignant neoplasms of the pancreas. It remains the fourth major cause of death due to cancer in the Western world. There is a direct correlation between pancreatic cancer and increasing age with a peak incidence of the disease occurring in the 65–75 year‐old age group. Untreated metastatic pancreatic cancer has a median survival rate of 3–5 months and 6–10 months for locally advanced disease. The majority of cases are diagnosed in the malignant stages, making curative therapy impossible and leading to poor prognosis and incidence equalling mortality.
kinase binding site
Kinases are enzymes that catalyse phosphorylation reactions on protein substrates. It has been estimated that there are up to 2000 protein kinases in a cell. Protein kinases are present in the cytoplasm and protein kinase receptors play a dual role as receptor and enzyme. Overexpression can result in cancer. There are kinases specific to particular residues, e.g. tyrosine kinases, serine‐threonine kinases and histidine kinases. ATP is used as the enzyme cofactor andphosphorylating agent.
EGFR kinase active site contains the binding site for the protein substrate and the binding site for the ATP cofactor. Clinically useful inhibitors target the ATP binding site: the ATP binding site is similar but not identical for all protein kinases and allows selectivity of inhibitor action. The purine base is buried deep into the binding site and forms two hydrogen bonding interactions to the binding site. The ribose residue binds to a ‘ribose binding pocket’. The triphosphate chain lies along a cleft towards the enzyme surface and interacts with two metal ions and amino acids.
The specificity surface is an area of unoccupied binding site. An empty hydrophobic pocket lies opposite the ribose binding pocket. The gatekeeper residue is an amino acid situated at the entrance to the hydrophobic pocket: the size of the gatekeeper residue is important in drug design, as is the nature of amino acids in the binding pockets.
Gefitinib
Gefitinib was developed by Astra Zeneca. It inhibits the kinase active site of the epidermal growth factor receptor: The EGF‐receptor is a tyrosine kinase receptor.
Gefitinib is a 4‐anilinoquinazoline structure
Struture (I) The secondary amine, electron‐donating substituents and small lipophilic group are all important for activity; useful in vitro activity but lower in vivo activity due to rapid metabolism by cytochrome P450 enzymes.
Structures (II) and (III) The methyl group and para‐position of the aromatic ring are susceptible positions for metabolism. A blocking metabolism should improve the half life of the drug.
Structure (IV) The fluoro substituent blocks para‐hydroxylation of the aromatic ring:
Fluorine is similar in size to hydrogen and has no steric effect but carbon‐fluorine bonds are very strong. The methyl group is replaced by a chloro substituent: Chlorine and methyl group have similar sizes and lipophilicities so chlorine acts as a bio‐isotere for the methyl group. Chlorine is also resistant to oxidation. The compound is less active in vitro, but more active in vivo
In gefitinib the morpholine ring increases water solubility: The morpholine nitrogen allows generation of water soluble amine salts. A spacer allows morpholine to protrude out of the active site and it remains solvated when the drug is bound.
Binding interactions were identified by a molecular modelling experiment: Gefitinib is docked with a model ATP binding site. The aniline ring occupies the normally vacant hydrophobic pocket opposite the ribose binding pocket. The quinazoline binds to the same region as the purine ring of ATP .
Imatinib
Imatinib was the first protein kinase inhibitor to reach the market. It is a selective inhibitor for a hybrid tyrosine kinase (Bcr‐Abl) which is active in certain tumour cells
Struture (I) The phenylaminopyrimidine structure was identified by random screening of compound libraries and originally identified as a protein kinase C inhibitor: PKC is a serine‐threonine kinase.
Structure (II) showed increased inhibition of PKC.
Structure (IV) inhibits tyrosine kinases as well.
CGP53716 showed increased activity against tyrosine kinases but no activity against serine‐threonine kinases.
In Imatinib the piperazine increases activity, selectivity and water solubility. A spacer was inserted to avoid creating an aniline structure.
imitanib binding…
Binding interactions were identified from a crystal structure of an inhibitor‐Abl kinase complex.
The amide serves as an ‘anchoring group’ by binding to Glu and Asp and orientates the molecule; Glu and Asp are important to the catalytic mechanism.
Other interactions determine target selectivity: A hydrogen bond to the gatekeeper Thr is essential to activity; N‐alkylation has been shown to eliminate activity.
Molecular modelling studies suggest that the piperazinyl group interacts with a glutamate residue and Imatinib inhibits protein kinases containing this glutamate residue (Abl, c‐Kit and PDGF‐R)
A conformational blocking group aids selectivity; it binds to a hydrophobic pocket that is not accessible if a larger gatekeeper residue was present.
Drug resistance has arisen due to mutation of the gatekeeper residue to isoleucine (T315I mutation); isoleucine is unable to form an important hydrogen bond to the amine.
what are nitrogen mustards useful in
They are useful anti‐tumour agents preventing replication and transcription; alkylation of nucleic acid bases can result in miscoding.
what are nitrogen mustards side effects
Toxic side effects include alkylation of proteins.
Examples of nitrogen mustards
Chlormethine - first used medicinally in 1942. Uracil mustard Melphalan Estramustine Chlorambucil Ifosfumide Bendamustine
Structure of chlormethine
In chlormethine the aromatic ring is electron withdrawing which lowers the nucleophilic strength of nitrogen. This is a less reactive alkylating agent with fewer side reactions and is less toxic.
How does melphalan work
Melphalan mimics phenylalanine and is transported into cells by normal transport proteins.
Structure of uracil
The uracil ring in uracil mustard is electron withdrawing and so this is a less reactive alkylating agent. It mimics a nucleic acid base and is concentrated in fast growing cells.
Structure of estramustine
The urethane group in estramustine is electron withdrawing. The alkylating group is attached to estradiol which is hydrophobic and capable of crossing cell membranes.
by‐product of cyclophosphamide
Acrolein, the by‐product from activation of the alkylating agent, is associated with toxicity
anthracyclines examples
Doxorubicin
Epirubicin
Daunorubicin
Idarubicin
Inhibitors of DNA polymerases examples
Cytarabine
Gemcitabine
Fludarabine
Chemotherapy for pancreatic cancer
Gemcitabine
Gemcitabine
Gemcitabine is the fluorinated analogue of 2’‐deoxycytidine and it interacts with DNA and RNA polymerases within the cancerous cell which inhibits nucleic acid replication and repair.
However, gemcitabine treatment only proves effective in 23.8% of patients with the only alternative being surgical removal of the localized cancerous tissue.
There is, therefore, a huge clinical need for an increase in the efficacy of treatment with gemcitabine as well as exploring alternative therapies. For example, the co‐administration of gemcitabine and capecitabine, a prodrug of 5‐fluorouracil, has been investigated in the treatment of advanced pancreatic cancer
Thiopurines examples
Azathiopurine
Mercatopurine
