Lecture 16: Therapeutics to Treat Cancer

Transcription

1 Lecture 16: Therapeutics to Treat Cancer Therapeutic Objectives Primary prevention stop the risk of cancer before it occurs o Smoking cessation o Coxibs COX2 selective cyclooxygenase inhibitor PGE2 facilitates the growth of the vessels Polyps can develop into a tumour Elimination of tumour cells (cure) o Tumour has been detected Reoccurring theme most attempts of removing cancer is unsuccessful due to resistance May require a combination of approaches o Surgical resection of solid tumour o Organ specific radiotherapy o Cycles of chemotherapy o Targeted therapy small molecules and biologicals o Immunotherapy o Cell therapy Cancer Biology Cancer cells characterised by: Uncontrolled proliferation o May or may not be rapid proliferation o Uncontrolled not capped De-differentiated state o Arise from a cell with a particular function o Loss of normal characteristics of parent cell o Undifferentiated appearance Causes of Cancer Mutations leading to: Inactivation of tumour suppressor genes o Release control on proliferation Activation of proto-oncogenes to oncogenes o Initiation phase o Sustained proliferating signal due to inappropriately persistent activation of proliferation o Loss of hand break allowing for cell turnover to be too great Tumour Suppressor Genes Acts as sentinels to detect DNA damage Eg/ p53 Typically, these gene products survey DNA to determine whether it is in a fit state for replication o Decision point to divide or not to divide p53 can be activated by DNA damage p53 activates two pathways from cell cycle arrest o DNA repair o Irreversible DNA damage and apoptosis Cell is not allowed to replicate if DNA is damaged If p53 is damaged, the cell is able to replicate Gene most commonly mutated in human cancer >50% of cancers

2 If DNA is damaged, p53 arrests replication Induces genetic instability Cancer Biology Cancer cells characterised by: Invasiveness o Loss of recognition of normal restraints o Inability to break through the basement membrane o Expression of EC matrix-degrading enzymes Ability to metastasise o Secondary tumour distant from primary o Often through lymphatic system Tumour Invasiveness and Metastasis Epithelial tumour mass Epithelial cells usually grow in a mono-layer surround a hollow entity Loss of growth/replicative control Cells acquire invasive characteristics Epithelial mesenchymal transition EMT o Less epithelial like o More mesenchymal o o Able to migrate Secrete MMPs that breakdown basement membrane matrix MMP mediated BM penetration Come into proximity of blood vessels angiogenesis o Formation of new blood vessels to sustain new growth Migration Intravasation o Invade the vessel Intravascular/intralymphatic dissemination Platelet adhesion/masking o Tumour cells hide themselves from the IS o Travel to a point where they will leave the blood vessels Extravasation o Leave the blood vessels and site themselves at a distant place favourable for growth o Draining lymph node o Distant target Dormancy/distant metastatic growth o Dormant for a period of time o Can be reactivated by local events o Or grow straight away o Repeat process where new growth has to attract blood vessels o metastasis formation Proto-Oncogenes Necessary for normal cell differentiation Normally responsible for cell growth and differentiation Mutation leads to abnormal growth oncogenic o Become uncontrolled Examples o Growth factors o Growth factors receptors

3 o Members of growth factor signalling pathways o Cell cycle transducers Becomes autonomous Eg/ ras gene Mutated in up to 20% of tumours Codes for Ras, a guanine nucleotide binding protein GTPase o Ras-GTP on o Ras-GDP off GTPase activity is important for hydrolysing the Ras-GTP to GDP to turn it off Common component of many signal transduction pathways leading to proliferation RAS is activated by GTP binding Mutation in RAS stabilises GTP form by reducing GTPase activity o Normally RAS controlled by growth receptor ligand o Mutated form is autonomous active regardless of what is happening with the receptor Mutations in Ras o Found in 20-30% human tumours Reduced inactivation of Ras o Reduced intrinsic GTPase activity o Reduced susceptibility to GTPase activating proteins Ras-GTP becomes predominant form o Constitutively activated Cell divides in absence of growth factor binding Triggers downstream events Ras Signalling Elk-1 = TF for Cyclin-D Ras/Raf/Erk phosphorylation of ERk-1 Leads to increase in production of cyclin D1 Autonomous signalling is able to drive the production of factors that are important to move through the cell cycle RAS important in migration turnover of the cytoskeleton Effectors downstream of RAS o NFkB, JNK o Important TF in inflammatory responses Activates PI3K that signals cell survival Drive to proliferate and drive to survive Tumour Cell Elimination Concept of selective cytotoxicity Exploiting the high fraction of tumour cells cycling by targeting cell cycle related events that renders cells in cycle more sensitive to cytotoxicity o Results in apoptosis or cytotoxicity What defines a live cell? An ability (potential) to replicate (useful in context) Only a fraction of cells are cycling at one time Some residual cells are not turning over less likely to undergo cytotoxicity in response to chemotherapy Amongst these might be cancer stem cells give rise to further proliferating pools of tumour cells Log Cell Kill Model The higher the kill fraction, the fewer the residual cells, longer cell population takes to regrow tumour Cell killing is first order constant fraction of cells killed Want cycles of chemotherapy that engenders high kill fractions Limitations on dosing and duration of chemotherapy o Chemotherapy not necessarily given at the optimal dose or over the optimal period of time o Needs to be manageable to the patient o Typically chemotherapy is given in cycles with times of management

4 Severity of adverse effects necessitates intermittent dosing Intermittent dosing allows for o Tumour growth cancer stem cells o Expansion of resistant cells Resistant cells grow exponentially regardless of treatment Principles of Cancer Chemotherapy In practice, growth limited by blood supply to tumour angiogenesis o Tumour grows to 1-2mm o Requires its own blood supply o Before this size it can use the perfusion of the tissue Resting cells may be resistant to treatment tumour stem cells o Not in cell cycle Metastases often the most significant survival factor o Usually the aspect of the tumour that limits life o Requires lifelong and different treatments o Survive treatment of primary tumour but need to treat rest of the life for suppression of metastases Therapeutic window is often non-existent dose for treatment is same or greater than dose giving side effects o Toxicity at the same doses of the agents that there is benefit Toxicity limits the dose and duration of therapy o Limits fractional kill and limits efficacy of chemotherapy If selective toxicity Is via rapid cell division Toxicity will also result from effects on other fast-dividing cells o Bone marrow suppression G-CSF boost bone marrow after cycle of chemotherapy Turns over rapidly so is more susceptible to chemotherapy A falling neutrophil count makes you more susceptible for infection Co-morbidity is sepsis GCSF treats the suppression o Impaired wound healing o Hair loss o Gut epithelium damage Lack of selectivity engenders adverse effects Nausea Vomiting Diarrhoea o Neuroendocrine cells in the gut Tiredness Cachexia also caused by tumour cytokine response, loss of appetite Susceptibility to infection/certain tumours o Immunosuppression increases the risk to leukaemia and lymphomas Anti-Emetic Prophylaxis for Chemotherapy Induced Nausea and Vomiting Mainstay of anti-nausea chemotherapy that allows higher doses of chemotherapy to be administered Research on mediators of nausea Serotonin activate 5-HT3 receptors on the afferent nerve o Trigger of nausea and vomiting o Ondansetron Central effects dopaminergic mechanisms release Substance P on NK receptors Dexamethasone can be administered once o GC allows higher doses of chemotherapy to be administered o Save money don t need to use it four times Cheaper and used less frequently o Improves compliance o Improve QOL

5 Classes of Anticancer Drugs Cytotoxic drugs Alkylating agents cisplatin, bleomycin Antimetabolites 5-Fluorouracil, Methotrexate Anthracyclines doxorubicin Microtubule inhibitors vincristine/vinblastine, paclitaxel Drugs that exploit other aspects of tumour biology Hormone antagonists and related agents tamoxifen, fulvestrant Anti-angiogenesis agents bevacizumab Kinase inhibitors imatinib Immune mechanisms pembrolizumab, CAR T cell therapy Cytotoxic Anticancer Drugs Kill cells o Damage DNA o Interfere with DNA synthesis o Interfere with mitosis Cells that are traversing cell cycle are targeted more frequently will be most affected o Rapidly dividing cells Alkylating Agents Bind covalently with nucleophilic cell components o N7 and O6 of guanine Many are bifunctional o Cross linking of DNA within a strand or between strands Cisplatin Discovered by chance while examining effects of electric currents on bacteria o Platinum electrode didn t support the growth of bacteria o Agent interferes with DNA synthesis o Bacteria at platinum electrode had reduced DNA synthesis Used for testicular and ovarian cancer Revolutionised treatment of testicular cancer in 1970s Causes intra-strand cross linking o Denaturation of DNA o Cell can t replicate cell with intrastrand covalent linkage o Inhibition of transcription can t open up for RNA polymerase Two leaving groups Cl

6 Bis-alkylation causes intra-strand crosslinks between adjacent guanine residues o Causes local denaturation of DNA Trans isomer inactive can still bind to DNA, doesn t form cross link o Cross-linking makes it effective Cell death via apoptosis Oxidative stress with a depletion of glutathione o Used to protect against ROS DNA damage induces repair mechanisms Damage can lead to p53 activation o Leads to a raft of biochemistry and apoptosis o Leads to apoptosis o Features that activate the mitochondria to release cychrome C o Formation of apoptasome and the activation of caspases o apoptotic death of tumour cell o Cleared my macrophage infiltrating the tumour Nephrotoxic, neurotoxic o Cause dose-limiting sensory neuropathy Associated with severe nausea and vomiting o Enhances nephrotoxicity o Can be controlled with ondansetron serotonin receptor 5-HT3 o In combination with Dexamethasone and Arepitant NK1 antagonist o Maximise doses that can be tolerated by the patient Resistance Increased expression of DNA repair enzymes Decreased drug uptake Enhanced drug inactivation increased GSH nucleophile Bleomycin Natural glycopeptide from steptomyces Bleomycin orange molecule Binds DNA and chelates Fe2+ o Chelated iron can trigger formation of ROS Radical formation causes single and double strand breaks in DNA DNA damaged engenders apoptotic response Used curatively in testicular cancer or lymphoma Pulmonary fibrosis is dose limiting o ROS to epithelium o Triggers in 2% of patients fibrosis Cumulative and irreversible used to model IPF rodents Actions of anti-fibrotic agents Used in lab administered to mice Given intranasally to reach airway epithelium Induce ROS burden in epithelium Damages epithelium that goes on to form fibrosis Fibrosis is at a level that has obscured most of the gas exchange zones o Normal parenchymal lung o Thin alveolar membranes with capillaries associated o Worse fibrosis and get deposition of collagen o Occupation of airspace so there is little gas exchange zone left

7 Serious adverse effect when used as chemotherapy DNA Synthesis Amino acids and PRPP are conjugated to form precursors of nucleotides Folate is required in AMP, GMP and TMP Inhibitors of Pyrimidine Nucleotide Synthesis 5-FU Sufficient analogy to uracil that enters the pathway and is initially converted by the same enzymes that are going to work on uracil Converted into a deoxy version that inhibits thymidylate synthase Inhibits formation of thymidine monophosphate that is the precursor of thymidine triphosphate used in the formation of DNA Inhibitor is able to inhibit thymidylate synthesis by acting as a false substrate but forming a stable complex with the enzyme target making it unavailable for the conversion of natural precursor into dttp 5-Flurouracil FdUMP directly inhibits thymidylate sunthetase o Inhibition of DNA synthesis o Engenders apoptotic response Administered with folate o Forms long lived complex persistent inhibition of the enzyme o Increases toxicity, as inhibited enzyme is a complex with FdUMP and cofactor derived from folate Adverse effects gut damage and bone marrow toxicity Predictable effects of inhibition of rapidly dividing populations of cells Methotrexate Inhibits dihydrofolate reductase DHFR o Enzyme that is strongly induced by the action of Cyclin-D o Turn on enzymes necessary for DNA synthesis Has higher affinity for DHFR than natural substrate analogue of folate Prevents generation of tetrahydrofolate FH4 o FH4 is a cofactor in thymidylate production Blocks thymidylate production In turn, disrupts DNA synthesis Low lipid solubility, therefore does not cross the blood brain barrier o Not useful for brain cancers Causes bone marrow depression, gut epithelium damage o Rapidly dividing normal cell population Resistance common Use high dose and rescue with leucovorin modified tetrahydrofolate o Folic acid analogue that works downstream o By-passes lack of tetrahydrofolate o Feeds into production of thymidylate o Restore cell capacity to duplicate their DNA High kill fraction and rescue the patient from the extent of damage Exploit selectivity for rapidly diving cell populations Anthracyclines Eg/ doxorubin Binds to DNA by intercalation sliding between base pairs

8 Inhibits the enzyme topoisomerase 2 o Required for DNA unwinding and rewinding to relieve supercoiling o Essential in the process of making DNA available for duplication o Nicks and re-joins DNA Doxorubicin Stabilises the DNA/topoisomerase II complex o DNA remains nicked o Inhibits replication Cardiotoxicin o Generates free radicals which damage cardiac tissue can induce fibrosis (potentially lethal) o Can be reduced by co-administration of dexrazoxane o Fe chelator reduce burden of free radicals o Reduced Fe2+ mediated free radical production Fe2+ catalyses formation of OH- radical from hydrogen peroxide OH- is extremely toxic and reacts within microseconds o Toxic response is repaired by fibrosis over time o Replacement of cardiomyocytes with fibrotic cells engenders a loss of CO and some dysrhythmia Microtubule Targets Beta subunits at (+) end hydrolyses GTP GDP at (+) end inhibits polymerisation (+) end grows 2x rate of These features called dynamic instability Dynamic instability required in the M phase Key in cell division by allowing the chromosomes to segregate according to the poles of the cell preparing to undergo mitosis Microtubules act as motors that are able facilitate the separation of the chromosomes necessary for the formation of daughter cells Involved in mitosis, IC trafficking, vesicular movement, cell structure and shape Formation of the positive end of the microtubule is with the GTP bound form of the b-tubulin The a- tubulin dominates at the negative end The positive end grows twice the rate of the negative end Growth is supported by the hydrolysis of GTP Little GTP or it has been hydrolysed appearance of GDP at the positive end starts to destabilise the structure o Dynamic instability o Required in the M phase for separation of chromosomes Low concentration of GTP bound b-tubulin Reduces the size of a GTPcap When GTPcap disappears, the structure becomes unstable OR a lot of b-tubulin bound to GTP, the cap at the positive end grows Stability and growth of microtubule is able to move chromosomes around Vinca Alkaloids There are binding sites on b-tubulin for vinca alkaloids o From periwinkle plant o Bind b-tubulin on GTP site o Reduce polymerisation and (+) end o Prevents (+) end extension o Depolymerisation of microtubule and loss of functions Vinblastine

9 Used with bleomycin and cisplatin for testicular cancer Nausea, vomiting, myelosuppression limits dosing Vincristine Used for some lymphomas Nausea, vomiting, peripheral neuropathy limits dosing o Neuropathy associated with the function of the microtubules in moving vesicles down the axon of the nerve o Interfere with dynamic behaviour of microtubules, interfere with vesicular traffic and get nerve dysfunction as a result Taxanes Bind b-tubulin away from GTP site Stabilise polymers Arrests cells in mitosis Leads to apoptotic death Paclitaxel Used for NSCLC, breast cancer Nausea, vomiting, hypersensitivity reaction-reduced with Dexamethasone/Histamine H1 receptor antagonists, pulmonary toxicity vascular permeability peripheral neuropathy limits dosing (microtubule role in trafficking in axon) myelosuppression Abraxame form that is conjugated with albumin-bound paclitaxel o No hypersensitivity burden and less myelosuppression Cancer Chemotherapy Cytotoxic drugs still provide mainstay of therapy Newer agents o More selective o Fewer, less dose-limiting side effects Resistance is still a problem