Mechanisms of anticancer agents ; drug resistance Flashcards
what are some current cancer treatments?
- surgery; highly successful for localised primary disease
- RT; external beam, intracavity, radioimmunotherapy
- Chemotherapy; applicable to systemic metastasised disease
- Immunotherapy; incl. vaccines
- Gene therapy; e.g. gene transfer
what is neoadjuvant chemotherapy?
to reduce the bulk of primary tumors prior to surgery or RT
what is adjuvant chemotherapy?
following primary surgery
what is fractional cell kill hypothesis?
a given drug conc. applied for a defined time period will kill a constant fraction of the cell population, independent of absolute number of cells
implications of drugs in cancer treatment
tumors best treated when they are small and treatment should continue until last cell is dead
what is the kineetic classification of anticancer drugs based on?
their effect on the cell cycle and if they are phase dependent or phase independent
what are some S-phase dependent drugs?
- ara C
- hydroxyurea
- methotrexate
- 6-thiogunaine
- raltitrexed
what are some G2/mitosis dependent drugs?
- etoposides
- vincas
- taxanes
what are somephase independent drugs?
- alkylating agents
- nitrosoureas
- mitomycin C
- anthracyclines
- 5FU
why do old drugs not work as well?
- limited by poor selectivity for tumors
- dose limiting effects on proliferating tissues
- drug resistance
haematological toxicity
most important dose-limiting toxicity for majority of cytotoxics
-myelosuppression ; risk of infection
- thrombocytopaenia (platelets) risk of haemorrhafe; may be delayed with some drugs (mitomycin C, nitrosoureas) or cumulative (chlorambucil)
GI toxicity
- nausea and vomiting; early onset (6 hours) or delayed up to two weeks (cisplastin) maybe alleviated by 5HT3 receptor antagonists (ondansetron) with dexamethasone
- diarrhoea (5FU, mitomycin C)
- mucositis (doxorubicin, 5FU, methotrexate)
other toxicities of chemotherapy
- alopexoa
- pulmonary toxicity
- cardiac toxicity
- renal
- bladder
- neurological
- local toxicity (at injection site)
tumor response; complete response
complete resolution of all measurable disease for at least one month
tumor response; partial response
50% reduction in product of two perpendicular diameters for on month or more
tumor response; stable disease
no change in size of measurable tumor over a period of one month or more
pathway of finding a new cancer drug
- target identification & validation ; molecular biology
- hit identification; screening, design
- lead optimisation; chemistry pharmacology; PK/PD
- preclinical development; manufactor, formulation, toxicology
- clinical trials; regulatory approval
how long does finding a new cancer drug take?
typically 12-15 years and >$500 million
what is looked at during target identification?
- prevalence and role in cancer vs normal organs
- does target provide a tractable drug target
what is looked at during validation of the drug?
genetics
- KO or KI mice
- RNA inerference
- dominant negatives
- antisense oligonucleotides
- inhibitory peptides/antibodies
what does hit identification of the drug involve?
compound acquisition
- natural products
- synthetic libraries of small molecules, peptides
- rationally designed molecules, antibodies
screening
- high throughput cell free assays (96, 384 well plates)
- cell based assays
what is the general lead optimisation cascade?
- cell free molecular screen; permeability, solubility, In-vitro, protein binding
- in vitro cell-based assays; paired cells with defined molecular pathologu, engineered isogenic pairs, diverse tumor panel
- phamacokinetics; blood levels, tissue levels
- maximum tolerated dose
- [hollow fibre tumor test (rapid in vivo acitvity read-out)]
- in vivo solid tumor; xenograft
- preclinical and clinical development
phase I clinical trials
- regulatory filing
- generally performed in cancer patients rather than healthy volunteers, usually because of low TI
- usually 20-30 patients
- what is max. tolerated dose? (PK important)
- what is dose limiting toxicity?
- antitumor activity NOT primary aim
- increasing emphasis on pharmacodynamics
what is phase II of clinical trials?
assess probabiliy of +ve risk to benefit ratio in Phase III.
generally single agent, single tumor type
- randomised control
- randomised disontinuation; all patients get same treatment, at 12 weeks , patients with stable disease randomised to placebo or active, evaluate at 24 weeks
dose escalation in phase I trials
- fundamental conflict; too fast = sudden jump from safe to life-threatening , too slow = large numbers of patients treated at ineffective doses, slower entry to phase II
- pharmacodynamics; molecular target
what is phase III
- is drug more effective than established therapy
- randomised clinical trials
what does current cancer drug development entail?
- chemical design
- circumvention of drug resistance
- molecuar targeted therapy; hallmarks of cancer
evading apoptosis
Bcl2 inhibitors
self-sufficiency in growth signals
kinase inhibitors; imatnib, elotinib, trastuzumab, sorafenib
tissue invasion and metastasis
MMPIs
limitless replicative potential
telomerase/telomere targeting
sustained angiogenesis
avastin, angiogenesis, inhibitors, vascular targeting agents
future directions of drugs
- individualisation of chemotherapy ; based on tumor genotype
- better detection
- better prevention
drugs used in pancreatic cancer
erlotinib
drugs used in renal cell carcinoma
FDA approved sorafenib and sunitinib
drugs used in NSCLC
erlotinib and bevacizumab
drugs used in breast cancer
trastuzumab
what is drug resistance?
- genetic instability of tumors = adaptation to environmental changes
- heterogeneity, low growth fraction & slow doubling time of most solid tumors = low fractional cell kill
- hypoxia reduces drug acces and tumor sensitivity to many drugs and radiation
chemosensitivity of cancer; group 1
sensitive, cures common
- burkitts lymphoma
- acute lymphoblastic leukaemia in children
- choriocarcinoma
- germ cell tumors
- Hodgkins disease
- wilms tumor
chemosensitivity of cancer; group 2
moderately sensitive may prolong survival
- ovarian cancer
- breast cancer
- GI cancer
- SCLC
- AML
chemosensitivity of cancer; group 3
resistant, no definite effect on survival
- NSCLC
- melanoma
- pancreatic
- renal
- gliomas
- metastatic colorectal cancer
- soft tissue sarcoma
pharmacological drug resistance
- increased drug efflux (MRP)
- decreased drug influx (RFC)
- cytoplasmic drug inactivation (GSH)
- gene amplificatioon of target (DHFR, TS)
- mutation of target (tubulin, topoisomerase II)
post target drug resistance
- increased DNA repair (AGT, NER)
- increased tolerance (loss of mismatch repair)
- failure to undergo apoptosis (loss of p53, increased BCL-2)
what are some cellular mechanisms of resistance? (7)
- decreased intracellular drug concentration
- increased metabolism and detoxification
- altered expression of target proteins
- enhanced DNA repair
- decreased drug activation
- salvage pathways
- failure to engage cell death pathways
what is decreased intracellular drug concentration?
- permeability glycoprotein P-glycoprotein (P-Gp)
- energy dependent efflux transporter, multidrug resistance (MDR)
- lung resistance protein (LRP)
- breast cancer resistance protein (BCRP)
- methotrexate, via defect in membrane carrier protein
- platinum drugs
what are some anticancer drugs that interact with p-glycoprotein? (12)
- doxorubicin
- mitoxantrone
- paclitaxel
- etoposide
- vinblastine
- topotecan
- mitomycin C
- daunorubicin
- taxotere
- teniposide
- vincristine
- actinomycin D
MRP family multidrug resistance
- 190kDa protein, 7 members identified to date
- organic ion transporters
- MRP1 expressed in most normal tissues, no strong correlation with clinical drug resistance, no modulators in clinic yet.
- MRP2 (cMOAT) may contribute to resistance to cisplastin
increased metabolism and detoxification
- tihols, tripeptide glutathione (GSH), glutathione transferases (GSTs), metallothioneins)
- alkylating agents
- cisplastin, carboplatin
- anthracyclines
- increased levels of cytidine deaminase
enhanced DNA repair
- DNA nucleotide excision repair
- 0^6-alkylguanine repair
- DNA mismatch repair
Repairing DNA ds breaks
NHEJ
- Ku dimers stabilise lesion and recruit DNA-PK
- alignment and ligation using DNA ligase IV and XRCC4
altered expression of target proteins
- dihydrofolate reductase (DHFR); methotrexate
- thymidylate synthase (TS); overexpression due to gene amplification
- altered tubulin
- altered topoisomerase
resistance mechanisms ; cisplastin
- reduced membrane transport
- increased DNA repair
- increased GSH/metallothionein (detoxification)
- increased tolerance
resistance mechanisms ; methotrexate/tomudex
- defect in reduced folate carrier (RFC) protein
- decrease in folypolyglutamate synthetase activity (FPGS)
- gene amplification of target; DHFR for methotrexate
resistance mechanisms; etoposide
- increased drug efflux (due to P-glycoprotein or MRP1)
- altered topoisomerase II (mutation, phosphorylation)
multidrug resistance
- ABC transporter efflux pu,ps
- increased GSH
- toposiomerase II mutation/lower activity
- loss of p53/ increased BCL2 (less apoptosis)
cisplastin mechanism of action
binding to DNA and forming intrastrand DNA adducts = inhibition of DNA synthesis and cell growth
clinical methods of overcoming drug resistance
- overcoming cytokenetic resistance
- biochemical modulation of drug resistance
- reduction in host toxicity
- novel approaches
overcoming cytokinetic resistance
- increased dose intensity
- combination chemotherapy
- alternating non-cross resistant chemotherapy
- schedule guided treatment
combination chemotherapy ; principles
- drugs should all be active when used alone
- have different mechanism of action
- have minimally overlapping toxicities
examples of combination chemotherapy (5)
- AL leukaemia; vincristine/prednisone/doxorubicin
- Hodgkin’s; MOPP
- ABVD; doxorubicin/bleomycin/vinblastine
- diffuse large cell lymphoma
- testicular cancer; bleomycin/cisplastin
drugs to circumvent multidrug resistance
- calcium channel blockers
- cyclosporins
- calmodulin inhibitors
- antioestrogens
- quinolines
reduction in host toxicity
- alteration in route of drug administration
- normal tissue rescue
- haematopoetic growth factors
- peripheral stem cell rescue
NOVEL approaches
- ADEPT antibody-directed prodrug therapy
- GDEPT gene directed enzyme prodrug therapy
- modulation of tumor oncogene / suppressor gene expression
- modulation of signal transduction pathways
- ribozyme or antisense inhibition of resistance mechanisms
- protective gene therapy- ex vivo transfer of mdr1 into bone marrow
