drug resistance mechanisms in cancer

germline mutatoin

occur in every cell as comes from mutation in sperm or egg

somatic mutations

not in every cell occur after birth in body cells

angiogenic growht factors

fibroblast growth factors, vegf, chemokines and cytokines

proto oncogenes

normal proteins that stimulate cell division and growth or regulated apoptosis, essential for normal function. only expressed when required.

examples of proto oncogenes

peptide growth factors, growth factor receptors, protien kinases, proteins regulating the cell cycle, proteins that affect apoptosis, transcriotion factoes.

oncogenes

mutated/defective version of proto oncogene

definition of cancer drug resistance

reduction of effectiveness of antineoplastic drug in curing a cancerious state.

intrisic cancer resistance

lack of tumour response to initioal therapy, primary resistance,

acquired resistance

tumour initally responds to drug then relapses. atttributed to acquired mutations caused by the treatment itself.

6 major types of cancer drug resistance

alteration of drug targets

increased expression of efflux pumps

increased drug metabolism

pro cell survival/ anti-apoptosis

altered proliferation

increased dna repair

alteration of drug targets example

imatinib

imatinib mechanism of action

for CML: BCR-ABL (philadelphia chromosome) protein. small molecule tyrosine kinase inhibitor. binds to Thr315 in ATP binding pocket of kinase domain (hydrogen/van der waals binding). prevents ATP binding, blocks enzyme fucntion.

BCR-ABL fusion protein

ABL protein proro-oncogene (RTK), involved in survival, differentiation, division etc. tightly regulated but once fused with BCR loses regulation, always active.

imatinib acquired resistnce mechanism

cytosine to thymine change at position 315 in kinase region. threonine→isoleucine. removal of hydrogen bonding site, blocks drug access, imatinib reistance.

other possible intrinsic resistance for imatinib

overexpression of efflux transporters e.g. p-glycoprotein

amplification of BCR-ABL gene.

2nd example of alteration of drug targets for reistance mechanism

gefitinib/erlotinib targeting EGFR in NSCLC

gefitinib moa

binds to atp-binding site of egfr, interrutps signalling (proliferation and sirvival pathways).

EGFR mutations involved in nsclc

all in exons 18-21.

gefitinib resistance example: threonine replaced with methionine at codon 790, exon 20.

gefitinib sensitivity example: leucine replaced with arginine at codon 858 exon 21.

acquired resitance to gefitinib

missense mutation in exon 20, threonine to methionine change at codon 790. blocks bidning of gefitinib to EGFR but allows egfr to continue signalling.

mechanism responsible for gefitinib resistance mutation

gefitinib treatment leads to activation of NF-KB, induces expression of AICDA which induces the T790M mutation.

increased expression of drug pumps example

p-glycoprotein: can be expressed intrinsically by cancer cell or show higher expression after treatment (acquired)

decreased expression of solute carrier transporters

OATs and OCTs cause influx of drug from blood. can be mutated in cancer to reduce function and uptake from blood. e.g. OATP1A2 genetic variant.

example of altered drug metabolism in resistance

tamoxifen prodrug converted by CYP2D6 and CYP3A4/5. SNPs in CYP2D6→no enzyme acrivity, no endoxifen production. 4 main metaboliser phenotypes (poor, intermedicate, extensive, ultra).

pro-survival mechanisms of drug resistance example

BCL2 gene determines cell survival or death. most cancer cells overexpress anti-apoptotic BCL2 proteins (e.g. bcl2) after stress (chemo).

examples of pro and anti-apoptotic bcl2 proteins

pro apoptotic: bid, bax, bad, bim

anti-apototic: bcl2

hwo does bcl2 prevent apoptosis

bind and sequester bid and bim (pro-apoptotic) to prevent them binding and activating bax and bak proteins (which normally promote release of cytC causing apoptosis)

how is bcl2 overexpressed (multiple ways)

translocation between chromosomes 14 and 18 brings bcl gene under control of Ig enhancers leading to overexpression

mutations in BCL-2 family proteins

BCL2 gene amplification

venetoclax

binds to bcl2 protein and prevents its binding to bim/etc which allows apoptosis to take place.

acquired venetoclax resistance mechanisms

upregulation of antiapoptotic protein MCL-1 and BCL-XL → can bind to bim and prevent apoptosis.

bcl2 protein glycine substitution to valine point mutation → bulkier, less affinity to venetoclax but still allows binding to bim etc.

how does p53 link with bcl2 family

p53 promotes expression of pro-apoptotic genes after cellular stress, upregulates bax, downregulated bcl2. so loss of p53 prevents apoptosis.

pten role

antagonises PI3K signalling. pi3k increases pip3, akt → pro survival.

pten dephosphorylates pip3 at d3 position to suppress cell survival.

pten mutations

2nd most common mutation in cancer.

many types of mutation, mostly in coding region of phosphatase domain, less phosphatase activity, less antagonism.

ras protein family role

GTPase. inactive when bound to gdp and active when bound to gtp. signalling molecules

mutations in ras protiens

intrinsic resistance mechanism.

permanently active ras proteins (e.g. H- ras, N-ras, K-ras). reduced GTPase activity so GTP remains bound and ras remains active.

colorectal cancer drugs

cetuximab and panitumumab, anti-egfr mabs

cetuximab and panitumumab resistance type

intrinsic.

cetuximab and panitumumab resistance mechansim

mutations in KRAS codons 12 and 13, constitutive action downstream of egfr