Drug resistance mechanisms in cancer

what is cancer drug resistance?

cancer cells learn to adapt or gain the ability to survive and grow despite the presence of anti-cancer therapy

intrinsic - lack of tumour response to initial therapy - primary resistance and not due to the presence of drug - important to get the right treatment

acquired - tumours initially respond to treatment, later relapse due to acquired mutations caused by the treatment itself

types of cancer therapies

targeted - cytostatic, block tumour growth, targets cancer-specific surface proteins

chemotherapy - cytotoxic, kill tumour cells, doesn’t discriminate and more likely to produce serious side effects

types of cancer drug resistance

alteration of drug targets

increased expression of efflux pumps

increased drug metabolism

pro-cell survival and anti-apoptosis

altered proliferation

increased DNA repair

alteration of drug targets - imatinib

BCR-ABL: ABL1 is a protooncogene, it becomes an oncogene when fused with BCR (BCR-ABL, Philadelphia chromosome) → unchecked proliferation and reduced apoptosis → CML

ABL kinases are non-receptor tyrosine kinases (nRTK)

imatinib: RTK inhibitor for chronic myeloid leukaemia (CML) - binds to the ATP-binding pocket of the ABL kinase domain, preventing dimerisation and BCR-ABL nRTK enzyme is permanently switched on, adding phosphate groups to substrates downstream → uncontrolled signalling and growth

imatinib resistance:

• point mutations - T315I (gatekeeper residue) → removed critical hydrogen-bonding site required for TKI binding → blocks imatinib access

• overexpression of ATP-binding cassette efflux transporters - main route of exit for imatinib → less effective

• amplification of the BCR-ABL gene

alteration of drug targets - gefitinib

EGFR in cancer: RTK activation → cell proliferation (MAPK), survival (PI3K/Akt), angiogenesis, migration

non-small cell lung cancer (NSCLC): some NSCLC are due to mutations in EGFR gene cause activation of receptor without ligand binding

gefitinib: first-generation EGFR TKI - binds reversibly to ATP-binding site of EGFR’s intracellular tyrosine kinase domain → blocks autophosphorylation → inhibits downstream signalling → cell cycle arrest and apoptosis in cancer cells

gefitinib resistance: acquired mutations “activating mutations” result in ligand-independent activation of the receptor

• EGFR T790M mutation in exon 20 (gatekeeper mutation) increases ATP affinity, reduces gefitinib binding

• point mutation → L858R mutation in exon 21 “sensitising mutation”

around 50% of NSCLC cases that initially respond to EGFR TKIs eventually acquire resistance

increased expression of drug efflux transporters - MDR1

multidrug resistance protein 1 (MDR1): transmembrane efflux pump, belongs to the ABC transporter family. MDR1 gene encodes P-gp. transports anti-cancer drugs out of the cell

intrinsic: increased MDR1/P-gp expression in cancers such as renal, adrenocorticoid, liver, pancreatic, colorectal

acquired: breast and SCLC with low base like MDR1/P-gp expression

decreased expression of solute carrier family (SLC) transporters

SLC transporters mediate the influx of substances into the cell

organic anion/cation transporters (OATs/OCTs)

there is evidence that some SLCO1A2 genetic variants caused a reduced or complete loss of the ability to transport specific drugs such as imatinib/methotrexate

increased drug metabolism

CYP2D6 plays a crucial role in the metabolism of tamoxifen, an inhibitor of the oestrogen receptor to treat ER positive breast cancer. it is a prodrug, CYP2D6, tamoxifen → endoxifen.

it is highly polymorphic → different individuals have different enzyme activities

in poor metabolisers, tamoxifen becomes less effective → increasing cancer recurrence (intrinsic resistance)

pro-cell survival/anti-apoptosis - Bcl-2

Bcl-2 is part of a family of anti- and pro- apoptotic regulators - it blocks pro-apoptotic proteins such as BAX and BAK from releasing cytochrome c → cell death

drug resistance: bcl2 overexpression - translocation between chromosomes 14 and 18 or bcl2 gene amplification

venetoclax: selective bcl-2 inhibitor which mimics pro-apoptotic BH3 proteins → displaces BAX/BAK → triggers apoptosis. used in chronic lymphocytic leukaemia

venetoclax resistance: acquired mechanisms -

• upregulation of MCL-1 and BCL-XL which bind to BH3 → inhibits activation of mitochondrial apoptotic pathway → reduced drug efficacy → continued growth

• G101V point mutation → bulkier side chain within the binding groove of BCL2 protein → loss of affinity to venetoclax → still allows binding of anti-apoptotic proteins → no apoptosis

pro-cell survival/anti-apoptosis - TP53

p53 is a tumour suppressor - loss of p53 in cancer cells could result in reduced BAX protein expression as well as increased bcl2 expression → limiting apoptosis

pro-cell survival/anti-apoptosis - PTEN

PTEN is a regulator that antagonises the PI3K/Akt signalling pathway - dephosphorylates PIP3 to PIP2

PTEN intrinsic mutations leads to enhanced cell survival, cancer progression, and resistance → loss of tumour suppressor function

trastuzumab - mAb that targets HER2 (prevents RTK activation)

trastuzumab resistance: intrinsic resistance - loss of PTEN function → cells continue to proliferate even when trastuzumab is bound to HER2

pro-cell survival - Ras

Ras catalyses GTP hydrolysis to become inactive GDP - mutations in Ras genes inhibit this activity and also impairs its binding to GAP proteins → MAPK signalling pathway still activated

KRAS oncogene mutation in some colorectal cancers - point substitutions in codons 12 and 13 in exon 2 - cetuximab for EGFR blockade, KRAS mutation causes resistance

alternative treatment: TKIs?