PSC2002/L12 ABC Transporters & Multidrug Resistance

Question 1

Describe BCRP. (3)

Answer 1

Breast cancer-related protein
Contributes to drug resistance in tumours
First identified in a breast cancer cell line
4q22.1
Expressed in other tissues and relevant to drug excretion
Aka MXR (mitoxantrone-resistance protein)

Question 2

Describe the structure of ABCG2/BCRP.

Answer 2

Single spanning transporters
Smaller than ABCB & ABCC families
1 set of MSD + 1 ATP-binding domain
Half-transporter
Form homodimers (BCRP) or heterodimers (ABCG5, ABCG8)

Question 3

Describe the location of ABCG2/BCRP. (2)

Answer 3

Similar location to P-gp
High levels in lactating breast
Secretion of xenobiotic into milk which has implications for breast-fed infants
Restrictions on use of certain drugs by nursing mothers

Question 4

Describe the findings from BCRP KO mice. (2)

Answer 4

Greater absorption of topotecan following oral dose into plasma mice expressing BCRP1
In (-/-) double KO mice, higher levels in plasma and fetus protected from therapeutic drugs or xenobiotics

Question 5

Describe the results of MDRI and BCRP KO mice. (2)

Answer 5

Evidence of functional redundancy
P-gp and BCRP most important transporters in BBB

Question 6

Describe ABCG2 expression in cancer.

Answer 6

As an MDR, can pump out wide range of chemotherapeutic agents
Reduce intracellular concentrations of drugs lowering effectiveness
E.g., breast cancer, leukemia, lung cancer, colon cancer

Question 7

Give 4 factors that contribute to a CML Leukemia initiating cell.

Answer 7

Quiescence
ABCG2 upregulation
Elevated BCR-ABL
Upregulated CXCR4
Hypoxic microenvironment

Question 8

How does inherent resistance of CML-initiating cells to imatinib affect the brain? (3)

Answer 8

Barrier to drugs treating brain tumours
Can’t cross BBB
Cancer stem cells resistant to imatinib due to upregulation of ABC transporters

Question 9

Describe self-renewal of cancer stem cells and tumour initiation. (3)

Answer 9

ABCG2 implicated in regulating self-renewal
Pool of undifferentiated cells by protecting from environmental stress or modulating signalling pathways in stemness
In some cancers, ABCG2 expression is associated with increased tumorigenic potential and ability of CSCs to repopulate tumours after treatment
Contributes to relapse and metastasis

Question 10

What is the role of ABCG2 in imatinib resistance? (2)

Answer 10

Imatinib substrate for ABCG2
Some evidence that imatinib resistance involves decreased levels of regulatory miRNA-212 (increased ABCG2 levels in leukemic cells)
Imatinib-mediated inhibition of BCR-ABL - downregulation of BCRP levels post-transcriptionally via PI3K-Akt pathway

Question 11

Describe imatinib resistance in CML stem cells. (2)

Answer 11

Higher levels of ABCG2 than more mature CML cells
Less sensitive to imatinib and likely to remain after mature cells eliminated

Question 12

What are RTKs?

Answer 12

Enzyme-coupled receptors which mediated growth factor signalling

Question 13

What are TKIs?

Answer 13

Targeted treatments for cancers

Question 14

How do TKIs work? (2)

Answer 14

Target specific signalling pathways deregulated in cancers
Non-toxic & more specific compared with traditional cytotoxic chemotherapies
Problems with acquired resistance
Important considerations if TKIs used in combination with traditional chemotherapy agents

Question 15

What was shown by GWAS about BCRP?

Answer 15

A genetic polymorphism resulting in amino acid change
Risk factor for gout

Question 16

Describe gout.

Answer 16

Accumulation of uric acid as crystals in joints occurs resulting in pain and inflammation

Question 17

Describe further work on BCRP and gout.

Answer 17

Shows uric acid is a substrate for BCRP and unstable form of this protein results in poor ability to excrete this compound
Uric acid generated by purine metabolism
Accumulation causes joint effects and kidney (stones)

Question 18

What mutation causes gout? Which second mutation can compensate for the unstable protein?

Answer 18

Q141K causes gout
H155A compensates

Question 19

Describe ABCG2 pharmacokinetics.

Answer 19

Ethnic differences should be considered during drug development phase and clinical phase
Reverse-S shaped curve between topotecan level (X) and cell survival (Y)

Question 20

Give 3 strategies to overcome ABC-mediated MDR.

Answer 20

Chemical inhibitors
Natural compounds
Antibodies
Reduced expression (siRNA, miRNA)
Agents that bypass transporters
Novel delivery systems (nanotechnology)

Question 21

What is the function of MDRI inhibitors?

Answer 21

Overcome MDR in cancer

Question 22

Give a first generation MDRI inhibitor.

Answer 22

Verapamil
Quinidine
Amiodarone
Cyclosporine A

Question 23

Give a second generation MDRI inhibitor.

Answer 23

Valspodar
Dexverapamil

Question 24

Give a third generation MDRI inhibitor.

Answer 24

Dofequidar
Zosuguidar
Tariquidar
Elacridar
Biricodar

Question 25

What is the role of verapamil (1st gen MDR I inhibitor)?

Answer 25

Sensitises MDR cells to chemotherapy

Question 26

Give 2 kinds of collateral sensitivity.

Answer 26

Blocked chemotherapy influx
Stimulated GSH efflux

Question 27

Describe blocked chemotherapy efflux.

Answer 27

Drug accumulation within MDR cell

Question 28

Describe stimulated GSH efflux. (4)

Answer 28

GSH not replenished
GSH efflux
Collateral sensitivity
Chemo-sensitisation

Question 29

What is the role of new compounds 5681014?

Answer 29

Modulator of MRP1
Inhibits efflux of chemotherapeutics
Drives glutathione efflux

Question 30

What occurs when 5681014 is combined with BSO?

Answer 30

BSO inhibits GTH synthesis
Enhances cell death and anti-cancer mechanisms

Question 31

Give 2 difficulties of targeting ABC transporters.

Answer 31

Physiological role of ABC transporters
Ubiquitous expression
Transporter redundancy
Dose adjustment monitoring when combining inhibitors with drugs with a narrow therapeutic window