6 - Oncology 3

Question 1

What is azathioprine converted into and where in the body does this take place?

Answer 1

• Must be rapidly converted to 6-mercaptopurine by RBC

* *Azathioprine is a prodrug of 6-mercaptopurine b/c of 6-MP’s poor BA

Question 2

Describe the metabolism of 6-mercaptopurine and its metabolites

Answer 2

• 6-MP converted to 6-thioguanine triphosphate and deoxy-6-thioguanine triphosphate via multiple steps
• 6-thioguanine triphosphate and deoxy-6-thioguanine triphosphate inhibit DNA synthesis and kill rapidly growing cells
• • Also appears to induce apoptosis in T-cells; hence its immunosuppressive effect

Question 3

What is the most significant SE of azathioprine?

Answer 3

Myelosuppression

Question 4

Specific AZA mechanism and metabolism

Answer 4

• 6-thioguanine triphosphate and deoxy-6-thioguanine triphosphate -> inhibit nucleic acid synthesis and kill rapidly growing cells and induce apoptosis in T-cells
• 6-methyl-thio-inosine di and triphosphate inhibit de novo purine synthesis
• TPMT and XO reduce pool of 6-MP resulting less of the active metabolites
• • Inhibition of XO w/ allopurinol increases toxicity of azathioprine

Question 5

Azathioprine – allopurinol interaction

Answer 5

• Xanthine oxidase (XO) metabolizes 6-MP
• Allopurinol inhibits XO and causes severe toxicity w/ normal doses of AZA
• Taking the 2 together usually requires a 70% AZA dose reduction
• Using allopurinol concurrently w/ AZA is similar to having 1/def genotype

Question 6

What affect do TPMT and XO have on AZA and 6-MP?

Answer 6

Reductions in the activities of either TPMT and XO could profoundly increase toxicity of AZA by increasing 6-MP plasma concentrations

Question 7

TPMT genotype and myelosuppression w/ AZA tx

Answer 7

• Majority of population is 1/1; very rare to be def/def (0.6%)
• 1/def considered intermediate enzyme activity
• Risk of myelosuppression w/ normal AZA dose increases w/ 1/def and is very high w/ def/def (100% compared to normal which is 8-43%)
• *def = *2, *3A, *3B, *3C, or *4 (all have reduced activity)

Question 8

AZA monitoring

Answer 8

• Even after picking a dose based on TPMT genotype, still need to monitor AZA
• TPMT testing can’t substitute for a CBC monitoring in px receiving AZA b/c of risk of severe myelosuppression
• All px getting AZA must get a regular CBC, even those w/ TPMT 1/1
• • Dosing based on TPMT genotype alone only reduces the # of cases of severe myelosuppression by 1/3 (this study only relates to those taking AZA for IBD, not cancer)
• • Includes those taking AZA for IBD

Question 9

Gefitinib

Answer 9

• EGFR tyrosine kinase inhibitor
• Indicated for 1st line tx of metastatic non-small cell lung cancer (NSCLC) who have activating mutations of the EGFR receptor tyrosine kinase domain
• Most of what we discuss regarding gefitinib is also applicable to erlotinib

Question 10

What is EGFR?

Answer 10

Transmembrane receptor tyrosine kinase required for growth and differentiation of epithelial cells

Question 11

EGFR function

Answer 11

• Binding of ligands cause EGFR dimerization and tyrosine kinase activity resulting in auto-phosphorylation of EGFR tyrosine residues
• This activates several signal transduction cascades which result in DNA synthesis and cell proliferation

Question 12

Gefitinib mechanism

Answer 12

• Gefitinib and erlotinib inhibit tyrosine kinase domain to prevent auto-phosphorylation of EGFR tyrosine residues, preventing signal transduction cascades and subsequent DNA synthesis and cell proliferation
• • Gefitinib binds to the ATP binding site of the EGFR TK domain competitively inhibiting ATP binding; ATP is required for kinase activity to act as a source of phosphate groups

Question 13

Gefitinib SE

Answer 13

• Diarrhea
• Pustular rash (~50% of px on gefitinib)
• Interstitial lung disease (associated w/ sx of cough and dyspnea; may be fatal; occurs in < 2% of px)

Question 14

Cons to gefitinib

Answer 14

• Was approved as a 3rd line tx for NSCLC
• Initially produced rapid improvement w/ minimal SE; however, large trials failed to show an effect on survival
• No clear association between expression of EGFR and effect of gefitinib
• • Except in a small subset of px (female, never smoked, having Japanese ancestry -> these individuals showed a 70% response)

Question 15

Why does gefitinib work in a specific group of px?

Answer 15

• Several mutations in the TK domain of EGFR appear to make it more sensitive to TKI (ie: gefitinib and erlotinib)
• • EGFR TK domain divided into ATP binding loop, P-loop, and activation-loop
• • ATP binding loop (several deletions happen here), P-loop (G719 AA substitutions) and activation loop (L858 AA substitutions)
• ATP binding loop mutations increase survival better than activation loop mutations

Question 16

ATP binding loop deletions

Answer 16

• Many deletions are found in the ATP binding loop, most of these are missing the sequence LREA
• Deletions make EGFR more susceptible to gefitinib inhibition

Question 17

P-loop substitution

Answer 17

• This part of the protein forms the binding site for the triphosphate of ATP hence the name P-loop
• Major mutations = G719S, G719C, G719A (considered mutations b/c don’t know if they occur in > 1% of the population, which would make them polymorphisms)
• All mutations increase sensitivity of EGFR to gefitinib

Question 18

How do mutations make gefitinib/erlotinib more effective

Answer 18

• Activating mutations in the EGFR TK domain decrease IC50 for gefitinib
• Decreased affinity for ATP (easier for gefitinib or erlotinib to compete)
• Increased affinity for gefitinib/ erlotinib
• Decreased catalytic efficiency (ex: TK activity already reduced compared to WT
• Altered substrate specificity (ex: tyrosine residue getting phosphorylated)

Question 19

Mutations that decrease effect of gefitinib

Answer 19

• Can happen w/ mutations that increase gefitinib effect
• • Mutations increasing gefitinib effect, called primary mutations
• • Mutations decreasing gefitinib effect, called secondary mutations
• Major secondary mutations = D761Y, T790M

Question 20

Secondary mutations regarding gefitinib

Answer 20

• D761Y and T760M reduce the effectiveness of gefitinib in those w/ activating primary mutations
• This effect is greater w/ the T790M mutation than the D761Y mutation
• T790M mutation blocks binding of gefitinib w/ EGFR TK
• Not clear how D761Y reduces sensitivity to gefitinib