Antifolates Flashcards
Methotrexate mechanism of action?
Inhibition of DHFR: leads to depletion of reduced folates
How is methotrexate eliminated?
Unchanged in urine
Which of the following options have been described as a possible cause of tumor resistance to methotrexate?
i. Overexpression of MRP-1
ii. Mutation in the reduced-folate carrier system
iii. Decreased concentrations of NADPH
iv. Dihydrofolate reductase gene polymorphism
v. Increased production of FPGS
a. i, ii, v
b. iii, iv
c. i, ii, iv
d. iii, v
e. ii, iv, v
c.
- MRP-1 is an ATP-binding cassette transporter that results in active efflux of MTX. Thus, its overexpression may result in resistance.
- The reduced-folate carrier system (RFC) is the primary means by which MTX is transported intracellularly. Functional defects in this system can result in MTX resistance.
- Binding of MTX to DHFR occurs in the presence of NADPH. When MTX normally binds to DHFR, this results in an increase in dihydrofolate PGs and depletion of reduced-folate pools.
- DHFR gene polymorphism results in decreased MTX binding affinity.
- FPGS is responsible for polyglutamation, which increases MTX cytotoxicity.
What medication has been shown to enhance renal excretion of methotrexate?
a. Cephalexin
b. Ciprofloxacin
c. Aspirin
d. Probenecid
e. Increased dietary phosphate to acidify the urine
a.
Cephalosporins enhance renal excretion of MTX, likely through competition for tubular resorption. The other drugs are weak acids that inhibit MTX excretion. As a side note, alkalinization of the urine is a protective mechanism against MTX nephrotoxicity.
Which of the following chemotherapeutics inhibits conversion of methotrexate to polyglutamated forms?
a. Doxorubicin
b. Vincristine
c. Cyclophosphamide
d. L-Asparaginase
e. Prednisone
d.
L-asparaginase inhibits conversion of MTX to its PG form, as does leucovorin. On a side note related to drug interactions, alkylating agents and doxorubicin increase expression of DHFR which may contribute to MTX resistance.
Which of the following has the highest affinity for methotrexate?
a. reduced folate carrier b. folate receptor c. proton-coupled transporter d. dihydrofolate reductase e. thymidylate synthase
a.
A 12-year-old dog is being treated for lymphoma with methotrexate. He has concurrent stage 3 CKD. He was treated in the last 24-hours with l-asparaginase and dexamethasone. Which of the following is most correct about this patient’s treatment?
a. The use of l-asparaginase with methotrexate will cause a synergistic kill of the lymphoma cells
b. Methotrexate is primarily metabolized by tissue esterases and this will impact the metabolism of dexamethasone.
c. The use of all three of these medications together has been associated with severe pneumonitis in the canine.
d. Due to methotrexate’s renal excretion and concurrent kidney disease, it may be recommended to reduce the dose of this medication.
e. hepatotoxicity is the most common toxicity associated with this medication combination.
d.
L-asparaginase can cause antagonism of methotexate’s antitumor activity, methotrexate is metabolized by 7-hydroxylation in the liver but is primarily excreted as intact drug in the urine, C is just wrong, myelosuppression and mucositis are the primary toxicities.
Identify the main mechanism of action of methotrexate.
a. Induces cytotoxic effects by covalently binding to purine DNA bases and disrupting the normal functions of cellular DNA
b. Inhibition of dihydrofolate reductase leads to partial depletion of reduced folates and polyglutamates inhibit purine and thymidylate biosynthesis
c. Induces cytotoxicity by interacting with tubulin & disrupting microtubule function
d. Inhibits DNA polymerase α, is incorporated into DNA, and terminates DNA chain elongation
b.
In actively proliferating tumor cells, inhibition of DHFR by MTX or other 2,4-diamino antifolates leads to an accumulation of folates in the inactive FH2 form, with variable depletion of reduced folates. Additional factors may contribute to MTX-associated cytotoxicity, including metabolism of the parent compound to polyglutamate derivatives and the accumulation of dihydrofolate polyglutamates as a consequence of DHFR inhibition. MTX and dihydrofolate PGs represent potent direct inhibitors of the folate –dependent enzymes of thymidylate and purine biosynthesis.
Answer A is the mechanism of action of cisplatin (platinum analogue) Chabner Ch. 15
Answer C is the mechanism of action of vinc and vinbl (vinca alkaloids) Chabner Ch. 13
Answer D is the mechanism of action of cytosine arabinoside (cytidine analogue) Chabner Ch. 9
Which of the following may reduce the incidence of severe myelosuppression related to administration of antifolates such as pemetrexed or pralatrexate?
a. Vitamin D supplementation
b. Taurine supplementation
c. Addition of an NSAID
d. Folate supplementation
d.
Low folate conditions may increase the toxicity of antifolates as a consequence of the upregulation of folate receptor in normal tissues in response to folate starvation. Folate supplementation of patients receiving either pemetrexed or pralatrexate reduces the incidence of sever myelosuppression related to these drugs. It is important to know that NSAID administration decreases renal clearance and increase toxicity. Vitamin D and taurine will not reduce the incidence of myelosuppression in these patients.
What are the two metabolites identified in subjects receiving high dose MTX?
a. 7-OH-MTX and DAMPA
b. 2-CdATP and oxipurinol
c. Acrolein and azo-PCB
d. 6-MP and ara-U
a.
Both of these metabolites are known to accumulate in plasma, and at 24 to 48 hours after high dose MTX administration, they account for most of the MTX-derived material found in plasma and may give spuriously high MTX values when using immunoassays.
Which enzyme is responsible for polyglutamation of folate substrates?
A. Dihydrofolate reductase
B. Polyglutamate synthetase
C. γ-glutamyl hydrolase
D. Folylpolyglutamyl synthetase
d.
FPGS can add up to 8 glutamyl groups, which facilitates the accumulation of intracellular folates in a selectively retained form. MTX PGs are slightly more potent inhibitors of DHFR, but significantly more potent antagonists of TS (thymidylate synthase).
DHFR – reduces intracellular folate compounds to form tetrahydrofolate. These reduced-folate compounds (i.e. tetrahydrofolate) act as coenzymes needed for synthesis of DNA. Antifolates (i.e. MTX) inhibit DHFR → accumulation of folates in the inactive FH2 form.
GGH – is a γ-glutamyl-secific peptidase that removes terminal glutamyl groups and return MTX PGs to their monoglutamate form. GGH and FPGS are important enzymes for regulating intracellular content of PG derivatives.
Which of the following lists of substrates is in the most correct order for substrate affinity of folylpolyglutamyl synthetase?
A. dihydrofolate> tetrahydrofolate> leucovorin> MTX
B. MTX> leucovorin > 5-methyltetrahydrofolate> dihydrofolate
C. leucovorin > tetrahydrofolate > MTX > dihydrofolate
D. aminopterin > MTX > dihydrofolate > leucovorin
a.
Dihydrofolate is the most avid substrate for FPDS. Then in descending order of affinity: tetrahydrofolate > 10-formyltetrahydrofolate or 5-methyltetrahydrofolate > aminopterin > leucovorin > MTX.
Which of the following is correct regarding methotrexate?
A. Methotrexate is a better substrate than Aminopterin for folylpolyglutamyl synthetase (FPGS) and is therefore a more cytotoxic agent.
B. Reductions in FPGS activity may have little effect on folate polyglutamate pools, but will reduce methotrexate polyglutamate formation and methotrexate cytotoxicity.
C. Low intracellular NADPH/NADH ratio promotes methotrexate cytotoxicity.
D. Methotrexate is a non-cell-cycle specific chemotherapeutic agent.
b.
- Aminopterin is more cytotoxic than MTX
- NADPH increases MTX binding to DHFR and NADH does not promote binding to DHFR, but still acts as a co-substrate. Therefore, INCREASED NADPH/NADH ratio should promote methotrexate cytotoxicity.
- Methotrexate is S phase specific.
Which of the following would be expected to increase methotrexate toxicity?
a. Concurrent L-asparaginase therapy
b. Alkalinization of the urine
c. Concurrent use of piroxicam
d. Concurrent leucovorin therapy
c.
Piroxicam decreases renal clearance and increases toxicity; the other three are protective and decrease toxicity.
A mutation that causes which of the following would contribute most to the development of resistance to methotrexate?
a. Decreased activity of the reduced folate carrier
b. Decreased activity of the folate receptor system
c. Decreased activity of the pH sensitive transporter
d. Decreased activity of the ABCG2 transporter
a.
The reduced folate carrier is primarily responsible for transport of methotrexate into the cell, and mutations that caused decreased uptake contribute to MTX resistance. (b) and (c) play a lesser role. (d) is involved in export of MTX out of the cell, so increased functionality would decrease cellular exposure to the drug.
