Antimetabolites Flashcards
1
Q
Which DNA bases are pyrimidine? which ones are purines?
A
Pyrimidines: uracil, thymine, and cysteine
Purines: adenine and guanine
2
Q
What’s the MOA of methotrexate?
A
- inhibition of dihydrofolate reductase (DHFR): increased dihydrofolate/ decreased reduced folate
- also inhibits thymidine synthase once it’s in a poly-glutamate form
3
Q
What’s the normal folate cycle?
A
4
Q
What are some mechanisms of resistance for methotrexate?
A
- transport carrier issues (mutation)
- decreased affinity to DHFR (need NADPH) or DHFR gene amplification
- low levels of thymidine synthase (can be done by 5-fluorodeoxyuridylate) or by depletion of its substrate dUMP
- increased efflux: MRP-1,2,3 and BCRP (breat cancer resistance protein)
- defects in polyglutamation (ex. with folypolytuamyl synthetase , FPPGs) (this is the part that’s inhibited by Elspar)
5
Q
What’s the mechanism of cell death for methotrexate?
A
Inhibition of thymidine monophosphate (TMP) and purine synthesis leads to a cessation of DNA synthesis = DNA strand breaks, resulting from nucleotide depletion or misincorporation of dUMP for dTMP, and cell death
- DNA chain maturation arrest
- DNA strand breaks: Ineffective repair from lack of nucleotides
- High levels of dUMP – Misincorporation into DNA/ uracil-DNA-glycosylase
6
Q
What’s the role of Leucovorin (folinic acid) in methotrexate?
A
It’s a rescue drug in case of severe myelosuppression –> it’s an active metabolite of folic acid –> essentially by passes the need for DHFR to make reduced folate
7
Q
What’s the major toxicity associated with methotrexate in dogs?
A
nausea
8
Q
What are some side effects of methotrexate?
A
- GI > myelosuppression
- Nephrotoxic
- Pneumonitis
- anemia (macrocytic)
- hepatotoxicity (mostly reversible/ brief increase in hepatic enzymes)
- neurotoxicity, enhanced BBB with RT
- hypersensitivity
9
Q
What are some drug interactions with methotrexate?
A
- Leucovorin: rescue for severe toxicity (didn’t work in one paper with canine OSA)
- L-asparaginase: blocks the toxicity and anti-tumour effects of methotrexate
- NSAID: decreased renal clearance and enhances toxicity
10
Q
What exotic species has used methotrexate to treat LSA?
A
ferret! combination protocol lived for 10 months
11
Q
How is methotrexate metabolized by the kidneys?
A
active secretion of MTX takes place in the proximal renal tubule, with reabsorption in the distal tubule
12
Q
What’s the MOA of 5-FU?
A
- inhibits thymidylate synthase, TS (by FdUMP)
- inhibits transcription by incorporation into RNA (FUTP)
- inhibits DNA synthesis by incorporation into DAN (FdUTP). It can’t be repaired due to depletion of dTTP via inhibition of TS
13
Q
How is 5-FU metabolized?
A
deactivated by dihydropyrimidine dehydrogenase (DPD) –> 90% elimination, the rest is excreted in the urine
14
Q
What happens is there is a deficiency in dihydropyrimidine dehydrogenase with 5-FU?
A
less DPD = less deactivation = increased toxicity
15
Q
What’s the role of Leucovorin in 5-RU
A
increases efficacy and toxicity!
16
Q
What are some drug interactions with 5-FU?
A
- Leucovorin –> increases efficacy, and toxicity (enhances the binding of fdUMP to TS)
- Radiation –> possible sensitizer
- Cimetidine –> decreases clearance of 5-FU
17
Q
What are some toxicities associated with 5-FU
A
- GI
- myelosuppression
- neurotoxic –> esp in cats –> contraindicated
- palmar-plantar dysesthesia
- ocular
- cardiac (not reported at clinical doses)
18
Q
What are some indication of 5-FU in horses?
A
mostly used as either intralesional (sarcoids) or topical (SCC).
Good response rate and duration of response
19
Q
What are some indications of 5-FU in dogs?
A
more so for carcinomas
- mammary carcinoma combined with cyclophosphamide (stage III) DFT/MST around 24m, sig better than no adjuvant therapy post-po
- with various carcinomas combined with carboplatin - had CR with colorectal carcinoma/ intestinal adenocarcinoma
- one case report of topic 5-FU for corneal SCC. No recurrence after 23 months
20
Q
What’s capecitabine?
A
oral prodrug of 5-FU
preferential accumulation in neoplastic cells
21
Q
How is capecitabine used in dogs?
A
As immunosuppressive for renal transplant
22
Q
What are some toxicities encountered with combination therapy involving capecitabine in dogs for renal transplants?
A
6/7 acute neurotoxicity –> death in 2
another study had 2/8 death due to neurotoxicity
2/8 had keratitis
23
Q
What is 5-FU?
A
halogenated analogue of uracil
24
Q
How is 5-FU transported into cells?
A
active transport with uracil transporter
requires intracellular activation to generate flurouridine (FU) and fuorodeoxyuridine (fdU) species
25
Can cats receive 5-FU?
no! contraindicated --> severe neurotoxicity.
Deficient DPD?
26
Can the dUTP that's been incorporated into DNA be corrected?
The mistake is recognized by uracil-DNA-glycosylase (UDG) and dUTP can be removed, but there is no thymine to replace it due to dTTP depletion from TS inhibition of the 5-FU
27
How important is the length of administration for base analogues?
important! it effects the dosing.
length of administration is part of the dosing can’t be changed!
28
Generally, what's the effect of deaminase and kinase on base analogues?
deaminase = deactivation --> increases resistance
Kinase = activate --> helps with efficacy
29
What are the 2 cytidine analogues?
Cytosine arabinoside/ cytarabine/ ara-C
Gemcitabine
30
What's the MOA of cytarabine?
Ara-CTP
- incorporation into DNA --> chain chain maturation arrest
- inhibits DNA polymerase --> effects DNA replication and repair
- Ara-CDP-choline --> inhibits synthesis membrane glycoprotein glycoplipids --> Not going to have normal membrane for cells
31
What carrier is used by Ara-C?
hENT1
32
What is the rate limiting step in Cytosar metabolism?
deoxycystidine kinase (the first step)
it's negatively controlled by the final product, d-CTP (but lack feedback inhibition with Ara-CTP)
33
What are some mechanisms of resistance for Cytosine arabinoside?
- hENT1: saturation, inhibition (can be done by RTKIs)
- presence of deaminase
- overexpression of Bcl-2 and Bcl-Xl (antiapoptoic proteins)
- deoxycytstidine (CdR) kinase mutation
34
What are the cellular response of Ara-CTP incorporation into DNA?
- Activation of ATR and Chk1 checkpoint kinases --> cell cycle arrest and repair
- Absence of either checkpoint sensitizes to apoptosis
35
What are some known drug interactions of Ara-C?
- Cyclophosphamide
- Cisplatin
Inhibition of repair of DNA adducts
- Hydroxurea
Inhibitor RR – decreased dCTP pool, increased
ara-CTP formation
dCTP inhibits CdR kinase and activates dCMP kinase
dCTP and ara-CTP compete for same active site of DNA polymerase
- Methotrexate
- Etoposide
Ara-C increases level of Topoisomerase II, enhance DNAsb
- THU (tetrahydrouridine)
Inhibitor of cytidine deaminase
36
What are some major toxicities of cytarabine?
GI
myelosuppression
37
What's the outcome of COAP in naive canine LSA?
It had an increased risk of relapse and death (HR 1.9). But CHOP was associated with more severe neutropenia and GI signs
38
How is Ara-c as a LSA rescue?
- Having Ara-C with Carboplatin was significantly better than carboplatin alone but more likely to be neutropenic and thrombocytopenic.
OST 28 days
- combination with bleomycin = 36.8% RR
39
What's the response of DMAC in canine LSA rescue?
RR 72% (44 CR, 28% PR); 11% SD
Median remission duration 61 days (CR 112 days, PR 44 days, and SD27 days). But a new study in 2019 found a lower RR (35%; 21% CR, 14% PR)) and shorter remission duration ( 62 d CR, 32 days PR)
40
What's the experience of cytarabine in canine AML?
- 1 dog had CR with ara-C,6-TG, vinc, and doxo
- AML-M5 acute monocytic leukemia: 3 dogs responded to doxo/ara-c/pred: lived for 62, 103, 112 days
- AML-M7 acute megakaryoablastic leukemia, 1 dog treated with vinc, daunorubicin, ara-C and prednisolone. With reinduced ara-C, lived for 24m
41
What's the outcome for DMAC for relapsed feline LSA?
26%RR, 17 day survival
42
What's the outcome of ara-C in cats with AML?
M1: 1 cat treated with COPA, ST 14 weeks (98 days)
M5: 1/3 responsded to ara-C, ST 78 days
1 cat with ara-C, doxo, vinc, pred - CR for 67 days, ST 95 days
M7: 1 cat with ara-C, CR, and ST 122 days
43
What's the outcome of ara-C in cats with ALL?
Ara-C single agent, CR, 10 weeks (70 days)
44
What other conditions has ara-c been used for cats?
case reports:
- myelodysplastic syndrome. PR, ST 51 days
- histiocytic SA with CNS involvement (with doxo): neuro signs resolved, short response
45
What's the MOA of gemcitabine?
- inhibits ribonucleotide reductase (diphosphate form, DfdCDP)
- inhibits DNA polymerase (dFdCTP)
- incorporation into DNA --> chain termination arrest
46
What enzyme will deactivate gemcitabine?
cytidine deaminase
47
Is gemcitabine cell cycle specific?
the incorporation into DNA is not limited to S phase
48
What does ribonucleotide reductase do?
Ribonucleotide reductase (RR) is the enzyme responsible for the conversion of ribonucleotides to 2′-deoxyribonucleotides and thereby provides the precursors needed for both synthesis and repair of DNA
- RNR = the rate-limiting reaction in the regulation of DNA synthesis
49
What are some drug interactions with gemcitabine?
- radiation sensitizer (due to it's RR inhibition)
- cytotoxicity increased by platinum and taxanes
50
What are some mechanisms of resistance of gemcitabine?
- Tumor levels of deoxycystidine kinase (CdR) kinase
- Induction of cytidine deaminase
- High concentration of HSP (heat shock protein)
- Increased expression of RR
- Inhibition of nucleoside transporters
51
What's the DLT for gemcitabine?
myelosuppression
increases with longer infusion time
52
What are some toxicities of gemcitabine?
- thrombotic microangiopathy
- acute respiratory distress (with bleomycin)
- RT sensitization
53
What's the outcome of gemcitabine + RT (6 x 6 Gy twice weekly) for feline oral SCC?
Gemcitabine given at 25mg/m2 IV
25% CR, 50% PR
MDFI = 42.5d, MST = 111.5 days
54
What's the MTD for gemcitabine single agent in dogs?
900mg/m2 IV weekly over 30min
DLT = neutropenia
no non-hematological toxicities
55
What's the appropriate CRI for gemcitabine in cats?
2.5-5mg/m2 per minute
56
What's the outcome of Gem/Carbo for carcinoma in dogs?
- 1 CR prostatic; 2 PR intestinal ACA; 12 SD tonsillar SCC
- 32% grade 3 or 4 neutropenia, 24% grade 3 or 4 thrombocytopenia, GI toxicity mild/self limiting
57
What's the outcome of Gem/Carbo for carcinoma in cats?
Gem Day 1, 8, 15; Carbo Day 1, 21-day cycle
- 33% grade 3 or 4 neutropenia, 16% grade 4 thrombocytopenia, 16% grade 3 GI toxicity
Gem Day 1, 8; Carbo Day 1, 21-dat cycle
- 14% grade 3 or 4 neutropenia, 7% grade 3 or 4 GI toxicity
1 CR pancreatic carcinoma; 1 PR SCC
Another study with pancreatic carcinoma showed MST of 165 days
58
What's the outcome of single agent gemcitabine with canine hepatocellular CA?
Gemcitabine at 350-400 mg m2 IV over 20 min weekly for 5 weeks
- MPFI 971 days (incompletely excised tumours was 971 days and 150 days nonresectable tumours)
- MST 983 days (massive 1339 days, nodular 983 days, diffuse disease 113 days)
59
What's the outcome of single agent gemcitabine with canine mammary CA?
no improvement post-op
60
What's the outcome of single agent gemcitabine with canine iUC?
Gemcitabine 800 mg/m2 IV over 30 to 60 minutes q 7 d and piroxicam 0.3 mg/kg PO q24 h
- 5% CR, 21% PR, 50% SD
- MST 230 days
all dogs had improvement in clinical signs
61
What's the outcome of single agent gemcitabine with canine LSA?
no OR
62
What's the outcome of gemcitabine + carboplatin with canine OSA?
no improvement over carboplatin single agent
63
What's the MOA of hydroxyurea?
inhibition of ribonucleotide reductase
64
What's the predominate route of elimination of hydroxyurea?
renal and hepatic metabolism
65
What are some drug interactions with hydroxyurea?
- Ara-C: increases conversion to active metabolite
- enhance activation of purine antimetabolites
- radiation sensitizer due to inhibition of DNA repair
66
What's the main indications of hydroxyurea?
- leukemia
- polycythemia vera
- essential thrombocythemia
- myeloproliferative diseaes
- MCT
67
How does hydroxyurea enter the cell?
passive diffusion
68
Is hydroxyurea cell cycle specific?
yes, S phase --> deficient deoxynucleotides --> accumulates in S phase --> apoptosis
69
What's the main mechanism of resistance of hydroxyurea?
increased synthesis of ribonucleotide reductase
70
Can ara-CTPP be excised once incorporated into DNA?
no --> will inhibit the function of the DNA template and subsequent synthesis
71
What's the primary mode of metabolism of ara-C?
deamination by the liver and extrahepatic tissues
72
What's the DLT for ara-C
myelosuppression
occasionally GI signs
73
How is ara-C administered?
Ideally 600mg/m2 IV CRI over 2-5 days
more convenient to do 4 SQ injection twice daily
74
What's the response of ara-C for canine LSA?
not good as single agent,
but maybe useful in other combination therapies (VCAA)
75
What can be used for MEU in dogs?
Low-dose subcutaneous Ara-C (50 mg/m2 BID for 2 days or 100 mg/m2 as a constant rate infusion for 1 day) has been reported to improve clinical signs in dogs with meningoencephalitis of unknown origin when combined with prednisone
76
What's the bioavailability of methotrexate?
high at low doses, and variable at high doses.
So it's usually dosed orally at low doses, and IV at high doses.
77
Why does methotrexate have GI side effects at doses that don't cause hematological side effects?
it has enterohepatic recycling
78
What's the main elimination route for methotrexate?
renal
79
Can dFdCTP be excised once incorporated into DNA?
no, this is critical for triggering of apoptosis
80
What's the mode of transport of transport for hydroxyurea?
passive diffusion
81
What are some toxicities of hydroxyurea?
- GI
- myelosuppression (including anemia) --> cats are more susceptible (methemoglobinemia at higher doses)
- rarely pulmonary fibrosis
- onycholysis in dogs with chronic treatment
