Term
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Definition
antimetabolites are a class of drugs for treatment of cancer, infection, gout, and other diseases
the cell synthesizes nucleotides from precursors and then incorporates the nucleotides into DNA (these cellular pathways can be referred to as "precursors to DNA")
all cancer antimetabolites act somewhere in "precursors to DNA" |
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Term
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Definition
adenine and guanine (derived from purine)
[image]
planar (stack in DNA)
heterocyclic base with purin core structure |
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Term
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Definition
cytosine, thymine, and uracil (derived from pyrimidine)
[image]
planar
heterocyclic base with pyrimidine core structure |
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Term
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Definition
base + (deoxy)ribose sugar
a purine or pyrimidine base linked to a 5 carbon sugar (ribose)
also deoxyribonucleosides (deoxyribose instead of ribose)
also commonly referred to as nucleosides (it is assumed that the pentose is ribose) |
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Term
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Definition
base + (deoxy)ribose + phosphate
a purine or pyrimidine base linked to a 5 carbon sugar (ribose) with a phsophate (the only difference between ribonucleoside and ribonucleotide is the presence of phosphate)
also deoxyribonucleotides
commonly referred to as nucleotides (it is assumed that the pentose is ribose)
[image]
above is the general structure of nucleosides and nucleotides (nucleoside phosphates)
nucleosides refer to a base linked to a pentose (5 carbon sugar) via a glycosidic bond
if the pentose is ribose the proper name is ribonucleoside
commonly, nucleoside is assumed to be synonymous with ribonucleoside
if the nucleoside is linked to phosphate(s) then it becomes a nucleotide or also called a nucleoside phosphate |
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Term
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Definition
2 strand polymer of deoxyribonucleotides
bonded together by hydrogen bonds between complementary purine/pyrimidine base pairs (adenine-thymine and guanine-cytosine) |
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Term
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Definition
interfere with synthesis of nucleotides or incorporation of nucleotides into DNA
when referring to cancer chemotherapy, any compound that interferes with production of DNA through acting as analogs of nucleotide precursors or analogs of nucleotides
these analogs inhibit enzymes involved in nucleotide synthesis or they are incorporated into DNA resulting in DNA damage or chain termination |
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Term
| nomenclature of purine nucleosides and nucleotides |
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Definition
[image]
antimetabolites are often close structural analogs of bases and nucleotides
1. adenine is a purine base
2. ribonucleoside = adenosine = adenine + ribose
3. ribonucleotide = adenylate (AMP, adenosine monophosphate) = adenosine + phosphate
4. deoxyribonucleoside = deoxyadenosine = adenine + deoxyribose (reduction at 2 prime position)
5. deoxyribonucleotide = deoxyadenylate = deoxyadenosine + phosphate |
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Term
| nomenclature of pyrmindine nucleosides and nucleotides |
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Definition
[image]
1. cytosine is a pyrimidine base
2. ribonucleoside = cytidine = cytosine + ribose
3. ribonucleotide = cytidylate (CMP, cytidine monophosphate) = cytidine + phosphate
4. deoxycytidine = cytosine + deoxyribose (reduction at 2 prime position)
5. deoxyribonucleotide = deoxycytidylate = deoxycytidine + phosphate
notice that NONE is present for the ribonucleoside and ribonucleotide derivatives of thymine
dTMP is produced from dUMP (by methylation) |
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Term
| overview of path between nucleotide precursors and protein |
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Definition
[image]
1. precursors (amino acids, others)
2. folate (IMP and dTMP)
3. replication, transcription, translation
de novo synthesis of purine ribonucleotides requires several precursors (include glycine, aspartate, glutamine, and CO2) and the essential cofactor folate
the precursors and folate form inosine monophosphate (IMP) from which both purine ribonucleotides are forms (adenosine monophosphate and guanosine monophosphate)
although not shown in this figure, folate is needed for base methylation of dUMP to form dTMP
ribonucleotides (base + ribose + phosphates) are used to form RNA or can be reduced on ribose 2 prime position to form deoxyribonucleotides which are incorporated into DNA
pyrimidine precursors include aspartate, NH4, and bicarbonate
[image]
Synthesis of purine deoxyribonucleotides:
inosine monophosphate (IMP) is the common precursor for both adenosine monophosphate (AMP, adenylate) and guanosine monophosphate (GMP, guanylate)
IMP is synthesized denovo from amino acids, phosphoribosylpyrophosphate (PRPP) and folate
IMP is aminated to form AMP or oxidized and aminated to form GMP
ribose is reduced during conversion of AMP to dAMP and GMP to dGMP through action of ribonucleotide reductase
the deoxyribonucleotides (in triphosphate form) are incorporated into DNA
Synthesis of pyrimidine deoxyribonucleotides:
amino acids and PRPP combine to form uridine monophosphate (UMP, uridylate)
UMP is the common precursor for both pyrimidine deoxyribonucleotides
UMP is aminated for form CMP
ribose in DMP is reduced and the resulting dCMP is incorporated into DNA (after converted to triphosphate)
UMP can also be reduced to dUMP and then methylated to form dTMP which is then used for DNA synthesis |
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Term
| major sites of action of antimetabolites |
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Definition
[image]
through examination of the chemical structure of antimetabolites and knowledge of nucleotide synthesis you can predict the biochemical site of action of antimetabolites
the final result of antimetabolites is interference with DNA synthesis (antimetabolites are S phase specific)
antimetabolites may interfere with the step(s) in the production of purine and/or pyrimidine deoxyribonucleotides or acts at the step of DNA polymerization (analogs may compete with DNA polymerase and/or become incorporated into DNA resulting in interference with elongation)
many antimetabolites must first be bioactivated (ribosylation and phosphorylation common)
purine synthesis and sites of drug action: IMP conversion, ribonucleotide reductase, DNA polymerase
[image]
start with IMP
dGTP synthesis:
IMP is oxidized to XMP (xanthylate, xanthosine monophosphate) by IMP dehydrogenase (IMPDH) which is blocked by 6-mercaptopurine or thioguanine
XMP aminated to GMP
GMP can be phosphorylated to GTP and incorpoated into RNA
for DNA synthesis, GMP is converted to dGMP by the enzyme ribonucleotide reductase (blocked by hydroxyurea)
dGMP is phosphorylated to dGTP and then incorporated into DNA
dATP synthesis:
IMP is aminated for formation of AMP (this is blocked by 6-mercaptopurine)
AMP can be phosphorylated and then incorporated into RNA
AMP can be reduced to dAMP by the enzyme ribonucleotide reductase (blocked by hydroxyurea)
dAMP can be phosphorylated to dATP and then incorporated into DNA (inhibited by fludarabine and cladribine, analogues of adenosine)
pyrimidine synthesis and sites of drug action: ribonucleotide reductase, folate cycle, DNA polymerase
[image]
the amino acid aspartate combines with carbamoyl phosphate to form orotate which combines with phosphoribosylpyrophosphate (PRPP) to form UMP (uridine monophosphate, uridylate)
dTTP synthesis:
UMP is reduced by ribonucleotide reductase to form dUMP (blocked by hydroxyurea)
dUMP can be converted to dTMP by action of thymidylate synthase and presence of MTHF (methylene tetrahydrofolate)
5-fluorouracil blocks thymidylate synthase
MTHF is regenerated by action of dihydrofolate reductase (DHFR)
mehtotrexate blocks conversion of DHF to THF thus interfering with dTMP production
dTMP is phosphorylated twice to form dTTP which is used for DNA synthesis
dCTP synthesis:
UMP is phosphorylated to UTP which may be used for RNA
UTP can be aminated for form CTP (cytidine triphosphate)
CTP can be reduced by ribonucleotide reductase to dCTP (blocked by hydroxyurea)
dCTP may be used for DNA synthesis (blocked by cytarabine) |
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Term
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Definition
methotrexate
pemetrexed
pralatrexate
folates are required by the body for not only production of purines and deoxythymidylate (dTMP) but also conversion of homocysteine to methionine, conversion of serine to glycine, and formation of glutamic acid |
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Term
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Definition
cell uptake via RFC (reduced folate carrier)
POLYGLUTAMATED inside cell
inhibits dihydrofolate reductase (DHFR)
blocks formation of IMP and dTMP
interferes with S PHASE
methotrexate requires transport into the tumor cell via the reduced folate carrier
once inside the cell methotrexate is polygutamated which is better retained intracellularly
inhibits DHFR, the enzyme that converts dihydroflate (DHF) to tetrahydrofolate (THF)
THF is needed for production of methylene tetrahydrofolate which is the cofactor that donates methyl group to dUMP for conversion to dTMP
methotrexate also blocks formation of IMP which is the common precursor to both purine nucleotides |
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Term
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Definition
thymidylate synthase inhibition
also inhibits DHFR
main toxic effect comes from inhibition of thymidylate synthase (catalyzes methylation of dUMP) which resulte in less dTMP and thus less dTTP available for DNA
polyglutamated - proposed to occur more in tumor cells versus normal cells |
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Term
| folic acid analogs - mechanism of resistance |
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Definition
1. LESS TRANSPORT INTO CELLS
analogs use the folic acid transport system for entry into the tumor cell (mainly reduced folate carrier, RFC)
mutation could result in less drug transport
2. DECREASE DRUG TARGET AFFINITY (I.E. DHFR)
especially for methotrexate, mutation of the dihydrofolate reductase (DHFR) enzyme could result in lower affinity thus requiring higher concentration of the drug to maintain same effectiveness (such a mutation must selectively decrease drug affinity)
3. LESS POLYGLUTAMATED-DRUG PRODUCTION (LESS CELL RETENTION)
after folic acid analog enters the tumor cell they are glutamated
the polyglutamated form of the drug is highly charged and is retained inside the cell
mutation in this biochemical pathway may result in less polyglutamation and therefore less intracellular drug concentration
4. MORE DRUG EFFLUX
tumor cells may express an efflux pump that removes intracellular methotrexate
a whole class of efflux pumps exist (MRPs, multidrug resistance proteins) that increase removal of drugs
5. INCREASE DHFR PRODUCTION; MULTIPLE GENE COPIES COMMON
after continuous drug exposure tumor cells contain high DHFR levels
this is attributed to multiple gene copies
gene amplification of the target protein is a common mode of anti-cancer drug resistance
for example if there is a concentration of methotrexate in the tumor that binds (inhibits) 90% of the DHFR and then a mutant cell arises that produces 10x more DHFR, there will be 10x amount free (non-inhibited DHFR) which may be enough for dTMP production
higher concentration of drug will be required to kill the tumomr cell that produces more DHFR
production of multiple gene copies of a drug target is common mode of drug resistance |
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Term
| folic acid analogs: toxicity and other considerations |
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Definition
bone marrow suppression
gastrointestinal
ACUTE AND CHRONIC liver toxicity
pulmonary and RENAL
skin (TEN, SJS)
accumulation in "THIRD SPACE" cavities
rapidly dividing cells (bone marrow and epithelial cells) are more susceptible
acute hepatic toxicity (elevated liver enzymes) and chronic toxicity (fibrosis and cirrhosis) may occur
renal toxicity is attributed to precipitation of parent drug and metabolites in tubules
toxic epidermal necrolysis and Stevens-Johnson syndrome are serious skin reactions
methotrexate accumulates slowly in third space fluid cavities (peritoneal, pleural, pericardial)
when the volume of these fluid spaces becomes large (ascites, pleural effusions) drug accumulation can be very significant
most of a methotrexate dose is recovered in the urine unchanged |
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Term
| use of leucovorin (folinic acid) |
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Definition
toxicity rescue
enhances 5-fluorouracil action
folinic acid is fomyltetrahydrofolate
it enters the folate cycle independent of dihydrofolate reductase activity (not affected by methotrexate)
this drug is used for rescue from methotrexate toxicity due to nucleotide deficiency
[image]
rescue:
bone marrow
gastrointestinal
NOT liver, renal, lung effects
folinic acid must have some differential action on tumor cells, otherwise it would cancel the anti-tumor effect of methotrexate
it is thought that folinic acid is concentrated more in normal cells compared to cancer cells (especially when RFC is mutated in cancer cell)
another hypothesis is methotrexate creates a window of time where apoptosis is irreversible in cancer cells
folinic acid is effective only against toxicity resulting from inhibition of DNA synthesis (bone marrow suppression, GI epithelial damage)
ENHANCED 5-FLUOROURACIL EFFECT
fluorouracil action is enhanced by folinic acid
reaction is limited by MTHF, there is more than enough thymidylate synthase - increased supply of rate limiting factor |
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Term
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Definition
thioguanine
mercaptopurine
cladribine
fludarabine phosphate
pentostatin
purine analogs interfere with endogenous pruine mucleotide synthesis and/or become incorporated into DNA causing DNA damage |
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Term
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Definition
1. after ribosylation and phosphorylation (thioGMP) inhibits IMPDH enzyme (INHIBITS IMP TO XMP) conversion
2. thio GTP -> DNA INCORPORATION -> stops strand elongation
[image]
structures of guanine and thioguanine
thioguanine is a structural analogue of purines
thioguanine resembles guanine and can be ribosylated and phosphorylated in parallel with endogenous nucleotides
the nucleotide form of thioguanine irreversibly inhibits IMPDH (inosine monophosphate dehydrogenase) and upon incorporation into DNA inhibits DNA replication
inhibition of IMPDH results in less dGTP production needed for DNA synthesis |
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Term
| azathioprine and mercaptopurine |
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Definition
azathioprine released after -SH reaction
mercaptopurine converted to thioinosine monophosphate (T-IMP)
T-IMP INHIBITS GMP AND AMP PRODUCTION
DNA incorporation
[image]
azathioprine and mercaptopurine are structural analogues of purines
azathioprine is a prodrug form of mercaptopurine
azathioprine reacts with sulfhydryl compounds in the liver (e.g. glutathione) to release mercaptopurine
the nucleotide form of mercaptopurine, thioinosine monophosphate (T-IMP, formed by action of HGPRT) inhibits the enzymes that convert IMP to AMP and GMP
azathioprine (currently used for prevention of renal transplant rejection and rheumatoid arthritis) is a prodrug that reacts with sulfydryl groups (glutathione) to release mercaptopurine (indicated for leukemia) which is then transported into cells
mercaptopurine is the ribosylated and phosphorylated by the enzyme HGPRT (hypoxanthine-guanine phosphoribosyl transferase) to form T-IMP (thioinosine monophosphate) which inhibits conversion of IMP to GMP and AMP
T-IMP is also further phosphorylated and incorporated into DNA causing DNA damage and interference with replication |
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Term
| thioguanine and mercaptopurine mechanisms of resistance |
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Definition
1. LESS HYPOXANTHINE-GUANINE PHOSPHORIBOSYL TRANSFERASE (HGPRT)
a major mechanism of resistance is decreased production of the activating enzyme HGPRT
this enzyme is needed for conversion of thioguanine to thioGMP and mercaptopurine to T-IMP
[image]
2. DECREASED INFLUX OR INCREASED EFFLUX
common to many other drug resistance mechanisms are decreased uptake (influx) or increased efflux
3. DNA DAMAGE RESPONSE
also common to other anti-cancer drug resistance is altered DNA repair mechanisms
this could mean the tumor cell has enhanced ability to repair damaged DNA or may not undergo apoptotic response to damaged DNA |
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Term
| thioguanine and mercaptopurine toxicity and other considerations |
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Definition
bone marrow suppression (MORE DELAYED)
liver (jaundice, veno-occlusion)
mercaptopurine metabolized by xanthine oxidase (allopurinol interaction)
thiopurine methyl transferase (TPMT) gene polymorphism
principle concern with these 2 drugs is bone marrow suppression (onset is more gradual compared to methotrexate)
they can also cause elevated hepatic transaminase enzymes, jaundice, and veno-occlusion
mercaptopurine is metabolized in part by xanthine oxidase to 6-thiouric acid
since allopurinol inhibits xanthine oxidase, dose of mercaptopurine should be reduced by ~75% in patients concurrently taking allopurinol
about 10% of patients having sequence polymorphisms that reduce activity of thiopurine methyl transferase resulting in less drug inactivation through methylation (greater toxicity) |
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Term
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Definition
hairy cell leukemia
high affinity inhibitor of ADENOSINE DEAMINASE (ADA)
intracellular accumulation of ADENOSINE AND DEOXYADENOSINE
accumulation of adenosyl homocysteine toxic to lymphocytes
also, DNA incorporation
[image]
pentostatin inhibits ADA (adenosine deaminase) the enzyme that converts adenosine and 2' deoxyadenosine to inosine and 2'-deoxyinosine, respectively
pentostatin binds to ADA with very high affinity b/c it structurally resemples the intermediate (transition state) in this enzymatic reaction
hairy cell leukemia is a relatively rare type of leukemia
it is a malignancy of the B cell lineage
diagnosis is often made by presence of characteristic "hairy cells" in bone marrow sample or peripheral blood
adenosine deaminase (ADA) is found mainly in lymphocytes
the enzyme catalyzes the conversion of adenosine to inosine and deoxyadenosine to deoxyinosine
inhibition of ADA causes intracellular accumulation of adenosine and deoxyadenosine
increased adenosine and deoxyadenosine blocks ribonucleotide reductase and blocks adenosyl homocysteine hydrolase resulting in increased adenosyl homocysteine which is toxic to lymphocytes
pentostatin tri-phosphate form incorporated into DNA
administration of this drug mimics the symptoms of genetic ADA deficiency |
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Term
| cladribine and fludarabine phosphate |
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Definition
DNA polymerase competition
triphosphate forms INCORPORATED INTO DNA
strand breakage or termination
DEMAINASE resistant
[image]
cladribine and fludarabine phosphate are also adenosine analogues
cladribine is a chlorinated purine analogue that is incorporated into DNA and causes DNA strand breaks
fludarabine phosphate is a fluorinated purine analogue that is incorporated into DNA; this drug also inhibits DNA polymerase and ribonucleotide reductase
cladribine is converted to triphosphate form by action of deoxycytidine kinase enzyme
triphosphate form inhibits ribonucleotide reductase and is inocorporated into DNA causing DNA strand breaks
cladribine is also indicated for hairy cell leukemia
in extracellular space fludarabine phosphate is dephosphorylated and then transported into cells where it is converted to triphosphate form (by action of deoxycytidine kinase) and then incorporated into DNA cuasing chain termination or strand breakage; also incorporated into RNA |
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Term
| cladribine and ludarabine phosphate mechanism of resistance |
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Definition
loss of deoxycytidine kinase
[image]
these drugs must be activated through phosphorylation by the enzyme deoxycytidine kinase
resistance to these drugs is associated with loss of tumor enzyme activity |
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Term
| cladribine and fludarabine phosphate toxicity and other considerations |
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Definition
myelosuppression
depletion of CD4 CELLS
PENTOSTATIN + FLUDARBINE = high risk lung damage
myelosuppression is dose limiting
low CD4 cell count correlates with high risk of opportunistic infection |
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Term
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Definition
fluorouracil
floxuridine
capecitabine
cytarabine
gemcitabine HCl |
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Term
| fluorouracil and fluorodeoxyruidine |
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Definition
converted to fluorodeoxyuridine monophsphate (FdUMP)
FDUMP INHIBITS THYMIDYLATE SYNTHASE
triphosphate derivative incorporated into RNA and DNA
[image]
note the structural similarity between uracil and 5-fluorouracil
uracil is the base in dUMP, the endogenous substrate for thymidylate synthase and 5FU is metabolized to FdUMP, an irreversible inhibitor of thymidylate synthase
5-fluorodeoxyuridine is ribosylated fluorouracil
fluorouracil must first be converted (ribosylated and phosphorylated) to fluorodeoxyuridine monophosphate (FdUMP)
FdUMP blocks formation of dTMP (and thus dTTP) by inhibition of thymidylate synthase
after FdUMP is converted to triphosphate form (FdUTP) it may be incorporated into both RNA and DNA
incorporation into DNA is thought to be a result in dTTP deficiency combined with elevated dUTP
the consequency of FdUTP incorporation into DNA is uncertain |
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Term
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Definition
high oral bioavailability
CONVERTED TO FLUOROURACIL
plasma protein binding (WARFARIN INTERACTION)
[image]
capecitabine is an orally available prodrug of fluorouracil
the mechanism of action is the same |
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Term
| fluorouracil, fluorodeoxyuridine, and capecitabine mechanism of resistance |
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Definition
1. LOSS OF ACTIVATION ENZYMES
there are several enzymes required for activation of these drugs (uridine phosphorylase, uridine kinase, phosphoribosyl transferase)
loss of activity of these enzymes through mutation will cause drug resistance
2. GENE AMPLIFICATION OF THYMIDYLATE SYNTHASE
similar to dihydrofolate reductase, thymidylate synthase gene can undergo amplification
3. MUTATION OF THYMIDYLATE SYNTHASE RESULTING IN LOWER DRUG AFFINITY
mutation can decrease drug binding affinity |
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Term
| fluorouracil, fluorodeoxyuridine, and capecitabine toxicity and other considerations |
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Definition
mucosal membrane ulcerations
HAND-FOOT SYNDROME
severe mucosal membrane ulceration can develop (preceded by stomatitis and esophagopharyngitis)
ulceration of GI can cause severe diarrhea
hand-foot syndrome is also called palmar plantar erythrodysesthesia syndrome (PPES)
this syndrome is associated with several anti-cancer drugs
the exact mechanism by which some chemotherapeutic agents cause hand-foot syndrome is not known
proposed to involve leakage of drug into tissues in pressure areas (hands, feet) |
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Term
| cytarabine and azacytadine |
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Definition
analogs of the nucleoside cytidine
cytarabine stops DNA strand elongation
azacytadine INTERFERES WITH CYTOSINE METHYLATION
cell differentiation
[image]
cytarabine and azacytidine are both analogues of the nucleoside cytidine
cytarabine has an arabinose sugar in place of ribose (note the chirality of the hydroxyl group highlighted in blue)
the incorporation of cytarabine triphosphate (araCTP) into DNA inhibits further nucleic acid synthesis b/c the replacement of 2'-deoxyribose by arabinose interrupts strand elongation
azacytidine has an azide group (highlighted in blue) within the pyrimidine ring
this drug is incorporated into nucleic acids and interferes with the methylation of cytosine bases
cytarabine has an arabinose sugar instead of ribose
this drug converted to triphosphate form and incorporated into DNA (stops strand elongation)
this is thought to result from formation of a stable tri-complex consisting of polymerase, drug, and DNA
azacytidine is converted to triphosphate form and then incorporated into RNA and DNA
after incorporation it inhibits methylation (C5 is the position of cytosine methylation; in azacytidine this position is N) of cytosine which alter gene expression and promotes cell differentiation |
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Term
| cytarabine and azacytidine mechanism of resistance |
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Definition
1. LOSS OF ACTIVATION ENZYMES
these drugs, like many other antimetabolites, are dependent on enzymatic activation
2. HIGH EXPRESSION OF CYTOSINE DEAMINASE
cytarabine can be inactivated through deamination to arauridine |
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Term
| cytarabine and azacytidine toxicity and other considerations |
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Definition
myelosuppression
cytarabine LIPOSOME FORMULATION (DepoCyt) - chemical arachnoiditis common
primary concern with these drugs is bone marrow suppression
liposome formulation injected intrathecally for lymphomatous meningitis (infiltration of malignant cells)
liposome formulation provides sustained release of cytarabine |
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