39 Personalized Medicine in Cancer Treatment Flashcards
What is pharmacogenetics?
The study of the genetic determinants of drug response variability
Germline or constitutional variants can impact disease treatments
What can polymorphic variant alleles do to the drug metabolism?
- activate a prodrug to its active form
- catalyze the inactivation and elimination of a drug or metabolite
What is the most important enzyme complex in drug metabolism?
cytochrome P450 superfamily (CYP)
It directly affect the pharmacokinetics of many drugs.
What is Cytochrome P450?
Cytochrome P450 is a hemeprotein
Plays a key role in the metabolism of drugs and other xenobiotics
It directly affect the pharmacokinetics of many drugs.
Besides CYP450 and enzymes, what other proteins are important for drug metabolism?
Cell receptors –> play a role in pharmacodynamics
Genetic variants in cell receptors can influence drug transport
What are the phases of drug metabolism?
phase I reactions
phase II reactions
What is the phase I of drug metabolism?
introduce reactive or polar groups
(-OH, -COOH, -NH2, -SH, etc.) into drugs, including oxidation, reduction, and hydrolysis,
where drugs cannot be excreted from bodies.
What is the phase II of drug metabolism?
The phase I modified drugs are conjugated to polar compounds by a variety of transferase enzymes, such as
uridine diphosphate (UDP)-glucuronosyltransferases,
sulfotransferases,
glutathione S-transferases
General pathways of drug metabolism
Phase I reactions –> Phase II reactions –> The conjugated drugs may be further processed, before being recognized by efflux transporters and pumped out of cells.
See the figure.
Contribution of different enzymes to drug metabolism:
CYP450
UGT, UDG glucuronosyl transferase;
FMO, flavin-containing monooxygenase;
NAT, N-acetyltransferase;
MAO, monoamine oxidase.
see the figure:
73% by CYP450
15% by UGT
What drugs are Fluoropyrimidines?
Fluoropyrimidines are a class of anti-cancer drugs, or more specifically antimetabolites.
Examples:
Capecitabine
Fluorouracil (5-FU)
Tegafur (Ftorafur®)
What type of cancers are treated by Fluoropyrimidines?
Solid tumors, such as breast and colorectal cancers.
What is an antineoplastic agent?
An antineoplastic agent is a chemotherapeutic agent that controls or kills cancer cells.
1) They are cytotoxic (inhibit or prevent cell function)
2) They are generally more damaging to dividing cells than resting cells.
3) Antineoplastic drugs are a subset of hazardous drugs.
What are the inactive prodrugs of 5-FU?
Capecitabine
Tegafur
What is the main mechanism of 5-FU activation?
Conversion from 5-FU to fluorodeoxyuridine monophosphate –>
inhibits thymidylate synthase
What is thymidylate synthase?
an important part of the folate-homocysteine cycle
and
purine and pyrimidine synthesis
What is the effect of 5-FU?
increased base excision repair damage–>DNA fragmentation and cell death.
the fluorouridine triphosphate metabolite can be incorporated into RNA in place of uridine triphosphate –> interfering with RNA processing and protein synthesis
What is the rate-limiting step of 5-FU catabolism?
Dihydropyrimindine dehydrogenase (DPD) conversion of 5-FU to dihydrofluorouracil
DPYD gene
1) Gene name for dihydropyrimidine dehydrogenase (DPD)
2) located at chr. 1p21.3
DPYD gene mutations
1) decrease DPD activity
2) increase drug half-life
3) severe or fatal 5-FU toxicity
What are the recommendations made by Clinical Pharmacogenetics Implementation Consortium (CPIC) for the DPYD gene mutations and 5-FU dosing?
1) 50% reduction in starting dose for patients who are heterozygous for a nonfunctional DPYD variant
2) an alternate therapy for patients with two nonfunctional DPYD variants (homozygous or compound heterozygous)
DPD deficiency
– homozygous loss-of-function
– an autosomal recessive disorder
– can be symptomatic and asymptomatic
– characterized by a wide range of severity : can have severe convulsive disorders with motor and mental retardation
– >50 mutations identified for DPD deficiency
– 3-5% Caucasian population has partial DPD deficiency
DPYD*2A
The mutation IVS14 + 1 G > A, DPYD*2A, is the most common mutation associated with DPD deficiency.
A G > A base change at the splice recognition sequence of intron 14, leads to exon skipping and results in a 165-bp deletion in the DPD mRNA
DPYD mutations detection
– Specimen: whole blood or tissues
– genotyping target: DPYD*2A decreased activity allele
– sequencing
Effect of heterozygous loss-of-function DPYD alleles
– reduction of 5-FU clearances: 40% to 80% reduction
Alternatives to DPYD genotyping are?
1) assessing DPD enzyme activity by dihydrouracil to uracil ratio in plasma
2) uracil breath test
3) measure DPD activity in peripheral mononuclear cells
Other genes influence 5-FU response:
1) ABCB1
2) MTHFR
3) TYMS
Which cancer is Irinotecan used for?
Irinotecan is used to treat metastatic colorectal cancer(CRC)
Irinotecan can be used with what anticancer agent to treat CRC?
5-FU
leucovorin leu-co-vo-rin
Irinotecan can be used with what anticancer agent to treat small cell lung cancer?
cisplatin
5-FU + Irinotecan are used to treat for what cancer?
colorectal cancer
leucovorin + Irinotecan are used to treat for what cancer?
colorectal cancer
cisplatin + Irinotecan are used to treat for what cancer
small cell lung cancer
How does Irinotecan works in treating cancers?
Binding to the topoisomerase I-DNA complex and prevent DNA replication
thus causes double strand DNA breakage and cell death
What is the active form of Irinotecan?
SN-38
How is the Irinotecan metabolized and eliminated?
SN-38 is glucoronized to SN-38 glucuronic acid (SN-38G)
and detoxified in the liver via conjugation by the UGT1A family, which releases SN-38G into the intestines for elimination.
what % of SN-38 is converted to SN-38G?
Approximately 70% of SN-38 becomes SN-38G, which has 1/100 of the antitumour activity and is virtually inactive.
What does UGT1A1 stand for?
the uridine diphosphate glucuronosyltransferase family 1 member A1
UGT1A1 gene is located on chr. 2q37.1
What are the symptoms of severe toxicities caused by impaired elimination of cytotoxic SN-38?
myelosuppression
diarrhea
neutropenia
chromosomal position of UGT1A1
chr. 2q37.1
What is the most important variant UGT1A1 allele?
UGT1A1*28
it is a promoter polymorphism comprised of 7 thymidine-adenine (TA) dinucleotide repeats:
[(TA)7TAA]
normal UGT1A1 allele
UGT1A1*1
it has 6 TA repeats: [(TA)6TAA]
How does the length of TA repeats affect UGT1A1 enzyme activity?
inversely correlated with UGT1A1 expression and activity
UGT1A1*28 heterozygotes has 25% reduction in enzyme activity
UGT1A1*28 homozygotes has 70% reduction in enzyme activity, and increased cytotoxicity
The function of the UGT family
it is responsible for the glucuronidation of hundreds of compounds:
– hormones,
– flavonoids,
– environmental mutagens,
– pharmaceutical drugs
Where the UGTs are expressed?
most in the liver
also in intestine, stomach, breast
Gilbert Syndrome
Caused by UGT1A1*28 or other UGT1A1 missense variants
An autosomal recessive unconjugated hyperbilirubinemia 高胆红素血症
No liver damage
can present with jaundice, mild abdominal pain, nausea triggered by fasting or infections
Which inherited disease is caused by UGT1A1*28 or its other missense mutations?
Gilbert Syndrome
Genetic testing of UGT1A1 method
Fluorescent PCR amplification and size separation capillary electrophoresis
Normal allele:
1) TA(6)TAA –UGT1A*1
Increased UGT1A1 activity allele:
1) TA(5)TAA – UGT1A1*36 - considered as normal phenotype
Reduced UGT1A1 activity allele:
1) TA(7)TAA – UGT1A128
2) TA(8)TAA – UGT1A137
Other gene affects Irinotecan toxicity risk
CYP3A4
What is Rasburicase for?
for prophylaxis 预防 and treatment of hyperuricemia during chemotherapy in patients with lymphoma, leukemia, and solid tumors
What happened during chemotherapy?
cell death and release of uric acid into the blood
Function of Rasburicase (AKA. Eliteck)
– A recombinant urate oxidase enzyme
– Break uric acid to allantoin (尿囊素) and hydrogen peroxide
— Allantoin and hydrogen peroxide are eliminated by kidney
— hydrogen peroxide can cause oxidative stress and hemolytic anemia
What is Pegloticase?
– A pegylated form of urate oxidase
– For treatment of refractory gout
What is the common FDA warning for Rasburicase and Pegloticase?
Contraindicated (禁忌) in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency due to G6PD gene mutations.
what is G6PD?
– glucose-6-phosphate dehydrogenase
– G6PD gene is located on chr.Xq28
– G6PD catalyzes the first step in the pentose phosphate pathway
G6PD deficiency
– X-linked autosomal recessive disorder
– population with high risk of G6PD deficiency: patients of African or Mediterranean ancestry
– happens 1 in 10 African American males
– G6PD deficiency has the protection against malaria infection
The benefit of G6PD deficiency
– Protection against malaria
– thus it occurs most frequently in the region with malaria endemic
what is the predominant founder mutation for G6PD in Mediterranean?
c.563C>T
Function of the pentose phosphate pathway
produces antioxidants to protect cells against oxidative stress
Symptoms of hemolytic anemia in G6PD deficiency patients?
dark urine
enlarged spleen
fatigue
rapid heart rate
shortness of breath
jaundice
Does people with G6DP deficiency have the same amount of response to a drug? if not, what else need to be tested?
No. Different mutations might affect the protein differently thus enzyme activity might vary.
Need to test enzyme activity levels
Classifications of G6PD mutations
most G6PD mutations are missense mutations that affect the protein stability
Classified by the level of enzyme activity
Class I: severe
Class II: <10% severely deficient
Class III: 10-60% moderate deficiency
Class IV: 60-150% normal activity
Class V: 150% enhanced activity
Class I G6PD variant phenotype
Congenital non-spherocytic hemolytic anemia (CNSHA) 先天性非球形细胞溶血性贫血
Class II G6PD variant phenotype
Risk of acute hemolytic anemia 溶血性贫血
Class III G6PD variant phenotype
Risk of acute hemolytic anemia 溶血性贫血
Class IV G6PD variant phenotype
No clinical manifestations
Class V G6PD variant phenotype
rare, only 1 case found, no clinical manifestations
For which Classes of G6PD variants Rasburicase is contraindicated or should not be used?
Class I, II, and III
Rasburicase is contraindicated in patients who are glucose-6-phosphate dehydrogenase deficient because these patients cannot break down hydrogen peroxide, a byproduct of rasburicase, which can lead to hemolysis.
Specimen can be used to detect G6PD variants
DNA extracted from whole blood or tissues
If Rasburicase is contraindicated, what is the alternative drug for it?
Allopurinol can be used in patients with G6PD deficiency.
Allopurinol can decrease the production of uric acid in tumor lysis syndrome but is ineffective in the treatment of hyperuricemia associated with tumor lysis syndrome.
Allopurinol is a very useful agent to prevent the development of tumor lysis syndrome.
What is Tamoxifen used for?
– treatment and prevention of estrogen receptor positive (ER-positive) breast cancer
– decrease recurrence by 50%
– decrease mortality by 30%
– inhibits ER
What caused Tamoxifen interindividual response variability?
differed Tamoxifen metabolism
Side effects of Tamoxifen?
80% women have hot flash
2.5 fold increased risk of endometrial cancer
increased risk of thromboembolic events and depression
If a patient has hot flashes and depression during the Tamoxifen treatment, what drug can be used to treat these symptoms?
Selective serotonin reuptake inhibitors (SSRIs)
What do Tamoxifen metabolites can do to treat breast cancer?
Inhibit aromatase, thus decrease the amount of estrogen in the body
Pathways involve in the Tamoxifen metabolism
1) 4-hydroxylation pathway (7% of Tamoxifen metabolized by this pathway)
2) N-demethylation pathway (92% of Tamoxifen metabolism)
What Tamoxifen metabolite is produced by 4-hydroxylation and N-demethylation pathways?
endoxifen, a potent secondary metabolite
Enzyme involved in 4-hydroxylation of Tamoxifen to produce 4-hydroxy-Tamoxifen
CYP2D6
Enzyme involved in N-demethylation of Tamoxifen to produce N-desmethyltamoxifen
CYP3A4
Fig. 1. Metabolism of Tamoxifen and role of various cytochromes. The conversion of tamoxifen to N-desmethyl-tamoxifen is mainly mediated by cytochrome CYP3A 4/5. The cytochrome CYP2D6 has a major role in conversion of Tamoxifen to 4-hydroxy-tamoxifen. The 4-hydroxy-tamoxifen is converted to endoxifen by CYP3A4/5 while N-desmethyl-tamoxifen is converted to endoxifen by CYP2D6.
Function of endoxifen
1) inhibition of ER
2) target ERα for proteasomal degradation
gene encodes ERα
ESR1
CYP2D6
located on chr. 22q13.2
Specimen for CYP2D6 genetic testing
DNA extracted from whole blood or tissues
Types of CYP2D6 mutations
SNP, deletion, insertion, and duplication
Four CYP2D6 predicted phenotypes for Tamoxifen metabolism
1) ultrarapid metabolizer
2) extensive (normal) metabolizer
3) intermediate metabolizer
4) poor metabolizer
Is CYP2D6*1 a normal allele?
Yes, wild type
For post-menopausal women, what inhibitors are used for CYP2D6 poor or intermediate Tamoxifen metabolizers?
Aromatase inhibitors
Prevent relapse of breast cancer
CYP2D6 duplicated functional CYP2D6 alleles are?
*1xN, *2xN, *35xN
increased activity
CYP2D6 duplicated nonfunctional CYP2D6 alleles are?
*4xN
CYP2D6 duplicated reduced function CYP2D6 alleles are?
*10xN
Other genes influence Tamoxifen response are?
other CYP450 family members
UGT family members
SULT family members
What are the members of UGT family?
In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8.
Function of UGT family
catalyze the covalent addition of sugars to a broad range of lipophilic molecules.
UGTs
UDP-glycosyltransferases
SULT family
Sulfate conjugation catalyzed by cytosolic sulfotransferase (SULT) enzymes
particularly SULT1A1, is a major pathway for drug metabolism in humans.
SULT
Sulfotransferase
Examples of Thiopurines
azathioprine
mercaptopurine
thioguanine
These drugs are analogs of guanine.
Thiopurines can be used to treat what diseases?
Childhood acute lymphoblastic leukemia
autoimmune diseases
inflammatory bowel diseases
lupus
transplantation
Mercaptopurine and azathioprine are used for what disease?
nonmalignant immunologic disorders
What Thiopurines can be used to treat lymphoid malignancies?
Mercaptopurine
What Thiopurines can be used to treat myeloid leukemias?
thioguanine
Thiopurines are inactive precursors and metabolized to active thioguanine nucleotides (TGNs) by what enzyme?
hypoxanthine guanine phosphoribosyl transferase
Active thioguanine inactivated by which enzyme?
thiopurine methyltransferase (TPMT)
How does active thioguanine nucleotides (TGNs) induce apoptosis?
TGNs are incorporated into RNA and DNA by phosphodiester linkages.
Inhibit several metabolic pathways and induce apoptosis
TGNs
thioguanine nucleotides
analogs of nucleic acid guanine
Besides induce apoptosis, what else does mercaptopurines do to cause cytotoxicity?
Mercaptopurines are metabolized to methly-thioinosine monophosphate, which inhibits de novo purine synthesis and cell proliferation.
what is the % of population have intermediate levels of TPMT activity?
10%
what is the % of population have low or undetectable levels of TPMT activity?
0.3%
31 TPMT variants are identified on which chromosomal region?
chr. 6p22.3
many variants are missense and associated with decreased activity
Most commonly tested TPMT variants
TPMT*2, *3A, *3B, *3C
Describe TPMT*3A
has two missense variants in cis
p.Ala154Thr
p.Tyr240Cys
The most common TPMT variant associated with low TPMT activity:
— moderate (heterozygous) to extremely high (homozygous) concentrations of TGN metabolites
— fatal toxicity possible without dose decrease
frequency in Caucasians is about 5%
Major side effect of TPMT variants can cause
Myelosuppression
Bone marrow toxicity
Initial genetic testing or PCR-based assays for TPMT variants prior of dosing are for what variants?
TPMT*2, *3A, *3B, *3C alleles
What medicine is used to relief mild to moderate pain in cancer patients?
Codeine - an opioid analgesic
What is the role of CYP2D6 in metabolism of Codeine?
Conversion of opioid to morphine
CYP2D6 poor metabolizer –> less morphine –> less pain relief
CYP2D6 ultra-rapid metabolizer –> too much morphine –> morphine intoxication and toxicity
What is the role of CYP2D6 in metabolism of tricyclic antidepressants?
CYP2D6 poor metabolizer:
– amitriptyline and nortriptyline metabolism will be impaired and increased risks of side effects
CYP2D6 ultra-rapid metabolizer
– reduce drug efficacy due to rapid elimination
