AssessmentsAntimetabolites for cancer treatment
Antimetabolites for cancer treatment
USMLE® Step 1 style questions USMLE
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A 63 year-old man comes to the rheumatology clinic because of a follow-up appointment. He was seen two weeks ago because of a several month history of fatigue and malaise accompanied by worsening arthralgias. This was most pronounced in the proximal interphalangeal and metacarpophalangeal joints without distal interphalangeal joint involvement. Prior laboratory tests showed elevated erythrocyte sedimentation rate and rheumatoid factor levels. Which of the following best describes the mechanism of action for the likely initial treatment?
Content Reviewers:Yifan Xiao, MD
Antimetabolites are a diverse group of medications that are used for the treatment of various conditions including cancer, infections and autoimmune disorders.
In this video, we are focusing on the antimetabolites used in cancer treatment.
DNA is composed of a sequence of deoxyribonucleotides and each deoxyribonucleotide is made out of a phosphate group, a five carbon sugar like deoxyribose, and a nucleobase, which can be either a pyrimidine like cytosine, or thymidine, or a purine like adenine or guanine.
Now, nucleotide synthesis starts with ribose-5-phosphate, which is specific for RNA, and an enzyme called ribose phosphate pyrophosphokinase uses an ATP to remove two phosphate groups from it, attaching them to ribose-5-phosphate, creating a phosphoribosyl pyrophosphate, or PRPP.
Because it catalyzes the synthesis of PRPP, the enzyme ribose phosphate pyrophosphokinase is also known as PRPP synthetase.
Next step is to make pyrimidines. The amino acid glutamine, bicarbonate, and water are used to form a molecule called carbamoyl phosphate which is then joined to aspartate and together, they form a ringed molecule called carbamoyl aspartic acid, which gets dehydrated to create a molecule called orotate.
Next, an enzyme moves the phosphoribose unit from PRPP to orotate and that forms orotidine monophosphate, or OMP.
Next, the enzyme UMP synthase converts orotidine monophosphate into uridine monophosphate, or UMP.
That UMP gets phosphorylated twice by nucleoside diphosphate kinase, to become uridine triphosphate, or UTP.
Finally, the enzyme CTP synthase, converts uridine triphosphate into cytidine triphosphate, or CTP.
Now, purine synthesis starts with the amino acids glutamine, aspartate, and glycine, together with bicarbonate and formate, which is the anion derived from formic acid.
These undergo a ten-step pathway and the result is inosine monophosphate, or IMP, which is sort of a generic purine.
IMP can be converted to AMP and GMP.
Okay, RNA nucleotides are usually in the monophosphate form, but to get to DNA nucleotides, we need them in the diphosphate form, so CDP, UDP, ADP, and GDP.
Next, an enzyme called ribonucleotide diphosphate reductase will reduce the ribose within them into deoxyribose, creating dCDP, dUDP dADP, and dGDP.
After this, they just need to lose a phosphate group, and we’ll have dCMP, dUMP, dAMP, and dGMP.
But, something is missing - dTMP. And here comes the folic acid, or vitamin B9, which is converted to tetrahydrofolic acid, or THF.
THF acts as a mediator and accepts a “methylene” group from the amino acid serine and transfers it to dUMP or deoxyuridine monophosphate.
Then, an enzyme called thymidylate synthetase can convert dUMP to dTMP or deoxythymidine monophosphate, and at that point we’re all set to make DNA.
Now, pyrimidine rings can be degraded completely back down to carbon dioxide (CO2) and ammonia (NH3,) which can then be excreted from the lungs and into urine.
In contrast, purine rings, or G and A are degraded down to the metabolically inert uric acid which is then excreted into urine.
For GMP to become uric acid, the enzyme purine nucleoside phosphorylase, first removes the ribose and the phosphate from it, turning it into guanine.
Next, another enzyme called guanase removes an amine group turning guanine into xanthine.
Finally, xanthine is oxidized into uric acid by the enzyme xanthine oxidase.
On the other hand, for AMP to become uric acid, first the enzyme AMP deaminase removes an amine group from it, turning it into IMP.
Then purine nucleoside phosphorylase comes in and removes the phosphate and the ribose from IMP, making hypoxanthine.
Hypoxanthine is then oxidised twice by xanthine oxidase - first to become xanthine, and then finally, to uric acid.
Now, it turns out that those intermediate molecules in purine degradation, guanine and hypoxanthine, can be restored into fresh new nucleic acids, through what is known as a salvage pathway.
The enzyme hypoxanthine-guanine phosphoribosyl transferase, or HGPRT for short, returns ribose and phosphate back to guanine to form GMP, and to hypoxanthine to form IMP.
Alright, now the cancer cells pretty much do nothing but divide all day long and so they are very sensitive to cytotoxic medications that block DNA synthesis.
The bad news is that plenty of normal cells in our body, like the cells lining the GI tract, and the precursors to blood cells and platelets, are also actively dividing and this explains why anticancer medications are toxic to these tissues.
So the antimetabolites usually disrupt the pathway responsible for DNA synthesis by mimicking nucleobases or folic acid, and cause DNA replication and cell proliferation to come to a halt.
Medications that mimic purine include azathioprine and cladribine, while medications that mimic pyrimidine include cytarabine and 5-fluorouracil. Finally, there's folic acid analogues like methotrexate.
Alright, let’s start with azathioprine which is the prodrug of 6-mercaptopurine, or 6-MP.
Azathioprine is converted to 6-MP by the enzyme thiopurine S-methyltransferase and some 6-MP is converted to 6-thioguanine, or 6-TG.
Both 6-MP and 6-TG act as purine analogs, and can conjugate with ribose and then get phosphorylated to form nucleotides.
These nucleotides can mimic normal nucleotides and incorporate into DNA halting DNA replication.
Also, active metabolites of 6-MP inhibit two important enzymes in the purine synthesis: PRPP synthetase and AMP deaminase.
PRPP synthetase converts PRPP to IMP and AMP deaminase converts AMP to IMP and so the end-result is the decreased production of IMP which, remember, is the generic purine, and thus, nucleotide synthesis comes to a halt.
Notice also that allopurinol, which is an antigout medication, inhibits xanthine oxidase, which is the enzyme that metabolizes 6-MP and when azathioprine and allopurinol are used together, 6-MP increases to toxic levels.
Now, an important side effect of azathioprine is bone marrow suppression which leads to pancytopenia.
When DNA synthesis is inhibited, megaloblastic anemia occurs.
This and bone marrow suppression are common to all the antimetabolites used for cancer treatment.
The decrease in white blood cells leads to immunosuppression, increased risk for infections and exacerbation of chronic infections such as hepatitis B infection and herpes zoster virus infection.
Also, azathioprine is contraindicated during pregnancy due to its teratogenic effects.
Acute toxic effects of azathioprine include gastrointestinal disturbances like nausea and vomiting.
Now, cladribine is another purine analog which can inhibit DNA proliferation through various mechanisms.
Cladribine gets phosphorylated to a triphosphate form which can be incorporated into newly synthesized DNA strands.
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