Antimetabolites for cancer treatment

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Introduction0:00–0:17

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. Alright, during the S phase of the cell cycle, the cell performs DNA replication.

Physiology0:17–5:06

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.

Mechanism of Action5:06–5:58

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.

Azathioprine5:58–8:17

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.
Moving on to indications. Azathioprine is used for the treatment of leukemias, such as acute lymphoblastic leukemia, or ALL, and chronic myelogenous leukemia, or CML.
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.
Other serious adverse effects include liver toxicity which manifests as cholestasis, or decreased bile flow, and acute pancreatitis.
Also, azathioprine is contraindicated during pregnancy due to its teratogenic effects. Acute toxic effects of azathioprine include gastrointestinal disturbances like nausea and vomiting.

Cladribine8:17–9:04

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.
Cladribine can also inhibit DNA polymerase, the key enzyme of DNA replication. Either way DNA synthesis comes to a halt.
Cladribine is also resistant to adenosine deaminase which is the enzyme that normally degrades purines and so it can reach very high levels inside the cells, decreasing the de novo purine synthesis.
Moving on to indications, cladribine is the drug of choice for hairy cell leukemia and can also be used for other types of leukemia.
Side effects include bone marrow suppression and megaloblastic anemia. Alright, now let’s move on to pyrimidine analog.

Cytarabine9:04–9:30

Cytarabine gets phosphorylated to Ara-CTP, a triphosphate metabolite, that binds and competitively inhibits DNA polymerase and thus, DNA synthesis.
Cytarabine is indicated for hematologic malignancies like leukemias and lymphomas. The main side effect of cytarabine is bone marrow suppression with megaloblastic anemia, like the purine analogs.

5-fluorouracil9:30–11:17

Now 5-fluorouracil, or 5-FU, is a pyrimidine analog that interferes with folic acid cycle. 5-fluorouracil gets metabolized to its active form which is 5-FdUMP and forms a complex with THF.
This complex can inhibit thymidylate synthase which normally binds to dUMP and converts it to dTMP. Inhibition of thymidylate synthase results in decreased synthesis of thymidine which is a DNA component.
Other active metabolites of 5-fluorouracil can also be incorporated into DNA and RNA resulting in inhibition of DNA and RNA synthesis.
Now, unlike the other antimetabolites, 5-fluorouracil is indicated for solid tumors rather than hematologic malignancies.
Common indications include colorectal cancer and pancreatic cancer, but it’s also indicated for head and neck cancer, breast cancer, ovarian cancer, bladder cancer and hepatocellular cancer.
What’s more, 5-fluorouracil can also be applied topically for the treatment of basal cell carcinoma. Now, keep in mind that the effects of this medication can be enhanced with the addition of leucovorin, which is also called folinic acid and it’s a reduced form of folic acid.
Moving on to the adverse effects. Like the rest of the antimetabolites, 5-fluorouracil can cause bone marrow suppression and megaloblastic anemia.
Other side effects include gastrointestinal disturbances like severe diarrhea, mucositis, alopecia, CNS toxicity, cutaneous reactions such as photosensitivity, and palmar-plantar erythrodysesthesia, or hand-foot syndrome.
Now, in case of an overdose, uridine can be an effective antidote. Finally, an important antimetabolite is methotrexate which is a folic acid analog.

Methotrexate11:17–13:53

Alright, DHFR normally converts DHF to THF which provide methyl groups necessary for the synthesis of dTMP from dUMP and for purine synthesis.
Now methotrexate has a higher affinity than DHF for DHFR, and so it binds and irreversibly inhibits this enzyme, resulting in decreased levels of THF.
THF is also involved in synthesis of purines and amino acids as well. The result is a decrease in synthesis of DNA, RNA, and proteins and thus, inhibition of cell division.
Alright, methotrexate can be used for the treatment of leukemias, especially acute lymphoblastic leukemia, or ALL, lymphomas such as primary CNS lymphoma and non Hodgkin’s lymphoma, as well as solid tumors like breast cancer, head and neck cancer, and bladder cancer.
Also, methotrexate can be used for conditions where there is abnormal proliferation of trophoblastic tissue. It’s the drug of choice for early unruptured ectopic pregnancy, and choriocarcinoma, which is a germ cell tumor.
Apart from its use as an anticancer agent, methotrexate is also used for inflammatory conditions like rheumatoid arthritis, psoriasis, inflammatory bowel disease, systemic lupus erythematosus, and dermatomyositis.
Now, let’s move on to side effects. Like all the antimetabolites, the major side effect of methotrexate is bone marrow suppression and megaloblastic anemia.
The good news is that myelosuppression can be reversed with folinic acid, or leucovorin, and this strategy is called “leucovorin rescue”.
Leucovorin is already a reduced form of folic acid and thus, it doesn’t require dihydrofolate reductase to get activated.
Also, leucovorin reverses the action of methotrexate by displacing it from its binding sites. Another important side effect of methotrexate is liver toxicity which manifests as macrovesicular fatty change, a severe form of hepatic steatosis where fat vesicles displace the nucleus to the periphery of the hepatocyte, and pulmonary toxicity which manifests as pulmonary fibrosis.
Also, use of methotrexate in pregnancy can lead to neural tube defects and congenital heart defects. So, methotrexate is contraindicated in pregnancy and characterized as category X.
Other side effects include alopecia, and mucositis. Now, let’s make a simple and fun mnemonic that’ll help you efficiently memorize these pharmacology facts.

Memory Palace13:53–17:56

We can set the scene in a port city. The stormy ocean with its pouring rain is where we’ll put the purine analogues, the city with its large pyramide will contain the pyrimidine analogues, and behind the city there’s is a nice field for frolicing for the folic acid analogues.
Now, in the stormy sea, we have a 19th century iron-clad battleship for cladribine. On top of the ship, there’s a majestic zebra prince for azathioprine.
Since both are used to treat leukemia, a cancer of the blood and bone marrow, let’s paint a bone with a crab eating the marrow on the side of the ship.
Cladribine is the drug of choice for hairy cell leukemia so we can also have some hair growing out of the gun turrets of the iron-clad.
For side effects, we can put some broken bones floating in the ocean for bone marrow suppression, and there’s a bomb with a red blood cell drawn on it for megaloblastic anemia.
Azathioprine causes liver damage and pancreatitis so we’ll have the Zebra prince toss a damage liver inside a flaming frying pan into the sea.
This drug is also teratogenic so let’s have the Zebra’s pregnant wife scold him for polluting. Okay let’s head to the city for the pyrimidine analogues.
First notice the largest building in the city is Arabic styled for cytarabine. 5 flower girls are coming out of his building to welcome the return of the prince and they represent 5-fluorouracil.
Cytarabine is used to treat leukemia so let’s put a big flag on the building with the same “crab eating bone marrow” design as seen on the iron-clad.
On the flag pole, there’s a little bump that looks like a lymph node. There’s a little crab hanging off of it since this drug can also treat lymphomas.
5-fluorouracil is only used to treat solid tumors, so the 5 flower girls are dragging a large, solid stone crab statue. The crab is wearing sunglasses and holding a bottle of suntan lotion to help you remember basal cell carcinoma.
Since the side effects of both drugs include bone marrow suppression and megaloblastic anemia, let’s put the bomb and the broken bones between the girls and the building.
For the drug specific side effects for 5-fluorouracil, let’s have one of the girls wearing sunglasses for photosensitivity.
The girl next to her is holding a toilet for severe diarrhea, The next girl is holding a brain in a jar for neurotoxicity, and the 4th girl has large, swollen hands and feet for hand-foot syndrome.
The last girl’s got a big smile on her face since she has the antidote, uridine. So she’s carrying a urinal with dinner plates in it.
Finally, let’s head to the fields of frolicing for our folic acid analogues. Under a tree, there’s a T-rex teaching math, for methoT-rexate.
It’s standing on a stone crab statue since it’s effective against solid tumors, and it’s holding a bone with a crab eating the marrow for leukemias.
A crab is pinching a lymph node on its neck for lymphomas. On the tree, there’s a fruit that looks kind of like a fetus to help you remember it’s used for ectopic pregnancies.
On the same branch, there’s a bunch of little crabs singing in a choire for choriocarcinoma. The dinosaur’s little claws are on fire to help you remember it’s also used to treat inflammatory conditions like rheumatoid arthritis.
For side effects, let’s look at the students sitting around the tree. One’s holding a broken bone and the bomb with the red blood cell, just like the ones we saw in the other areas.
The kid next to him also have a broken bone, but luckily, he’s got a warranty, so he’s smiling. This is a reminder that leucovorin, or lucky warranty, can reverse the bone marrow suppression caused by methotrexate.
The kid next to him is holding a damaged liver with a strip of bacon on top for hepatic steatosis, and the last kid dozed off, so a spider is building a web on his chest for pulmonary fibrosis.
His pregnant mother is yelling at him to wake up and also to help you remember this drug is teratogenic. All right, as a quick recap.

Review17:56–19:11

The antimetabolites used for cancer treatment include azathioprine and cladribine which are purine analogs, cytarabine and 5-fluorouracil which are pyrimidine analogs, and methotrexate which is a folic acid analog.
They all inhibit DNA synthesis and so they act in the S-phase and are cell-cycle specific. Azathioprine is used for the treatment of leukemias.
Cladribine is the drug of choice for hairy cell leukemia. Cytarabine is indicated for hematologic malignancies like leukemias and lymphomas.
5-FU is indicated for solid tumors like colorectal cancer and pancreatic cancer, but it’s also indicated for head and neck cancer, breast cancer, ovarian cancer, bladder cancer and hepatocellular cancer.
5-FU can also be applied topically for the treatment of basal cell carcinoma. Methotrexate can be used for leukemias, lymphomas, as well as solid tumors.
Also, it’s the drug of choice for early unruptured ectopic pregnancy, and for the choriocarcinoma. The major side effect of all the antimetabolites used for cancer treatment is bone marrow suppression but in the case of methotrexate, it can be reversed with leucovorin.

Mind Map19:11–19:26

But wait, there’s more: Here’s a mind map with all of the mnemonics. Go ahead and pause the video so you can test yourself to see what you remember.
Stay tuned for the answers after the credits.