Nucleotide analogues

Question 1

Give an example of a pyrimidine analogue and a purine analogue

Answer 1

Pyrimidine: 5-Fluorouracil
Purine: 6-Mercaptopurine

Question 2

Metabolism and Mechanism of action

1. 5-FU
2. 6-MP

Answer 2

1. 5-FU is activated via a series of enzymes to form 5-Fluorodeoxyuridine monophosphate.
   It is a potent inhibitor of Thymidylate synthase (TS). TS converts deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) - dTMP interrupts the action of this enzyme blocks synthesis of the pyrimidine thymidine, which is a nucleoside required for DNA replication.

It has been demonstrated that 80% to 85% of 5-FU is catabolized to inactive metabolites by dihydropyrimidine dehydrogenase (DPD), and only 1 to 3% of the original dose of 5-FU mediates the cytotoxic effects on tumor cells and normal tissues through anabolic actions, thereby inhibiting DNA synthesis and RNA processing and function

2. The leukaemia drug mercaptopurine (MP) is converted by the enzyme (HPRT)
   - to thio-inosine monophosphate (TIMP). TIMP ave been reported to inhibit purine ribonucleotide synthesis
   - to 6-Thioguanine nucleotides: has antileukimic and myelosuppressive effects

The enzyme thiopurine methyltransferase (TPMT) competes for the MP substrate, catabolizing it to methylmercaptopurine (MeMP), an inactive metabolite.

Question 3

Pharmacogenetics

1. 5-FU
2. 6-MP

Answer 3

Relling and Dervieux 2001
1. Some individuals have polymorphism in DPD gene and therefore unable to inactivate drug - get DPD syndrome.
In the liver, more than 80% of 5-FU is inactivated by dihydropyrimidine dehydrogenase (DPD)38. DPD activity varies fold among individuals. Patients with low DPD activity cannot efficiently inactivate 5-FU, and form excessive amounts of active metabolites, which lead to haematopoietic, neurological and gastrointestinal toxicities (e.g. e myelosuppression, neurotoxicity and hand-foot syndrome) that than can be fatal. Approximately 3% of the population are thought to carry heterozygous mutations that inactivate DPD, and 0.1% are homozygous for inactivating mutations

2. TPMT polymorphisms have been associated with mercaptopurine’s therapeutic efficacy and also its toxicity. Three NONSYNONYMOUS SNPS account for over 90% of the clinically relevant TPMT mutations. About 1 in 300 individuals carries two mutant TPMT alleles, and does not express functional TPMT. These individuals produce high levels of TGNs after mercaptopurine therapy , experience life-threatening myelosuppression and require doses to be reduced to as little as a tenth of the normal dose in order to tolerate therapy.
   TPMT, however, has no known endogenous substrates, and individuals with defective TPMT activity are not known to exhibit any phenotype in the absence of drug challenge. Patients HAVE to be screened for genetic difference before taking drug.

Question 4

Drug resistance

1. 5-FU
2. 6-MP

Answer 4

1. 5-FU
   A. Increase target expression (thymidylate synthase)
   B. Increased expression of anti-apoptotic proteins
2. 6-Mercaptopurine is used in the treatment of acute leukemias.
   A. However, resistant tumor cells develop rapidly, probably because of altered specificity or lack of HPRT, so that thio-IMP (the active inhibitor) is not formed.
   B. Other mechanisms may include altered cell permeability and an increased rate of destruction of 6-mercaptopurine.

Question 5

Tumor types

1. 5-FU
2. 6-MP

Answer 5

1. IV = colon cancer, esophageal cancer, stomach cancer, pancreatic cancer, breast cancer, and cervical cancer. Cream = BCC
2. ALL, CML

Question 6

Toxicity

1. 5-FU
2. 6-MP

Answer 6

1. inflammation of the mouth, low blood cell counts, inflammation of the skin, myelosuppression
2. bone marrow suppression, liver toxicity