Drugs which Interfere with Nucleic Acid and DNA Metabolism (DONE)

Question 1

Examples of inhibition of nucleic acid and DNA metabolism

Answer 1

Nucleotide biosynthesis e.g. sulphonamides and DHFR inhibitors
DNA polymerisation and transcription e.g. quinolones and aminoacridines
Nucleic acid catabolism e.g. allopurinol

Question 2

Nucleic acid biosynthesis

Answer 2

Purine and pyrimidine biosynthesis requires a cofactor called folic acid
Folic acid derivatives are carriers of one carbon units. They are important in the biosynthesis of a number of metabolites, for examples purines and pyrimidines

Question 3

Inhibitors of folic acid metabolism as drugs

Answer 3

Folic acid metabolism is integral in nucleic acid biosynthesis. Inhibition of folic acid metabolism can be used in drug design.
Inhibition of folic acid biosynthesis
Inhibition of folic acid processing

Question 4

Inhibitors of folic acid biosynthesis

Answer 4

Sulphonamides- bacterial infections, leprosy, malaria, toxoplasmosis, pneumocystis carinii infections

Question 5

Inhibitors of folic acid processing

Answer 5

DHFR inhibitors- cancer, bacterial infections, malaria, P. carinii infections, toxoplasmosis, rheumatoid arthritis
Thymidylate synthase inhibitors- cancer

Question 6

Sulphonamides

Answer 6

Sulphonamides inhibit the biosynthesis of folic acid
They are competitive antagonists of para-aminobenzoic acid- inhibit dihydropteroate synthase, act as false substrates of dihydropteroate synthase to produce inactive folic acid
Sulphonamides cross the cell membrane as the neutral form and inhibit the dihydropteroate synthetase as the ionised form

Question 7

Similarity between p-aminobenzoic acid and sulphonamides

Answer 7

Size
Charge distribution
Strongly electron withdrawing sulphonyl group with high electron density on oxygen
Large dipole movement
Can be a salt at physiological pH

Question 8

Basis of selectivity of sulphonamides

Answer 8

Sulphonamides are only effective in organisms where de novo folic acid biosynthesis occurs, and there is no mechanism for uptake of preformed folates
Bacteria biosynthesise folic acid and have no folate uptake mechanism from their environment
Humans cannot biosynthesise folates, folic acid is an essential nutrient
Inhibition of folic acid biosynthesis has potential for antibacterial treatment in humans

Question 9

Important criteria in drug design

Answer 9

Potency against target enzyme
Correct physicochemical properties to reach target site
Reach target site without other interactions e.g. binding to protein
Reach lethal concentration for microorganism
Minimum side effects

Question 10

Structure activity relationships of sulphonamides

Answer 10

P-amino group is essential and must be unsubstituted
Aromatic ring and sulphonamide are both required
Aromatic ring must be p-substituted
Sulphonamide nitrogens must be secondary
R can be varied

Question 11

pKa

Answer 11

The pKa determines the potency of sulphonamides against the dihydropteroate synthase and also whether the drugs have the correct physicochemical properties to reach the target enzyme
The pKa of sulphonamides can be altered by varying the structure of R. The more electron withdrawing R is, the lower the pKa
The optimum pKa for sulphonamides is 7 at physiological pH

Question 12

Changing pKa

Answer 12

Increasing pKa leads to a greater proportion of unionised sulphonamide which increases cellular permeation but lowers enzyme activity
Decreasing pKa leads to a greater proportion of ionised sulphonamide which increases sulphonamide but lowers cellular permeation
For compounds of pKa<6, the rate limiting step is permeation across the cell membrane

Question 13

Analogues of sulphonamides

Answer 13

By altering the nature of R it is possible to alter the properties of the drug, R is usually an aromatic or heterocyclic structure
The features that it is possible to vary are: ionisation, protein binding and water/lipid solubility

Question 14

Types of sulphonamide

Answer 14

Unabsorbed after oral administration- used for gastrointestinal infections
High solubility, quick absorption, rapid excretion- used for UTIs
Absorbed rapidly, excreted slowly- used for chronic infections and prophylaxis

Question 15

Sulphones

Answer 15

Diarylsulphones are active against dihydropteroate synthase although they cannot ionise. They have a similar mode of action to sulphonamides

Question 16

Mode of action of sulphones

Answer 16

Increasing the charge density on the sulphone oxygen increases activity

Question 17

Therapeutic use of sulphonamides

Answer 17

Bacterial infections- UTI, eye lotions, mucous membranes, gut infections
Sulphonamides do not actually kill the bacteria, they prevent growth and cell division, this allows the human immune system to kill the bacteria
Sulphonamides are also used in the treatment of leprosy and malaria
Sulphonamides are often used in combination with DHFR inhibitors

Question 18

Problems in the use of sulphonamides

Answer 18

Low activity, side effects, toxicity, resistance
In general sulphonamide have been superseded by other drugs such as penicillins which show better activity, lower toxicity and less problems with resistance

Question 19

Other modes of action of sulphonamides

Answer 19

Sulphonamides have other modes of action which are not related to their ability to inhibit folic acid metabolism
Inhibit carbonic anhydrase, diuretics, treatment of glaucoma
Antidiabetics act on the pancreas to stimulate insulin production, potentiating the action of insulin

Question 20

Dihydrofolate Reductase Inhibitors

Answer 20

DHFR inhibitors block folic acid processing
DHFR is a vital enzyme in the biosynthesis of the nucleoside thymidine
The only source of thymidine is the folate mediated addition of methyl to deoxyuridine monophosphate
Inhibition of this step deprives the cell of thymidine and leads to cell death

Question 21

Inhibition of folic acid cycle

Answer 21

Inhibition of the folic acid cycle prevents formation of thymidine monophosphate
The cycle has been inhibited at the enzymes DHFR and thymidylate synthase
DHFR inhibitors have been used to treat cancer, bacterial infections, malaria, rheumatoid arthritis, infections due to P. carinii and T. gondii
Thymidylate synthase inhibitors have been used to treat cancer

Question 22

DHFR is a good drug target

Answer 22

There is significant structural variation of DHFR between species e.g. man, bacteria, protozoa
Structural variation occurs in the size, shape and charge distribution of the enzyme active site. Therefore inhibitors will bind differently to DHFR enzymes from different species
The only source of thymidine required for DNA synthesis is the folate mediated addition of methyl to dUMP
Rapidly dividing cells require large amounts of nucleic acids e.g. bacteria, cancer

Question 23

Common DHFR inhibitors

Answer 23

Methotrexate, pyrimethamine, trimethoprim

Question 24

Structure activity relationship of methotrexate

Answer 24

2,4 diamino is essential for potent inhibition
Replacement of 4-amino leads to loss of activity
4-amino group increases the basicity of the 1-N
1-N coordinates to the DHFR, the more basic it is the stronger the binding
5-N and 8-N not essential for activity

Question 25

Interaction between 2,4-diaminopyrimidine and DHFR

Answer 25

The 2,4 diaminopyrimidine interacts with a glu or asp side chain in the active site of DHFR, the more basic 1-N the stronger the interaction

Question 26

Synergism between sulphonamides and DHFR inhibitors

Answer 26

Sulphonamides and DHFR inhibitors act on the same biosynthetic pathway. The effect of one enhances that of the other.
Using them together allows reduction of individual doses reducing toxicity and resistance

Question 27

Methotrexate

Answer 27

This is one of the best inhibitors of DHFR. It has a very similar structure to dihydrofolate, the natural substrate
Used clinically as an anti-cancer agent, anti-arthritic agent and treatment of acute psoriasis

Question 28

Clinical use of methotrexate

Answer 28

Treatment of various leukaemias and a number of solid tumours
Potentially very toxic as it does not discriminate between normal and tumour cells
With large doses of methotrexate, leucovorin rescue is used to reverse the effects of methotrexate in healthy cells

Question 29

Leucovorin

Answer 29

Leucovorin can reverse the effects of methotrexate and other DHFR inhibitors in certain cells
Leucovorin is metabolised to methylene tetrahydrofolate, this by-passes the need for tetrahydrofolate and hence DHFR
Leucovorin is selectively taken up by normal cells and not cancer cells, hence it makes the action of methotrexate selective
Leucovorin is administered after the methotrexate

Question 30

Resistance

Answer 30

Methotrexate uptake into cells occurs via a specific folate carrier
In resistant cells this transport is impaired

Question 31

Pyrimethamine

Answer 31

Used as an anti-malarial in combination with sulphonamides
Maloprim is pyrimethamine and dapsone, and used for prophylaxis
Fansidar is pyrimethamine and sulfadoxine and is used for treatment

Question 32

Trimethoprim

Answer 32

Used on its own occasionally, most often in conjunction with sulphonamides
Most common combination is co-trimoxazole, a combination of trimethoprim and sulfamethoxazole

Question 33

New DHFR inhibitors

Answer 33

New DHFR inhibitors are being sought for cancer and microbial infections
A particular need is for agents to treat infections due to pneumocystis carinii and toxoplasmosis gondii which are major problems in AIDS patients
Leucovorin is transported into mammalian cells but not parasitic cells

Question 34

Thymidylate synthase

Answer 34

This enzyme shows little structural variation between species
It is not therefore a good antimicrobial target
However, it is a very useful target for cancer chemotherapy as cancer cells divide rapidly and require a large amount of thymidine. Depriving them of this slows or prevents growth