Drugs which Interfere with Nucleic Acid and DNA Metabolism (DONE) Flashcards
Examples of inhibition of nucleic acid and DNA metabolism
Nucleotide biosynthesis e.g. sulphonamides and DHFR inhibitors
DNA polymerisation and transcription e.g. quinolones and aminoacridines
Nucleic acid catabolism e.g. allopurinol
Nucleic acid biosynthesis
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
Inhibitors of folic acid metabolism as drugs
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
Inhibitors of folic acid biosynthesis
Sulphonamides- bacterial infections, leprosy, malaria, toxoplasmosis, pneumocystis carinii infections
Inhibitors of folic acid processing
DHFR inhibitors- cancer, bacterial infections, malaria, P. carinii infections, toxoplasmosis, rheumatoid arthritis
Thymidylate synthase inhibitors- cancer
Sulphonamides
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
Similarity between p-aminobenzoic acid and sulphonamides
Size Charge distribution Strongly electron withdrawing sulphonyl group with high electron density on oxygen Large dipole movement Can be a salt at physiological pH
Basis of selectivity of sulphonamides
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
Important criteria in drug design
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
Structure activity relationships of sulphonamides
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
pKa
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
Changing pKa
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
Analogues of sulphonamides
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
Types of sulphonamide
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
Sulphones
Diarylsulphones are active against dihydropteroate synthase although they cannot ionise. They have a similar mode of action to sulphonamides
Mode of action of sulphones
Increasing the charge density on the sulphone oxygen increases activity
Therapeutic use of sulphonamides
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
Problems in the use of sulphonamides
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
Other modes of action of sulphonamides
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
Dihydrofolate Reductase Inhibitors
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
Inhibition of folic acid cycle
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
DHFR is a good drug target
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
Common DHFR inhibitors
Methotrexate, pyrimethamine, trimethoprim
Structure activity relationship of methotrexate
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
Interaction between 2,4-diaminopyrimidine and DHFR
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
Synergism between sulphonamides and DHFR inhibitors
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
Methotrexate
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
Clinical use of methotrexate
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
Leucovorin
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
Resistance
Methotrexate uptake into cells occurs via a specific folate carrier
In resistant cells this transport is impaired
Pyrimethamine
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
Trimethoprim
Used on its own occasionally, most often in conjunction with sulphonamides
Most common combination is co-trimoxazole, a combination of trimethoprim and sulfamethoxazole
New DHFR inhibitors
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
Thymidylate synthase
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
