LEAD DISCOVERY AND FORCES INVOLVED IN DRUG INTERACTIONS Presented by: SANCHIT KUMAR SRIVASTAV M.PHARM (PHARMACEUTICAL CHEMISTRY)
CHOOSING DISEASE & DRUG TARGET
IDENTIFYING A BIOASSAY
FINDING A LEAD COMPOUND
ISOLATION & PURIFICATION
IDENTIFICATION OF PHARMACOPHORE
FORCES INVOLVED IN DRUG-RECEPTOR INTERACTIONS:
INTRODUCTION: Various efforts were made to isolate and purify the active principles of various remedies by using following principles involved in drug discovery & drug development as:
Improve pharmacokinetic properties
Study drug metabolism
Tests for toxicity
Design a manufacturing process
General Principles Involved
Choose a disease
Choose a drug target
Identify a bioassay
Find a lead compound
Isolate & Purify the lead compound
Determine the structure of the lead compound
Identify a pharmacophore
Improve target interactions
Once a particular area of medical need has been determined, the next stage is to identify the suitable drug target and it involves following:
Identification of Receptor, enzyme, or nucleic acid
Identification of whether agonist or antagonist should be designed for a particular receptor
For e.g. : Agonist of serotonin receptors are useful for treatment of migraine, while antagonist of dopamine receptors are useful as antidepressants.
Target specificity & selectivity (to reduce side effects)
Targeting drugs to specific organs & tissues
Identifying a bioassay
Choosing a right bioassay plays crucial role in Lead Discovery which involves following factors:
Choice of bioassay: The test implemented should be simple, quick, relevant.
In vivo tests: Tests on animals involve including a clinical conditions in an animal to produce observable symptoms. The animal is then treated to see whether the drug alleviates the problem by eliminating observable symptoms.
For e.g. development of NSAIDS was carried out by inducing inflammation on test animals and then testing to see whether the drugs relieved the inflammation.
In vitro tests: It involves specific tissues, cells, or enzymes. Receptor agonists & antagonists can be tested on isolated tissues or cells which express the target receptor on their surface. This can also be performed to tests the drugs for physiological effects.
Screening by NMR: To detect whether a compound binds to a protein target , in this a compound is radiated with a short pulse of energy & its nuclei are promoted to excited state giving off energy, this energy can be measured to produce an spectrum. Now the NMR spectrum of drug, then the protein is added. If the drug binds to a protein it essentially become a part of protein.
Finding a Lead compound
Once a target & a testing system has been chosen, the next is to find a Lead Compound” which shows the desired pharmaceutical activity. There are many ways by which a lead compound might be discovered.
Screening of natural materials (e.g. Artemisinin): Plants & trees have always rich source of lead compounds like morphine, cocaine, digitalis, quinine, nicotine, & many others. For e.g.: Local anesthetics from cocaine , Anticancer agent from taxol.
Various agents can also be determined from following:
Microbiological world: bacteria, fungi for Cephalosporin, Tetracycline.
Marine world: coral, sponges for Curacin A.
Combinatorial synthesis: It involves the production of chemical libraries or pools, one of which may prove to be a useful compound.
CADD: To determine the structure of the protein & its binding site & to determine the molecule which will fit and bind.
Computerized screening of structural databanks: This is related to pharmacophore of the drug.
Designing lead compounds by NMR technique: To detect whether a compound binds to a protein target , in this a compound is radiated with a short pulse of energy & its nuclei are promoted to excited state giving off energy, this energy can be measured to produce an spectrum. Now the NMR spectrum of drug, then the protein is added. If the drug binds to a protein it essentially become a part of protein.
Isolation & Purification
It is mainly related with chromatographic technique.
Structure Activity Relationships
By synthesizing compounds where one particular group of the molecule is removed or altered, it is possible to find out which groups are essential and which are not. This involves testing all the analogues for biological activity and comparing them with the original compound. If an analogue shows a significantly lowered activity, then the group which has been modified must have been important.
Drug Metabolism It involves the Phase-1 and Phase-2 metabolism. Identification of Pharmacophore Once it is established which groups are important for a drug’s activity, we move to the next step- the identification of the lead compound. The pharmacophore summarizes the important functional group which are important for the activity and their relative position in the space with respect to each other.
Before the drugs move for the clinical trials it is tested for its toxicity. Safety assessment starts with in vitro and in vivo testing on genetically engineered cell culture or transgenic mice to examine any effects on the cell reproduction and to identify potential carcinogens.
Toxicity of a drug used to measured by its LD-50 value (the lethal dose required to kill the 50% of a group of animals).
For e.g., UK-47265, an antifungal agent was an extremely promising one, but in vivo tests on mice, dogs, and rats showed that it had liver toxicity and was potentially teratogenic agent .
There are four phases of clinical trials:
Phase-1: Healthy volunteers take the drug to test whether the drug has the effect claimed including desired potency, its pharmacokinetics, side effects.
Patients may also be used.
Phase-2: Testing of drugs on a small group of patients
Phase-3: Drug is to be tested on a much larger sample of patients and compared with other available treatments, i.e. they may be compared with placebo (a preparation which has no effect at all).
There is random selection of the patients.
Phase-4: The drug is now placed on the market and can be prescribed. However, the drug is still monitored for its effectiveness & for any unexpected side-effects.
Forces Involved In Drug Receptor Interactions
Following types of the Bonding or Forces involves in the Drug-Receptor interactions:
Covalent Bonding: The stability of this type of bond permits the formation of an easily reversible drug-receptor complex. When the receptor is activated by an irreversible antagonist is said to be formation of covalent bond. E.g. acetyl-cholinesterase.
Hydrogen bonding: H-bonds are a type of dipole-dipole interaction formed between the proton of a group X-H (X: electronegative atom).
Electrostatic bonding: The charged ions produced by the drug molecules may be attracted to charged groups within the receptor sites. For e.g. acetylcholine
The positively charged quaternary nitrogen of Ach may be attracted to the negative charge of an ionized carbonyl group at receptor site.
Dipole-Dipole & Ion-Dipole interaction: These forces are generally associated along with electrostatic bonding.
The C-X bonds in the drugs & receptor will have asymmetric distribution of electrons;- produces electron dipoles.
Note: a dipole-dipole interaction is weaker than the ion-dipole interaction.
Hydrophobic Forces: When 2 non-polar groups such as lipophillic group on a drug and a non-polar receptor group each surrounded by the ordered water molecules become disordered in an attempt to associate with each other, this increases the entropy- results in decrease in free energy that stabilizes the drug receptor complexes. This stabilization known as Hydrophobic interaction.
Van der Waals or London Attractive forces: Van der Waals bonds exists between all atoms- are based on induction of asymmetry in the electron cloud of an atom by a molecules of neighboring atom. Such forces operate within an effective distance of about 0.4 to 0.6 nm & exert an attractive forces of less than 2 kJ/mol.