LEAD DISCOVERY AND FORCES LEAD DISCOVERY AND FORCES INVOLVED IN DRUG INTERACTIONSINVOLVED IN DRUG INTERACTIONS

Presented by:Presented by:

SANCHIT KUMAR SANCHIT KUMAR SRIVASTAVSRIVASTAV

M.PHARM M.PHARM

(PHARMACEUTICAL (PHARMACEUTICAL CHEMISTRY)CHEMISTRY)

CONTENTSCONTENTS► LEAD DISCOVERY:LEAD DISCOVERY:

• INTRODUCTIONINTRODUCTION• CHOOSING DISEASE & DRUG TARGETCHOOSING DISEASE & DRUG TARGET• IDENTIFYING A BIOASSAYIDENTIFYING A BIOASSAY• FINDING A LEAD COMPOUNDFINDING A LEAD COMPOUND• ISOLATION & PURIFICATIONISOLATION & PURIFICATION• STRUCTURE DETERMINATIONSTRUCTURE DETERMINATION• SARSAR• IDENTIFICATION OF PHARMACOPHOREIDENTIFICATION OF PHARMACOPHORE• MISC. FACTORSMISC. FACTORS

► FORCES INVOLVED IN DRUG-RECEPTOR INTERACTIONS:FORCES INVOLVED IN DRUG-RECEPTOR INTERACTIONS:

INTRODUCTION:INTRODUCTION:Various efforts were made to isolate and purify the Various efforts were made to isolate and purify the

active principles of various remedies by using active principles of various remedies by using followingfollowing principles principles involved in drug discovery & drug involved in drug discovery & drug

development as:development as:

Miscellaneous Principles:Miscellaneous Principles:

• Improve pharmacokinetic Improve pharmacokinetic propertiesproperties

• Study drug metabolismStudy drug metabolism• Tests for toxicityTests for toxicity• Design a manufacturing Design a manufacturing

processprocess• Clinical trialsClinical trials

General Principles General Principles InvolvedInvolved

• Choose a diseaseChoose a disease• Choose a drug targetChoose a drug target• Identify a bioassayIdentify a bioassay• Find a lead compoundFind a lead compound• Isolate & Purify the lead Isolate & Purify the lead

compoundcompound• Determine the structure of the Determine the structure of the

lead compoundlead compound• SARsSARs• Identify a pharmacophoreIdentify a pharmacophore• Improve target interactionsImprove target interactions

DRUG TARGETDRUG TARGET

Once a particular area of medical need has been Once a particular area of medical need has been determined, the next stage is to identify the suitable determined, the next stage is to identify the suitable drug target and it involves following:drug target and it involves following:

• Identification of Receptor, enzyme, or nucleic acidIdentification of Receptor, enzyme, or nucleic acid• Identification of whether agonist or antagonist Identification of whether agonist or antagonist

should be designed for a particular receptorshould be designed for a particular receptor• For e.g.For e.g.:: Agonist of serotonin receptors are useful for Agonist of serotonin receptors are useful for

treatment of migraine, while antagonist of dopamine treatment of migraine, while antagonist of dopamine receptors are useful as antidepressants.receptors are useful as antidepressants.

• Target specificity & selectivity (to reduce side Target specificity & selectivity (to reduce side effects)effects)

• Targeting drugs to specific organs & tissuesTargeting drugs to specific organs & tissues

Identifying a bioassayIdentifying a bioassay

Choosing a right bioassay plays crucial role in Lead Choosing a right bioassay plays crucial role in Lead Discovery which involves following factors:Discovery which involves following factors:

Choice of bioassay:Choice of bioassay: The test implemented should be The test implemented should be simple, quick, relevant.simple, quick, relevant.

In vivo tests:In vivo tests: Tests on animals involve including a Tests on animals involve including a clinical conditions in an animal to produce clinical conditions in an animal to produce observable symptoms. The animal is then treated observable symptoms. The animal is then treated to see whether the drug alleviates the problem by to see whether the drug alleviates the problem by eliminating observable symptoms.eliminating observable symptoms.

For e.g. development of NSAIDS was carried out by For e.g. development of NSAIDS was carried out by

inducing inflammation on test animals and then inducing inflammation on test animals and then testing to see whether the drugs relieved the testing to see whether the drugs relieved the inflammation.inflammation.

In vitro tests:In vitro tests: It involves specific tissues, cells, or It involves specific tissues, cells, or enzymes. Receptor agonists & antagonists can be enzymes. Receptor agonists & antagonists can be tested on isolated tissues or cells which express the tested on isolated tissues or cells which express the target receptor on their surface. This can also be target receptor on their surface. This can also be performed to tests the drugs for physiological performed to tests the drugs for physiological effects.effects.

Screening by NMR:Screening by NMR: To detect whether a compound To detect whether a compound binds to a protein target , in this a compound is binds to a protein target , in this a compound is radiated with a short pulse of energy & its nuclei radiated with a short pulse of energy & its nuclei are promoted to excited state giving off energy, this are promoted to excited state giving off energy, this energy can be measured to produce an spectrum. energy can be measured to produce an spectrum. Now the NMR spectrum of drug, then the protein is Now the NMR spectrum of drug, then the protein is added. If the drug binds to a protein it essentially added. If the drug binds to a protein it essentially become a part of protein.become a part of protein.

Finding a Lead compoundFinding a Lead compound Once a target & a testing system has been chosen, the next is to Once a target & a testing system has been chosen, the next is to

find a Lead Compound” which shows the desired pharmaceutical find a Lead Compound” which shows the desired pharmaceutical activity. There are many ways by which a lead compound might activity. There are many ways by which a lead compound might be discovered.be discovered.

Screening of natural materials (e.g. Artemisinin):Screening of natural materials (e.g. Artemisinin): Plants & trees Plants & trees have always rich source of lead compounds like morphine, have always rich source of lead compounds like morphine, cocaine, digitalis, quinine, nicotine, & many others. For e.g.: cocaine, digitalis, quinine, nicotine, & many others. For e.g.: Local anesthetics from Local anesthetics from cocainecocaine, Anticancer agent from , Anticancer agent from taxol.taxol.

Various agents can also be determined from following:Various agents can also be determined from following: Microbiological world: bacteria, fungi for Microbiological world: bacteria, fungi for Cephalosporin, Cephalosporin,

Tetracycline.Tetracycline. Marine world: coral, sponges for Marine world: coral, sponges for Curacin A.Curacin A.

Combinatorial synthesis:Combinatorial synthesis: It involves the production of chemical It involves the production of chemical libraries or pools, one of which may prove to be a useful libraries or pools, one of which may prove to be a useful compound.compound.

CADD:CADD: To determine the structure of the protein & To determine the structure of the protein & its binding site & to determine the molecule which its binding site & to determine the molecule which will fit and bind.will fit and bind.

Computerized screening of structural databanks:Computerized screening of structural databanks: This is related to pharmacophore of the drug.This is related to pharmacophore of the drug.

Designing lead compounds by NMR technique:Designing lead compounds by NMR technique: To To detect whether a compound binds to a protein detect whether a compound binds to a protein target , in this a compound is radiated with a short target , in this a compound is radiated with a short pulse of energy & its nuclei are promoted to pulse of energy & its nuclei are promoted to excited state giving off energy, this energy can be excited state giving off energy, this energy can be measured to produce an spectrum. Now the NMR measured to produce an spectrum. Now the NMR spectrum of drug, then the protein is added. If the spectrum of drug, then the protein is added. If the drug binds to a protein it essentially become a part drug binds to a protein it essentially become a part of protein.of protein.

Isolation & Purification Isolation & Purification • It is mainly related with chromatographic It is mainly related with chromatographic

technique.technique.

Structure Activity RelationshipsStructure Activity Relationships

• By synthesizing compounds where one particular By synthesizing compounds where one particular group of the molecule is removed or altered, it is group of the molecule is removed or altered, it is possible to find out which groups are essential and possible to find out which groups are essential and which are not. This involves testing all the which are not. This involves testing all the analogues for biological activity and comparing analogues for biological activity and comparing them with the original compound. If an analogue them with the original compound. If an analogue shows a significantly lowered activity, then the shows a significantly lowered activity, then the group which has been modified must have been group which has been modified must have been important.important.

Drug MetabolismDrug Metabolism

It involves the Phase-1 and Phase-2 metabolism.It involves the Phase-1 and Phase-2 metabolism.

Identification of PharmacophoreIdentification of Pharmacophore

Once it is established which groups are important for a drug’s Once it is established which groups are important for a drug’s activity, we move to the next step- the identification of the lead activity, we move to the next step- the identification of the lead

compound. The pharmacophore summarizes the important compound. The pharmacophore summarizes the important functional group which are important for the activity and their functional group which are important for the activity and their

relative position in the space with respect to each other.relative position in the space with respect to each other.

Toxicity TestingToxicity Testing

► Before the drugs move for the clinical trials it is tested Before the drugs move for the clinical trials it is tested for its toxicity. Safety assessment starts with for its toxicity. Safety assessment starts with in vitroin vitro and and in vivo in vivo testing on genetically engineered cell testing on genetically engineered cell culture or transgenic mice to examine any effects on culture or transgenic mice to examine any effects on the cell reproduction and to identify potential the cell reproduction and to identify potential carcinogens.carcinogens.

► Toxicity of a drug used to measured by its LD-50 Toxicity of a drug used to measured by its LD-50 value (the lethal dose required to kill the 50% of a value (the lethal dose required to kill the 50% of a group of animals).group of animals).

► For e.g., UK-47265, an antifungal agent was an For e.g., UK-47265, an antifungal agent was an extremely promising one, but extremely promising one, but in vivo in vivo tests on mice, tests on mice, dogs, and rats showed that it had liver toxicity and dogs, and rats showed that it had liver toxicity and was potentially teratogenic agentwas potentially teratogenic agent..

Clinical TrialsClinical Trials

There are four phases of clinical trials:There are four phases of clinical trials:►Phase-1:Phase-1: Healthy volunteers take the drug to test whether the Healthy volunteers take the drug to test whether the drug has the effect claimed including desired potency, its drug has the effect claimed including desired potency, its pharmacokinetics, side effects.pharmacokinetics, side effects.

Patients may also be used.Patients may also be used.

►Phase-2:Phase-2: Testing of drugs on a small group of patients Testing of drugs on a small group of patients

►Phase-3:Phase-3: Drug is to be tested on a much larger sample of Drug is to be tested on a much larger sample of patients and compared with other available treatments, i.e. patients and compared with other available treatments, i.e. they may be compared with they may be compared with placebo placebo (a preparation which has (a preparation which has no effect at all).no effect at all).

There is random selection of the patients.There is random selection of the patients.

►Phase-4: Phase-4: The drug is now placed on the market and can be The drug is now placed on the market and can be prescribed. However, the drug is still monitored for its prescribed. However, the drug is still monitored for its effectiveness & for any unexpected side-effects.effectiveness & for any unexpected side-effects.

Forces Involved In Drug Receptor Forces Involved In Drug Receptor InteractionsInteractions

Following types of the Bonding or Forces involves in Following types of the Bonding or Forces involves in the Drug-Receptor interactions:the Drug-Receptor interactions:

► Covalent Bonding:Covalent Bonding: The stability of this type of bond permits The stability of this type of bond permits the formation of an easily reversible drug-receptor complex. the formation of an easily reversible drug-receptor complex. When the receptor is activated by an irreversible antagonist When the receptor is activated by an irreversible antagonist is said to be formation of covalent bond. E.g. acetyl-is said to be formation of covalent bond. E.g. acetyl-cholinesterase.cholinesterase.

► Hydrogen bonding:Hydrogen bonding: H-bonds are a type of dipole-dipole H-bonds are a type of dipole-dipole interaction formed between the proton of a group X-H (X: interaction formed between the proton of a group X-H (X: electronegative atom).electronegative atom).

► Electrostatic bonding:Electrostatic bonding: The charged ions produced by the drug The charged ions produced by the drug molecules may be attracted to charged groups within the molecules may be attracted to charged groups within the receptor sites. For e.g. acetylcholinereceptor sites. For e.g. acetylcholine

The positively charged quaternary nitrogen of Ach may be The positively charged quaternary nitrogen of Ach may be attracted to the negative charge of an ionized carbonyl group attracted to the negative charge of an ionized carbonyl group at receptor site. at receptor site.

► Dipole-Dipole & Ion-Dipole interaction:Dipole-Dipole & Ion-Dipole interaction: These forces are These forces are generally associated along with electrostatic bonding.generally associated along with electrostatic bonding.

The C-X bonds in the drugs & receptor will have The C-X bonds in the drugs & receptor will have asymmetric distribution of electrons;- produces electron asymmetric distribution of electrons;- produces electron dipoles.dipoles.

Note:Note: a dipole-dipole interaction is weaker than the ion-dipole a dipole-dipole interaction is weaker than the ion-dipole interaction.interaction.

► Hydrophobic Forces:Hydrophobic Forces: When 2 non-polar groups such as When 2 non-polar groups such as lipophillic group on a drug and a non-polar receptor group lipophillic group on a drug and a non-polar receptor group each surrounded by the ordered water molecules become each surrounded by the ordered water molecules become disordered in an attempt to associate with each other, this disordered in an attempt to associate with each other, this increases the entropy- results in decrease in free energy that increases the entropy- results in decrease in free energy that stabilizes the drug receptor complexes. This stabilization stabilizes the drug receptor complexes. This stabilization known as Hydrophobic interaction.known as Hydrophobic interaction.

► Van der Waals or London Attractive forces:Van der Waals or London Attractive forces: Van der Waals Van der Waals bonds exists between all atoms- are based on induction of bonds exists between all atoms- are based on induction of asymmetry in the electron cloud of an atom by a molecules of asymmetry in the electron cloud of an atom by a molecules of neighboring atom. Such forces operate within an effective neighboring atom. Such forces operate within an effective distance of about 0.4 to 0.6 nm & exert an attractive forces of distance of about 0.4 to 0.6 nm & exert an attractive forces of less than 2 kJ/mol.less than 2 kJ/mol.