INDUSTRIAL PHARMACEUTICAL CHEMISTRY (CHEM606/612) DRUG DISCIVERY DR. GHULAM ABBAS

Medication Substance administered for the diagnosis, treatment, or relief of a symptom or for the prevention of diseases. Used interchangeably with the word drug. Drug also has the undertone of an illegally obtained substance. Pharmacy is Art of preparing, compounding, and dispensing drugs. Also refers to the place where drugs are prepared.

Drug discovery: Finding a lead It has been divided into several steps Generally, when a pharmaceutical company or university research group initiates a new medicinal chemistry project with aim to identify a lead compound, they will consider the following steps in order: 1-Choosing the disease

Drug discovery: Finding a lead 2-Choosing a drug target Drug targets Discovering drug targets Target specificity and selectivity between species Target specificity and selectivity within the body Targeting drugs to specific organs and tissues Pitfalls (a hidden or unsuspected danger or difficulty)

Drug discovery: Finding a lead 3-Identifying a bioassay Bioassay selection In vitro test In vivo tests Test validity High-throughput screening Screening by NMR Affinity screening Surface Plasmon resonance Scintillation proximity assay

Drug discovery: Finding a lead 4-Finding a lead compound Screening of natural products (the plant kingdom, the microbial world, the marine world, animal sources, venoms and toxins) Medical folklore (tradition) Screening synthetic compound “ libraries” Existing drugs

Continue……… Starting from natural ligand or modulator (natural ligands for receptors, natural substrates for enzymes, enzyme products as lead compounds, natural modulators as lead compounds)

Drug discovery: Finding a lead 4-Finding a lead compound Combinatorial synthesis Computer aided design Serendipity (destiny) and prepared mind Computerized searching of structural databases Designing lead compounds by NMR

Drug discovery: Finding a lead 5-Isolation and purification 6-Structural determination 7-Herbal medicine

I- Choosing the disease Pharmaceutical companies tend to concentrate on developing drugs for diseases which are prevalent in developed countries, and aim to produce compounds with better properties than existing drugs. Pharmaceutical companies have to consider economic factors as well as medical ones when they decide which disease to target when designing a new drug. A huge investment has to be made towards the research and development of a new drug.

I- Choosing the disease Therefore, companies must ensure that they get a good financial return for their investment. As a result, research projects tend to focus on diseases that are important in the developed world, because it is the best market for new drugs. Thus, research is carried out on ailments such as migraine, depression, ulcers, obesity, flu, diabetes, cancer and cardiovascular disease.

Conti….. Less is carried out on the tropical diseases of the developed world. Only when such diseases start to make an impact in richer countries, the pharmaceutical companies sit up and take notice. Example: research in antimalarial drugs has increased due to increase in tourism to more exotic countries and the spread of malaria into southern states of US.

II- Choosing a drug target Choosing which disease to tackle is usually a matter for company’s market strategists. The science becomes important at the next stage. A molecular target is chosen which is believed to influence a particular disease when affected by a drug. The greater the selectivity that can be achieved, the less chance of side effects.

II- Choosing a drug target 1- Drug targets Once a therapeutic area has been identified the next stage is to identify a suitable drug target (e.g. receptor, enzyme or nucleic acid) Understanding which biomacromolecules are involved in a particular disease state is very important. This will allow the medicinal chemist whether agonist or antagonist to be designed for a particular receptor or whether inhibitors should be designed for a particular enzyme.

Conti….. For example, tricyclic antidepressants such as Desipramine are known to inhibit the uptake of NA from nerve synapses. However, these drugs also inhibit uptake of serotonin, so the possibility arose that inhibiting serotonin uptake might be beneficial. A search for selective serotonin uptake inhibitors has led to the discovery of Fluoxetine, the best selling antidepressant.

II- Choosing a drug target 2- Discovering drug targets If a drug or a poison produces a biological effect, there must be a molecular target for that agent in the body. In the past, the discovery of drug targets depends on finding the drug first. Then, natural chemical messengers started to be discovered. But many targets still stay hidden (orphan receptors i.e,novel receptors whose endogenous ligand is unknown ) and their chemical messengers are also unknown.

Conti…. The challenge is to find a chemical that will interact with these targets in order to find their function and whether they will be suitable as drug targets. This is one of the main driving forces behind the rapidly expanding area of Combinatorial synthesis (synthesis of a large number of compounds in a short period of time using different reagents and starting material and are tested for activity.)

II- Choosing a drug target 3- Target specificity and selectivity between species Target specificity and selectivity is a vital factor in modern medicinal chemistry research. The more the selective a drug is for its target, the less chance that it will interact with different targets and have less undesirable side effects. For example, penicillin target an enzyme involved in bacterial cell wall biosynthesis. Mammalian cells does not have a cell wall, so this enzyme is absent in human cells and penicillin has few side effects.

II- Choosing a drug target 4-Target specificity and selectivity within the body Selectivity is also important for drug acting on targets within the body Enzyme inhibitors should only inhibit the target enzyme and not some other enzyme. Receptors agonist/ antagonist should ideally interact with a specific kind of receptor (adrenergic receptor) rather than a variety of different receptors, or even a particular receptor type ( such as β- receptor) or even a particular receptor subtype β2- receptor. Ideally, enzyme inhibitors should show selectivity between the various isozymes of an enzyme.

II- Choosing a drug target 5-Targeting drugs to specific organs and tissues Targeting drugs against specific receptor subtypes often allows drugs to be targeted against specific organ or against specific areas of brain. This is because the various receptor subtypes are not uniformly distributed around the body, but are often concentrated in particular tissues. For example, adrenergic receptors in the heart are predominantly β1 while those in the lungs are β2. If a drug acts on either, less side effects would be observed.

II- Choosing a drug target 6-Pitfalls The body is a highly complex system. It is possible to identify whether a particular enzyme or receptor plays a role in a particular aliments. For any given function, there are usually several messengers, receptors, and enzymes involved in the process

II- Choosing a drug target 6-Pitfalls For example, there is no one simple cause for hypertension, there are variety of receptors and enzymes which can be targeted in its treatment. These include β1-adrenoceptors, calcium ion channels, angiotessin-converting enzyme (ACE), and potassium ion channels. Sometimes, more than one target may need to be addressed for a particular ailment. For example, most of the current therapies for asthma involve a combination of bronchodilator (β2 agonist) and an anti-inflammatory agent such as a corticosteroid

III-Identifying a bioassay 1-Choice of bioassay Choosing the right bioassay or test system is crucial to the success of a drug research program. The test should be simple, quick and relevant as there are usually a large number of compounds to be analyzed. Human testing is not possible at such early stage, so the test has to be done in vitro first. Because in vitro tests are cheaper, easier to carry out, less controversial and can be automated than in vivo one. In vivo tests needed to check the drugs interaction with specific target and to monitor their pharmacokinetics properties.

III-Identifying a bioassay 2-In vitro tests They do not involve live animals. Instead, specific tissues, cells, or enzymes are isolated and used. Enzyme inhibitors can be tested on pure enzyme in solution. Receptor agonist and antagonists can be tested on isolated tissues or cells. Antibacterial drugs are tested in vitro by measuring how effectively they inhibit or kill bacterial cells in culture

III-Identifying a bioassay 3-In vivo tests In vivo tests on animals often involve inducing a clinical condition in the animal to produce observable symptoms. The animal is then treated to see whether the drug alleviates the problem by eliminating the observable symptoms. For example, the development of non-steroidal inflammatory drugs was carried out by inducing inflammation on test animals.

III-Identifying a bioassay 3-In vivo tests The animals used may be transgenic i.e,some mouse genes are replaced by human genes so the mouse produces the human receptor or enzyme. Or the mouse’s gene may be altered to be susceptible for some disease such as breast cancer.

III-Identifying a bioassay 3-In vivo tests There are several problems associated with in vivo testing. It is slow and it also causes animal suffering. There are also many problems of pharmacokinetics and the result obtained may be misleading. For example, penicillin methyl ester is hydrolyzed in mice into active penicillin, while it is not hydrolyzed in humans or rabbits. Also, thalidomide is teratogenic in rabbits and humans while it is not in mice.

III-Identifying a bioassay 4-Test validity Sometimes the validity of testing procedure is easy and clear. For example, the antibacterial drug can be tested by its effect on killing bacteria. Local anaesthetics are tested by their effect on blocking action potential in isolated nerve. In other cases, the testing procedure is more difficult. For example, there is no animal model for antipsychotic drug. Thus, validity of the test should be carried out.

III-Identifying a bioassay 5-High throughput screening (HTS) HTS involves the miniaturization and automation of in vitro tests such that a large number of tests can be carried out in a short period of time. It involves testing of large number of compounds versus a large number of targets. The test should produce easily measurable effect. This effect may be cell growth, an enzyme catalyzed reaction which produces a color change (may be a dye) or displacement of radioactive labelled ligand from its receptors.

III-Identifying a bioassay 6-Screening by NMR NMR was used as a tool for determining the molecular structures of compounds Recently, compounds can be tested or screened for their affinity to a macromolecular target by NMR spectroscopy. The relaxation times of ligands bound to a macromolecule are shorter than when they are unbound (can’t be detected). In NMR spectroscopy the compound is radiated with a short pulse of energy which excites the nuclei of specific atoms (H,N,C) afterwards, the excited nuclei slowly relax back to the ground state giving off energy as they so.

III-Identifying a bioassay 6-Screening by NMR There are, several advantages in using NMR as a detection system: 1-It is possible to screen 1000 small molecular weight compounds a day with one machine. 2-The method can detect weak binding which would be missed by conventional screening methods. 3-It can identify the binding of small molecules to different regions of binding site. 4-It is complementary to HTS. The later may give false-positive results, but these can be checked by NMR to ensure that the compounds concerned are binding in the correct binding site.

III-Identifying a bioassay 6-Screening by NMR 5-The identification of weakly binding molecules allows the possibility of using them as building blocks for the construction of larger molecules that bind more strongly. 6-Screening can be done on a new protein without needing to know its function. NMR screening also has limitations, the main one being that at least 200 mg of the protein required.

III-Identifying a bioassay 7-Surface Plasmon resonance (SPR) & scintillation proximity assay (SPA) SPR (change in refractive index)& SPR (reduction of emission of light) are two visual methods of detecting whether ligands bind to macromolecular targets . Chapter Continueeee……………………………