Medicinal chemistry, as the name indicates, deals with the chemistry of medicines.
It is one of the essential branches of chemistry and it directly relates to health care.
Even other chemistry branches like organic, inorganic, and analytical chemistry also contribute to medicines, but their scope varies.
Medicinal chemistry tries to interlinks health, disease, and medicine scientifically.
The subject emphasizes chemistry in medicine, mechanism of action, synthesis.
What is Medicinal chemistry?
Medicinal chemistry deals with the chemistry of medicines in the following ways.
- Describes Medicine Chemical structure
- Structure-activity relationship
- Its Physical and chemical properties
- The Synthesis process (preparation)
- Possibilities of new drug molecules
- Causes of drug toxicity.
Medicine Structure
The medicinal chemistry subject discusses all the classes of drugs of modern medicine. It describes the structure of medicines, their relative chemical differences in terms of therapeutic efficiency.
It also defines the type of compound like acidic or basic, its source, functional groups, bonds within, etc.
For example: When it discusses the anti-epileptic drugs, it classifies the drugs, their chemical differences, the mechanism of action like what receptor the drug binds, the changes it brings in terms of treatment.
It also explains which antiepileptic class is suitable for a specific type of epilepsy based on their activity.
Structure-activity relationship (SAR)
This part is another key component of medicinal chemistry. Drugs of the same class have similar structures yet having different pharmacological effects.
With the structural-activity relationship, medicinal chemistry clearly explains the reasons for those differences.
For example, both morphine and codeine are alkaloids derived from opium poppy plants. Yet, morphine is a strong analgesic (pain killer), while codeine is a strong anti-cough agent and weak analgesic.
If you notice their chemical structures in the above picture, both molecules appear similar but with only an addition of methane functional group in codeine encircled in red.
So SAR describes the role of that moiety in structure and what happens with further changes.
2. Physical and chemical properties
It analyzes the physical and chemical properties of medicines.
Physical properties like solubility, melting point, light sensitivity, crystalline, or amorphous are studied. Solubility decides how the medicine is to be formulated. If it is water-insoluble, it is converted to some salt form.
Crystalline or amorphous nature indicates the flow properties which are needs during making tablets, capsules, etc.
Light sensitivity indicates how medicine should be preserved. If light-sensitive, it is packed in opaque packs.
A melting point indicates how far the compound is stable at room temperature and how it should be handled during medicine formulation.
If the melting point is so low, it may need to be stored in cold temperatures. 
Chemical properties like pH, internal bonds, saturation help in different ways.
If a compound is acidic, it means it is insoluble in an acidic medium like the stomach but soluble in a basic medium like the intestine.
So the pH of the compound helps design a simple tablet or sustained-release one to release medicine in the intestine.
Chemical saturation indicates its reactive tendencies with other molecules.
3. It also describes the functional groups present in the drug and their relation to the rest of the molecule. This helps us know which part of the molecule contributes to the medicinal effects. And suitable alterations can be done for better effect.
4. It helps to differentiate which stereo-isomeric forms are effective for treatment. The organic compounds have isomers like Dextro and levorotatory forms. Among medicines, one isomer is active than others. For example, (Levo) DOPA is used in Parkinson’s disease. While its (Dextro) DOPA form is inactive.
Similarly, Amphetamine is active in Dextro form and less active in Levo.
The Synthesis process (preparation)
Synthesis of medicine is one of the critical factors of the availability of drugs in the market.
A drug could be synthesized in multiple ways. But, the one method which is economical as well yielding quality drug is preferred.
So, medicinal chemistry covers the synthesis procedure for established drugs. Every drug molecule is synthesized properly.
Possibilities of new drug molecules
As we have seen before, the slight variation in chemical structure could lead to new drug development.
With the knowledge of existing drug structures, scientists can make new drugs suitable for new diseases.
Causes of drug toxicity.
Drug toxicity is a major concern for doctors as well as patients. This medicinal chemistry tries to explain the structural reasons for the toxicity and the required modification to overcome it.
Further, it explains the mechanism of action of drugs in a disease condition. A drug acts to bring the desired effect by suitable means. Medicinal chemistry tries to establish the mechanism of action of all drugs.
It can also predict the metabolites of a drug after metabolism.
Applications of medicinal chemistry
It helps to develop effective medicines, minimize toxicity, produce the drug in an inexpensive manner.
1) Efficient medicines: A medicine is effective when it can kill the parasite, rectify a deficiency or alter a physiological state to the desired level of well-being. The effectiveness of medicine depends on factors like
a) Reach the remote parts of disease by solubility in non-aqueous regions of the body.
b) kill the parasite or produce the desired effect at low concentrations so as to minimize the bulk of medicine preparation.
c) Avoid the development of resistance by the disease-causing agent or system.
2) Minimal toxicity: Any medicine we take is a foreign substance to the body. So it is metabolized in the liver and excreted by the urine. So the medicine prepared should safe for the body and preferably less toxic. Medicinal chemistry strives to prepare water-soluble drug molecules that will be easily metabolized. Or, if lipid solubility is desired, then the molecule which can easily be converted to water-soluble forms is created. Thus the accumulation of drug toxins is controlled by medicinal chemistry.
3) Inexpensive drugs: Most drugs used in life-threatening diseases are expensive. Medicinal chemistry can help find alternative drug molecules or manufacturing methods to minimize the cost of the drug. This helps the drug to reach people of all economic statuses.
4) Discovery of new drug molecules: This is done by using software where the qualitative structure-activity relationship (QSAR) is modeled.
Since people develop insensitivity to drugs or there is resistance to drugs by microbes, medicinal chemistry helps to keep adding new molecules to treat diseases more effectively.
Alternatively, when a substance is found to show some therapeutic effects, that substance is isolated and tested for its chemical characteristics.
Then a similar molecule is made with all the desirable properties to make a better drug molecule.
It also minimizes the time required for drug discovery. In the past, all the molecules with expected therapeutic value were synthesized and then tested on animals. This is quite an expensive and time taking procedure.
Medicinal chemistry currently uses computer-aided drug discovery (CAD) to minimize the time taken for drug discovery.
It avoids the need to synthesize all the test molecules and also test them on many animals. By the use of CAD, the drug molecule and its ability to work on a receptor are predicted.