(i) Classification of Drugs

Drugs - are substances that alter the biochemical processes in the body. Drugs that have beneficial effects are used in medicines.

Medicines - usually contain the drug plus other ingredients such as fillers to add bulk or sweeteners to improve the taste.

Drugs generally work by binding to specific protein molecules. These protein molecules can be found on the surface of a cell (receptor) or can be specific enzyme molecules within a cell.

Drugs that act on receptors can be classified as agonists or antagonists.

Agonists mimic the natural compound and bind to the receptor molecules to produce a response similar to the natural active compound.

e.g. salbutamol in asthma inhalers.

Antagonists prevent the natural compound from binding to the receptor, and so blocks/inhibits the natural response from occurring.

e.g. propranolol used to treat high blood pressure.

(ii) How drugs work

Agonists and antagonists work by binding to receptors.

A receptor is a part of a very large protein molecule that has evolved naturally to interact with a specific small, biologically active molecule. The shape, size and structure of the receptor site are such that the biologically active molecule can bind reversibly with the receptor.

Agonists match the shape of the receptor and encourage a similar response.

Antagonists bind to the receptor but do not trigger a response. They block the receptor, so the natural compound cannot bind.

Such binding, between functional groups in the active molecule and functional groups in the complex protein, can involve van der Waals forces (including hydrogen bonding) and/or ionic bonds. The overall shape and size of the drug must fit the receptor binding site or the active site of the enzyme.

Many drugs that act on enzymes are classified as enzyme inhibitors and act by binding to the active site of the enzyme and blocking the reaction normally catalysed there.

Drug Interactions

The bonding at the receptor site is helpful for selecting the type of drug that will bind to the receptor.

Drugs can bind to receptors by ionic bonds or intermolecular forces of attraction.

The diagram below shows a drug with a negatively charged head group binding to a receptor with a positively charged head group.

The diagram below shows a drug with a positively charged head group binding to a receptor with a negatively charged head group.

The diagram below shows a drug binding to a receptor through hydrogen bonding.

Pharmacophores

The structural fragment that is involved in the drug action is known as the pharmacophore. This can be identified by comparing the structures of different drugs that have similar effects on the body.

The pharmacophore normally consists of different functional groups, correctly orientated with respect to each other, that allow the drug to interact with the receptor binding site or enzyme active site.

Summary

All of the above is summarised in the video below.

(iii) Calculations associated with pharmaceuticals

Percentage mass of solute

Percentage by mass is the mass of solute in every 100 g of solution.

% mass = (mass of solute ÷ mass of solution) x 100

Example 1

100 g of a salt solution contains 40 g of dissolved salt. Calculate the percentage by mass of salt.

Answer

Percentage volume

Percentage solution by volume is the number of cm³ of solute made up to 100 cm³ of solution.

% volume = (volume of solute ÷ volume of solution) x 100

Example 2

A bottle of Vodka has 40 ml alcohol per 100 ml of solution. Calculate the % volume of ethanol in solution.

Answer

Converting ppm to mg

The unit ppm stands for parts per million and refers to 1 mg per kg or 1 mg per litre.

Example 3

Answer

(iv) Pharmaceutical Chemistry Quiz