1. General features

Semi-synthetic medicines are derived from natural products, which often provide a useful starting point for the discovery of physiologically effective molecules and therefore play an important role in the development of new medicines.

Both fully synthetic and fully natural compounds have some limitations that can be overcome by using a semi-synthetic alternative. Many naturally occurring molecules could potentially be used as medicines but may not be effective because they are chemically unstable, not easily absorbed by the human body or induce unwanted side effects. Potentially useful fully synthetic molecules may also fail as medicines for the same reasons. In addition, fully synthetic molecules are technically challenging to create ‘from scratch’ in the laboratory, particularly if scientists do not know the precise details of how they bind to target cells to achieve their effects.

Modification of a natural product can therefore be a useful alternative when fully synthetic compounds would be too complex or expensive to produce, and the natural product alone does not meet all the criteria of a useful medicine. The modifications made to natural compounds can improve factors such as chemical stability or solubility in water, and the strength of interaction between a medicine and the cells that it binds to in the body.

Like chemical medicines, semi-synthetic medicines are relatively small and simple molecules compared with biologic medicines (see section on biologics), which may contain proteins such as enzymes and antibodies.