Classification of drugs

Drugs are classified in different ways depending on where and how the drugs are being used. The methods of interest to medicinal chemists are chemical structure and pharmacological action, which includes the site of action and target system. However, it is emphasised that other classifications, such as the nature of the illness, are used both in medicinal chemistry and other fields depending on what use is to be made of the information. In all cases, it is important to bear in mind that most drugs have more than one effect on the body and so a drug may be listed in several different categories within a classification scheme.

1.8.1 Chemical structure

Drugs are grouped according to the structure of their carbon skeletons or chemical classifications, for example steroids, penicillins and peptides. Unfortunately in medicinal chemistry this classification has the disadvantage that members of the same group often exhibit different types of pharmaceutical activity. Steroids, for example, have widely differing activities: testosterone is a sex hormone, spironolactone is a diuretic and fusidic acid is an antibacterial agent (Fig. 1.16).

(CH3)2C=CHCH2CH:

COOH

(CH3)2C=CHCH2CH:

COOH

Testosterone

Spirolactone

Fusidic acid

Figure 1.16 Examples of the diversity of action of compounds belonging to the same class (Ac=acetyl)

Testosterone

Spirolactone

Fusidic acid

Figure 1.16 Examples of the diversity of action of compounds belonging to the same class (Ac=acetyl)

Classification by means of chemical structure is useful to medicinal chemists who are concerned with synthesis and structure-activity relationships.

1.8.2 Pharmacological action

This classification lists drugs according to the nature of their pharmacodynamic behaviour, for example diuretics, hypnotics, respiratory stimulants and vasodilators. This classification is particularly useful for doctors looking for an alternative drug treatment for a patient.

1.8.3 Physiological classification

The World Health Organization (WHO) has developed a classification based on the body system on which the drug acts. This classification specifies seventeen sites of drug action. However, a more practical method but less detailed system often used by medicinal chemists is based on four classifications, namely:

1. Agents acting on the central nervous system (CNS). The central nervous system consists of the brain and spinal cord. Drugs acting on the CNS are the psychotropic drugs that effect mood and the neurological drugs required for physiological nervous disorders such as epilepsy and pain.

2. Pharmacodynamic agents. These are drugs that act on the body, interfering with the normal bodily functions. They include drugs such as vasodilators, respiratory stimulants and antiallergy agents.

3. Chemotherapeutic agents. Originally these were drugs such as antibiotics and fungicides that destroyed the microorganisms that were the cause of a disease in an unwitting host. However, the classification has also now become synonymous with the drugs used to control cancer.

4. Miscellaneous agents. This class contains drugs that do not fit into the other three categories, for example hormones and drugs acting on endocrine functions.

1.8.4 Prodrugs

Prodrugs are compounds that are pharmacologically inert but converted by enzyme or chemical action to the active form of the drug at or near their target site. For example, levodopa, used to treat Parkinson's syndrome, is the prodrug for the neurotransmitter dopamine. Dopamine is too polar to cross the blood-brain barrier but there is a transport system for amino acids such as levodopa. Once the prodrug enters the brain it is decarboxylated to the active drug dopamine (Fig. 1.17).

Blood-brain barrier

Figure 1.17 A schematic representation of the formation of dopamine from Levodopa

Blood-brain barrier

Figure 1.17 A schematic representation of the formation of dopamine from Levodopa

+1 0

Post a comment