Antimicrobial Chemotherapeutic Drugs

Basics of application and the principle of selective toxicity

Unlike surface antiseptics, chemotherapeutic agents are administered systemically into the body. This imposes a strict requirement on them: they must possess a dual action. On the one hand, the drug must destroy the pathogen as effectively as possible, while on the other hand, it must remain safe and non-toxic to the tissues of the macroorganism (human or animal).

This fundamental idea was first formulated by the outstanding German immunochemist and Nobel laureate Paul Ehrlich. He introduced the concept of selective toxicity into science. The essence of this concept lies in the colossal difference between how toxic a specific substance is to the parasite versus the host cells.

The mechanism of selective action is realized through the drug hitting a specific "target" — a unique anatomical structure or a specific metabolic link of the microbe that is completely absent in the cells of the macroorganism. It is important to understand that human safety and high selectivity, rather than simply the ability to kill all living things, determine whether a chemical compound will become a full-fledged drug. Because of this rigorous selection, out of thousands of active substances known to science, only a few dozen are used in clinical practice.

Complexity of therapy depending on the pathogen

How easy it is to select a safe and effective drug directly depends on the biological differences between the pathogen and the host cell. The more differences there are, the easier it is to find a unique target.

1. Bacteria (prokaryotes): Therapy is the easiest to select (low complexity). The bacterial cell has many fundamental morphological and metabolic differences from human cells, which provides pharmacology with a huge choice of targets.
2. Fungi and protozoa (eukaryotes): Complexity ranges from medium to high. Since these are eukaryotic microorganisms, their cellular structure and metabolism are largely similar to our cells. Finding a structure that they possess but we lack is significantly more difficult.
3. Viruses (non-cellular forms): Represent an extremely high complexity for therapy development. The paradox is that viruses are cardinally different from us (they have no cell wall or own metabolism at all), yet they are obligate intracellular parasites. Affecting a virus that has already penetrated inside the host cell and uses its resources without disrupting the vital activity of the human cell itself is a most complex microbiological task.

Classification of antimicrobial agents

The totality of microbes affected by a specific drug forms the concept of "spectrum of activity". Chemotherapeutic agents are conventionally divided according to several key criteria.

By object of action (target):

• Acting on cellular forms of life: antibacterial, antifungal, and antiprotozoal agents.
• Acting on non-cellular forms: antiviral drugs.
• Additionally, this classification distinguishes drugs with antitumor activity.

By spectrum of antibacterial action:

• Narrow spectrum: active against a strictly limited range of pathogens (for example, affecting predominantly only Gram-positive or only Gram-negative bacteria).
• Broad spectrum: their activity extends to a large number of varieties, covering representatives of both groups.

By character of effect (type of action on the cell):

• Microbicidal (bactericidal, fungicidal): cause immediate death of microbes due to the irreversible destruction of their vital structures.
• Microbiostatic: cause only a delay in growth and reproduction (inhibition) without instantly killing the microbial cell.

By origin and chemical structure:

• Antibiotics: act only on cellular forms (antitumor antibiotics are also included here).
• Synthetic chemotherapeutic agents: artificially synthesized molecules of diverse structures capable of targeting both cellular and non-cellular (viral) forms of microorganisms.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed mechanisms of selective toxicity (specific bacterial cell targets)
• Principles of rational antibiotic therapy
• Development of bacterial resistance to chemotherapeutic agents
• Mechanism of action and classification of synthetic chemotherapeutic agents
• Specifics of antifungal and antiprotozoal drugs

Other topics in the section "Chemotherapeutic Antimicrobial Drugs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.