Basics of Application and the Principle of Selective Toxicity
Unlike surface antiseptics, chemotherapeutic agents are administered systemically. This imposes a strict requirement: they must exhibit a dual action. On the one hand, the drug must destroy the pathogen as effectively as possible; on the other hand, it must remain safe and non-toxic to the host tissues (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 achieved because the drug targets a specific "target"—a unique anatomical structure or a specific metabolic pathway of the microbe that is completely absent in host cells. It is crucial to understand that safety for humans and high selectivity, rather than simply the ability to kill all living matter, determine whether a chemical compound becomes a fully-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 depends directly on the biological differences between the pathogen and the host cell. The greater the differences, the easier it is to find a unique target.
- Bacteria (prokaryotes): Therapy is easiest to select (low complexity). The bacterial cell has numerous fundamental morphological and metabolic differences from the human cell, providing pharmacology with a vast choice of targets.
- Fungi and Protozoa (eukaryotes): Complexity ranges from medium to high. Since these are eukaryotic microorganisms, their cellular structure and metabolism are largely similar to human cells. Finding a structure that they possess, but humans do not, is significantly harder.
- Viruses (non-cellular forms): Represent extreme complexity for drug development. The paradox is that viruses are cardinally different from us (they completely lack a cell wall and their own metabolism), 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 tremendously difficult microbiological task.
Classification of Antimicrobial Agents
The collection of microbes against which a specific drug acts forms the concept of the "spectrum of activity." Chemotherapeutic agents are generally classified according to several key criteria.
By object of action (target):
- Acting on cellular life forms: antibacterial, antifungal, and antiprotozoal agents.
- Acting on non-cellular forms: antiviral drugs.
- Additionally, this classification includes agents with antitumor activity.
By spectrum of antibacterial action:
- Narrow spectrum: active against a strictly limited range of pathogens (e.g., predominantly affecting 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 mechanism of action (type of effect on the cell):
- Microbicidal (bactericidal, fungicidal): cause immediate death of microbes due to the irreversible destruction of their vital structures.
- Microbiostatic: cause only an arrest of 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 drugs: artificially created molecules of diverse structures capable of acting on both cellular and non-cellular (viral) forms of microorganisms.