Concept and fundamental differences
In the early 20th century, the outstanding researcher Paul Ehrlich introduced a new term into medical practice. Today, we understand this concept as a specialized method for treating infectious and parasitic diseases, as well as malignant neoplasms. Special chemical substances, commonly referred to as chemotherapeutic agents, are used to implement this method.
The fundamental principle underlying this entire branch of pharmacology is termed "selective toxicity". The essence of this principle lies in using substances that act strictly selectively: they exert a powerful, lethal effect on the parasite or tumor cell while causing no significant harm to the host's (patient's) body.
It is crucial for students and physicians to understand the key difference between this approach and classical pharmacotherapy. In standard pharmacotherapy, the drug's target is always located within the human's own tissues—drugs are used to correct impaired functions of organs and systems. In the case of chemotherapy, however, the target (or specific receptor) is located outside human tissues. The site of drug action is directly within the foreign microorganism's cell or the structure of a malignant tumor.
Rule #1: Pathogen identification
When prescribing treatment, a physician must follow a strict sequence of actions. This is necessary to ensure maximum effectiveness and safety of therapy. The first crucial step is identifying the pathogen.
The main challenge at this stage is that many severe and life-threatening diseases (e.g., pneumonia, meningitis, or sepsis) are polyetiological. This means they can be caused by completely different microbes. At the same time, a strict time factor comes into play: full microbiological identification takes a significant amount of time, yet delaying treatment in severe patient conditions is strictly prohibited.
To resolve this dilemma, the following clinical tactics are applied:
- Prior to receiving accurate culture results, the patient is immediately prescribed empirical therapy. Broad-spectrum drugs capable of covering the most likely pathogens are used for this purpose.
- As soon as the laboratory completes the identification of the microorganism, transitioning to targeted, precision therapy becomes both possible and necessary.
Rule #2: Determining susceptibility
Once the enemy has been identified, its vulnerabilities must be determined. For this purpose, susceptibility is assessed (an antibiogram is compiled). The main goal of this stage is to select the most effective and safest agent not just for the bacterial species, but for the specific strain isolated from the given patient.
Choosing the optimal drug is a complex analytical process in which the physician relies on several baseline criteria:
- Maximum proven activity of the drug against the identified pathogen.
- Safety of the selected agent for the patient.
- Mandatory consideration of the macroorganism's individual characteristics (the patient themselves) and the specifics of the disease course.
- Clinical pharmacology parameters of the drug in question.
Rules #3 and #4: Start time and routes of administration
Treatment success largely depends on the speed of response. Therapy must be initiated at the earliest stages. The biological rationale for an early start lies in the life cycle of pathogens: microbial cells are most sensitive to drug action precisely during their phase of active growth and reproduction. In addition, there is a direct dependence on quantity: the smaller the initial microbial mass, the easier it is to suppress the infectious process. Early treatment also has colossal epidemiological significance, as it effectively limits the spread of infection both within the patient's body and in the external environment.
The concluding principle is the judicious choice of the route of administration and dosage form. The physician's main task here is to create an adequate therapeutic concentration of the drug directly at the infection site. It is precisely the localization of the pathological process that determines which route of administration will be optimal. The primary mechanism and goal of this rule is ensuring maximum, tight contact between the chemotherapeutic drug and the disease pathogen.
Briefly: answers to topical questions
What factors determine the choice of administration route for a chemotherapeutic drug?
The choice of administration route for a chemotherapeutic drug is determined by the need to create an adequate therapeutic concentration directly at the infection site. The primary factor is the localization of the pathological process, which dictates the drug delivery method to ensure maximum contact with the pathogen. Additionally, the drug's bioavailability and the feasibility of oral administration by the patient are taken into account.
What are the main mechanisms of action of antibacterial chemotherapeutic agents on the microbial cell?
The main mechanisms of action of antibacterial chemotherapeutic agents are related to their effects on various structures and processes of the microbial cell. The following sites of action are distinguished:
- Inhibition of cell wall synthesis — characteristic of $\beta$-lactams, glycopeptides, and bacitracin (bactericidal effect).
- Disruption of cytoplasmic membrane permeability — caused by polymyxins and polyene antibiotics.
- Inhibition of intracellular protein synthesis — drugs act at the translation stage (macrolides, tetracyclines, aminoglycosides).
- Inhibition of RNA synthesis — characteristic of rifampicin.
What mechanisms of bacterial resistance to chemotherapeutic agents exist?
The development of bacterial resistance to chemotherapeutic agents is realized through several biochemical mechanisms. The following main pathways are distinguished:
- Enzymatic inactivation — destruction of the antibiotic by bacterial enzymes.
- Structural target modification — alteration of the site of action, causing the drug to stop "recognizing" the target.
- Impaired drug accumulation in the cell — decreased drug uptake or its active efflux.
There is also natural resistance, the mechanism of which lies in the absence of a target for antibiotic action; an example is mycoplasmas, which lack a cell wall and are resistant to drugs acting on cell wall synthesis.
What typical side effects and complications are characteristic of antimicrobial chemotherapy?
Typical side effects and complications of antimicrobial chemotherapy are divided into allergic and non-allergic. All adverse reactions are categorized into four main groups:
- Allergic — associated with the drug's effect on the immune system.
- Toxic — caused by the direct chemotherapeutic action of the drug.
- Organotropic — manifested by damage to specific organs of the patient.
- Mixed — combine various mechanisms of damage.
Among specific complications, antibiotic-associated colitis is also highlighted, for the prevention of which probiotics are used.
The main difference is easy to remember through the target: pharmacotherapy "fixes" its own cells (target inside tissues), while chemotherapy "kills" foreign ones (target outside human tissues).
Class and exam questions
What is the difference between empirical and targeted therapy?
Empirical therapy is prescribed before culture results are obtained and uses broad-spectrum drugs. Targeted therapy is used after precise identification of the microbe.
Why is it impossible to wait for culture results in severe infections?
Diseases such as sepsis or meningitis develop rapidly. Waiting for microbiological identification can cost the patient's life, so treatment is initiated immediately.
What is the purpose of an antibiogram?
It allows determining the susceptibility of the specific pathogen strain isolated from the patient in order to select the most effective and safe drug.
What else to study on this topic
- Empirical therapy: criteria for selecting broad-spectrum drugs.
- Methods of microbiological identification of polyetiological diseases.
- Antibiogram: methods for determining strain susceptibility.
- Pharmacokinetics: establishing a therapeutic concentration at the infection site.
- Growth phases of a microbial cell and their impact on treatment efficacy.
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