Combination therapy

Rational combinations (synergism)

Proper combination of drugs allows achieving a pronounced clinical effect unattainable with monotherapy. A classic example of bactericidal synergism is the combination of aminoglycosides and penicillins (β-lactams). The mechanism is simple: β-lactams destroy the bacterial cell wall, increasing its permeability. This opens the "gates" for aminoglycosides, making it much easier for them to penetrate inside the cell to their main target—ribosomes. Such a regimen saves lives in severe infections, including sepsis and endocarditis.

Other examples of a rational approach:

• Antihypertensive therapy: combining drugs from different pharmacological classes enhances blood pressure reduction and minimizes side effects.
• Tuberculosis treatment: the combination of rifampicin and isoniazid demonstrates powerful synergism in vitro.

Irrational combinations and their risks

Errors in combination lead to neutralization of treatment or severe complications. Three main problems are distinguished:

1. Pharmacodynamic antagonism Occurs when combining bactericidal and bacteriostatic agents. For example, penicillins kill only actively dividing cells. If tetracycline (a bacteriostatic) is added, cell division stops, and penicillin loses its effectiveness. Another option is competition for the target: macrolides and lincosamides compete for binding to the 50S ribosomal subunit. The result is a weakened effect and cross-resistance.

2. Summation of toxicity It is strictly forbidden to combine drugs with the same side-effect profile. The combination of aminoglycosides and polymyxins threatens severe nephrotoxicity, neurotoxicity, and neuromuscular blockade. Similarly, several aminoglycosides (streptomycin, gentamicin, kanamycin, monomycin) must not be prescribed together—this repeatedly increases the burden on the kidneys and hearing.

3. Chemical antagonism Drugs can react even before reaching the target. Local anesthetics (PABA ester derivatives) release PABA itself upon hydrolysis, which competes with sulfonamides, nullifying their action. A similar conflict arises between cationic detergents and anionic soaps, which sharply reduces the effectiveness of antiseptic treatment.

Principles of antibacterial chemotherapy

The prescription of antibiotics is subject to strict rules, where the priority is the microbiological principle. The ideal scenario is choosing a drug based on bacteriological culture and antibiogram.

If the causative agent is unknown and the patient's condition requires immediate action (e.g., fever has appeared), empirical therapy is initiated. The physician relies on a retrospective analysis of typical pathogens in a specific hospital. Primarily, main antibiotics (drugs of choice) are prescribed. Reserve agents are kept for extreme cases—in case of intolerance to the main drugs or detected flora resistance.

When choosing the spectrum of action, preference is always given to narrow-spectrum drugs. A broad spectrum is justified only in severe infections prior to exact microorganism identification. If a combination is required, the drugs must have different spectra of action, and the microflora must be sensitive to each of them.

Prevention of resistance and special cases

Bacteria are capable of developing group specificity (cross-resistance). If a microbe survives the attack of one drug, it will most likely be resistant to its chemical analogs as well.

To slow down this process, it is necessary to:

• Prescribe antibiotics strictly according to indications (they do not replace surgery).
• Perform drug rotation every 10–15 days.
• Limit the use of antibiotics in veterinary medicine (especially as growth promoters).

Special indications for combination therapy:

• Severe infections of unknown etiology: 2–3 drugs are prescribed for maximum coverage.
• Complicated forms: carbapenems (meropenem, imipenem) are used due to their ultra-broad spectrum.
• Mixed infections: when several pathogens are involved at once (anaerobes, chlamydia, gonococci).
• Tuberculosis: requires long-term treatment (from 6 to 12 months or more) and a strictly mandatory combination of two or more drugs. Monotherapy is unacceptable here due to the rapid development of mycobacterial resistance.

Class and exam questions

What else to study on this topic

• Mechanisms of development of cross-resistance in microorganisms
• Classification of antibiotics into main and reserve groups
• Features of carbapenem use in complicated infections
• The impact of chemical antagonism on antiseptic efficacy
• Combination of antibiotics with sulfonamides and nitrofuran derivatives

Other topics in the "Additional Topics" section

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