Classification and pharmacokinetics
In clinical pharmacology, systemic sulfonamides are strictly categorized by the duration of their half-life ($t_{1/2}$). Sulfamethizole belongs to the short-acting drug group ($t_{1/2} < 10$ hours). This category also includes sulfanilamide (streptocide), sulfathiazole (norsulfazole), sulfacarbamide (urosulfan), and sulfadimidine (sulfadimezine).
For comparison, there are intermediate-acting drugs (10–24 h, e.g., sulfamethoxazole), long-acting (24–48 h, sulfadimetoxin), and ultra-long-acting (> 48 h, sulfalene). Sulfamethizole's short half-life imposes strict requirements on the dosing regimen: to maintain a therapeutic concentration in the blood, the drug must be taken frequently — 4–6 times a day.
Following oral administration, the drug demonstrates good absorption from the gastrointestinal tract, rapidly penetrating the systemic circulation and providing a resorptive effect.
Use in urinary tract infections (UTIs)
Sulfamethizole is one of the drugs of choice in the treatment of UTIs due to its unique metabolic properties.
Most sulfonamides undergo biotransformation in the liver via acetylation, turning into inactive compounds. However, sulfamethizole (like ursulfan) undergoes little acetylation. As a result, it is filtered by the kidneys predominantly in its free (active) form. This allows for exceptionally high active concentrations of the antibacterial agent directly in the urine, effectively suppressing infection pathogens.
Role in the treatment of intestinal infections
In the treatment of intestinal infections (dysentery, shigellosis, gastroenteritis, enterocolitis), as well as for the prevention of purulent complications in intestinal surgeries, sulfonamides acting in the intestinal lumen (poorly absorbed drugs) are traditionally used.
However, such monotherapy has two significant drawbacks:
- Pathogens localize not only in the lumen, but also deeply invade the intestinal wall, where "luminal" drugs do not penetrate.
- Bacterial resistance develops rapidly to monotherapy.
To solve this problem, combination therapy is used. Poorly absorbed drugs are combined with well-absorbed systemic sulfonamides — sulfamethizole or sulfadimidine. Once absorbed into the blood, sulfamethizole reaches the intestinal wall "from the inside" (via the systemic circulation), ensuring the eradication of intramural bacteria.
Administration features and concomitant therapy
Administration of sulfamethizole requires strict adherence to rules to minimize side effects:
- Alkaline drinking: The drug is taken orally 30 minutes before meals, and each dose must be washed down with plenty of alkaline drink. This is a classic requirement for sulfonamides to prevent drug crystallization in the urinary tract (crystalluria).
- Vitamin therapy: A mandatory condition for long-term treatment is the prescription of B vitamins (thiamine, riboflavin, nicotinic acid). Sulfonamides suppress the normal intestinal microflora, primarily Escherichia coli. It is this bacterium that is responsible for the endogenous synthesis of these vitamins in the human body. Without replacement therapy, the patient will rapidly develop hypovitaminosis.
Dosing schedule and prescription
The standard course of treatment is 7–10 days and requires the use of high starting doses to rapidly establish a therapeutic concentration in the blood:
- First dose: 2.0 g (loading dose).
- Subsequent doses: 1.0 g every 4 hours.
- From the third day: 1.0 g 4 times a day.
Example of a tablet prescription:
Rp.: Tab. Aethazoli 0.5 D.t.d. N. 40 S. Take orally 30 min before meals, with an alkaline drink. First dose 2.0 g (4 tablets), then 1.0 g (2 tablets) every 4 hours. From the 3rd day — 1.0 g 4 times a day.
Briefly: answers to topical questions
What is the mechanism of the antibacterial action of sulfamethizole?
The mechanism of the antibacterial action of sulfamethizole consists of competitive antagonism with para-aminobenzoic acid (PABA) and enzymatic blockade of dihydropteroate synthase.
- Competitive antagonism — due to structural similarity, the drug competes with PABA for binding to dihydropteridine.
- Enzymatic blockade — competitive inhibition of the dihydropteroate synthase enzyme occurs at an early stage of synthesis.
- Disruption of synthesis — the formation of dihydropteroic and dihydrofolic acids (precursors of folic acid) is blocked, which disrupts nucleotide synthesis in the bacterial cell.
What side effects are characteristic of sulfamethizole?
Sulfamethizole, as a short-acting sulfonamide, is characterized by a number of systemic and organotoxic side effects.
- Allergic reactions — fever, pruritus, rash, as well as photosensitization.
- Organotoxicity — dyspeptic disorders and hepatotoxicity.
- Hematopoiesis disorders — anemia, leukopenia, thrombocytopenia.
- Kidney damage — crystalluria (formation of crystals in urine under acidic reaction), the risk of which is increased for short-acting drugs.
- Complications in newborns — development of kernicterus due to the displacement of bilirubin from its bond with albumin.
What is the antimicrobial spectrum of sulfamethizole?
Sulfamethizole, as a representative of the sulfonamide group, has a broad spectrum of antimicrobial activity. The drug is effective against gram-positive and gram-negative bacteria, covering both aerobic and anaerobic microorganisms.
The pathogens most sensitive to the drug are:
- Haemophilus influenzae;
- Neisseria meningitidis;
- Pneumococci;
- Certain strains of bacteroides.
What are the contraindications to the use of sulfamethizole?
According to the cited sources, contraindications and restrictions for the use of sulfamethizole as a sulfonamide include the following:
- Newborns — sulfonamides are contraindicated due to the risk of kernicterus: displacement of bilirubin from albumin binding and the development of severe toxic brain damage.
- Hypersensitivity — a general contraindication for antibacterial drugs indicated in the source.
For pregnancy, sources indicate that sulfonamides belong to category C, except in late terms, and in late terms — to category D; category D drugs are used if the clinical effect outweighs the potential risk.
Sulfa-ETHY-zole (Ethazole) — IT is short-acting (<10 h), IT means clean kidneys (low acetylation), IT strikes the intestinal wall.
Class and exam questions
Why is sulfaethidole effective in urinary tract infections?
The drug undergoes little acetylation in the liver. Due to this, it is excreted by the kidneys in unchanged (free) form, creating high therapeutic concentrations of the active substance directly in the urine.
Why is sulfaethidole combined with poorly absorbable sulfonamides in intestinal infections?
Poorly absorbed drugs act only in the intestinal lumen. Sulfamethizole is well absorbed into the blood and destroys pathogens that have penetrated deep into the intestinal wall, and also helps overcome bacterial resistance.
Why are B-complex vitamins prescribed during sulfaethidole therapy?
The drug suppresses normal intestinal microflora, particularly Escherichia coli. Since it is responsible for the endogenous synthesis of thiamine, riboflavin, and nicotinic acid, the patient requires their additional intake.
How to properly take sulfaethidole?
Orally, 30 minutes before meals, be sure to wash down with an alkaline drink to prevent crystallization in the kidneys. Due to the short half-life (<10 h), frequent administration is required — every 4 hours in the first days.
What else to study on this topic
- Mechanism of competitive antagonism between sulfonamides and para-aminobenzoic acid (PABA).
- The process of sulfonation/acetylation of sulfonamides in the liver and the pathogenesis of crystalluria.
- Comparative pharmacokinetics of short-, intermediate-, and long-acting sulfonamides.
- Principles of combined antibacterial therapy for intestinal infections.
- The role of normal intestinal microflora (Escherichia coli) in vitamin synthesis.
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