Pharmacokinetics and pH Dependence
Chemically, Acidum acetylsalicylicum is a weak acid with $pK_a = 3.5$. The graph of its ionization versus ambient $pH$ exhibits a pronounced S-shape. In the acidic environment of the stomach (at $pH$ 1–2), the molecules exist predominantly in a lipophilic, non-ionized state, which is why the initial phase of absorption occurs there.
As the $pH$ shifts toward alkalinity, the degree of ionization increases sharply: already at $pH$ 4.5, more than 90% of the substance converts into hydrophilic anions. However, the small intestine remains the primary site of absorption due to its massive surface area and anatomical features, which fully override the $pH$ factor. In the systemic circulation, the drug undergoes hydrolysis by plasma esterases, resulting in a very short half-life (only 15–20 minutes). The acid is excreted via renal tubular secretion. If accelerated excretion is required, the urine is artificially alkalinized; in an alkaline environment, the substance becomes ionized, and its passive reabsorption is blocked.
Dosing Paradox and Antiplatelet Effect
The key pharmacological challenge when using the drug for thrombosis prevention is navigating its dual action. Cyclooxygenase (COX) is responsible for the synthesis of two functional antagonists: thromboxane $A_2$ (triggers aggregation in platelets) and prostacyclin (inhibits aggregation in the endothelium). The therapeutic goal is to selectively shut down thromboxane.
Selectivity is achieved through differences in cellular biology:
- Platelets are anucleate elements. Irreversible COX blockade deprives them of function for their entire lifespan (7–10 days), until new cells enter from the bone marrow (complete pool turnover takes about 2 days).
- Endothelial cells possess a nucleus. They rapidly synthesize new proteins, and prostacyclin production recovers within several hours.
Additionally, about 30% of the administered dose undergoes presystemic metabolism (deacetylation) during its first pass through the liver. The concentration in the portal vein is high, which reliably 'shuts down' platelets. A lower concentration reaches the systemic vessels, sparing the endothelium. Increasing the dose erases this gradient, suppressing prostacyclin synthesis and diminishing the therapeutic effect.
Safety Profile: Gastrointestinal Tract and Bronchi
The most common adverse effect is ulcerogenicity. By suppressing COX, the drug decreases the production of gastroprotective prostaglandins $E_2$ and $I_2$. As a result, mucus and bicarbonate secretion drops, while hydrochloric acid production rises. This leads to erosions and gastrointestinal bleeding even at low doses.
To protect the GI tract, clinicians use:
- Enteric-coated formulations (pass through the stomach in transit).
- Combinations with antacids (e.g., magnesium hydroxide, which neutralizes acid).
The second major hazard is 'aspirin-induced asthma'. Due to COX blockade, arachidonic acid metabolism switches entirely to the lipoxygenase pathway. The result is massive overproduction of leukotrienes, which provoke severe bronchospasm.
Contraindications
Aside from peptic ulcer disease and bronchial asthma, the drug has several strict limitations:
- Age under 5 years: extremely high risk of Reye's syndrome (progressive encephalopathy with cerebral edema, potentially fatal).
- Genetic defects: glucose-6-phosphate dehydrogenase deficiency carries a risk of erythrocyte hemolysis due to plasma $pH$ shifts.
- Obstetrics: third trimester of pregnancy and lactation.
- Organ pathologies: renal dysfunction and risk of uncontrolled bleeding.