Inhalation administration (Inhalatio)
Gaseous substances, vapors of volatile liquids, and aerosols (with a particle size dispersion of 1 to 10 µm) are administered through the lungs. This route provides two types of effects:
- Systemic (resorptive): due to the huge surface area of the alveoli and abundant blood supply, lipophilic substances enter the blood very rapidly. Used, for example, for inhalation anesthesia.
- Local: aerosols act on the respiratory mucosa and bronchi (e.g., treatment of bronchial asthma). The depth of penetration depends on particle size: particles smaller than 2 µm reach the alveoli, while larger ones (> 6 µm) settle in the upper respiratory tract.
To prevent unwanted absorption into the blood during asthma therapy, poorly absorbed hydrophilic compounds (ipratropium bromide) or drugs with high presystemic elimination in the liver (salbutamol, beclometasone) are used.
Transdermal and topical administration
Transdermal administration involves the use of patches (TTS) or metered ointments containing lipophilic drugs. The substance penetrates through the skin directly into the systemic circulation, bypassing the liver. This ensures a prolonged effect and constant drug concentration (this is how nitroglycerin, scopolamine, and fentanyl work).
Absorption is influenced by the structure of the skin:
- Epidermis (contains keratin) blocks hydrophilic substances.
- Dermis is permeable to all compounds.
Hydrophilic substances can be absorbed through skin appendages (sweat/sebaceous glands), but their surface area is small (about 1%). To enhance absorption, enhancers are used: alcohol, surfactants, or dimethyl sulfoxide (Dimexidum).
Topical administration (ointments, creams) targets a local effect; systemic drug absorption is considered undesirable here.
Intranasal administration
Drugs (drops, sprays) are absorbed from the nasal mucosa via passive diffusion. Peptide hormones are often administered this way in very small doses (e.g., desmopressin for diabetes insipidus — 10–20 µg).
Important feature: the nasal cavity has its own metabolic barrier (cytochrome P-450 isoforms: CYP1A1, CYP4B1, CYP2B1). This can reduce drug bioavailability directly at the site of administration. Absorption also depends on blood supply, mucus viscosity, pH, humidity, and temperature.
Intracavitary and specific methods
These routes deliver the drug directly to the pathology zone:
- Intraperitoneally (cavitas peritonei): between the visceral and parietal layers of the peritoneum, often for administering antibiotics during surgeries.
- Subarachnoid and epidural spaces: subarachnoidally and epidurally. Used for spinal anesthesia or delivery of antibiotics in CNS infections (to bypass the blood-brain barrier).
- Pleural cavity (cavitas pleuralis): e.g., antituberculosis agents.
- Joint cavity (cavitas articularis): hydrocortisone in rheumatoid arthritis.
- Iontophoresis: administration of ionized substances under the influence of an electric field (used in dentistry).
To remember the skin barriers: Epidermis (keratin) — shield against water (hydrophilic drugs); Dermis — lets everything through (lipo- and hydrophilic); Appendages (1%) — backup entrance for ions.
Class and exam questions
What is the main advantage of transdermal therapeutic systems (TTS)?
They allow the drug to enter the systemic circulation, bypassing the first-pass metabolism in the liver (presystemic metabolism), and provide a prolonged, steady effect.
How does aerosol particle size affect its action during inhalation?
Small particles (< 2 µm) penetrate deeply into the alveoli, while large particles (> 6 µm) settle on the epithelium of the upper respiratory tract and terminal bronchioles.
Why is the absence of a resorptive effect important in the treatment of bronchial asthma?
To avoid systemic adverse effects. For this purpose, hydrophilic substances that are poorly absorbed or those that are rapidly degraded in the liver upon entering the bloodstream are used.
What else to study on this topic
- Mechanisms of absorption across cell membranes
- Presystemic elimination and first-pass metabolism
- Structure and functions of the blood-brain barrier (BBB)
- Pharmacokinetics of peptide hormones
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