Inhalation Route (Inhalatio)
Gaseous substances, vapors of volatile liquids, and aerosols (particle size 1 to 10 µm) are administered through the lungs. This route provides two types of effects:
- Systemic (resorptive): Due to the massive alveolar surface area and rich blood supply, lipophilic substances rapidly enter the bloodstream. This is utilized, for example, in inhalation general 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, whereas larger ones (> 6 µm) deposit in the upper airways.
To prevent unwanted systemic absorption in asthma therapy, poorly absorbed hydrophilic compounds (ipratropium bromide) or drugs subject to high presystemic hepatic elimination (salbutamol, beclomethasone) are used.
Transdermal and Topical Administration
Transdermal administration involves the use of patches (transdermal therapeutic systems, TTS) or metered ointments containing lipophilic drugs. The drug penetrates through the skin directly into the systemic circulation, bypassing the liver. This provides a prolonged effect and a steady plasma concentration (utilized by nitroglycerin, scopolamine, and fentanyl).
Absorption is influenced by skin structure:
- Epidermis (containing keratin) blocks hydrophilic substances.
- Dermis is permeable to all compounds.
Hydrophilic substances can be absorbed through skin appendages (sweat and sebaceous glands), but their total surface area is small (about 1%). Absorption enhancers such as alcohols, surfactants, or dimethyl sulfoxide (Dimexidum) are used to facilitate penetration.
Topical application (ointments, creams) aims for a local effect; systemic absorption of the drug is generally undesirable here.
Intranasal Administration
Drugs (drops, sprays) are absorbed from the nasal mucosa via passive diffusion. Peptide hormones are frequently administered this way in very low doses (e.g., desmopressin for diabetes insipidus — 10–20 mcg).
An important feature is the presence of a metabolic barrier within the nasal cavity (cytochrome P-450 isoforms: CYP1A1, CYP4B1, CYP2B1). This can decrease drug bioavailability directly at the administration site. Absorption also depends on local blood supply, mucus viscosity, pH, humidity, and temperature.
Intracavitary and Specific Methods
These routes deliver drugs directly to the pathological site:
- Intraperitoneal (cavitas peritonei): between the visceral and parietal peritoneum, often used for antibiotic administration during surgeries.
- Intrathecal (subarachnoid and epidural): used for spinal anesthesia or to deliver antibiotics for CNS infections (bypassing the blood-brain barrier).
- Intrapleural (cavitas pleuralis): e.g., administration of anti-tuberculosis agents.
- Intra-articular (cavitas articularis): hydrocortisone for rheumatoid arthritis.
- Iontophoresis: introduction of ionized substances via an electric field (used in dentistry).