Inhalers are a cornerstone of Novel Drug Delivery Systems (NDDS), providing targeted delivery for respiratory diseases and potential systemic drug delivery.
The two primary types of inhalers are Dry Powder Inhalers (DPIs) and Metered Dose Inhalers (MDIs). Below is a structured overview of their formulation aspects.
Dry Powder Inhalers (DPIs)
Mechanism and Components
DPIs deliver drugs as a powder formulation inhaled into the lungs.
Device Types: Single-dose, multi-dose, or capsule-based inhalers.
Dispersion Mechanism: Relies on the patient’s inhalation flow to deagglomerate drug particles and ensure effective delivery into the airstream.
Formulation Challenges
Particle Size:
Optimal particle size: 1–5 micrometers for deep lung deposition.
Size distribution significantly impacts inhaler performance.
Morphology:
Shape and surface properties affect flow, dispersion, and deposition efficiency.
Agglomeration:
Tendency of drug particles to cluster, which can hinder performance.
Excipients
Lactose: Improves flowability and deagglomeration.
Magnesium Stearate: Enhances powder flow.
Glidants/Surfactants: Prevent static charge buildup.
Advantages
Breath-actuated, reducing the need for coordination during inhalation.
Disadvantages
Requires adequate inspiratory flow from the patient for effective drug delivery.
Metered Dose Inhalers (MDIs)
Mechanism and Components
MDIs deliver drugs as an aerosol using a propellant.
Key Components:
Canister: Metal container housing the formulation.
Valve: Releases a fixed volume per actuation, ensuring consistent dosing.
Actuator: Plastic mouthpiece directing the aerosolized drug into airways.
Counters: Indicate remaining doses in modern MDIs.
Formulation Challenges
Propellant Selection:
Transition from CFCs to environmentally friendly HFAs (e.g., HFA 134a, HFA 227).
HFAs require careful consideration of drug solubility and stability.
Coarse and Fine Particle Fraction:
Fine particle fraction (<5 micrometers) determines deep lung deposition efficiency.
Valve Crystallization:
Drug crystallization on valve components may affect dosing consistency.
Excipients
Ethanol: Cosolvent for drug dissolution.
Surfactants: Maintain suspension uniformity.
Stabilizers: Prevent degradation.
Comparison of DPIs and MDIs
Aspect | DPI | MDI |
Delivery Form | Powder | Aerosol |
Actuation | Breath-actuated | Propellant-actuated |
Particle Size | 1–5 micrometers | <5 micrometers (fine fraction) |
Coordination Need | Minimal | Requires coordination |
Excipients | Lactose, magnesium stearate | Ethanol, surfactants, stabilizers |