Patches, injections and inhalers

Question 1

Describe the different types of patches

Answer 1

1. Matrix Patch
   - Matrix patches hold the drug in a matrix, usually made of long chain polymers (much like a gel but with much less water mixed in), the drug diffuse through the matrix along the concentration gradient delivering the drug to the skin.
   - relatively new, common
   - benefits: they can be cut in half where dose reduction is required
2. Rate limiting membrane patches
   - much older concep, much less commonly used
   - The drug is made as a solution, suspension or gel and is held in a reservoir behind a rate limiting membrane. - - The drug then diffuses across the rate limiting membrane to be delivered to the skin
   - These patches cannot be cut if dose reduction is required as cutting the patch would result in the destruction of the reservoir system.

Question 2

PATCHES: Describe common excipients and their purposes,

Answer 2

Characteristics of successful adhesives;
– Stick to the skin for the patch’s lifetime
– Non-irritating and non-allergenic
– Compatible with drug and other excipients
– Allow painless removal and leave no residue

Other excipients in patches

• Penetration enhancers - can help the drug pass through the skin e.g. mineral oils
• Long chain polymers - used to enhance the viscocity of the liquid in a rate limiting membrane patch or to create the matrix in a matrix patch
• Viscous liquids - used to thicken the solution in a rate limiting membrane patch e.g. silicone
• Co-solvents - used to help dissolve the drug in solution e.g. ethanol

Question 3

PATCHES: Describe the stability and packaging of the formulations

Answer 3

Unlike most traditional formulations, much of the packaging of the patch is also its excipients as they play an important role in the delivery of the drug. Here are the main components/layers of a patch listed in order from outside to inside;

1. Occlusive backing layer – usually made of plastic. Usually either skin colour or transparent. This layer protects the patch and holds it together (the bit visible after application to the skin)
2. The drug - this will be in solution or suspension in the matrix/reservoir
3. Rate limiting membrane – this will only be present in patches using a rate limiting membrane - e.g. polyethylene-vinyl acetate with or without a plasticizer
4. Adhesive layer – this layer creates a diffusion gradient and adheres to the skin
5. Removable strip – this protects the adhesive and is only removed before application (much like a plaster)

Question 4

Describe common advantages and disadvantages of the formulations

Answer 4

A

Question 5

Advantages and Disadvantages of transdermal drug delivery

Answer 5

Advantages
- By passes the GIT – this does not necessarily mean faster action, but it will avoid the drug being broken down by enzymes and different pHs of the gut

• No 1st pass metabolism – as the drug is not absorbed in the GIT the drug does not undergo first pass metabolism which means more of the drug will reach systemic circulation intact.
• Systemic effects with a controlled constant rate of delivery – this means that once the drug has reached the steady state concentration in the blood as long as the patient replaces the patch in a timely manner the drug concentration will remain constant – no peaks (causing side effects) and troughs (causing symptom breakthrough)
• Long duration of action, up to 7 days – this can help massively with patient adherence

Disadvantages
- Can cause skin sensitisation/irritation – usually a result of the adhesive. Advice to reduce this effect is to rotate the site of application each time so the affected skin has a chance to recover.

• Patch may fall off – this will result in systemic levels gradually coming down, the patient should apply another patch as soon as they realise this has happened.
• Not suitable for all drugs – not all drugs can pass through the skin into systemic circulation and so the chemical properties of the drug need to be considered.
• Appearance – patches are visible on the skin and this can be a problem for some patients
• Hairy areas – patches will not adhere properly to the skin when applied to hairy areas of the body, patients need to pick the most appropriate site for application.