NEURO 20: Transdermal Drugs

1. Physicochemical Properties of Drugs for Transdermal Delivery

Basically, not every drug can be used in a patch. The drug needs to:
✅ Be small enough (Molecular weight < 500 g/mol) – because big molecules can’t get through the skin.
✅ Have both water and fat solubility – it needs to dissolve in both to pass through skin layers.
✅ Preferably be non-ionized – because charged molecules don’t pass through the skin easily.
✅ Be potent (works at low doses) – skin absorption isn’t super efficient, so we need drugs that work at tiny doses.

For example, fentanyl (MW 336.5 g/mol) and buprenorphine (MW 467.6 g/mol) fit these criteria!

2. When Are Transdermal Opioid Medications Preferred?

Patches are great when:
🔹 Patients have trouble swallowing (e.g., nausea, difficulty swallowing pills).
🔹 We need steady pain control (avoids peaks and crashes like pills).
🔹 We want to avoid first-pass metabolism (pills go through the liver first; patches don’t).
🔹 Patients have GI issues (vomiting, diarrhea, absorption issues).

Key Benefits:
✔ Consistent drug levels → avoids ups and downs in pain control.
✔ Long-acting → fentanyl = 3 days, buprenorphine = 7 days.
✔ Convenient → less frequent dosing = better compliance.

3. Composition & Pharmacokinetics of Fentanyl & Buprenorphine Patches

There are two types of transdermal systems:
1⃣ Reservoir (multilayer) system – drug is in a liquid/gel and is released at a controlled rate.
2⃣ Matrix (monolithic) system – drug is mixed inside a solid matrix and slowly releases over time.

Fentanyl (Duragesic) Patch:

• Type: Matrix system.

• Onset: Slow (takes ~12-24 hours to start working).

• Duration: 3 days.

• Steady drug release → forms a "depot" in the skin before entering the bloodstream.

Buprenorphine (BuTrans) Patch:

• Type: Multi-Layer system.

• Onset: Slow (also takes time to kick in).

• Duration: 7 days.

• Less constipation compared to fentanyl.

Both drugs are highly potent opioids, meaning only small doses are needed for pain relief.

4. Patient Selection & Safe Dosing

🚨 NOT for opioid-naïve patients (fentanyl) – could cause fatal respiratory depression.
✅ Use in opioid-tolerant patients who need stable, long-term pain control.
✅ Buprenorphine can be used in opioid-naïve patients, but at low doses.
⚠ Start low & go slow – patches take time to reach full effect (~24 hrs).

Dosing Guidelines:

• Fentanyl patch: Replace every 3 days (72 hours).

• Buprenorphine patch: Replace every 7 days.

• Do NOT cut the patch – it messes up drug release.

• Monitor after removal – drug stays in the skin for 24 hours after taking the patch off.

5. Counseling Points for Patients

💡 Application:
✅ Apply to clean, dry, non-hairy skin (upper arm, chest, back).
✅ No lotion, oil, or wet skin – affects absorption.
✅ Rotate sites – avoid applying to the same spot repeatedly.

⚠ Safety Tips:
🚫 Do NOT apply heat (e.g., heating pads, hot showers) – increases drug release = overdose risk.
🚫 Do NOT cut the patch – changes how the drug is released.
⚠ Dispose of properly – fold patch in half (sticky side together) before throwing away.
🚨 Signs of overdose:

• Extreme sleepiness

• Slow breathing

• Confusion

• Pale or blue lips → SEEK HELP IMMEDIATELY!

Extra Tip: If a patient is cognitively impaired, apply the patch to the upper back so they don’t remove it accidentally.

Quick Summary of Key Points

✔ Only small, potent drugs work for transdermal delivery.
✔ Patches provide steady pain relief and avoid GI issues.
✔ Fentanyl = 3-day patch, Buprenorphine = 7-day patch.
✔ Do NOT use fentanyl in opioid-naïve patients.
✔ Apply correctly, avoid heat, and monitor for overdose.