What is a PAMORA?
A PAMORA (Peripherally Acting μ-Opioid Receptor Antagonist) is a class of medications that selectively blocks opioid receptors in the gastrointestinal tract without crossing the blood-brain barrier, thereby reversing opioid-induced constipation while preserving central pain relief. 1, 2
Mechanism of Action
PAMORAs work by antagonizing μ-opioid receptors specifically in the enteric nervous system. When opioids bind to these peripheral receptors, they cause:
- Reduced gastrointestinal motility and peristalsis
- Decreased fluid secretion into the intestinal lumen
- Increased fluid reabsorption from the bowel
- Increased sphincter tone
The critical feature of PAMORAs is their restricted ability to cross the blood-brain barrier, which means they reverse the constipating effects of opioids in the gut without interfering with centrally-mediated pain relief or precipitating opioid withdrawal symptoms 1, 2.
FDA-Approved PAMORAs
Three PAMORAs are currently FDA-approved for opioid-induced constipation (OIC):
1. Naldemedine (oral)
- Structurally related to naltrexone
- Approved March 2017
- Dosing: 0.2 mg once daily
- Strongest evidence: High-quality evidence from 4 randomized trials (>2,400 patients) showing 52% response rate vs 35% placebo 2
2. Naloxegol (oral)
- Pegylated derivative of naloxone
- First PAMORA approved (2014)
- Dosing: 12.5-25 mg once daily
- Moderate-quality evidence supporting efficacy 1, 2
- Substrate for P-glycoprotein transporter, limiting CNS entry
3. Methylnaltrexone
- Available in both subcutaneous injection and oral tablet formulations
- First approved in 2008 for terminally ill patients
- Only PAMORA approved for both cancer and non-cancer pain 1
- Unique quaternary amine structure prevents blood-brain barrier crossing
- Not metabolized via CYP3A4, reducing drug-drug interaction potential 1
- Subcutaneous form: most patients achieve defecation within 90 minutes 3
Clinical Context and Use
When to Use PAMORAs
PAMORAs are indicated when laxatives fail to adequately control OIC. The Bowel Function Index score ≥30 (on a 0-100 scale) indicates clinically significant constipation warranting escalation from laxatives to PAMORAs 2.
Important Distinction: Alvimopan
While alvimopan is technically a PAMORA, it is NOT FDA-approved for OIC. It is only approved for short-term in-hospital management of postoperative ileus, and the doses studied for OIC are not commercially available 1.
Key Clinical Advantages
- Preserve analgesia: Do not interfere with opioid pain relief 4, 5, 6
- No withdrawal symptoms: Limited CNS penetration prevents opioid withdrawal 1, 2
- Rapid onset: Particularly with subcutaneous methylnaltrexone (within 90 minutes) 3
- Well-tolerated: Most adverse events are mild, transient, and gastrointestinal in nature 2, 4, 7
Common Pitfalls
Drug interactions matter for some PAMORAs but not others: Naloxegol and naldemedine are CYP3A4 substrates, so their levels are affected by CYP3A4 inhibitors/inducers. Methylnaltrexone is NOT metabolized by CYP3A4, making it the preferred choice when significant drug interactions are a concern 1, 5.
Cost considerations: PAMORAs are significantly more expensive than laxatives (estimated monthly costs range from hundreds to thousands of dollars) 8, which is why guidelines recommend laxatives as first-line therapy 2.
Population-specific evidence: Most PAMORA trials excluded cancer patients or included them as small subgroups. Methylnaltrexone has the most robust evidence in advanced cancer and palliative care populations 3.