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Table 2 Expected response rate to methylnaltrexone

From: Clinical evaluation of the efficacy of methylnaltrexone in resolving constipation induced by different opioid subtypes combined with laboratory analysis of immunomodulatory and antiangiogenic effects of methylnaltrexone

Opioid Probability that constipation is opioid induced Occupation of the peripheral μ receptor by opioid subtype Expected response rate to methylnaltrexone
Morphine Sulphate 70% High 60%
Oxycodone 75% Intermediate-high 60%
Fentanyl 40% Low 25%
  1. Based on the data in Table 1, we expect a high probability that the constipation is opioid induced for morphine sulphate and oxycodone. This is in contrast with a low probability that it is opioid induced for fentanyl. Because morphine sulphate primarily acts on the peripheral mu receptor, we expect a good response rate to methylnaltrexone. The combination of a high probability that the constipation is opioid induced in the oxycodone group, with its activity on both the central and the peripheral mu, kappa, and gamma receptor leads to an expected response rate that equals that of morphine sulphate. For the fentanyl group both the low probability that the constipation is opioid induced, and the sequestration in the central nerve system lead to a very low expected response rate to methylnaltrexone.