Methylnaltrexone for the treatment of opioid-induced constipation in the setting of
palliative or hospice care, is significantly more effective than placebo (1). However, in
both the randomized and the open-label phase of the multi center trial showing this favorable
outcome, the drug produced rescue-free laxation in only about half of the patients (2). There
may be several reasons for this result, since constipation in palliative care patients often
has multiple simultaneously occurring causes.
Assuming that constipation of the non-responders is still opioid-induced, one of the possible
reasons for not responding to methylnaltrexone could be that central actions of opioids
contribute to constipation by reducing motility of the intestines through direct actions in
the spinal dorsal horn (2). However, as methylnaltrexone is a µ-receptor antagonist and not
all opioids are solely µ-receptor agonists another reason may well be that successful
laxation is determined by the receptor-profile of the specific opioid the patient is using.
Opioids do not only influence bowel functioning, but also immune system functioning and
angiogenesis. Methylnaltrexone possibly antagonizes these changes, therefore this study will
also investigate the influence of methylnaltrexone on immunologic and angiogenic parameters.