The two drugs are used differently. Relistor is used to induce relatively quick bowl movement while NKTR118 is to make people who take opiate on regularly basis to have regular bowl movement.
I see jq answered more clearly and succinctly then I could have. I was not as familiar with Relistor but since I can't get you or mcbio to do the work for me :-)...
RELISTOR is administered as a subcutaneous injection. The usual schedule is one dose every other day, as needed, but no more frequently than one dose in a 24-hour period. (2.2) ... RELISTOR is indicated for the treatment of opioid-induced constipation in patients with advanced illness who are receiving palliative care, when response to laxative therapy has not been sufficient. Use of RELISTOR beyond four months has not been studied. (1)
The adjusted mean change from baseline in the number of weekly RFBMs was 3.1 in the methylnaltrexone QD group (P < .001 versus placebo) and 2.1 in the methylnaltrexone alternate-day dosing group (P = .01 versus placebo) compared with 1.5 in the placebo group
In the Phase 2 NKTR-118 trial 3.6 for the 25mg arm vs. 1.9 for the placebo group. Now AZ is using a lower dose and different formulation for the Phase 3 trial. So even if one assumes slightly lower efficacy my prior contention is very much in tact namely that there is not a huge different (at least in efficacy) of these two therapies. Now if NKTR-118 is intended for regular use by those taking opioids (who are prone to constipation) I think that is a bigger opportunity then use to induce a bowl movement.