The primary objectives of the study are: • To evaluate the effects of two doses of nalbuphine HCl ER tablets on the 7-day average daily worst itch (i.e., most severe) intensity during the last 7 days of the Fixed Dose Period (“Evaluation visit”; Visit 5) compared to baseline, as measured by daily electronic patient diary records for the Numerical Rating Scale (NRS) and reported as the proportion of patients with at least a 30% reduction in the baseline 7-day average daily worst itch NRS score • To evaluate the safety and tolerability of nalbuphine HCl ER in the study population.