Axelopran for Advanced Cancer in Patients Receiving Opioids

The primary objective of this single arm, open label, phase II trial is to determine if axelopran use impacts cancer control in patients with advanced cancers of the lung, breast, pancreas, and prostate. The primary study period for assessing the primary aim is through day 43 (6 weeks). The main questions it aims to answer are: * Does axelopran show a signal for efficacy in slowing tumor progression? * Is axelopran safe and tolerable for long-term use in this patient population? * Does axelopran show a signal for efficacy in improving bowel function and quality of life? * Does axelopran show a signal for efficacy in reducing systemic inflammation, cachexia, and prognostic serum biomarkers of inflammation? Patients will take axelopran as monotherapy after relapse or progression on or after standard systemic therapy. Clinician and patient must be willing to attempt a delay in next line of systemic cancer therapy (if available) until day 43 to assess change in cancer status on repeat imaging. Clinician can move to the next line of therapy whenever deemed clinically necessary. Participants will: * take oral axelopran capsules daily for up to 1 year, or longer if deriving benefit * attend 10 in-person study visits, each lasting approximately 1-2 hours * complete study procedures including but not limited to imaging exams, blood draws, electronic health surveys, and physical assessments