2.3. Procedures

MRI sessions were conducted immediately prior to (Pre-XRNTX), as well as approximately 2 weeks (12 ± 7 days) after the first XRNTX injection (On-XRNTX). Subjective craving for and withdrawal from opioids was recorded prior to each fMRI session using self-report on a 0–9 scale. Plasma concentrations of naltrexone and 6-beta-naltrexol (an active metabolite) were ascertained at the second MRI session 12 ± 7 days after injection, using established liquid chromatography and tandem mass spectrometry techniques (Langleben et al., 2014; Slawson et al., 2007). During the study, continuation of care was discussed with the participants and they were given referrals to treatment providers in the community. A urine drug screen (UDS, Redwood Toxicology Laboratory, Santa Rosa, CA) for opiates (e.g. heroin and codeine metabolites), oxycodone, methadone, buprenorphine, cocaine, amphetamine, methamphetamine, benzodiazepines and phencyclidine was conducted prior to each MRI scan and each XRNTX injection.