Psilocybin-assisted Existential, Attachment and RelationaL (PEARL) Therapy for Patients With Advanced Cancer

Inclusion Criteria: 1. \>18 years of age. 2. Ability to speak and read English (patient to provide written informed consent and participate in PEARL intervention, as determined by study personnel). 3. Resident of Ontario. 4. No cognitive impairment indicated in medical record or by attending oncologist or palliative care physician. 5. Confirmed diagnosis of stage IV solid tumour cancers, sarcoma, endocrine, melanoma cancers, or stage 4 lymphoma with expected survival of greater than 6 months as determined by their oncologist or palliative care physician. 6. At least mild depressive symptoms, defined as \>8 on the Patient Health Questionnaire-9 (PHQ-9) (Kroenke et la., 2001). 7. Interest in and ability to participate in and complete the PEARL intervention and protocol as outlined. 8. Participants who are sexually active and could become pregnant must be using effective birth control (per their physician), prior to study entry, during study participation, and for the duration of the study. Participants who are sexually active and could inseminate a partner must agree to use effective birth control after psilocybin administration until the end of study. For participants of child-bearing potential, a negative serum pregnancy test result is required at screening. A urine pregnancy test will be administered on the morning of psilocybin administration for applicable participants. Participants cannot be pregnant or nursing through the duration of the study. 9. If using prescribed medications or other substances, participants must agree to refrain from taking them if instructed by study investigators. These include: * not using any non-prescription medication, nutritional supplement, or herbal supplement except when approved by the treatment team (exceptions will be evaluated by the Investigator and will include acetaminophen, non-steroidal anti-inflammatory drugs, and common doses of vitamins and minerals), * not using nicotine for at least 2 hours before psilocybin administration, and not again until approximately 7 hours after psilocybin administration, * consuming approximately the same amount of caffeine-containing beverages (e.g., coffee, tea) that they consume on a usual morning before arriving at the treatment centre for the psilocybin session day, * not taking any as needed medications on the mornings of psilocybin sessions (with the exception of daily and as needed opioid pain medication), * refraining from using any psychoactive drugs, including alcoholic beverages, within 24 hours of the psilocybin administration. 10. Participants must have someone drive them after the session to where they are staying (home, hotel or another location), because psilocybin may affect their alertness and concentration on the evening of the dosing session. Exclusion Criteria: 1. Primary cancer of the brain, or metastasis to the brain associated with clinically significant symptoms (e.g., affective, cognitive, personality-related, psychotic, or other symptoms, including seizures). 2. Symptoms consistent with delirium, psychosis, or other symptoms judged to be incompatible with establishment of rapport or safe exposure to psilocybin. 3. A history of past intolerability of psilocybin or other psychedelics. 4. Past/present psychiatric diagnoses including bipolar I disorder, psychotic disorders, active substance use disorders or suicidality (as distinguished from desire for hastened death or readiness for death, per the discretion of the study team). 5. If participant is under 30 years of age and has first degree relative with a primary psychotic disorder. 6. Severe hypertension (defined as systolic blood pressure \>150/or diastolic pressure \>95) based on two readings on the same day. If the second reading remains over 150/95, the patient can be brought in for another reading on a different day. Patients can be re-screened for participation once blood pressure is adequately controlled. 7. Moderate or severe hepatic impairment, as defined by Child-Pugh class B or C, or elevations in AST or ALT greater than 3 times the upper limit of normal. 8. Severe renal impairment (defined as eGFR \< 30). 9. Known paraneoplastic syndrome or "ectopic" hormone production by the primary tumor if incompatible with psilocybin, determined in consultation with the study palliative care physician. Patients could be enrolled if it is determined that the patient's condition is compatible with psilocybin administration. 10. Cardiovascular conditions including uncontrolled hypertension, angina, a clinically significant ECG abnormality (e.g., atrial fibrillation without rate control), transient ischemic attack in the last six months, stroke, peripheral or pulmonary vascular disease (no active claudication). 11. Uncontrolled epilepsy or history of seizures in past 6 months. 12. Participants with diabetes who are unable to skip a meal (lunch), or whose diabetes requires administration of medication more than twice daily, or who have had symptomatic hypoglycemia within the prior 30 days. 13. GI bleed in last 6 months. 14. Use of other agents that would be inappropriate to take with psilocybin in the judgement of the investigator. These agents may include psychoactive prescription medications (e.g., benzodiazepines, lithium, Selective serotonin reuptake inhibitors), medications having a primary pharmacological effect on serotonin-2a (5-HT2A) receptors (e.g., olanzapine), or medications that are monoamine oxidase (MAO) inhibitors, any potent metabolic inducers (e.g. rifamycin, rifampin, rifabutin, rifapentine, carbamazepine, phenytoin, phenobarbital, nevirapine, efavirenz, taxol, dexamethasone, St John's wort) or inhibitors (e.g. HIV protease inhibitors, itraconazole, ketoconazole, erythromycin, clarithromycin, troleandomycin). 15. Any other medical condition or laboratory abnormality judged to be incompatible with safe exposure to psilocybin Of note, in suitable patients, these medications may be paused or tapered between study enrolment and prior to the start of the intervention when it is deemed safe to do so. A safe and appropriate tapering regimen will then be developed based on the particular medication, on a case-by-case basis. If taking an MAO inhibitor, the psilocybin session will not be conducted until at least 5 half-lives of the agent have elapsed after the last dose. Patients prescribed opioids will be allowed to take their usual dose regimen for analgesia, including the use of as needed analgesic medications on psilocybin session days.