A Study of Extending Relugolix Dosing Intervals Through Addition of Itraconazole or Ritonavir in Prostate Cancer Patients

Inclusion Criteria: * Must have histologically or cytologically confirmed prostate cancer with plans to start medical castration therapy. * Age 18 or older. Because no dosing or adverse event data are currently available on the use of relugolix in combination with itraconazole or ritonavir in patients \<18 years of age, children are excluded from this study. * Eastern Cooperative Oncology Group performance status ≤2 (Karnofsky ≥60%) * Participants must satisfy the following laboratory criteria: * Testosterone \> 200 ng/dl * QTc within normal limits * Aspartate Transferase (AST)/alanine transaminase (ALT) \< 3x upper limit of normal * Ability to take oral medication and be willing to adhere to the medication regimen. * Ability to understand and the willingness to sign a written informed consent document. Exclusion Criteria: * Need for combination systemic therapy for prostate cancer including docetaxel and/or second-generation androgen receptor pathway targeted agents (i.e. abiraterone, enzalutamide, apalutamide, darolutamide, etc). * Concurrent therapy with strong interacting drugs with CYP3A4 and P-gp (See Appendix list of strong inhibitors or inducers) due to concerning possible drug-drug interactions with itraconazole, ritonavir and relugolix. Potential drug-drug interactions will also be reviewed by study investigators prior to enrollment. * History of allergic reactions attributed to compounds of similar chemical or biologic composition to relugolix, itraconazole or ritonavir. * Patients receiving treatment with another investigational drug or other intervention within 3 expected half-lives of the agent. * Serious or uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, unstable angina pectoris, or psychiatric illness/social situations that would limit compliance with study requirements. * New York Heart Association Class III of IV congestive heart failure * Known gastrointestinal malabsorption. * Active psychiatric illness/social situations that would limit compliance with protocol requirements.