Phase I trial of enzalutamide (Enz) plus the glucocorticoid receptor antagonist relacorilant (Rela) for patients with metastatic castration resistant prostate cancer

5062 Background: Resistance to androgen receptor (AR) targeted therapies is common in mCRPC. Glucocorticoid receptor (GR) expression increases with AR inhibition in patients (pts) and blockade of GR signaling inhibits CRPC growth in preclinical models when combined with AR blockade. We thus conducted a phase I study of the combination of rela and enz in patients with mCRPC. Methods: The study used a 6+3 design to assess safety and pharmacokinetics (PK) of the 2-drug combination. The starting dose of rela 100mg/day was chosen based on current clinical studies indicating this dose was both safe for daily dosing and pharmacologically active and the starting dose for Enz was 120mg based on anticipated drug-drug interactions. Escalating doses of rela were combined with Enz 120 using pre-specified dose escalation rules. An additional 12 patients were to be assessed at the MTD to refine safety and PK profiles. Primary endpoint was safety of the combination and to establish safe and pharmacologically active doses of both agents. Key secondary endpoints were tumor response per RECIST 1.1 criteria and progression-free survival (PFS; defined as either PSA progression, radiographic progression or death). PSA was measured after 12 weeks or at the last time at which PSA data was available, including times <12 weeks. Results: 35 pts were enrolled. Pts had a median age of 74 (range 62-85) and baseline PSA of 89.5 (range 7.3-2391). 100% had prior potent AR signaling inhibitor (abi or enz) with 89% receiving prior enz, 94% prior abi, 83% prior both. 69% of pts received prior docetaxel. Median PSA change was 0.1%. Enz 120mg + Rela 100mg and Enz 120+Rela 150 were found to be safe and tolerable and achieved desirable Enz PK. DLTs were noted in 3 pts at the 200mg Rela dose, which was deemed to have exceeded the MTD, which was thus defined as 150mg Rela+120mg Enz. Most common ≥ grade 3 treatment related adverse events (TRAEs) were fatigue (11% of pts), anemia (9%), abdominal pain (6%) and pericardial effusion (6%). Median PFS was 2.5 months for 21 of 35 evaluable pts. Best response per RECIST 1.1 were Not Evaluable (40%), SD (34%) & PD (26%). No CR or PR were noted. Most common reasons for discontinuation from trial included progressive disease/hospice (82%), AEs (12%), physician discretion (3%) & death (3%). 2 pts remain on trial as of data cutoff (12/31/22). Conclusions: The combination of enzalutamide and relacorilant was safe & largely well tolerated. The majority of pts were discontinued from trial due to progressive disease or hospice. Further analyses of secondary & correlative endpoints such as translational/endocrine biomarkers, GR target gene expression and circulating tumor cells are ongoing. Clinical trial information: NCT03674814 .