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411 Background: Although immune checkpoint inhibitors (ICI) have improved outcomes in some pts with platinum-resistant mUC, many pts (eg, pts with TCGA luminal 1 tumors, many of whom are FGFRa) may not benefit. ERDA, a pan-FGFR (1-4) inhibitor, demonstrated promising phase 1 activity: 11 partial responses among 24 FGFRa mUC pts. We report efficacy and safety of ERDA in the ongoing global open-label phase 2 study BLC2001 (NCT02365597). Methods: Pts had measurable mUC with specific FGFR2/ FGFR3 mutations or translocations per central lab Janssen assay, ECOG 0-2, and were chemorefractory (progressed during/following ≥ 1 line of prior systemic chemo or ≤ 12 mos of [neo]adjuvant chemo). Cisplatin-ineligible, chemo-naïve pts, and prior ICI treatment were allowed. Pts were randomized 1:1 to 28-d cycles of oral 6 mg/d continuous dosing (6 C) or 10 mg/d intermittent 7 d on/7 d off dosing (10 I) ERDA; the dose was further uptitrated if no significant treatment-related adverse events (TRAEs) were observed. The primary end point was ORR. Results: 78 pts received 6 C and 33 pts received 10 I (10 I cohort stopped early) ERDA. 31 pts in 6 C arm were further uptitrated. Across arms, 50% had ≥ 2 prior lines of therapy; 93% were chemorefractory. Confirmed ORRs (RECIST 1.1) were 35% and 24%, and disease control rates (CR+PR+SD) were 74% and 73% in the 6 C and 10 I arms, respectively. Adverse events (AEs) were manageable, and there were no treatment-related deaths (Table). Treatment is ongoing in 10 pts. Conclusions: ERDA (6 C or 10 I) has promising efficacy and tolerability in pts with FGFRa mUC. Based on these results and ERDA pharmacometric modeling, dosing was optimized at 8 mg/d (continuous), and this cohort is ongoing. Phase 3 study is planned. Clinical trial information: NCT02365597. [Table: see text]

Background Exposure–response analyses were conducted to explore the relationship between selected efficacy and safety endpoints and serum phosphate (PO4) concentrations, a potential biomarker of efficacy and safety, in locally advanced or metastatic urothelial carcinoma patients with FGFR alterations treated with erdafitinib. Methods Data from two dosing regimens of erdafitinib in a phase 2 study (NCT02365597), 6 and 8-mg/day with provision for pharmacodynamically guided titration per serum PO4 levels, were analyzed using Cox proportional hazard or logistic regression models. Efficacy endpoints were overall survival (OS), progression-free survival (PFS), and objective response rate (ORR). Safety endpoints were adverse events typical for FGFR inhibitors. Results Exposure-efficacy analyses on 156 patients (6-mg = 68; 8-mg = 88) showed that patients with higher serum PO4 levels within the first 6 weeks showed better OS (hazard ratio 0.57 [95% CI 0.46–0.72] per mg/dL of PO4; p = 0.01), PFS (hazard ratio 0.80 [0.67–0.94] per mg/dL of PO4; p = 0.01), and ORR (odds ratio 1.38 [1.02–1.86] per mg/dL of PO4; p = 0.04). Exposure-safety analyses on 177 patients (6-mg = 78; 8-mg = 99) showed that the incidence of selected adverse events associated with on-target off-tumor effects significantly rose with higher PO4. Conclusions The exploratory relationship between serum PO4 levels and efficacy/safety outcomes supported the use of pharmacodynamically guided dose titration to optimize erdafitinib’s therapeutic benefit/risk ratio. Clinical trial registration number NCT02365597.

A population pharmacokinetic (PK)-pharmacodynamic (PD) model was developed using data from 345 patients with cancer. The population PK-PD model evaluated the effect of erdafitinib total and free plasma concentrations on serum phosphate concentrations after once-daily oral continuous (0.5-12 mg) and intermittent (10-12 mg for 7 days on/7 days off) dosing, and investigated the potential covariates affecting erdafitinib-related changes in serum phosphate levels.Phosphate is used as a biomarker for erdafitinib's efficacy and safety: increases in serum phosphate were observed after dosing with erdafitinib, which were associated with fibroblast growth factor receptor target engagement via inhibition of renal fibroblast growth factor 23-mediated signaling. PK-PD model-based simulations were performed to assess the approved PD-guided dosing algorithm of erdafitinib (8 mg once-daily continuous dosing, with up-titration to 9 mg based on phosphate levels [<5.5 mg/dl] and tolerability at 14-21 days of treatment). The serum phosphate concentrations increased after the first dose and reached near maximal level after 14 days of continuous treatment. Serum phosphate increased with erdafitinib free drug concentrations: doubling the free concentration resulted in a 1.8-fold increase in drug-related phosphate changes. Dose adjustment after at least 14 days of dosing was supported by achievement of >95% maximal serum phosphate concentration. The peak-to-trough fluctuation within a dosing interval was limited for serum phosphate concentrations (5.68-5.65 mg/dl on Day 14), supporting phosphate monitoring at any time relative to dosing. Baseline phosphate was higher in women, otherwise, none of the investigated covariateparameter relationships were considered clinically relevant. Simulations suggest that the starting dose of 8-mg with up-titration to 9-mg on Days 14-21 maximized the number of patients within the target serum phosphate concentrationsThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.