Introduction
The treatment of urothelial bladder cancer (UBC) has been transformed by the approval of immune checkpoint inhibitors (ICIs), fibroblast growth factor receptor (FGFR) inhibitors, and antibody-drug conjugates (ADCs). The pivotal EV-302/KEYNOTE-A39 trial establishing enfortumab vedotin (EV) plus pembrolizumab as a first-line standard-of-care option for locally advanced or metastatic UBC underscores the therapeutic advances now available 23. Yet the diversity of molecular drivers across patients means that biomarker-guided selection, monitoring, and trial design are now central requirements rather than aspirational goals. This narrative review provides a practical framework covering predictive, resistance, pharmacodynamic, and safety biomarkers, and translates the evidence into actionable recommendations for clinical development and regulatory strategy.
1. Predictive Biomarkers for Treatment Response
The four biomarkers with the strongest clinical validation across the current UBC therapeutic armamentarium are PD-L1, FGFR2/3 alterations, HER2, and Nectin-4. Table 1 summarizes assay platforms, cutoffs, and companion diagnostic (CDx) status for each.
Table 1. Predictive Biomarkers in Urothelial Bladder Cancer: Assay, Cutoff, and CDx Status
| Biomarker | Relevant Therapy | Assay Modality | Platform / Scoring System | Established Cutoff | CDx Status |
|---|---|---|---|---|---|
| PD-L1 | Pembrolizumab, atezolizumab, avelumab | IHC | SP142 (IC score), 22C3 (CPS), SP263 | SP142: IC ≥5% (IC2/3); 22C3: CPS ≥10; SP263: ≥25% TC+IC | Regulatory aid (FDA/EMA/NMPA); not universally mandatory |
| FGFR2/3 alterations | Erdafitinib | RT-PCR or NGS | therascreen FGFR RGQ RT-PCR (Qiagen); validated NGS panels | Any pathogenic FGFR3 alteration (mutation/fusion) | FDA-approved CDx (2024); EMA-approved CDx (2024) 28 |
| HER2 | Disitamab vedotin, T-DXd, MRG-002 | IHC (primary); FISH (supplemental) | Automated IHC; 0/1+/2+/3+ scoring | IHC 2+ or 3+ for ADC eligibility; FISH not required for disitamab vedotin | NMPA-approved (disitamab vedotin, China, December 2021) 2425 |
| Nectin-4 | Enfortumab vedotin | IHC; NGS (NECTIN4 amplification) | Modified HER2 scoring (0–3); NGS copy number | IHC ≥2 positive; NECTIN4 amplification (≥3 copies or ratio ≥2) has been reported in emerging studies | Not yet established as CDx; exploratory 34 |
PD-L1 expression, assessed by IHC, remains the most widely applied predictive biomarker in ICI-treated UBC; however, its predictive value is assay-dependent and setting-dependent. Analysis of the IMvigor130 trial using paired SP142 and 22C3 assays on 655 UBC samples demonstrated that SP142 IC2/3 status (≥5% immune cell positivity) predicted superior outcomes with atezolizumab (median OS 27.5 vs. 13.1 months; HR 0.56; p=0.0003), driven by enrichment for dendritic cell-predominant PD-L1 expression and effector T-cell signatures. By contrast, 22C3 CPS ≥10 captured a more heterogeneous population, including tumors with tumor-cell-dominant PD-L1 staining, and showed a smaller magnitude of benefit 1. In the adjuvant setting, pembrolizumab (AMBASSADOR trial) demonstrated disease-free survival benefit regardless of 22C3 CPS status, suggesting that PD-L1 is prognostic but not predictive in this context 1. A multi-cohort meta-analysis of 707 advanced UBC patients further confirmed that PD-L1 alone is insufficient for patient selection; tumor mutational burden (TMB), APOBEC mutational signature enrichment, and pro-inflammatory macrophage abundance independently predicted ICI response 6. In China, the SP263 assay with a ≥25% combined tumor cell and immune cell threshold is the standard IHC platform endorsed by the 2020 national consensus and current CSCO guidance 2526.
FGFR2/3 alterations are present in 15–20% of advanced UBC and represent the most robustly validated targetable alteration. The FDA (January 2024) and EMA (August 2024) both granted full approval to erdafitinib for FGFR3-altered locally advanced or metastatic UBC following progression on at least one prior line including a PD-1/PD-L1 inhibitor, based on Study BLC3001 Cohort 1 showing a median OS of 12.1 versus 7.8 months (HR 0.64; p=0.005), PFS of 5.6 versus 2.7 months (HR 0.58; p=0.0002), and confirmed ORR of 35.3% versus 8.5% 28. The therascreen FGFR RGQ RT-PCR kit (Qiagen) is the approved CDx; validated NGS panels are an acceptable alternative 2. Emerging data with TYRA-300, a highly FGFR3-selective inhibitor, show a 54.5% partial response rate and 100% disease control in FGFR3-altered metastatic UBC at therapeutic doses, with ctDNA reductions confirming molecular target engagement 4.
HER2 overexpression (IHC 2+ or 3+) is present in approximately 5–30% of advanced UBC depending on the assay and cutoff applied 1315. Disitamab vedotin (RC48), the leading HER2-directed ADC in UBC, received NMPA approval in China in December 2021, based on pooled data from the RC48-C005 and RC48-C009 trials showing an ORR of 50.5%, median PFS of 5.9 months, and median OS of 14.2 months in 107 previously treated HER2-overexpressing patients 24. The 2026 Chinese expert consensus standardizes HER2 IHC scoring in UBC as 0/1+/2+/3+, emphasizing that FISH is not required for ADC patient selection since clinical benefit is observed in both FISH-positive and FISH-negative IHC 2+ cases, and that attention to heterogeneity (including U-shaped staining in micropapillary variants) is essential 25. Globally, trastuzumab deruxtecan (T-DXd) is in phase I–II trials across multiple geographies, and additional China-developed HER2 ADCs (MRG-002/trastuzumab vedotin Phase III, BB-1701 Phase II, BL-M07D1 Phase II) are active 27.
Nectin-4 is highly expressed in urothelial carcinoma and is the molecular target of EV. In a retrospective cohort of 20 patients with metastatic UBC treated with EV, Nectin-4-positive primary tumors (IHC ≥2) correlated with significantly longer PFS compared with Nectin-4-negative tumors (6.2 vs. 1.4 months; p=0.005) 3. EV-302 exploratory analyses from 2025 ESMO demonstrate that NECTIN4 amplification by NGS further stratifies response in first-line mUC, supporting this as a prospective enrichment variable 23.
2. Resistance Biomarkers
Resistance in UBC operates at multiple biological levels. Table 2 categorizes resistance mechanisms by therapy class, evidence strength, and recommended detection modality.
Table 2. Resistance Biomarkers in UBC
| Therapy Class | Resistance Mechanism | Biomarker | Detection Method | Evidence Strength |
|---|---|---|---|---|
| FGFR inhibitors | Gatekeeper / kinase-domain mutations (e.g., V555M in FGFR3, V564F in FGFR2) | Secondary FGFR mutations | ctDNA or tissue NGS at progression | Clinically validated 184 |
| FGFR inhibitors | Paracrine NRG1/HER3 axis activation via adipocyte precursor-derived NRG1 | NRG1 expression (tumor microenvironment) | Single-cell RNA-seq or IHC | Preclinical; hypothesis-generating 19 |
| ICI (PD-1/PD-L1) | Cold (T-cell-depleted) tumor microenvironment; immune-suppressive mechanisms in high-infiltration tumors | T-cell exhaustion markers; pro-suppressive gene signatures | Bulk RNA-seq, multiplex IHC, ctDNA dynamics | Clinically demonstrated; not yet routinely actionable 610 |
| ICI | Pre-existing resistance clones | ctDNA variant allele fraction (VAF) at baseline and at progression | ctDNA plasma panel (e.g., 53-gene + TERT promoter) | Validated: pre-existing clones drive acquired resistance 10 |
| Nectin-4 ADC (EV) | ABC transporter-mediated MMAE efflux (MDR1/ABCB1, MRP1/ABCC1, BCRP/ABCG2) | ABC transporter expression | IHC | Clinically suggestive; requires prospective validation 3 |
| Nectin-4 ADC (EV) | Decreased Nectin-4 expression as disease advances | Nectin-4 IHC score | IHC on metastatic lesions | Retrospective cohort data 3 |
For FGFR inhibitors, acquired resistance is mediated by polyclonal secondary kinase-domain mutations emerging during therapy; serial ctDNA monitoring can detect these alterations before radiographic progression and inform transition to next-generation FGFR inhibitors (e.g., KIN-3248, an irreversible pan-FGFR1–4 inhibitor) 185. The NRG1/HER3 paracrine mechanism—wherein adipocyte precursor-derived NRG1 activates HER3 signaling to bypass FGFR blockade—is an emerging resistance pathway for which pertuzumab-based combination reversal has been demonstrated in preclinical models 19. For ICI, the finding that on-treatment increase in ctDNA fraction during pembrolizumab therapy is strongly predictive of failure (ORR 18.7% in ctDNA-rising vs. 76.1% in ctDNA-stable patients; median PFS 2.8 vs. 9.8 months; P<0.001) positions serial ctDNA as an actionable pharmacodynamic-resistance composite marker 10. For EV, the inverse relationship between Nectin-4 expression and ABC transporter expression across disease stages—with transporter levels rising as disease advances—represents the critical resistance axis to monitor 3.
3. Pharmacodynamic Biomarkers of Target Engagement
Pharmacodynamic (PD) biomarkers confirm that a drug reaches its target and produces the intended molecular effect. A well-designed trial sampling schema should include tissue, circulating, and functional markers. Table 3 summarizes recommended PD biomarkers with sampling timepoints.
Table 3. Pharmacodynamic Biomarker Sampling Schema for Key Therapy Classes in UBC
| Therapy Class | PD Biomarker | Specimen | Recommended Timepoints | Purpose |
|---|---|---|---|---|
| FGFR inhibitors | Serum phosphate (on-target FGFR1/2 inhibition) | Blood | Baseline; Cycle 1 Day 14–21; monthly thereafter 28 | On-target engagement and dose adjustment |
| FGFR inhibitors | ctDNA VAF reduction | Plasma | Baseline; Week 4; every 6–8 weeks; progression 54 | Early molecular response surrogate |
| FGFR inhibitors | Phospho-ERK (p44/42) reduction | Tumor tissue (paired biopsy) | Baseline; Day 7–14 on-treatment 5 | Proof-of-mechanism (early phase) |
| FGFR inhibitors | FGF23, parathyroid hormone | Blood | Baseline; Cycle 1–2 | Downstream pathway disruption |
| ICI | CD8+ T-cell infiltration, PD-1+ T-cell frequency, HLA-DR/Ki-67 | Tumor tissue / blood (flow cytometry) | Baseline; Week 3–4; Week 8–12 | Immune activation confirmation |
| ICI | Serial ctDNA fraction | Plasma | Baseline; Cycle 2 (early); Week 8–12; progression 10 | Pharmacodynamic response and resistance detection |
| ICI | MDSC enumeration | Blood | Baseline; 24–48 h post PI3K inhibitor (if used in combination) 7 | Immune microenvironment modulation |
| Nectin-4 ADC | Nectin-4 protein expression (on-treatment modulation) | Tumor tissue (where feasible) | Baseline; Week 2–4 | Target saturation / adaptive regulation |
| All | Urine-based ctDNA | Urine | Baseline; on-treatment (serial) 20 | Non-invasive complement to plasma ctDNA |
The serum phosphate rise serves dual purposes as both an on-target PD readout and a safety-limiting toxicity marker for FGFR inhibitors. In the KIN-3248 phase I trial, change in phosphate at Cycle 1 Day 8 correlated with drug exposure (Pearson R = 0.62; P = 7.1e-7), and ctDNA clearance at Cycle 2 associated with radiographic response (Pearson R = 0.45; P = 0.021) 5. TYRA-300 produces ctDNA reductions at therapeutic doses with minimal hyperphosphatemia, demonstrating that FGFR3-selective inhibition can decouple the PD signal (ctDNA) from the on-target toxicity (phosphate) 4. For immune combinations, MDSC enumeration 24–48 hours after PI3K inhibitor dosing—demonstrated in the copanlisib plus nivolumab phase Ib trial—provides an accessible early circulating PD readout for immune-modulatory agents 7. Urine-based ctDNA analysis is a validated, non-invasive complementary modality for longitudinal monitoring in UBC patients who shed DNA into urine 20.
4. Safety Risk Biomarkers and Monitoring
Biomarker-guided safety management is a regulatory expectation and a clinical necessity in UBC, where patients often have impaired renal function and older age. Table 4 summarizes safety risk markers by therapy class.
Table 4. Safety Risk Biomarkers and Monitoring Recommendations
| Therapy Class | Key Toxicity | Safety Biomarker | Monitoring Schedule | Management |
|---|---|---|---|---|
| FGFR inhibitors | Hyperphosphatemia (64–85% incidence) | Serum phosphate, renal function | Baseline; Day 14–21 of Cycle 1; monthly 285 | Phosphate binders; dose reduction (8 mg to 9 mg escalation protocol) |
| FGFR inhibitors | Stomatitis, palmar-plantar erythrodysesthesia (PPE) | Clinical exam; patient-reported outcomes | Each cycle | Oral mucosa care; topical agents; dose modification |
| FGFR inhibitors | Ocular toxicity (central serous retinopathy) | Ophthalmologic exam | Baseline; periodic (every 2–3 months) | Ophthalmologic referral; dose hold |
| ICI | irAEs (endocrinopathy, hepatitis, pneumonitis) | TSH, free T4, LFTs, creatinine, CRP, IL-6 | Baseline; each cycle 1 | Immunosuppression per grade; dose interruption |
| ICI | Immune-related toxicity prediction | Autoimmune disease history; HLA allotype (exploratory) | Baseline history; HLA testing if available | Enhanced monitoring in high-risk patients |
| HER2 ADC (T-DXd and others) | Interstitial lung disease / pneumonitis (reported across HER2 ADC studies, including rare fatal cases) | Baseline pulmonary function; chest CT | Baseline; at any new respiratory symptom 4 | Drug hold; corticosteroids; permanent discontinuation for grade ≥3 |
| HER2 ADC | Cardiotoxicity | Echocardiography, troponin, BNP | Baseline; periodic | Cardiology referral |
| Nectin-4 ADC (EV) | Peripheral neuropathy (dose-limiting) | Clinical neuropathy exam; CTCAE grading | Baseline; every 2–4 weeks 3 | Dose reduction or discontinuation per grade |
| Nectin-4 ADC (EV) | Skin reactions (rash, pruritus; grade ≥3 ~10%) | Clinical skin exam; dermatology consult | Baseline; each cycle 4 | Topical steroids; dose reduction |
| Nectin-4 ADC (EV) | Hyperglycemia | Fasting glucose, HbA1c | Baseline; periodic 4 | Glycemic optimization; dose modification |
EV-302 subgroup analyses presented at 2025 ESMO confirmed that EV plus pembrolizumab maintains efficacy and safety in patients ≥75 years (ORR 66%), with diabetes (ORR 70%), and with renal impairment GFR <60 mL/min (ORR 66%), with grade ≥3 adverse events lower than chemotherapy across all subgroups 23. This real-world-relevant safety data informs dose selection and monitoring intensity for these high-risk subgroups.
5. Trial Design and Label Strategy Integration
A coherent biomarker strategy must address eligibility, stratification, adaptive design, endpoint interpretation, and regulatory labeling at the program outset. Table 5 provides a decision framework.
Table 5. Biomarker Integration Framework for UBC Clinical Trials
| Biomarker | Role in Trial | Designation | Assay / Specimen | Action |
|---|---|---|---|---|
| FGFR2/3 alterations | Eligibility (mandatory) + stratification | Mandatory CDx | Tissue/plasma NGS or therascreen RT-PCR 24 | Restrict to altered tumors for FGFR inhibitor trials; allow ctDNA for prescreening |
| PD-L1 (22C3 CPS or SP263) | Stratification in ICI trials; NOT mandatory for adjuvant selection | Mandatory stratification; not restrictive in adjuvant | IHC (central lab) 116 | Stratify by CPS ≥10 or SP263 ≥25%; exploratory subgroup analysis |
| HER2 (IHC 2+/3+) | Eligibility for HER2-ADC trials | Mandatory CDx in China (disitamab vedotin) | IHC per 2026 consensus; FISH optional 2425 | Require IHC 2+ or 3+; document FISH status as exploratory |
| Nectin-4 (IHC + NECTIN4 amplification) | Eligibility + enrichment enrichment sub-cohort | Recommended + exploratory | IHC; NGS for amplification 323 | Require IHC positivity; prospectively stratify by NECTIN4 amplification |
| ctDNA (VAF dynamics) | Serial PD / early response / resistance detection | Exploratory (mandatory collection) | Plasma ctDNA panel (baseline, Week 4, Week 8–12, progression) 1017 | Collect at all timepoints; pre-specify Week 4 analysis as interim PD endpoint |
| TMB / APOBEC signature | Exploratory enrichment in ICI trials | Exploratory | Tissue NGS 6 | Collect; evaluate in biomarker-unselected enrichment cohorts |
| MDSC / immune profiling | Combination IO trial PD monitoring | Exploratory | Flow cytometry (blood); baseline + Day 2 post PI3K inhibitor 7 | Collect in PI3K+ICI combination arms |
| ABC transporters (MDR1, MRP1, BCRP) | EV resistance stratification | Exploratory | IHC (tumor) 3 | Collect in EV trials; validate prospectively before mandatory use |
| Serum phosphate | Mandatory safety monitoring; PD marker | Mandatory safety | Blood 28 | Assess pre-dose and Cycle 1 Day 14–21; monthly thereafter; guide dose escalation |
| ERCC2 functional mutations | Platinum sensitivity prediction | Exploratory | CRISPR-Select or functional NGS 22 | Collect in chemotherapy-containing arms; exploratory endpoint |
Adaptive Design Considerations: Basket designs should incorporate separate cohorts for FGFR3-altered, HER2-overexpressing, and Nectin-4-amplified disease in parallel to enable biomarker-specific efficacy analyses 4. Enrichment strategies—restricting phase 2b/3 enrollment to PD-L1-high or FGFR-altered populations—can increase response rates and reduce required sample size when phase 2 signals support this approach. Pre-specified interim biomarker analyses (e.g., ctDNA VAF reduction at Week 4 in FGFR programs) can serve as pharmacodynamic decision points for dose escalation, futility, or expansion cohort selection 45.
Regulatory Labeling: Erdafitinib labeling mandates therascreen FGFR RT-PCR testing with clear hyperphosphatemia management algorithms (dose escalation from 8 mg to 9 mg based on phosphate monitoring at Day 14–21) and is aligned across FDA and EMA labels 28. Disitamab vedotin labeling in China specifies HER2 IHC 2+ or 3+ with the 2026 standardized scoring framework as the selection basis 2425. For EV plus pembrolizumab, EAU, NCCN, and ESMO guidelines recommend PD-L1 and FGFR testing in all locally advanced and metastatic UBC; positive PD-L1 indicates ICI eligibility, and positive FGFR results indicate erdafitinib in post-ICI lines 16. ctDNA and NECTIN4 amplification remain exploratory biomarkers requiring prospective validation before label integration; post-marketing registries and real-world data should be structured to generate this evidence systematically 1723.
Conclusion
A comprehensive, multi-modal biomarker strategy is now inseparable from responsible drug development and clinical practice in urothelial bladder cancer. Assay standardization has been achieved for FGFR2/3 (RT-PCR/NGS) and HER2 (IHC per the 2026 consensus), and CDx frameworks are regulatory-grade for erdafitinib in the US and EU and for disitamab vedotin in China. PD-L1 retains value for stratification but not selection in all settings, and its predictive performance is critically assay-dependent. Nectin-4 IHC is a practical eligibility marker for EV-based programs, with NECTIN4 amplification emerging as a promising enrichment variable. Serial ctDNA monitoring, serum phosphate assessment, and immune profiling form the pharmacodynamic backbone that allows real-time evidence of target engagement, early detection of molecular resistance, and evidence-based dose management. Trial design that mandates these biomarkers at baseline, integrates serial sampling through treatment, and pre-specifies adaptive analyses will generate the regulatory-grade evidence necessary to refine labeling language, support CDx development, and ultimately extend precision medicine to every patient with urothelial bladder cancer.