Predictive and Prognostic Performance of IHC3 Immunohistochemistry-based Molecular Subtyping in Muscle-Invasive Bladder Cancer

Abstract

Luminal/basal molecular subtyping in muscle-invasive bladder cancer (MIBC) has shown important relationships to prognosis and chemosensitivity, where tumours of the basal subtype have been reported to be more aggressive, yet more sensitive to cytotoxic therapy. Despite this, molecular subtyping of MIBCs has yet to play a role in clinical treatment selection, largely limited by the complexity and artifacts associated with transcriptomic profiling methods. These shortcomings have in part been addressed by a group form Lund University who validated their subtyping using immunohistochemistry (IHC) to identify 3 key intrinsic subtypes of MIBC: Urothelial-like (URO), Genomically Unstable (GU) and Basal/Squamous cell carcinoma like (SCCL). We aimed to assess the ability of a simplified IHC-based algorithm, termed “IHC3” to identify key intrinsic molecular subtypes and explore the relationships of this classification to prognosis and treatment outcomes. 3 tissue microarrays (TMAs) were constructed from (n=133) individual samples from trans-urethral resection of bladder tumour (TURBT) and cystectomy specimens from 87 patients. TMAs were stained using clinical grade IHC assays for GATA3, KRT5 and p16. Scoring was conducted with visual and digital image analysis using percentage positive tumour cells, staining intensity and spatial localization. IHC3 identified 3 subtypes of MIBC with prognostic differences. Whether given before (NACT) or after surgery (ACT) no significant association between subtype and survival benefit from chemotherapy treatment. Nevertheless, because of their inferior prognosis, Basal/SCCL and GU subtypes might derive the most benefit from an effective systemic therapy. Using just three antibodies, IHC3 identified key molecular subtypes with prognostic significance, where GU and Basal subtypes showed poor prognosis compared to URO, which showed good prognosis. Application of IHC3 to an expanded cohort of 500+ patients will allow the extension of this work to further explore the predictive significance of IHC3 subtypes and confirm their potential clinical utility.