Carbonic anhydrase 9

Overview

Carbonic anhydrase 9 (CA9), also known as carbonic anhydrase IX, is a membrane-associated enzyme in the carbonic anhydrase family. It is widely recognized as a hypoxia-associated marker and a cancer-relevant protein because of its role in regulating tumor pH and supporting survival in low-oxygen microenvironments. By contributing to acid–base balance at the cell surface, CA9 can help tumor cells adapt to hypoxia, promote aggressive behavior, and influence response to therapy.

In biomedical research, CA9 is studied both as a biomarker and as a therapeutic target. Its expression is often linked to invasive phenotypes and treatment resistance in solid tumors, including renal cell carcinoma, triple-negative breast cancer, anal squamous cell carcinoma, and other malignancies such as colorectal, head/neck, melanoma, and prostate tumors. Because of its tumor-associated expression pattern, CA9 has also been explored for molecular imaging and for selective inhibition strategies, including combinations with other cancer-associated enzymes such as carbonic anhydrase XII.

Focus of Latest Publications

Recent publications have continued to examine carbonic anhydrase 9 (CA9/CAIX) as a hypoxia-associated target in cancer biology, therapy, and imaging. Several studies focused on its role in tumor aggressiveness and metabolic adaptation. In hepatocellular carcinoma cells, CA9 was identified among prognostic genes linked to glycolysis-related pathways, and CA9 knockdown reduced glucose uptake and lactate production while also suppressing HK2 and PKM2 expression. In triple-negative breast cancer, sequential invasion assays isolated invasive epithelial subpopulations that expressed CA9 as a druggable driver of invasiveness; downregulation of CAIX reduced proliferation, invasiveness, and chemotherapy sensitivity. Another study in pediatric gliomas and glioneuronal tumors examined CAIX expression together with immune response biomarkers, reflecting ongoing interest in its relationship to the tumor microenvironment and clinical outcome.

CA9 has also remained a prominent therapeutic target for inhibitor development. A medicinal chemistry study reported bis-triazole-linked benzenesulfonamides as selective carbonic anhydrase IX/XII inhibitors, with nanomolar activity against hCA IX and favorable selectivity over off-target isoforms. These compounds also enhanced the antiproliferative effects of azacitidine and sorafenib in breast and pancreatic cancer cells under normoxic and hypoxic conditions, supporting CAIX-targeted chemosensitization. In a separate natural-product-based study, a semi-synthetic sugar-based enone derivative from Moringa oleifera pericarp, S2M2, showed strong inhibitory activity against CAIX alongside EGFR-TK, with docking and molecular dynamics simulations supporting stable binding in the CAIX active site.

Imaging applications were also highlighted. A preclinical study of the SuFEx-modified PET probe [68Ga]Ga-SF-DPI-4452 for clear cell renal cell carcinoma showed that CAIX targeting was preserved while gastrointestinal uptake was markedly reduced compared with the parent tracer, improving tumor-to-background contrast in mice and confirming reduced off-target accumulation in cynomolgus monkeys. Together, these publications reinforce CA9/CAIX as a versatile target for cancer diagnosis and treatment, spanning metabolic regulation, invasion, chemosensitization, and molecular imaging.

Additional clinical correlative studies assessed CA9/CAIX expression as a prognostic biomarker. In anal squamous cell carcinoma treated with definitive chemoradiotherapy, CA9 was evaluated alongside DICER for prognostic value, and in pediatric CNS tumors CAIX expression was studied in relation to immune biomarkers and outcomes. Collectively, the recent literature continues to position CA9 as a hypoxia-linked marker with functional relevance across multiple tumor types and as a target of interest for both therapeutic and imaging strategies.