CAIX (carbonic anhydrase IX)

aka CA9, G250 antigen, MN antigen

A protein that clear cell kidney cancer makes almost all the time because of a gene fault. It helps the tumour survive in low-oxygen conditions, and antibodies that stick to it can be used to find or treat the cancer.

Carbonic anhydrase IX is a transmembrane enzyme with an extracellular catalytic domain that helps regulate intracellular and extracellular pH, allowing tumour cells to survive an acidic, hypoxic microenvironment. Its expression is switched on by the HIF-1 transcription factor: in clear cell renal cell carcinoma (ccRCC), loss-of-function mutation or deletion of the VHL tumour-suppressor gene (found in roughly 75-95% of ccRCC cases) leaves HIF-1α constitutively active even without hypoxia, so CAIX is expressed diffusely and strongly on almost all ccRCC cells, in contrast to normal kidney, which does not express it.

This expression pattern (near-ubiquitous in ccRCC, essentially absent in normal renal parenchyma and in non-clear-cell kidney tumours) is what makes CAIX attractive: it is a differential diagnostic marker for ccRCC as well as a therapeutic target. The chimeric monoclonal antibody girentuximab (cG250) binds a conformational epitope on the catalytic domain, internalises via clathrin-coated vesicles and recycles rather than being degraded, giving long intracellular retention. Because girentuximab is a full antibody, tumour-to-background contrast builds slowly and imaging is typically read several days after injection.

As a target, CAIX's strength is its consistency and specificity in ccRCC; its weakness for antibody-based approaches is slow pharmacokinetics (days, not hours) and the fact that HIF-2α-driven, VHL-independent disease later in progression can show falling CAIX expression, which has been linked to worse prognosis. Small-molecule and peptide CAIX ligands (for example 68Ga-DPI-4452) have since been developed to give same-day imaging with high tumour-to-background contrast.

CAIX (carbonic anhydrase IX): what it is and how drugs act on it · animated schematic, not to scale

Target · the protein and the cell it sits on
Drug · antibody, small molecule, cell or radioligand
Effect · signal, damage or kill

In plain words · A protein that clear cell kidney cancer makes almost all the time because of a gene fault. It helps the tumour survive in low-oxygen conditions, and antibodies that stick to it can be used to find or treat the cancer.

1 · What it is
A protein that clear cell kidney cancer makes almost all the time because of a gene fault. It helps the tumour survive in low-oxygen conditions, and antibodies that stick to it can be used to find or treat the cancer.
2 · What goes wrong in cancer
A transmembrane carbonic anhydrase whose extracellular catalytic domain hydrates CO2 to bicarbonate and protons, helping tumour cells regulate pH and survive hypoxia.
3 · How drugs use it
No product in this corpus aims at CAIX (carbonic anhydrase IX) yet. Inhibitors are shaped to fit the enzyme's active site so the reaction the cancer relies on stops.

External identifiers

Biology

A transmembrane carbonic anhydrase whose extracellular catalytic domain hydrates CO2 to bicarbonate and protons, helping tumour cells regulate pH and survive hypoxia. Its expression is switched on by the HIF-1 pathway, so it is expressed at very high, uniform density in VHL-mutant clear cell renal cell carcinoma while normal kidney does not express it.

Where it is found

Clear cell renal cell carcinoma: overexpressed in >90-95% of cases, driven by VHL/HIF-1 pathway loss
Normal kidney parenchyma: essentially absent
Some hypoxic non-renal tumours (expression follows tumour hypoxia rather than a fixed tissue pattern)
Normal gastrointestinal tract: low-level physiological expression

Class

enzyme · CA9