Aspirin has survival benefits in colorectal cancer – not quite so simple

The authors aimed to evaluate if survival benefits associated with aspirin are reliant on tumor expression of the HLA class I antigen (a protein involved in regulation of the immune system).

Previous studies have shown that aspirin use after a diagnosis of colorectal cancer improves colorectal cancer-specific survival and overall survival. Some studies show that taking aspirin can reduce the risk of distant metastasis (spread of the cancer to distant organs) by 31%, and by up to as much as 74% in colorectal cancer.

Platelets (cells involved in blood clotting) are believed to protect cancer cells that travel through the blood stream. The HLA class 1 antigen is a protein that can be found on these cancer cells. It is believed that the immune system attacks and eliminates cells with low or absent levels of this protein. As aspirin has antiplatelet activity (and can stop platelets clumping together around these cancer cells), it was thought that aspirin would be useful primarily in colorectal cancers with low or absent levels of the HLA class 1 antigen protein.

The authors aimed to demonstrate that the survival benefit associated with low-dose aspirin (75 – 300 mg once daily) use after a diagnosis of colorectal cancer might depend on the presence of the HLA class I antigen. The study included 999 patients with colorectal cancer who had tumor tissue removed during surgery that was available for examination. 963 patient tumor samples were analysed for HLA class I antigen; 33.2% did not express the protein while 66.8% did express the protein.

Of the 999 patients, 18.2% were defined as aspirin users after diagnosis. By the end of the study, of those who were aspirin users, 37.9% had died compared to 48.5% of those who were non-aspirin users. Aspirin use after diagnosis was associated with 36% improved likelihood of survival.

For those aspirin users whose tumors had the presence of HLA class I antigen, there was a 47% improved likelihood of survival, while no survival benefit was present in those whose tumors did not have the protein present.

The authors concluded that, contrary to belief, the survival benefit associated with low-dose aspirin use after diagnosis of colorectal cancer was present only in those who expressed the HLA class I antigen.