Interest in platelet-tumor interactions was reported as early as 1973 in the International Journal of Cancer. Experimental evidence is now able to demonstrate how platelets and coagulation pathways protect tumor cells from the immune system and enable them to attach to vascular endothelium and enhance metastatic spared. A recent paper by Professor Elwood and colleagues reviews the evidence up to August 2017 around the use of low dose aspirin in people with a diagnosis of cancer. A meta-analyses of 71 studies was performed with pooled data covering over 120 000 patients with cancer taking aspirin.
Of the studies included 29 observational studies explored colorectal cancer and post-diagnostic aspirin use, fourteen reported on aspirin and breast cancer and 16 looked at aspirin and prostate cancer. There is also data from 12 other cancer types. Ten of the studies in the meta-analysis also included evidence about metastatic spread.
The nature of observational studies limits the strength of the evidence and randomised controlled trials (RCTs) are now underway with aspirin as an adjunct cancer treatment but it will be 10 years or so before this data is available. As such this meta-analysis of the observational trials presents an important opportunity to start a discussion around whether there is now sufficient evidence to justify a recommendation to consider low-dose aspirin for cancer treatment.
The observational data for colorectal cancer suggests that aspirin is associated with a reduction in colorectal cancer mortality of around 25% and a probable reduction in metastatic spread. There may however be some publication bias in the data available.
The data for breast cancer also shows a 20% reduction in breast cancer mortality as well as a significant reduction in the incidence of metastatic spread. Evidence for a reduction in all-cause mortality is also suggested by the available data.
In addition prostate cancer studies also indicate a reduction in prostate cancer of around 15% as well as a reduction in all-cause mortality. One study was inconsistent with this but it has not been possible to find out the reason for this difference.
The authors conclude that whilst it is important to get data from RCTs and to weight up the risks and benefits of aspirin; “These results give extensive evidence consistent with reductions of about 15-25% in cancer mortality by aspirin.” and call for more discussion and a wider spread of knowledge about aspirin and cancer. For further information please see: Elwood PC, Pickering JE, Morgan G et al. Systematic review update of observational studies further supports aspirin role in cancer treatment: Time to share evidence and decision-making with patients? PLOS/ONE September 25th 2018 https://doi.org/10.1371/journal.pone.0203957