While aspirin has shown potential cancer prevention effects in some studies, a landmark Australian trial of low dose aspirin in older people has shown it may promote growth of existing or undiagnosed cancers.

Recent findings from the ASPREE (ASPirin in Reducing Events in the Elderly) study showed a surprising increase in the secondary endpoint of all-cause mortality, primarily due to excess cancer deaths.

Now a new analysis of the ASPREE data has shown that the increase in cancer deaths was not due to an increase in overall incident cancers but mostly to incident metastatic cancers.

Presented at the Digestive Disease Week 2019 meeting in San Diego, the findings come from a prospective randomised controlled trial that recruited 19,000  people over the age of 70, mostly from Australian primary care practices (16,703) in 2010-14. Participants were healthy elderly people but a previous cancer diagnosis was not a barrier to enrolment.

Participants were randomised to daily low dose aspirin (100 mg) or placebo. After a median of 4.7 years of treatment, there were 981 cancer events in the aspirin arm and 952 in the placebo arm. Cancer was the major contributor to the higher all-cause mortality in the aspirin group, and cancer-related death occurred in 3.1% of the participants in the aspirin group and in 2.3% of those in the placebo group.

The new analysis showed there was no significant difference between groups for all incident cancer, all solid tumours, or haematological cancer. There was also no significant difference between groups according to specific tumour types, including colorectal cancer.

Risk of incident non-metastatic cancer was the same in the aspirin and placebo arms (HR=0.99) However, the risk of incident metastatic cancer was elevated with aspirin (HR=1.18; 95% CI,0.96-1.46), although this could be attributable to chance.

The researchers said that if it was a true effect, the increase in the incidence of advanced cancer in the aspirin arm may account for the observed increase in cancer deaths.

Aspirin could have dual effects to prevent a cancer from forming, but also encourage tumour growth, particularly in the elderly, they postulated.

“These data support the possibility that aspirin may adversely affect short-term outcomes among the elderly with undiagnosed cancers (e.g. prevalent tumours at enrolment or early incident tumours),” the study authors said.

The findings mays also reflect differential effects of aspirin on tumour activity according to age, they added.

Longer term follow-up of the low-dose aspirin cohort is warranted, they advised.