Allergan, however, is in the process of merging with Abbvie in a US$63bn deal and to get EU approval a number of drugs have to be divested.
Brazikumab is one of these due to concerns it might be dropped as its new owner already has blockbuster Humira in its portfolio.
The new arrangement will see Allergan still fund the development costs incurred by AstraZeneca to take brazikumab through the trial process.
Analysts at Shore Capital said the decision was “sensible” as “there is clearly an opportunity in the event of promising clinical data”.
In a separate update, the FTSE 100-listed pharma giant said it sold rights to five medicines used to treat hypertension to private UK company Atnahs Pharma for US$350mln upfront, with optional instalments of up to US$40mln based on sales.
The five drugs together made annual sales of US$132mln in 2018.
Late-stage studies results
It was a busy Monday morning for AstraZeneca as it posted two further updates on ongoing trials.
The company shared “statistically significant and clinically meaningful” results for both Brilinta and Enhertu, which treat heart conditions and stomach cancer respectively.
Brilinta, used for people with coronary artery disease and type-2 diabetes, help people in the early stages of a stroke when used in conjunction with aspirin.
Analysts at Liberum said it is “a small positive” given Brilinta’s patent will remain exclusive to AZN for just another five years, while the Enhertu results were “encouraging” as it is “at least a US$3bn drug by peak sales”.
Cancer drug Enhertu, jointly developed with Japan's Daiichi Sankyo, showed it can improve the survival rate for people suffering from HER2-positive stomach cancer.
HER2 is a protein seen in many types of tumours including stomach, breast and lung.
Shares dipped 2% to 7,544p on Monday morning.