Background Adenosine is a metabolite that suppresses antitumor immune response of T and NK cells via extracellular binding to the two subtypes of adenosine-2 receptors, A(2)ARs. While blockade of the A(2A)ARs subtype effectively rescues lymphocyte activity, with four A(2A)AR antagonists currently in anticancer clinical trials, less is known for the therapeutic potential of the other A(2B)AR blockade within cancer immunotherapy. Recent studies suggest the formation of A(2A)AR/A(2B)AR dimers in tissues that coexpress the two receptor subtypes, where the A(2B)AR plays a dominant role, suggesting it as a promising target for cancer immunotherapy. Methods We report the synthesis and functional evaluation of five potent A(2B)AR antagonists and a dual A(2A)AR/A(2B)AR antagonist. The compounds were designed using previous pharmacological data assisted by modeling studies. Synthesis was developed using multicomponent approaches. Flow cytometry was used to evaluate the phenotype of T and NK cells on A(2B)AR antagonist treatment. Functional activity of T and NK cells was tested in patient-derived tumor spheroid models. Results We provide data for six novel small molecules: five A(2B)AR selective antagonists and a dual A(2A)AR/A(2B)AR antagonist. The growth of patient-derived breast cancer spheroids is prevented when treated with A(2B)AR antagonists. To elucidate if this depends on increased lymphocyte activity, immune cells proliferation, and cytokine production, lymphocyte infiltration was evaluated and compared with the potent A(2A)AR antagonist AZD-4635. We find that A(2B)AR antagonists rescue T and NK cell proliferation, IFN gamma and perforin production, and increase tumor infiltrating lymphocytes infiltration into tumor spheroids without altering the expression of adhesion molecules. Conclusions Our results demonstrate that A(2B)AR is a promising target in immunotherapy, identifying ISAM-R56A as the most potent candidate for A(2B)AR blockade. Inhibition of A(2B)AR signaling restores T cell function and proliferation. Furthermore, A(2B)AR and dual A(2A)AR/A(2B)AR antagonists showed similar or better results than A(2A)AR antagonist AZD-4635 reinforcing the idea of dominant role of the A(2B)AR in the regulation of the immune system.