The extracellular pH is lower inside solid tumors than in normal tissue. The acidic environment inhibits the cytotoxicity of lymphocytes in vitro and promotes tumor cell invasion. In the present study, both in vitro and in vivo experiments were conducted to investigate how NaHCO₃ would affect the antitumor activities of cytokine-induced killer (CIK) cells against hepatocellular carcinoma (HCC) cells. For the in vitro experiments, HepG2 cells were cultured at pH 6.5 and 7.4 in the presence of CIK cells or CIK cell-conditioned medium (CM_CIK). For the in vivo experiments, nude mice were xenografted with HepG2-luc cells and divided into four groups: i) CIK cells injection plus NaHCO₃ feeding; ii) CIK cells injection plus drinking water feeding; iii) normal saline injection plus NaHCO₃ feeding; and iv) normal saline injection plus drinking water feeding. The results indicated that the viability and growth rate of HepG2 cells were remarkably suppressed when co-cultured with CIK cells or CM_CIK at pH 7.4 compared with those of HepG2 cells cultured under the same conditions but at pH 6.5. In the xenograft study, a marked synergistic antitumor effect of the combined therapy was observed. NaHCO₃ feeding augmented the infiltration of cluster of differentiation 3-positive T lymphocytes into the tumor mass. Taken together, these data strongly suggest that the antitumor activities of CIK cells against HepG2 cells were negatively affected by the acidic environment inside the tumors, and neutralizing the pH (for example, via NaHCO₃ administration), could therefore reduce or eliminate this influence. In addition, it should be recommended that oncologists routinely prescribe soda water to their patients, particularly during CIK cell therapy.

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