Synergistic anti-tumor efficacy of doxorubicin and flavopiridol in an in vivo hepatocellular carcinoma model

A previous study showed that flavopiridol increased doxorubicin sensitivity in hypoxic hepatocellular carcinoma (HCC) cells by increasing apoptosis through suppressing hypoxia-inducible N-myc downstream-regulated gene-1 (NDRG1) expression. However, this has not been investigated in an in vivo HCC model. Therefore, we aimed to elucidate whether the combination of doxorubicin and flavopiridol has a synergistic anti-tumor effect in an in vivo HCC model. An HCC mouse model was established by implanting C3H/He mouse with MH134 cells. Then, doxorubicin with or without flavopiridol was injected. The anti-tumor efficacy was assessed by evaluating tumor volumes, and the underlying mechanism was investigated by quantifying apoptotic cells, the Ki-67 proliferation index, and microvessel densities (MVDs). Immunohistochemistry of NDRG1 was performed to determine the underlying mechanism. Tumor growth was significantly suppressed in the doxorubicin + flavopiridol combination group compared to the other three groups. The percentage of apoptotic cells was significantly higher, and Ki-67-positive proliferating cells were significantly lower in the combination group compared to the other groups; however, MVDs were not significantly different across the groups. Increased apoptosis by flavopiridol occurred by suppressing hypoxia-inducible NDRG1 expression. These results show that a combination of doxorubicin and flavopiridol has a synergistic anti-tumor effect in an in vivo HCC model. This synergistic effect of combination therapy was attributed to increased apoptosis and decreased proliferation of tumor cells rather than decreased angiogenesis. These findings suggest that flavopiridol might be an effective adjuvant therapy to doxorubicin-resistant HCC cells by inducing apoptosis through suppression of NDRG1 expression.