Recently, resistance of colon cancer cells to the HDAC-inhibitor butyrate has been demonstrated to be coupled to high Akt levels. The molecular mechanism responsible for VPA non-responsiveness is not yet clear. Based on in vivo results, two hypotheses are conceivable: 1) RCC cells are initially equipped with a huge mass of highly activated Akt, which counteracts the CP-690550 antigrowth potential of VPA exerted by HDAC-inhibition. Since altering the Akt level in RCC cells in vitro did not influence the efficacy of VPA to diminish growth, this hypothesis seems unlikely. 2) Chronic VPA application induces Akt elevation in RCC cells over time, finally leading to drug non-responsiveness. The in vitro studies presented here, conducted with therapeutically relevant VPA concentrations, provide evidence that Akt rises with longterm VPA treatment of RCC cells, which negatively correlates with the capacity of VPA to stop cancer growth. In another experimental setting, prolonged exposure of gastric cancer cells to increasing concentrations of butyrate resulted in the acquisition of resistance, which was accompanied by Akt up-regulation. Furthermore, the sensitivity of lung adenocarcinoma cells to the HDAC-inhibitor FK228 inversely depends on the Akt signaling pathway. Therefore, the RCC cells may establish undesired feedback loops in the presence of VPA. Akt may serve as the dominant counter regulator, finally enabling the cancer cells to restart their growth program. HDAC and histone analysis were done after 10 weeks of chronic VPA exposure. No differences in HDAC expression were seen in treated versus non-treated animals. A moderate reduction of total and acetylated histone H3 was evident in responders, whereas the H3 and aH3 level were not altered in the VE-822 clinical trial non-responders, compared to the control. The tumor suppressor and Akt-inhibitor PTEN, additionally evaluated, was down-regulated in VPA non-responders. Most importantly, a massive up-regulation of Akt was evoked in the non-responders. In responders there was a small increase of pAkt, whereas total Akt was strongly diminished. This opens the question of whether combined inhibition of HDAC and Akt may prevent VPA driven resistance induction. Chronic application of either VPA or the mTOR-inhibitor everolimus has caused drug non-responsiveness in RCC cells, which however, could be prevented when both agents were used in combination. A novel strategy has been provided by Qian and coworkers to overcome the dynamic and adaptive natures of tumor cells. They constructed a dual-acting compound by incorporating HDAC inhibitory functionality into an Akt inhibitor pharmacophore. Greater growth inhibition and proapoptotic activity than single-target Akt or HDAC inhibitors in both cultured and implanted cancer cells was shown.