CREB1 and neoplasm: However, significant reductions in phospho-GSK3b, CREB, MEK and RSK1 were observed in tumours that were growing in the presence of palbociclib (two-cycles) compared to vehicle (p = 0.03, p = 0.04, p = 0.04, p = 0.02, respectively), suggesting that there is activation of alternative pathways compared to untreated control tumours (Figure 8).