ALDH7A1 and glioblastoma: Notably, although all the PDE inhibitors with high antiproliferative potency in our experiments caused the elevation of both the cAMP and cGMP concentrations, a general increase in the cyclic nucleotide levels on their own was likely a necessary but not sufficient condition for suppressing glioblastoma cell proliferation, because several PDE inhibitors failed to affect cell survival despite they increased cAMP and cGMP levels.