ALDH7A1 and glioblastoma: Fewer PDE inhibitors altered cGMP levels: only trequinsin, sildenafil, tadalafil and zaprinast increased cGMP levels by >25%, whereas lower, but still significant, increases in cGMP concentrations were caused by ibudilast, MY-5445, TC3.6, BAY 73-6691, PQ10, papaverine, PF-04449613, PF-2545920, TC-E 5005, BC 11-38 and IBMX in at least one of the two glioblastoma cell lines tested (Figure 3B).