PAC-1 activates procaspase-7 in a less efficient manner with EC50 of 4.5 μM. Elevated caspase 3 level in cancer cell lines allows PAC-1 to selectively induce apoptosis in a manner proportional to procaspase-3 concentration with IC50 of 0.35 μM for NCI-H226 cells to ~3.5 μM for UACC-62 cells. PAC-1 induces apoptosis in the primary cancerous cells with IC50 values of 3 nM to 1.41 μM, more potently than in the adjacent noncancerous cells with IC50 of 5.02 μM to 9.98 μM, which is also directly related to the distinct procaspase-3 concentration. PAC-1 activates procaspase-3 by chelating zinc ions, thus relieving the zinc-mediated inhibition and allowing procaspase-3 to auto-activate itself to caspase-3. PAC-1 is capable to induce cell death in Bax/Bak double-knockout cells and Bcl-2 and Bcl-xL-overexpressing cells with the same efficacy as its wild-type counterpart in a delayed manner. PAC-1 induces cytochrome c release in a caspase-3 independent manner, which subsequently triggers downstream caspase-3 activation and cell death. PAC-1 can not induce cell death and caspase-3 activation in Apaf-1 knockout cells, suggesting that apoptosome formation is essential for caspase-3 activation by PAC-1-mediated cell death.
In vivo
Administration of PAC-1 at 5 mg with low and steady releasing significantly inhibits the growth of ACHN renal cancer xenograft in mice. Oral administration of PAC-1 (50 or 100 mg/kg) significantly retards tumor growth of NCI-H226 lung cancer xenograft in a dose-dependent manner, and markedly prevents the cancer cells from infiltrating the lung tissue. The in vivo anti-tumor effect of PAC-1 is ascribed to procaspase-3 activation and subsequently apoptosis induction consistent with the activity in vitro.
