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Resistance to imatinib mesylate is an emergent problem in the treatment of Bcr-Abl expressing myelogenous leukemias and additional therapeutic strategies are required. We observed that galangin, a non-toxic, naturally occurring flavonoid was effective as anti-proliferative, and apoptotic agent in Bcr-Abl expressing K562 and KCL22 cells and in imatinib mesylate resistant K562-R and KCL22-R cells. Galangin induced an arrest of cells in G0–G1phase of cell cycle and a decrease in pRb, cdk4, cdk1, cycline B levels; moreover, it was able to induce a monocytic differentiation of leukemic Bcr-Abl+ cells. Of note, galangin caused a decrease in Bcl-2 levels and markedly increased the apoptotic activity of imatinib both in sensitive or imatinib-resistant Bcr-Abl+ cell lines. In contrast, flavonoids unable to modify the Bcl-2 intracellular levels, such as fisetin and chrysin, did not increase the apoptotic effect of imatinib. These data suggest that galangin is an interesting candidate for a combination therapy in the treatment of imatinib-resistant leukemias.

Pterostilbene and 3,5-hydroxypterostilbene are the natural 3,5-dimethoxy analogs of trans-resveratrol and piceatannol, two compounds which can induce apoptosis in tumor cells. In previous studies we demonstrated the importance of a 3,5-dimethoxy motif in conferring pro-apoptotic activity to stilbene based compounds so we now wanted to evaluate the ability of pterostilbene and 3,5-hydroxypterostilbene in inducing apoptosis in sensitive and resistant leukemia cells. When tested in sensitive cell lines, HL60 and HUT78, 3'-hydroxypterostilbene was 50-97 times more potent than trans-resveratrol in inducing apoptosis, while pterostilbene appeared barely active. However, both compounds, but not trans-resveratrol and piceatannol, were able to induce apoptosis in the two Fas-ligand resistant lymphoma cell lines, HUT78B1 and HUT78B3, and the multi drug-resistant leukemia cell lines HL60-R and K562-ADR (a Bcr-Abl-expressing cell line resistant to imatinib mesylate). Of note, pterostilbene-induced apoptosis was not inhibited by the pancaspase-inhibitor Z-VAD-fmk, suggesting that this compound acts through a caspase-independent pathway. On the contrary, 3'-hydroxypterostilbene seemed to trigger apoptosis through the intrinsic apoptotic pathway: indeed, it caused a marked disruption of the mitochondrial membrane potential delta psi and its apoptotic effects were inhibited by Z-VAD-fmk and the caspase-9-inhibitor Z-LEHD-fmk. Moreover, pterostilbene and 3'-hydroxypterostilbene, when used at concentrations that elicit significant apoptotic effects in tumor cell lines, did not show any cytotoxicity in normal hemopoietic stem cells. In conclusion, our data show that pterostilbene and particularly 3'-hydroxypterostilbene are interesting antitumor natural compounds that may be useful in the treatment of resistant hematological malignancies, including imatinib, non-responsive neoplasms.

Considering that the stereochemistry of the C9-C10 alkenyl portion of natural 9-cis-RA, as the one of the olefinic moiety of the previously described isoxazole retinoid 4, seems of particular importance for their apoptotic activity, we prepared a novel class of TTNPB analogues bearing both the cis or trans configuration of the alkenyl portion. The compounds were evaluated in vitro for their cytotoxic and apoptotic activities. We discovered that the cis-TTNPB 9c possesses apoptotic activity comparable with that of the retinoid 4. Moreover, the amino arotinoid 16c showed potent apoptotic activity in HL60 promyelocytic leukemia cells. Interestingly, 16c proved to be a particularly potent apoptosis-inducing agent active in multidrug resistant (MDR) cell lines. Therefore, to the best of our knowledge, 16c may represent the first known aminoarotinoid endowed with potent apoptotic activity in MDR cells. Taken together, these results seem to point out that the cis-stilbene motif of arotinoids may be at least an important feature in conferring cytotoxic and apoptotic activity to this class of compounds.