G3139 (oblimersen) may inhibit prostate cancer cell growth in a partially bis-CpG-dependent non-antisense manner.

G3139 is an 18-mer phosphorothioate oligodeoxyribonucleotide, which is targeted to the initiation codon region of the bcl-2mRNA. Although treatment of PC3 prostate cancer cells with G3139, which contains two CpG motifs, causes a dramatic decrease in bcl-2 protein expression after 3 days, it did not result in significant cellular apoptosis, as it does in many other cell lines. The absence of apoptosis was demonstrated by the absence of pro-caspase 3 cleavage products and of Annexin V cell surface expression. In addition, ATP production and the mitochondrial membrane potential DeltaPsim were preserved. Despite this, G3139 significantly inhibited the rate of cellular proliferation in complete media and blocked cloning in soft agar. G4232, a variant of G3139 that down-regulates bcl-2 expression to the same extent but has both CpG cytidines C5 methylated, was only minimally antiproliferative. A series of mismatched G3139-related oligomers were synthesized that could also substantially down-regulate bcl-2 protein expression, but only if the CpG motifs were preserved, demonstrating the presence of additional non-antisense mechanisms. G3139 caused production of reactive oxygen species in growth-arrested cells and oxidation of nuclear guanosine to 8-hydroxy-2'-deoxyguanosine, as determined by 1F7 monoclonal antibody staining. Bromodeoxyuridine incorporation studies demonstrated that G3139 induced a G1-S entry block and an intra-S-phase block in PC3 cells that persisted as long as 3 days. This finding coincides with the observation that expression of several proteins encoded by S-phase genes, including c-myb and poly(ADP-ribose) polymerase, were significantly reduced. These results illustrate the complexity of the mechanism of action of G3139 in PC3 cells.