PHA-848125 is a novel
cyclin-dependent kinase inhibitor under Phase I/II clinical investigation. In this study, we describe, for the first time, the effect of
PHA-848125 on human
melanoma cells in vitro. Seven
melanoma cell lines with different sensitivity to
temozolomide (TMZ) were exposed to
PHA-848125 for 5 days and then assayed for cell growth. In all cases, including TMZ-resistant cells,
PHA-848125 IC(50) values were significantly below the maximum plasma concentrations achievable in the clinic. In the most PHA-848125-sensitive cell line, the
drug caused a concentration-dependent G(1) arrest.
PHA-848125 also impaired phosphorylation of the
retinoblastoma protein at CDK2 and CDK4 specific sites, decreased
retinoblastoma protein and
cyclin A levels, and increased p21(Cip1), p27(Kip1) and p53 expression. Combined treatment with fixed ratios of TMZ plus
PHA-848125 was studied in three
melanoma cell lines.
PHA-848125 was added to the cells 48 h after TMZ and cell growth was evaluated after 3 additional days of culture. Parallel experiments were performed in the presence of
O(6)-benzylguanine (BG), to prevent repair of methyl adducts at O(6)-guanine induced by TMZ.
Drug combination of TMZ plus BG and
PHA-848125 produced additive or synergistic effects on cell growth, depending on the cell line. In the absence of BG, the combination was still more active than the single agents in the cell line moderately sensitive to TMZ, but comparable to
PHA-848125 alone in the two TMZ-resistant cell lines. When TMZ plus BG were used in combination with
PHA-848125 against cultured normal melanocytes, neither synergistic nor additive antiproliferative effects were observed. Our results indicate that
PHA-848125 can have a therapeutic potential in
melanoma patients, alone or combined with TMZ. Moreover this agent appears to be particularly attractive on the bases of its effectiveness against TMZ-resistant
melanoma cells.