10567 Background: Vorinostat, a Histone Deacetylase (HDAC) inhibitor is a novel targeted antineoplastic agent with promising activity when combined with carboplatin-paclitaxel against NSCLC. The exact molecular mechanism underlying its growth inhibitory and apoptotic effects is not well understood. We investigated the influence of p53 gene status on the interaction of vorinostat and carboplatin (a DNA targeting agent) in various NSCLC cell lines. Methods: NSCLC cells with wild type p53 (A549, 128.88T), mutant p53 (201T) and p53 null phenotype (Calu-1) were used. Cytotoxicity induced by serial dilution of carboplatin in the presence and absence of a fixed dose (1μM) of vorinostat was assessed by MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide] assay. In a separate experiment, cells were also transiently transfected with a p21 promoter-luciferase reporter construct to assess p53 activation following a 24 h exposure to vorinostat, carboplatin, or vorinostat/Carboplatin combination. Luciferase activity was quantified by luminometry and corrected for total protein. Results: Vorinostat displayed single agent activity in each cell line, with greater growth inhibition observed in the p53 mutant and null cells. Synergistic interactions between carboplatin and vorinostat were observed in p53 wild type cells and the IC50 for carboplatin was reduced 3- to 5-fold. In contrast, the interaction between vorinostat and carboplatin was additive or less than additive in p53 mutant and p53 null cells. Vorinostat also increased expression of the p21 reporter construct in each of the cell lines. Conclusions: Vorinostat regulates p21 gene expression and elicits anti-tumor activity in NSCLC cells independent of their p53 status. Vorinostat potentiates carboplatin-induced cytotoxicity in NSCLC with wild type p53 but not p53 deficient cells, suggesting involvement of a p53 dependent pathway. The addition of vorinostat may allow for a reduction in standard dose of carboplatin with improvement in overall therapeutic index. A phase II/III clinical trial is in progress to evaluate vorinostat in combination with carboplatin-based regimen in advanced NSCLC. Support: CA099168–01, ASCO Foundation CDA No significant financial relationships to disclose.