loss of PTEN expression somewhat improved the growth potential of BT474 cells when handled at clinically relevant doses of lapatinib, which correlates with a rise in AKT activity.loss of PTEN expression also abrogated trastuzumab sensitivity. Critically, a second non overlapping shRNA able to suppressing Ubiquitin conjugation inhibitor PTEN phrase, also conferred resistance to lapatinib and trastuzumab therefore arguing against an off-target impact. An shRNA targeting GFP was used as a negative control in every Eichhorn et al. Page 4 Cancer Res. Writer manuscript, available in PMC 2009 November 15. Findings. Apparently, treatment with both trastuzumab and lapatinib conferred an enhanced response to the growth potential of HER2 positive cells in comparison to either treatment alone, confirming the of others which have indicated that combining lapatinib with trastuzumab boosts their biological effect. However, while combination therapy with lapatinib and trastuzumab minimal cellular growth in PTEN knockdown cells, viable cells remained. To investigate the sensitivity of the PTEN knockdown cell lines to the various HER2 targeted therapies we analysed the growth potential RNA polymerase of PTEN knockdown cells when treated with trastuzumab, lapatinib or both for four weeks. Treatment with HER2 focused treatments fully inhibited the proliferation potential of control cells. However, the ablation of PTEN expression in BT474 cells reduced the growth inhibitory properties of both trastuzumab and lapatinib. Collectively these data claim that PTEN expression is required for both lapatinib and trastuzumab sensitivity in cells. As has previously been noted lapatinib progress inhibition correlates with downregulation of HER2 dependent PI3K signalling. Thus, in order to examine the results of lapatinib on PI3K signalling in cells which lack PTEN action, we handled BT474 cells or BT474 PTEN depleted cells with lapatinib. Indeed, related pifithrin alpha to trastuzumab, there was an important downregulation in AKT473 phosphorylation in lapatinib treated control cells compared to untreated control cells. In contrast downregulation of AKT phosphorylation was attenuated in lapatinib treated PTEN knockdown cells when compared with lapatinib treated controls. However, unlike trastuzumab, no change was noticed in MAPK phosphorylation upon treatment with lapatinib. In addition, treatment of cells with both trastuzumab and lapatinib resulted in an additive inhibitory influence on AKT activity suggesting that trastuzumab and lapatinib may function through partially non overlapping systems to disrupt HER2 dependent PI3K signalling. The accepted dose in patients of lapatinib when utilized in combination with capecitabine is just a daily dose of 1250mg. This serving in a small plasma drug concentration of approximately 500 nM. For that reason to try if lapatinib sensitivity can be overcome by PTEN loss at clinically relevant concentrations we conducted a colony formation assay.