Abstract AZD8186 is a novel potent small molecule that targets the lipid kinase PI3Kβ with selectivity vs PI3Kα. AZD8186 reduces pAKT-S473 in the PTEN deficient MDA-MB-468 cell line with an IC50 <5nM, while in the PI3Kα-dependent PIK3CA mutant line BT474 it is 200 fold less potent. AZD8186 also demonstrates cellular activity versus PI3Kβ, inhibiting IgM-dependent pAKT-S473 with an IC50 of 15nM. In cell line panels, sensitivity to AZD8186 is associated in part with loss of PTEN function suggesting that targeting tumours that have lost normal levels of PTEN function through deletion, mutation or down regulation might enrich for sensitivity to AZD8186. Deficiencies in PTEN are described in a number of tumour types such as prostate, squamous lung, breast, renal and glioblastoma. AZD8186 has single anti-tumour activity in pre-clinical models representing each of these settings, which is associated with dynamic regulation of a number of key pathway biomarkers. Clinically, agents such as AZD8186 are likely to be used in combination. The use of AZD8186 in conjunction with either androgen therapy or docetaxol has been explored in cell lines and pre-clinical disease models. Combining AZD8186 with MDV-3100 resulted in synergistic inhibition of LNCAP prostate tumour cell growth, demonstrating the potential to combine with androgen therapy. Moreover in HCC70 (breast) and PC3 (prostate) tumour xenografts AZD8186 shows increased benefit in combination with docetaxel, demonstrating the ability to combine with cytotoxic chemotherapy. Exploration of dose and schedule revealed that efficacy can be maintained in combination with docetaxol at a lower dose of AZD8186 than required for monotherapy activity. Moreover efficacy was maintained when the frequency of AZD8186 dosing was reduced using a shorter intermittent schedule. Understanding how AZD8186 combines with standards of care for PTEN null disease segments provides important insight into how agents targeting PI3Kβ-dependent tumours may be used most effectively in the clinic. [Current affiliation for S. Cosulich is Novartis, Basel, Switzerland.] Citation Format: Urs Hancox, Sabina Cosulich, Hannah Dry, Lynsey Hanson, Clare Crafter, Bernard Barlaam, Martina Fitzek, Lara Ward, Marie Cumberbatch, Steve Powell, Rebecca Ellston, Mandy Lawson, Steve Wedge, Liz Harrington, Stephen Green, Simon T. Barry. AZD8186: a potent selective inhibitor of PI3Kβ targeting PTEN-deficient tumours dependent on dysregulated PI3Kβ signalling. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3264. doi:10.1158/1538-7445.AM2013-3264