Indoleamine 2,3 dioxygenase 1 (IDO 1) is the major enzyme catabolising the Tryptophan outside the liver. It has been shown that its plays a important role in generating a immunosuppressive micro-environment in tumors. IDO expression has been shown by Hennequart et al. to be driven by Cyclooxygenase-2 (COX-2) expression. The investigator's team also shown that anti-COX2, celecoxib, can in a xenograft models of ovarian cancer decrease IDO1 expression and result in an infiltration of the tumor by T cells. The investigator proposed then to conduct a proof of concept study to evaluate the effect of pre-operative short administration of Celecoxib on IDO expression and Immune cells tumors infiltration, in patients with endometrial cancer. Indeed, this tumor type is well known to express frequently a high level of IDO.

Indoleamine 2,3-dioxygenase 1 (IDO1) is an enzyme that regulates immune responses by degrading tryptophan, which is required for efficient T-cell activation. IDO1 expression is limited in normal tissues but is induced by IFN-gamma in inflammatory tissues, to prevent excessive T-cell activity. The investigator's collaborators previously reported that IDO1 is expressed in many human tumours, and that pharmacological inhibition of IDO1 leads to immune rejection of IDO1+ mouse tumours. Based on these results, IDO1 inhibitors are now in clinical development. A first inhibitor, Epacadostat, showed encouraging results combined with anti- Programmed Cell Death -1 (PD-1) antibodies. Tumoural expression of IDO1 can be induced by IFN-gamma, which is produced by tumour-infiltrating lymphocytes (TIL), and represents a mechanism of adaptive immune resistance. However, other tumours express IDO1 in the absence of TIL and Interferon-gamma (INF gamma). This seems to be particularly frequent in endometrial carcinoma. This constitutive IDO1 expression prevents T-cell infiltration and represents a mechanism of intrinsic immune resistance. The investigator's collaborators recently showed that constitutive tumoural expression of IDO1 was triggered by cyclooxygenase-2 (COX-2) and mediated by autocrine prostaglandin-E2 (PGE2) signaling via the Protein Kinase C (PKC) and phosphoinositide 3-kinase (PI3K) pathways. Constitutive IDO1 expression was reduced by COX-2 inhibitors in vitro. Celecoxib is a well-known and widely used anti-inflammatory drug. It is a specific inhibitor of COX-2. In vivo, celecoxib induced immune rejection of IDO1-expressing human tumour xenografts, while reducing IDO1 expression and promoting T-cell infiltration. These preclinical results suggest that COX-2 inhibition in patients carrying tumours expressing IDO1 constitutively will decrease IDO1 expression, increase T-cell infiltration and might increase responsiveness to anti-PD-1/ Programmed Cell Death Ligand-1(PD-L1) therapy and thereby exert enhanced anti-tumour activity. The investigators wish to obtain clinical evidence that celecoxib can induce these first two effects

Key Inclusion Criteria:

• Women > 18 years of age.
• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2
• Adequate renal, hepatic and haematologic functions as defined by laboratory parameters.
• Agrees to undergo additional endometrial biopsies for scientific purposes.

Key Exclusion Criteria:

• Known hypersensitivity or intolerance to celecoxib
• Active, known or suspected autoimmune disease. Vitiligo, type I diabetes mellitus, residual hypothyroidism controlled by hormone substitution therapy, psoriasis not requiring systemic treatment, or conditions not expected to recur in the absence of an external trigger are allowed.
• Positive hepatitis B or C, HIV, and pregnancy tests.
• Immunosuppressive treatment