Predictive factors for successful growth of patient derived xenografts (PDX).

e15069 Background: PDXs have become a core component of translational cancer research. Developing these models can become challenging since little is known about which factors influence engraftment rates. We sought to determine which clinical, pathological, or molecular factors may predict better engraftment rates in PDXs. Methods: Between March 2017 and January 2021, biopsies obtained from patients with primary or metastatic cancer were implanted into athymic nude mice. Statistical analyses were performed to identify factors that could correlate with final engraftment defined as achievement of at least three passes and sampling of PDXs tumors. We focused on clinical (patient factors) pathological (patients’ tumor samples) and molecular characteristics (patients’ tumor samples) analyzed either by immunohistochemistry (IHC) or next generation sequencing (NGS). Results: 585 tumor samples were collected and implanted. 21 failed to engraft due lack of malignant cells. Of 564 tumor-positive samples, 187 (33.2%) PDXs achieved successful growth at time of analysis (Feb, 21). The following clinical characteristics were correlated with engraftment: systemic antibiotics within 2 weeks of sampling: (38.1% (72/117) antibiotics- group vs 30.7% (115/260) no-antibiotics) (p = 0.048); systemic steroids within 2 weeks (41.5% (34/48) the steroids-receiving group vs 31.7% (153/329) no-steroids) (p: 0.05). For women, menopausal status was predictive: 34.9% (95/177) in postmenopausal achieved growth, Vs 20,4% (10/39) for premenopausal (p = 0.031). Baseline LDH levels: 74.9% (140/187) LDH levels above the upper limit of normality (ULN) against 25.1% (47/187) with normal LDH (p = 0.034). Tumor grade: Grade 1: 25.4% (47/187); grade 2: 34.8% (65/187) and grade 3: 40.1% (75/187) tumors achieved successful growth (p = 0.043). Similarly, higher ki67 levels were also correlated with better engraftment rates: (low (Ki67 < 15%): 8.9% (9/45) achieved growth, Vs high (Ki67 > 15%): 31% (35/113) (p:0.002). Presence of lymphovascular invasion in tumor sample was also predictive: 42.2% (97/230) with lymphovascular Vs 26.9% (90/334) of samples with no invasion (p = 0.0001). Likewise, 41.8% (59/141) of neural invasion-positive samples achieved growth against 30.3% (128/428) (p = 0.008). Mismatch repair deficient tumors showed better engraftment rates: 62.1% (18/29) achieved growth vs 40.8% (75/184) of proficient tumors (p = 0.026). 84 PDX were breast models, among which 57.9% (11/19) ER negative models grew, Vs 15.4% (10/65) of ER positive models (p = 0.0001). Conclusions: tumors with higher grade and Ki67, lymphovascular and/or perineural invasion, with dMMR and ER expression negative have higher chance of PDX development. Some clinical characteristics can also interfere with PDXs development such as use of steroids or antibiotics prior sampling.