M. Okano1, M. Oshi1, K. Takabe1 1Roswell Park Cancer Institute,Surgical Oncology,Buffalo, NY, USA
Introduction: Patient-Derived Xenograft (PDX) has come into the limelight of breast cancer research to be used for pre-clinical studies. Some of its weaknesses are its poor engraftment rates and slow growths, which often limits its use as an avatar of the donor patient. Therefore, improvement of the models especially in engraftment and tumor growth are in urgent need. We hypothesized that orthotopically implanted tumors (Ortho) engraft better, grow faster and larger compared from subcutaneously implanted PDX (SQ), which is the standard model.
Methods: NSG mice were used to generate PDX. 2 tumors were derived from brain metastasis (B-met), and the others were from primary breast cancers. 3 tumors were ER(+)HER2(-) and 7 tumors were triple negative (TN). Both of B-met tumors are ER(+)HER2(-). Tumor “engraftment” was defined as tumorigenesis of palpable tumor after implantation regardless of time it took.
Results:The overall engraftment rate was significantly better in Ortho than SQ (77.8% (n=137/176) vs. 50.7% (n=79/156), p<0.01). Ortho tumors grew remarkably larger than SQ tumors. The mean tumor weight was significantly heavier in Ortho than SQ (0.75g vs 0.14g, p<0.01). Ortho tumors demonstrated more abundant mitotic figures compared with SQ tumors (19.2 vs 7.9, p<0.01). Ortho tumors had more Ki-67 positive cells than SQ tumors (31.5 vs 21.8, p=0.015). The tumor weight was significantly larger when implantation was made to the 2nd or to the 4th mammary fat pad (0.73g vs 0.96g, p=0.02). Tumor engraftment of 1st generation was low (24.8% (n=32/129)), but the rate of 2nd (82.2% (n=46/56)) and 3rd (80.6% (n=58/72)) generation was significantly increased (p<0.001). The time it took for the 1st generation to grow was the longest between 3 generations (1st; 152days, 2nd; 66days, 3rd; 63days, p<0.01). The mean tumor weight was significantly higher in Ortho sites among all generations (1st -3rd) of TN cancer (0.2g, 1.1g vs 0.8g, respectively, p<0.01). ER positive cancer xenograft revealed significantly lower engraftment rate (26.7% (n=12/45) vs 65.1% (n=216/332), p<0.01), slower tumor growth, and lighter tumor weight (0.18g vs 0.47g, p<0.001) than TN xenograft. The xenograft from brain-metastasized breast cancer also showed higher engraftment rate in MP than SQ (94.4% (n=68/72) vs. 69.4% (n=50/72), p<0.01) although was not the organ that brain metastasis originally grew. The brain metastasis tumors also demonstrated higher tumor weight in MP than SQ (0.28g vs 0.54g, p<0.001). The brain metastasis tumors grew faster than primary tumor (52days vs 95days, p<0.01).
Conclusion:Orthotopical implantation showed better take rate, greater tumor size and weight than heterotopic implantation, regardless of the cancer subtypes and their sources.