Pancreatic cancer is more malignant and aggressive than other cancers. 25. In cases of locally advanced or unresectable pancreatic cancer, chemotherapy is the mainstay of treatment. Surgery following neoadjuvant chemoradiotherapy has recently been used as the standard treatment, even for resectable pancreatic cancer 26. Therefore, chemotherapy is expected to play a more central role in the treatment of pancreatic cancer in the future. In clinical cases, pancreatic cancer cells are histologically surrounded by a dense stroma and are categorized as hypovascular tumors. These characteristics have been proposed as the cause of the poor prognosis of patients with pancreatic cancer. The results of the PDX-M assay revealed that apoptosis was induced in cells on the surface of tumors treated with PGV-1. On the other hand, viable tumor cells remained in the center of tumors. Furthermore, the results of the PDOX assay revealed no significant difference in terms of tumor weights between the control and PGV-1 groups. These results indicated that part of the tumor or the implantation site was affected by the low perfusion of blood. Moreover, histological findings obtained from the HE staining of tumors in the PDX-M assay revealed not only cancer cells, but also stroma components. Therefore, for a chemotherapeutic drug to reach and penetrate pancreatic cancer cells, a sufficient amount of the drug needs to be administered. However, clinical trials demonstrated that higher doses of chemotherapeutic drugs were associated with an increased incidence of side effects with negligible therapeutic benefits 27 28. Therefore, transformative chemotherapeutic agents with sufficient efficacy and fewer side effects need to be established in order to improve the prognosis of patients with pancreatic cancer.
Pancreatic cancer cells in a patient’s body differ from pancreas cancer cell lines cultured in vitro in terms of heterogeneity and the microenvironment, and, thus, it is crucial to assess the efficacy of novel compounds with discretion. The results of the CDX-M assay showed that PGV-1 suppressed tumor growth and TGI was 96.6%. In contrast, PGV-1 inhibited tumor growth and TGI decreased to 54.8% in the PDX-M assay. In the CDX-M assay with pre-formed tumors, TGI for PGV-1 monotherapy, gemcitabine monotherapy, and combination therapy with PGV-1 and gemcitabine were 64.4, 75.0, and 99.8%, respectively, whereas in the PDX-M assay with pre-formed tumors, TGI were 35.5, 40.3, and 81.3%, respectively. These results clearly showed differences in the efficacy of drugs between the CDX-M and PDX-M assays, and indicate that the results obtained from cell line-based experiments are not sufficient to evaluate the efficacy of chemotherapeutic candidates.
A previous study reported that neoadjuvant chemotherapy induced somatic mutations 29. Therefore, it is difficult to use chemotherapy-treated samples for the accurate evaluation of chemotherapeutic candidates because the additional mutations caused by chemotherapy may alter drug sensitivity. Regarding gemcitabine, since gemcitabine treatments induce resistance, it is inappropriate to use gemcitabine-treated specimens to evaluate the efficacy of gemcitabine. Resected samples of pancreatic cancer without neoadjuvant chemotherapy were used in the present study to establish PDX-M, thereby avoiding these issues. Moreover, the success rate of engraftment for PDX-M was shown to decrease to approximately 30% when a resected sample of pancreatic cancer that had been treated with neoadjuvant chemotherapy was used 30. Therefore, neoadjuvant chemotherapy may induce apoptosis in tumor cells and fibrosis. In terms of accurate evaluations and successful establishment, it is ideal to use a resected sample that has never been exposed to neoadjuvant chemotherapy.
In the CDX-M assay with pre-formed tumors, tumors were significantly larger in PGV-1-treated mice than in gemcitabine-treated mice after six days, whereas no significant differences were observed in tumor volumes between mice treated with PGV-1 and gemcitabine in the PDX-M assay with pre-formed tumors on day 6. These results may be attributed to different genetic mutations in the CDX- M and PDX-M assays; however, the genotypes of major pancreatic cancer driver genes did not markedly differ between cells used in the CDX- M and PDX-M assays (Supplementary Table 2), suggesting that differences other than genetic mutations may have contributed to these responses.
In translational cancer research, many chemotherapeutic candidates have been investigated for use in combination therapy with gemcitabine, and some have successful exerted synergistic effects as in preclinical models 31 32 33. Recent studies demonstrated that combination therapy with curcumin/curcumin-related compounds and gemcitabine exerted stronger anti-tumor effects than monotherapy with each reagent 34 35, while other studies reported no synergistic effects for combination therapy with gemcitabine 36. The present results from CDX-M and PDX-M assays showed that combination therapy with PGV-1 and gemcitabine suppressed tumor growth more effectively than each monotherapy.
There is currently no explanation for the high efficacy of combination therapy with PGV-1 and gemcitabine; however, PGV-1 may inhibit the acquisition of gemcitabine resistance in addition to its tumor-suppressing ability. PGV-1 and gemcitabine may also act on different points of the cell cycle (PGV-1 for the M phase and gemcitabine for the S phase), thereby preventing resistance. The combination of reagents that inhibit different points of the cell cycle, namely, the combination of gemcitabine and nab-paclitaxel, has been applied. However, since nab-paclitaxel causes severe neutropenia and peripheral neuropathy by inhibiting the effects of microtubulin, severe side effects caused by both drugs are intolerable. PGV-1 did not induce side effects, such as weight loss and myelosuppression. Furthermore, it did not enhance side effects caused by gemcitabine. Therefore, PGV-1 is a good candidate for combination therapy with gemcitabine. Furthermore, PGV-1 was effective when orally administered, which reduces the burden on patients. In clinical cases, PGV-1 as monotherapy is suitable for the prevention of postoperative recurrence in patients with pancreatic cancer. PGV-1 in combination therapy with gemcitabine is suitable for locally advanced or unresectable cases of pancreatic cancer. In conclusion, we conducted experiments using multiple xenograft assays to investigate the potential efficacy of PGV-1 as both monotherapy and combination therapy. PGV-1 is a highly potent chemotherapeutic candidate for pancreatic cancer. We propose that PGV-1 needs to be pharmaceutically developed as an orally administered drug for the treatment of pancreatic cancer.