Several folate-drug conjugates are currently undergoing clinical trials for application in oncology. However, the efficacy of folate-targeted therapy strongly depends on the folate receptor (FR) abundance at the surface of cancer cells. Recently, it has been postulated that up-regulation of FRα by means of chemo-sensitizing agents could enhance the anticancer activity of FR-drug conjugates. In this study, we demonstrate in vitro that a combination of dexamethasone (Dexa) and valproic acid (VPA) increases FRα expression selectively at the surface of FR-overexpressing cancer cells. The same stimulation was observed in vivo in KB-tumor xenografts when mice are treated with this combined treatment. This effect is reversible since treatment interruption induces the return of FR expression at basal level. When incubated with Dexa and VPA, the β-galactosidase-responsive folate-monomethyl auristatin E (MMAE) conjugate, called MGAF, exhibits higher cytotoxic activity on several FR-positive human cancer cell lines, compared to its administration as a single agent. This improved toxicity results from the enhanced concentration of MMAE released within cancer cells after internalization and subsequent enzymatic activation of MGAF. Higher deposition of MMAE is also observed in vivo after up-regulation of FR expression level in tumor xenografts, induced by the prior administration of the Dexa/VPA combination. In this model, MGAF/Dexa/VPA combined therapy results in an 81% inhibition of tumor growth compared to the control group, while MGAF used in monotherapy is inefficient. Since Dexa and VPA are currently used in humans, this finding could be of great interest for further development of folate-drug conjugates, in particular for those that are presently under clinical investigation.

几种叶酸-药物偶联物目前正在临床试验中用于肿瘤学。但是，以叶酸为靶标的疗法的疗效在很大程度上取决于癌细胞表面的叶酸受体（FR）丰度。近来，已经假定借助于化学敏化剂上调FRα可以增强FR-药物缀合物的抗癌活性。在这项研究中，我们在体外证明了地塞米松（Dexa）和丙戊酸（VPA）的组合在过表达FR的癌细胞表面选择性地增加FRα表达。在体内观察到相同的刺激当采用这种联合治疗方法治疗小鼠时，其在KB肿瘤异种移植中的作用。这种效果是可逆的，因为治疗中断会导致基础水平的FR表达恢复。与Dexa和VPA一起孵育时，与作为单一药剂给药的β-半乳糖苷酶反应性叶酸-单甲基澳瑞他汀E（MMAE）缀合物，称为MGAF，对几种FR阳性人类癌细胞系表现出更高的细胞毒活性。这种改善的毒性是由于内化和随后的MGAF酶促活化后癌细胞内释放的MMAE浓度增加所致。在体内也观察到更高的MMAE沉积事先给予Dexa / VPA组合诱导后，肿瘤异种移植物中FR表达水平上调。在该模型中，与对照组相比，MGAF / Dexa / VPA联合疗法对肿瘤生长的抑制作用为81％，而单药疗法中使用的MGAF效率低下。由于Dexa和VPA目前已用于人类，因此这一发现可能对叶酸-药物结合物的进一步开发具有重大意义，特别是对于目前正在临床研究中的叶酸-药物结合物。