Frankel A, Man S, Elliott P, Adams J, Kerbel R S
Biological Sciences Program, Cancer Biology Research, Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario, Canada.
Clin Cancer Res. 2000 Sep;6(9):3719-28.
Almost all known conventional cytotoxic anticancer drugs are less effective in killing tumor cells grown as multicellular spheroids than in killing tumor cells grown as monolayer cell cultures. This "multicellular resistance" reflects the relative intrinsic drug-resistant phenotype of most solid tumors growing in vivo and is due to factors such as limited drug penetration or reduced fractions of proliferating cells. Proteasome inhibitors such as PS-341, a dipeptide boronic acid analogue, represent an interesting new class of potential anticancer drugs, which are entering early-phase clinical trials. PS-341 has been found to have good broad-spectrum cytotoxic activity in the 60-monolayer cell line National Cancer Institute screen. However, because its relative potency has not been tested in spheroid systems, we analyzed the activity of PS-341 in a spheroid/solid tumor context using four different human ovarian carcinoma cell lines and three prostate carcinoma cell lines, respectively. We found, with one exception, that PS-341 showed equal or greater activity in spheroids than in the respective monolayer cell cultures, even in a prostate cancer spheroid model with a very low growth fraction. PS-341 induced apoptotic cell death in carcinoma cells in both culture systems. We also noted a decrease in XIAP protein, a member of the inhibitor of apoptosis (IAP) family of apoptosis inhibitors, and phosphorylation of Bcl-XL in PS-341-treated ovarian carcinoma cells. Furthermore, DNA fragmentation, a hallmark of apoptosis (in this case, induced by PS-341), was completely inhibited by the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD). Taken together, the results indicate that unlike most other known anticancer cytotoxic drugs, PS-341 appears to be as effective in killing tumor cells grown in the form of multicell spheroids as in killing tumor cells grown in monolayer cell culture. Hence, this compound has the potential to circumvent multicellular drug resistance and, as such, may show promising activity against solid tumors with low growth fractions in vivo, which are frequently intrinsically resistant to conventional cytotoxic anticancer drugs.
几乎所有已知的传统细胞毒性抗癌药物在杀死以多细胞球体形式生长的肿瘤细胞方面，都不如在杀死以单层细胞培养形式生长的肿瘤细胞有效。这种“多细胞抗性”反映了大多数在体内生长的实体瘤相对固有的耐药表型，并且是由诸如药物渗透受限或增殖细胞比例降低等因素导致的。蛋白酶体抑制剂，如二肽硼酸类似物PS-341，代表了一类有趣的新型潜在抗癌药物，它们正在进入早期临床试验阶段。在国立癌症研究所的60种单层细胞系筛选中，已发现PS-341具有良好的广谱细胞毒性活性。然而，由于其相对效力尚未在球体系统中进行测试，我们分别使用四种不同的人卵巢癌细胞系和三种前列腺癌细胞系，在球体/实体瘤环境中分析了PS-341的活性。我们发现，除了一个例外，PS-341在球体中的活性与在各自的单层细胞培养中相等或更高，即使在生长分数非常低的前列腺癌球体模型中也是如此。PS-341在两种培养系统中均诱导癌细胞凋亡性细胞死亡。我们还注意到，在PS-341处理的卵巢癌细胞中，凋亡抑制蛋白（IAP）家族成员XIAP蛋白减少，以及Bcl-XL磷酸化。此外，DNA片段化是凋亡的一个标志（在这种情况下，由PS-341诱导），被半胱天冬酶抑制剂N-苄氧羰基-Val-Ala-Asp-氟甲基酮（Z-VAD）完全抑制。综上所述，结果表明，与大多数其他已知的抗癌细胞毒性药物不同，PS-341在杀死以多细胞球体形式生长的肿瘤细胞方面似乎与在杀死以单层细胞培养形式生长的肿瘤细胞一样有效。因此，这种化合物有可能规避多细胞耐药性，并且因此可能对体内生长分数低的实体瘤显示出有前景的活性，这些实体瘤通常对传统细胞毒性抗癌药物具有内在抗性。
