title: Lack of multicellular drug resistance observed in human ovarian and prostate carcinoma treated with the proteasome inhibitor PS-341.
abstract: 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.
authors.affiliation: Biological Sciences Program, Cancer Biology Research, Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario, Canada.
authors.forename: A
authors.initials: A
authors.lastname: Frankel
authors.forename: S
authors.initials: S
authors.lastname: Man
authors.forename: P
authors.initials: P
authors.lastname: Elliott
authors.forename: J
authors.initials: J
authors.lastname: Adams
authors.forename: R S
authors.initials: RS
authors.lastname: Kerbel
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D002277:Carcinoma / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism; D002448:Cell Adhesion / Q000187:drug effects; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000096:biosynthesis; D019008:Drug Resistance, Neoplasm; D005260:Female; D006801:Humans; D008297:Male; D010051:Ovarian Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology*; D010766:Phosphorylation / Q000187:drug effects; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology*; D011480:Protease Inhibitors / Q000494:pharmacology*; D011506:Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D011719:Pyrazines / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D018874:Spheroids, Cellular / Q000187:drug effects* / Q000473:pathology; D014407:Tumor Cells, Cultured / Q000187:drug effects; D051636:X-Linked Inhibitor of Apoptosis Protein; D051020:bcl-X Protein
chemical_list: D000970:Antineoplastic Agents; C494811:BCL2L1 protein, human; D001897:Boronic Acids; C493956:CDKN1A protein, human; D061945:Caspase Inhibitors; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D011480:Protease Inhibitors; D011506:Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D011719:Pyrazines; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D051020:bcl-X Protein; D000069286:Bortezomib; D020169:Caspases