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.