ritonavir and Sarcoma

The biosynthesis of immunoglobulin leads to constitutive endoplasmic reticulum (ER) stress in myeloma cells, which activates the unfolded protein response (UPR). The UPR promotes protein folding by chaperones and increases proteasomal degradation of misfolded protein. Excessive ER stress induces apoptosis and represents a molecular basis for the bortezomib sensitivity of myeloma. Most solid malignancies such as sarcoma, by contrast, are poorly bortezomib sensitive and display low levels of ER stress. We hypothesized that pharmacologic induction of ER stress might sensitize malignancies to bortezomib treatment. We show that the HIV protease inhibitor ritonavir induces ER stress in bortezomib-resistant sarcoma cells. Ritonavir triggered the UPR, decreased the degradation of newly synthesized protein, but did not directly inhibit proteasomal active sites in the therapeutic dose range in contrast to bortezomib. Whereas neither bortezomib nor ritonavir monotherapy translated into significant apoptosis at therapeutic drug levels, the combination strongly increased the level of ER stress and activated PERK, IRE1, and ATF6, synergistically induced CHOP, JNK, caspase-4, and caspase-9, and resulted in >90% apoptosis. In summary, ritonavir increases the level of ER stress induced by bortezomib, which sensitizes bortezomib-resistant cells to bortezomib-induced apoptosis. Ritonavir may therefore be tested clinically to improve the sensitivity of solid malignancies toward bortezomib treatment.

Topics: Antineoplastic Agents; Apoptosis; Binding Sites; Boronic Acids; Bortezomib; Cell Death; Cell Line, Tumor; Drug Screening Assays, Antitumor; Endoplasmic Reticulum; G2 Phase; Humans; Neoplasm Proteins; Proteasome Inhibitors; Protein Biosynthesis; Protein Folding; Protein Subunits; Pyrazines; Ritonavir; Sarcoma

The anti-HIV protease inhibitors represent a new class of agents for treatment of HIV infection. Saquinavir, ritonavir, indinavir, and nelfinavir are the first drugs approved in this class and significantly reduce HIV RNA copy number with minimal adverse effects. They are all substrates of cytochrome P450 3A4, and are incompletely bioavailable. The drug transporting protein, P-glycoprotein (P-gp), which is highly expressed in the intestinal mucosa, could be responsible for the low oral bioavailability of these and other drugs which are substrates for this transporter. To determine whether these protease inhibitors are modulators of P-gp, we studied them in cell lines which do and do not express P-gp. Saquinavir, ritonavir and nelfinavir significantly inhibited the efflux of [3H]paclitaxel and [3H]vinblastine in P-gp-positive cells, resulting in an increase in intracellular accumulation of these drugs. However, similar concentrations of indinavir did not affect the accumulation of these anticancer agents. In photoaffinity labeling studies, saquinavir and ritonavir displaced [3H]azidopine, a substrate for P-gp, in a dose-dependent manner. These data suggest that saquinavir, ritonavir, and nelfinavir are inhibitors and possibly substrates of P-gp. Because saquinavir has a low bioavailability, its interaction with P-gp may be involved in limiting its absorption.

Topics: Anti-HIV Agents; Antibiotics, Antineoplastic; Antineoplastic Agents, Phytogenic; ATP Binding Cassette Transporter, Subfamily B, Member 1; Biological Availability; Cell Survival; Daunorubicin; Dose-Response Relationship, Drug; Drug Interactions; Drug Resistance, Multiple; Drug Resistance, Neoplasm; Female; HIV Protease Inhibitors; Humans; Indinavir; Leukemia, Erythroblastic, Acute; Nelfinavir; Paclitaxel; Ritonavir; Saquinavir; Sarcoma; Tumor Cells, Cultured; Uterine Neoplasms; Vinblastine