ABSTRACT
The proteasome inhibitors carfilzomib (Cfz) and bortezomib (Btz) are used successfully to treat multiple myeloma, but have not shown clinical efficacy in solid tumors. Here we show that clinically achievable inhibition of the ß5 site of the proteasome by Cfz and Btz does not result in loss of viability of triple-negative breast cancer cell lines. We use site-specific inhibitors and CRISPR-mediated genetic inactivation of ß1 and ß2 to demonstrate that inhibiting a second site of the proteasome, particularly the ß2 site, sensitizes cell lines to Btz and Cfz in vitro and in vivo. Inhibiting both ß5 and ß2 suppresses production of the soluble, active form of the transcription factor Nrf1 and prevents the recovery of proteasome activity through induction of new proteasomes. These findings provide a strong rationale for the development of dual ß5 and ß2 inhibitors for the treatment of solid tumors.
Subject(s)
Antineoplastic Agents/pharmacology , Nuclear Respiratory Factor 1/antagonists & inhibitors , Proteasome Endopeptidase Complex/metabolism , Proteasome Inhibitors/pharmacology , Triple Negative Breast Neoplasms/drug therapy , Antineoplastic Agents/chemical synthesis , Antineoplastic Agents/chemistry , Apoptosis/drug effects , Cell Survival/drug effects , Dose-Response Relationship, Drug , Humans , Nuclear Respiratory Factor 1/metabolism , Proteasome Inhibitors/chemical synthesis , Proteasome Inhibitors/chemistry , Structure-Activity Relationship , Triple Negative Breast Neoplasms/metabolism , Triple Negative Breast Neoplasms/pathology , Tumor Cells, CulturedABSTRACT
Downregulation of the unfolded protein response mediates proteasome inhibitor resistance in multiple myeloma. The Human Immunodeficieny Virus protease inhibitor nelfinavir activates the unfolded protein response in vitro. We determined dose-limiting toxicity and recommended dose for phase II of nelfinavir in combination with the proteasome inhibitor bortezomib. Twelve patients with advanced hematologic malignancies were treated with nelfinavir (2500-5000 mg/day p.o., days 1-14, 3+3 dose escalation) and bortezomib (1.3 mg/m(2), days 1, 4, 8, 11; 21-day cycles). A run in phase with nelfinavir monotherapy allowed pharmakokinetic/pharmakodynamic assessment of nelfinavir in the presence or absence of concomittant bortezomib. End points included dose-limiting toxicity, activation of the unfolded protein response, proteasome activity, toxicity and response to trial treatment. Nelfinavir 2×2500 mg was the recommended phase II dose identified. Nelfinavir alone significantly up-regulated expression of proteins related to the unfolded protein response in peripheral blood mononuclear cells and inhibited proteasome activity. Of 10 evaluable patients in the dose escalation cohort, 3 achieved a partial response, 4 stable disease for 2 cycles or more, while 3 had progressive disease as best response. In an exploratory extension cohort with 6 relapsed, bortezomib-refractory, lenalidomide-resistant myeloma patients treated at the recommended phase II dose, 3 reached a partial response, 2 a minor response, and one progressive disease. The combination of nelfinavir with bortezomib is safe and shows promising activity in advanced, bortezomib-refractory multiple myeloma. Induction of the unfolded protein response by nelfinavir may overcome the biological features of proteasome inhibitor resistance. (clinicaltrials.gov identifier: 01164709).
