Proteasomal inhibition has proven good clinical effect in Multiple Myeloma; however, some patients are resistant to treatment and most patients acquire resistance during the disease course. It is well known that cells degrade proteins by two distinct but interconnected pathways; one is protein degradation by proteasomes, which may be clinically targeted by proteasome inhibitors, the other is by formation of autophagosomes and lysosomal degradation, which can be inhibited by hydroxychloroquine (HCQ). Assuming that the two arms of protein degradation may compensate for each other, there is a rationale to simultaneously target both arms. We tested the combined effects of HCQ and the reversible proteasome inhibitor bortezomib or the irreversible inhibitor carfilzomib on myeloma cell lines and primary cells. As expected, HCQ potentiated carfilzomib-induced myeloma cell death, but surprisingly, HCQ had little or no effect on bortezomib activity. We also found that HCQ potentiated the effect of the irreversible proteasome inhibitor oprozomib, but not the effect of the reversible inhibitor ixazomib. Thus, the sensitivity towards the irreversible proteasome inhibitors carfilzomib or oprozomib is to a larger extent affected by the autophagic system than the reversible inhibitors bortezomib and ixazomib. In conclusion, we here suggest that in attempts to increase the clinical efficacy of proteasome inhibitors, inhibitors of autophagy and lysosomal degradation such as HCQ should be combined with an irreversible proteasome inhibitor such as carfilzomib, and not with bortezomib.

Citation Format: Kristine Misund, Katarzyna Anna Baranowska, Toril Holien, Kristian Starheim, Ida Johansson, Glenn Buene, Anders Waage, Geir Bjørkøy, Anders Sundan. Chloroquine potentiates carfilzomib but not bortezomib effects on myeloma cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1768. doi:10.1158/1538-7445.AM2015-1768