Abstract 921: Liposomal bortezomib improves chemosensitivity and inhibits cell proliferation in part through BCR/ABL downregulation in CML
Authors: Yang Xiaojuan , Zhang Mengzi , Peng Yong , Liu Heng , Zhu Jing , Pang Jiuxia , Lee Robert J. , Liu Shujun 
Published: August 2012

Abstract
The t(9;22) Philadelphia chromosome translocation fuses the BCR gene to the c-ABL proto-oncogene resulting in a chimeric BCR/ABL protein. Although BCR/ABL inhibitor imatinib has shown unprecedented efficacy for treatment of chronic myeloid leukemia (CML), resistance to this agent due to BCR/ABL mutations or increased protein expression poses a significant clinical problem, thereby arguing for novel therapeutic approaches. BCR/ABL gene amplification frequently occurs in blast crisis and BCR/ABL expression is required for its mutation. BCR promoter contains Sp1 binding sites and proteasome inhibitor bortezomib (BORT) disrupts Sp1 transactivation, therefore, we hypothesized that BORT could abolish BCR/ABL expression leading to disrupted kinase activity. However, the fast clearance and the inability to reach target cells in vivo have limited its application. As such, we designed a liposomal BORT (L-BORT) and transferrin receptor (TfR)-targeted L-BORT (Tf-L-BORT) for the objective of enhancing BORT delivery efficiency. We observed that there were 3.6-fold and 10.4-fold increases in cellular uptake of L-BORT and Tf-L-BORT, respectively, compared to that of the free drug. We evidenced a 2.7-fold augmented chemosensitivity to doxorubicin (DOX) in cells exposed to subtoxic concentration of BORT, with a decrease in IC50 from 8 μM to 300 nM. Tf-L-BORT further reduced IC50 of DOX to 90 nM. These data indicated the enhanced chemosensitizing efficacy of the L-BORT formulations. When single tumor cells from K562 tumor bearing mice were treated by subtoxic L-BORT combined with DOX, a synergy of toxicity was evidenced by 70-fold increased chemosensitivity (p&lt;0.05). Tf-L-BORT was more effective in chemosensitizing tumor single cells than L-BORT, with decreased IC50 of DOX to 94 nM. Pharmacokinetic study in mice after i.v. bolus injection showed that L-BORT had slower tissue distribution (T1/2α, 7.3 min vs. 2.9 min), increased elimination phase half-life (T1/2α, 82.9 h vs. 68.3 h), decreased clearance (CL, 4.2 mL/h vs. 27.7 mL/h), and increased area under the plasma concentration-time curve (AUC, 4714.1 h·ng/mL vs. 721.4 h·ng/mL) compared to the free drug. This indicated a significant and potentially beneficial change in BORT pharmacokinetics in vivo. Mechanistically, when K562 cells were exposed to L-BORT (0, 10, 30 and 60nM) or DOX (0, 100, 200 and 500nM) for 48h, BCR/ABL protein was significantly decreased in a dose-dependent manner. Interestingly, the combination of L-BORT with DOX synergistically abrogated BCR/ABL and Sp1 protein expression. Altogether, these results highlight the therapeutic relevance of disrupting BCR/ABL protein expression and strongly support the utilization of L-BORT alone or in combination with DOX to treat CML patients harboring overexpressed BCR/ABL.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 921. doi:1538-7445.AM2012-921