Abstract 4526: Inhibition of human multiple myeloma proliferation by naltrindole
Authors: Joshi Jyoti , Terskiy Alexandra , Howells Richard D. 
Published: June 2012

The purpose of this study was to test and characterize the antiproliferative activity of naltrindole (Nti), a selective delta opioid receptor antagonist, toward human multiple myeloma (MM) cells. Nti has been shown to inhibit the allogeneic mixed lymphocyte reaction in vitro, and to block graft rejection in vivo, similarly to cyclosporine A. Based on its immunosuppressive properties we decided to test the effects of Nti on proliferation of MM cells. MM is an invasive and incurable plasma cell neoplasm responsible for 10% of all hematological malignancies. We have screened a variety of opioid compounds for activity against MM and we found that Nti inhibited the proliferation of several human MM cell lines (U266, RPMI 8226, ARP1) with an EC50 of approximately 20 μM, whereas a variety of other human cells lines were substantially less sensitive. Co-culture of MM cells with human bone marrow stromal cells did not affect the antiproliferative activity of Nti. A 10-fold molar excess of naltrexone, a non-selective opioid antagonist, did not block the Nti-induced inhibition of U266 cell proliferation. [3H]-Nti exhibits saturable, low affinity binding to intact MM cells and the pharmacological properties of the Nti binding site differ significantly from the properties of the delta opioid receptor, leading us to hypothesize that Nti inhibits proliferation of MM cells through a non-opioid receptor-dependent mechanism. RT-PCR assays confirmed the lack of delta, kappa and mu receptor mRNA in U266 and RPMI 8226 MM cells. The identity of the naltrindole binding site is currently under investigation. Nti does not induce apoptosis in MM cells, based on FACS analysis and caspase cleavage assays. While investigating the mechanism of action of Nti, we have observed that it increases intracellular calcium levels in MM cells, and the calcium appears to be released from the endoplasmic reticulum, based on inhibition of the response following thapsigargin treatment. This effect is specific to Nti as other opioids such as naltrexone and morphine do not affect the levels of calcium in MM cells, nor do they block the activity of Nti. Based on the observed in vitro anti-proliferative activity of Nti toward MM cell lines, an in vivo study was conducted. Nti injected IP daily at 30mg/kg significantly decreased tumor volumes in a murine SCID/human RPMI 8226 xenograft model over a 39-day period compared with saline injected controls. Curcumin, a constituent of turmeric extracted from the rhizomes of the plant Curcuma longa) has been shown to induce apoptosis in MM cells. We have also observed that curcumin inhibits the proliferation of MM cells. Although the chemical structures of Nti and curcumin differ substantially, curcumin inhibits Nti binding with an EC50 of 58 µM in MM cells. It appears that Nti and curcumin warrant further study as potential therapeutic agents for the treatment of human MM. Supported by grants from NIDA, the NJ Commission on Cancer Research, and the Foundation of UMDNJ.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4526. doi:10.1158/1538-7445.AM2011-4526