Novel therapies for natural killer cell lymphoma

Abstract

Extranodal NK/T-cell lymphoma, nasal type (ENKL) represents the second most frequently diagnosed type of extranodal lymphoma among the Hong Kong Chinese. ENKL is a highly aggressive disease, and its relapsing or refractory cases are virtually incurable. With the overall aim of finding novel therapeutic options for ENKL, our group previously reported that the proteasome inhibitor Bortezomib administered at pharmacological concentrations has anticancer activity against ENKL. However, as serious concerns remain over the toxic side effects associated with high pharmacological dosages of Bortezomib used to treat different cancers, the first aim of the current study was to explore the potential use of drugs already approved to treat other conditions by the U.S. Food and Drug Administration (FDA) to potentiate the antitumor activity of Bortezomib at much lower pharmacological concentrations in ENKL. This approach would minimize the undesirable toxic side effects of Bortezomib. ENKL are tumors of NK cell origin characterized by a cytolytic phenotype and express IL-2 receptor α subunit (IL2Rα) (CD25). Therefore, this study investigated the ability of Basiliximab, a chimeric CD25 antagonist currently used as an immunosuppressive agent in transplantation, to inhibit cell growth in ENKL based on its specific binding to CD25 expressed ENKL cells and to potentiate the cytotoxic effect of Bortezomib against ENKL. First, the dose-and time-dependent cytotoxic effects of Bortezomib were examined in vitro in four ENKL tumor cell lines (SNK-6, NK-YS, KHYG, YT). The results confirmed the anticancer activity of Bortezomib used at pharmacological concentrations on all of the ENKL cell lines and showed that the cytotoxic effect on SNK-6 cells required a relatively higher pharmacological concentration. Further studies showed that the mechanism of action of Basiliximab was mainly cytostatic. It only inhibited ENKL cells proliferation for limited period of time by hindering cell cycle progression and was reversible. However, pretreatment of ENKL cell lines with Basiliximab significantly enhanced the cytotoxic effect of Bortezomib in the NK-YS and SNK-6 cell lines, thus significantly reducing the Bortezomib concentrations required for antitumor activity. The YT (IL-2-independent) and KHYG cell lines (IL-2-dependent, but lacking CD25) were used as negative controls for the Basiliximab experiments. Denileukin Diftitox is a fusion protein designed to direct the site-specific cytocidal action of diphtheria toxin to malignant cells expressing the CD25 component of the IL-2R (IL2Rα). Denileukin Diftitox has recently been shown to be effective in treating relapsed or refractory cutaneous T-cell lymphomas, which mostly express CD25 (IL2Rα), thus raising the possibility that this drug could be used to treat ENKL as these tumor cells also express CD25 (IL2Rα). Significantly, Denileukin Diftitox showed dramatic antitumor effects against ENKL cell lines (NK-YS, SNK-6 and YT) at very low pharmacological concentrations. The KHYG cell line, lacking CD25 (IL2Rα), was used as the negative control. The pre-clinical findings described in this thesis provide a foundation and rationale for clinical trials that combine Basiliximab with Bortezomib, and use of Denileukin Diftitox as a single therapeutic agent to treat ENKL.