In Vitro Activity of SYK and BCR-ABL Inhibitors in Aggressive Lymphomas.

The B cell receptor tyrosine kinase SYK is a critical component of the B-cell receptor signalling pathway in normal B cells. We have recently reported that SYK is amplified and over-expressed in mantle cell lymphoma (MCL) and that the growth of MCL and diffuse large B cell lymphoma (DLBCL) cell lines over-expressing SYK is inhibited by piceatannol, a known SYK inhibitor (Bertoni et al, ASH 2005; Rinaldi et al, BJH 2006). Others have reported important SYK expression in splenic marginal zone B cell lymphomas, in DLBCL and peripheral T cell lymphomas (PTCL) (Ruiz-Ballesteros et al, 2005; Mahadevan et al, w Streubel et al, 2006), suggesting SYK targeting agents could be useful for the treatment of various lymphoma subtypes. SYK inhibitors are already in clinical development for treatment of asthma. Here, we report on the activity on lymphoma cell lines and primary cells of the SYK/ZAP-70 inhibitor #1 (Novartis) and of the BCR-ABL inhibitors imatinib (Novartis) and nilotinib (Novartis), which could act as cross-selecting SYK inhibitors (Atwell et al, 2004).

We treated four human MCL and three DLBCL established cell lines with increasing doses of the SYK/ZAP-70 inhibitor #1, imatinib and nilotinib for 72 h. Cell viability was measured with the MTT assay. The two cell lines expressing high levels of SYK, JeKo-1 and SUD-HL-6, were sensitive to the compounds (IC50: Syk/ZAP-70 inhibitor #1, 1–5 μM; imatinib, 15–20 μM; nilotinib, 10 μM). Cells with lower SYK expression were generally less sensitive to all three compounds. To obtain further data on the relevance of SYK inhibition in lymphoma, we treated nine lymphoma primary cells with the piceatannol (Sigma), the SYK/ZAP-70 inhibitor #1 and nilotinib. Responses, defined as <50% decrease in viable cell number, were observed with piceatannol (4/9 samples), the Syk/ZAP-70 inhibitor #1 (3/9 samples) and nilotinib (3/9 samples). Immunoblotting experiments aimed to elucidate the mechanism of action are under-way and data will be presented at the meeting.

In conclusion, our data indicate that pharmacological inhibition of SYK is a therapeutic approach to be further investigated in subsets of aggressive lymphomas.