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Synergistic Inhibition of ErbB Signaling by Combined Treatment with Seliciclib and ErbB-Targeting Agents

Authors' Affiliation: Cyclacel Ltd., Dundee, United Kingdom

Requests for reprints: Ian N. Fleming, Cyclacel Ltd., 1 James Lindsay Place, Dundee, DD15JJ, United Kingdom. Phone: 44-1382-206062; Fax: 44-1382-206067; E-mail: ifleming{at}cyclacel.com.

Purpose: The aims of this study were to investigate whether the cyclin-dependent kinase inhibitor seliciclib could synergize with agents that target ErbB receptors and to elucidate the molecular mechanism of the observed synergy.

Experimental Design: Synergy between seliciclib and ErbB receptor targeted agents was investigated in various cell lines using the Calcusyn median effect model. The molecular mechanism of the observed synergy was studied in cultured cells, and the combination of seliciclib and the epidermal growth factor receptor (EGFR) inhibitor erlotinib was evaluated in an H358 xenograft model.

Results: Seliciclib synergized with the anti-HER2 antibody trastuzumab in a breast cancer cell line, which overexpresses the HER2 receptor, and with the erlotinib analogue AG1478 in non–small cell lung cancer cell lines. In the H358 non–small cell lung cancer cell line, synergy involved decreased signaling from the EGFR, with AG1478 directly inhibiting kinase activity while seliciclib decreased the levels of key components of the receptor signaling pathway, resulting in enhanced loss of phosphorylated extracellular signal-regulated kinase and cyclin D1. The combination of seliciclib and erlotinib was evaluated further in an H358 xenograft and shown to be significantly more active than either agent alone. An enhanced loss of cyclin D1 was also seen in vivo.

Conclusions: This is the first report that investigates combining seliciclib with an EGFR inhibitor. The combination decreased signaling from the EGFR in vitro and in vivo and was effective in cell lines containing either wild-type or mutant EGFR, suggesting that it may expand the range of tumors that respond to erlotinib, and therefore, such combinations are worth exploring in the clinic.