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Combination of a second generation genetically modified herpes simplex virus 1 (HSV1) with paclitaxel in the treatment of breast cancer in vitro.

723 Background: A genetically modified oncolytic HSV1 JS1/ICP34.5-/ICP47- encoding GM-CSF is in a Phase I clinical trial in a number of solid tumours. Preliminary results are encouraging and a Phase II clinical trial in combination with chemotherapy in breast cancer is planned. The aim of the work is to assess the effects of combining the virus and paclitaxel in breast cancer cell lines and to study the mechanism of action of the virus in combination with paclitaxel.


METHODS
Breast cancer cell lines MDA-MB-231 and MCF-7 were treated with virus, paclitaxel or in combination at different ratios. Cell viability was determined by MTS assays, dose response curves generated and IC50 values estimated and analysed using isobolograms. Cell cycle analysis was performed after propidium iodide (PI) staining using FACS. Apoptosis was assessed with annexin V and PI staining. Effects on viral replication were assessed by plaque assay.


RESULTS
The effects of combining JS1/ICP34.5-/ICP47- given prior to, with, or after paclitaxel were either synergistic or additive. For example, for MDA-MB-231, a combination IC50 value was 2x10-4pfu/ml (virus) with 5nM of paclitaxel, but 5x10-5pfu/ml and 20nM respectively alone. This indicated synergism at this ratio. Other ratios indicated at least additive effects.Work to elucidate the mechanisms of this additivity/synergy demonstrated that: - the virus blocks the cell cycle of MCF-7 cells at G1/S and causes some apoptosis; - paclitaxel blocks the cycle at G2/M and causes significant apoptosis; - apoptosis appears not to be enhanced by the combination of drug and virus; - paclitaxel does not affect virus replication.


CONCLUSIONS
The combination of JS1/ICP34.5/ICP47- with paclitaxel may be beneficial in the treatment of breast cancer as isobolograms show either additivity or synergy. However, synergy did not appear to result from an augmentation of virus replication and was not the result of an increase in the number of cells undergoing apoptosis. Further studies are underway to elucidate the molecular events underlying these findings. [Table: see text].