Sequential treatment with cisplatin (P) in combination with infusional 5-FU/LV (PFL) followed by Irinotecan (IR) + 5-FU/LV (IRFL) followed by Docetaxel (T) + 5-FU/LV (TFL) in patients (PTS) with metastatic (M) gastric carcinoma (GC): preliminary results of a phase II trial by the Gruppo Oncologico Nord Ovest (G.O.N.O.)