ROLE OF MOLECULAR DETERMINANTS IN PREDICTING RESISTANCE TO TRASTUZUMAB IN PATIENTS DIAGNOSED WITH HER-2 OVEREXPRESSING METASTATIC BREAST CANCER

Breast cancer (BC) is the second most cause of death worldwide. In Italy there are approximately 40,000 new cases per year and 11,000 deaths, representing one of the major causes of morbidity and mortality cancer in females. In Tuscany it has been estimated, in 2010 approximately 4100 breast cancer cases will be diagnosed and 850 deaths. Many preclinical and clinical trials have been performed to investigate and establish effective treatments. HER2 receptor is overexpressed in 20–30 % of BC and it has been chosen to be an effective therapeutic target. Trastuzumab (Herceptin), a humanized monoclonal antibody, has been approved by FDA to be anti-HER2 therapy, but the mechanism by which Trastuzumab exerts its antitumor activity is not fully understood. Nevertheless, Trastuzumab has been suggested to induce antibody-dependent cellular cytotoxicity (ADCC), inhibit HER2 extracellular domain cleavage, or inhibit PI3K/ AKT survival signalling, either by down-regulating HER2 signalling or by increasing PTEN membrane localization and phosphatase activity, leading to a decline in PI3K/AKT pathway activation and inhibition of proliferation. In spite of this, it is still largely unclear why almost half of the breast cancer patients overexpressing HER2 are initially do not respond to Trastuzumab based therapy even when combined with chemotherapy, or eventually become resistant to Trastuzumab during treatment. Mutational defects in HER2/PIK/AKT/PTEN signalling pathway and Fc--receptors polymorphisms have been suggested to be involved in Trastuzumab resistance. In addition, activation of HER-related receptors, such as insulin-like growth factor I receptor (IGF-R1) has been suggested in preclinical studies to increase PI3K/AKT signalling pathway, resulting in limited Trastuzumab efficacy. We proposed the involvement of PIK/AKTl/PTEN pathway and ADCC immune response in the mechanism of Trastuzumab resistance. We collected 77 samples of HER-2 positive metastatic breast cancer (MBC) patients. Mutational analysis have been performed on hot spots exons 9 and 20 of PIK3CA, exon 4 of AKT1 and exons 5, 7 and 8 of PTEN genes. Furthermore, F158V and R131H polymorphisms of FcγRIIIa and FcγRIIa genes have been performed. Our results showed a highly significant impact of PIK3CA/AKT1/PTEN pathway gaining mutation on overall survival (OS) of the study population (P<0.0001) and a high significant relationship between PIK3CA mutations and (PFS-I) (P=0.0001). Homozygous SNPs V158V/H131H of FcγRIIIa and FcγRIIa are significantly related with better OS of our study population (P=0.0037). In conclusion, our results suggest the important role of PIK3CA gene mutation and FcγRIIIa polymorphisms in the patient’s OS, which could be used as a molecular predicting marker for the Trastuzumab resistance. Moreover, we found that IGF1-R expression is related with patient’s response to Trastuzumab (p < 0.05). An understanding of Trastuzumab resistance mechanisms would be a helpful tool in the development of rational drug combinations to circumvent resistance and allow better selection of patients likely to respond