Second-line treatment of high-grade osteosarcoma (HGOS) patients is based on different approaches and chemotherapy protocols, which are not yet standardized. Although several drugs have been used in HGOS second-line protocols, none of them has provided fully satisfactory results and the role of rescue chemotherapy is not well defined yet. This article focuses on the drugs that have most frequently been used for second-line treatment of HGOS, highlighting the present knowledge on their mechanisms of action and resistance and on gene polymorphisms with possible impact on treatment sensitivity or toxicity. In the near future, validation of the so far identified candidate genetic biomarkers may constitute the basis for tailoring treatment by taking the patients' genetic background into account. Osteosarcoma is the most common tumor of bone, which can exibit different levels of malignancy [1,2]. High-grade osteo sarcoma (HGOS) is a very aggressive neoplasm that, in unselected populations, is responsible for death of approximately 40-50% of patients [2,3]. The subgroup of the so-called conventional HGOS includes patients with high-grade tumors located in the extremities, nonmetastatic at diagnosis and younger than 40 years [1]. In this subgroup of HGOS, prognosis is slightly better, reaching 60-65% when patients are treated with multiagent neoadjuvant chemotherapy [4][5][6].The major clinical problem limiting the cure rate of HGOS patients is relapse, which in most cases consists in development of lung metastasis within the first 24-36 months from diagnosis [7,8]. Treatment of relapsed HGOS is based on different approaches and second-line chemotherapy protocols which, however, are not standardized and can rescue no more than 20-25% of patients [4,5].
Methods of data searching & selectionA systematic literature search was performed using 'treatment' and 'osteosarcoma' as key words in PubMed [9]. We further restricted the selection to studies performed in humans. By using all the aforementioned different combinations and selections, the literature searches revealed a total of 13,976 results. The key terms 'polymorphism', 'toxicity', 'drug response', 'drug resistance', 'pharmaco genetic', 'pharmacogenomic', 'epigenetic', 'methylation' and 'clinical trial' were then used to refine the results of the previous search, in order to select the publications which were strictly related to each section of this review. Full-length articles and reviews in English were taken into consideration.Only drugs used in at least two different clinical trials with published results were included in this review and, consequently, we considered genes and gene poly morphisms with possible relevance for response and/or