Small cell lung cancer (SCLC) is characterized by prompt response to chemotherapy and radiotherapy but relapsing with drug resistance and distant metastasis, leading to poor overall prognosis. New anticancer agents and regimens are drastically needed for SCLC treatment. Arsenic trioxide (ATO), a traditional Chinese medicine used as a poison for thousands of years, has been tested in many hematological and solid cancers both in vitro and in vivo, with promising effects. In order to establish the scientific ground for future clinical application of ATO in SCLC, this study aimed to investigate the anticancer effect and mechanism of ATO in SCLC using in vitro and in vivo models, either as a single agent or in combination with standard chemotherapy. In addition, an ATO-acquired resistant cell line (H841-AR) derived from SCLC cell line H841 was used to explore potential mechanisms of ATO resistance and cross-resistance to other chemotherapeutic drugs. In the first part of this study, ATO was shown to exert cytotoxic effect in all of the chosen SCLC cell lines. Various cellular mechanisms were triggered upon ATO exposure: redox status disturbances (hydrogen peroxide (H2O2) generation, glutathione (GSH) depletion and thioredoxin 1 (Trx1) down-regulation), mitochondrial membrane depolarization (MMD), DNA damage, apoptosis and necroptosis. In concert with this, Bcl-2 was down-regulated accompanied by MMD, release of AIF and SMAC, DNA degradation, XIAP inhibition and caspases activation. Adoption of N-acetyl-L-cysteine (NAC) and buthionine sulfoximine (BSO) demonstrated GSH depletion and reactive oxygen species (ROS) generation played the pivotal role to mediate cytotoxic effect of ATO in SCLC. In the second part of this study, when combined with chemotherapeutic agents, ATO displayed synergistic and antagonistic interaction with cisplatin and etoposide respectively in SCLC cell line models. The beneficial combination of ATO and cisplatin was also substantiated by tumor xenograft models. Augmented GSH depletion and suppressed drug efflux mechanism were found to explain the synergistic effects. In the last part of this study, H841-AR was generated as an acquired multi-drug resistant (ATO, cisplatin and etoposide) cell line to investigate the potential resistance mechanisms and possible future drug combinations. Comparing H841-AR cells with parental H841 cells using cDNA microarray, a long list of genes was altered in ATO-resistant cells. At least 20 up-regulated and 45 down-regulated genes were short-listed as candidates with a cut-off at 5-fold change. Interestingly, qPCR data has shown that 5 selected up-regulated genes in H841-AR cells were also highly expressed in DMS79 cells with intrinsic ATO resistance compared to the relatively sensitive cell lines, indicating that these genes might be associated with ATO resistance in SCLC. In summary, ATO was shown to be an active anticancer agent in SCLC, either alone or in combination with cisplatin. The major mechanisms of action of ATO and its synergism with cisplatin in SCLC were elucidated. Genetic data derived from an acquired resistant (to ATO, cisplatin and etoposide) SCLC cell line may help to uncover the mechanisms of resistance to ATO, allowing possible future drug combinations. / published_or_final_version / Medicine / Doctoral / Doctor of Philosophy