Abstract Introduction. Despite improvement in current treatment strategies, including surgery, chemotherapy (CT) and radiotherapy (RT), survival of cancer patients in advanced stages has not improved significantly over the last decade. Organometallic complexes of platinum (cisplatin, carboplatin and oxaliplatin) are in use as chemotherapeutic agents worldwide. However, acquired drug resistance to these platinum complexes has necessitated the search for novel strategies to overcome this limitation, which includes combination therapies and the development of novel platinum or non-platinum complexes as drugs. Methods. Anti-proliferative effects of a bis-amidine complex (Os-Sh1) was determined by measuring the inhibition of cell proliferation using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry-based cell-cycle analysis, annexin V dye binding assay and TUNEL assay in endometrial cancer (Hec1A and KLE) and cervical cancer (HeLa) cells. Results: Cell viability (MTT) assays revealed inhibition of cell proliferation on treatment with the bis-amidine osmium complex (Os-Sh1) in HEC1A, KLE and HeLa cell lines in a dose- (1μM – 50μM) and time-dependent manner (24h – 96h). Cell-cycle analysis showed increased cell death, i.e. the sub-G0 fraction, with G0/G1-arrest and reduction in the G2/M phase of cell cycle on treatment with the Os-Sh1 in these cell lines. Results of annexin V assay suggested apoptosis as the mechanisms of Os-complex induced cell death. Apoptosis was further confirmed by TUNEL assay using flow cytometry analysis. Western blot analysis showed cleavage of caspase 9, caspase 3 and poly-ADP ribose polymerase (PARP), suggesting activation of the intrinsic mitochondrial pathway of apoptosis. Interestingly, Os-Sh1 was more effective in inhibiting cell proliferation at a lower dose and shorter period of time (LD50=20 μM, 48h) in comparison to cisplatin, carboplatin and oxaliplatin in HEC1A, KLE and HeLa cell lines. Conclusions: Our in vitro data clearly demonstrated anti-proliferative effects of a novel bis-amidine osmium complex in endometrial and cervical cancer cells at a lower dose and in shorter period of time in comparison to platinum drugs. Based on these results, we hypothesize that such osmium complex has the potential as effective cytotoxic agents wherein cancer cells acquire resistance to commonly used platinum drugs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2540. doi:10.1158/1538-7445.AM2011-2540