Natural and synthetic compounds of Formulae Ia-Ie having a lactone structure, in particular Securolide, have been determined to be effective anti-tumor compounds which target the hrad9 gene and/or protein encoded thereby or complex containing the protein and/or the p53 gene and/or protein. Securolide is cytoselective for mutants of hRad9 based on studies conducted in Rad9 mutant yeast strains. Securolide appears to interact with mutant hRad9 in cancer cells to produce DNA lesions which result in apoptosis. Studies have demonstrated that Securolide is useful for treating proliferation disorders such as melanoma, leukemia, breast cancer, lung cancer, ovarian cancer, colon cancer, esophagus cancer, liver cancer, and lymphatic cancer, and to alleviate pain associated with the cancer. Other compounds effective for the treatment of cancer and optionally pain associated therewith may also be identified using the same assays, for example, by screening for efficacy in assays using Rad9 and/or p53 defective mutant yeasts.