No abstract

The synthesis, cytotoxicity, and nucleoside binding of some platinum-acridinylthiourea conjugates derived from the prototypical compound [PtCl(en)(ACRAMTU)](NO3)2 ("PT-ACRAMTU"; en=ethane-1,2-diamine, ACRAMTU=1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, protonated form) are reported. To establish structure-activity relationships within this class of compounds, systematic changes were made to the thiourea nonleaving group, which links the intercalator to platinum. Three new derivatives of ACRAMTU, one di-, one tri-, and one tetraalkylated, were generated, where the degree of alkylation indicates the number of alkyl groups attached to the SCN2 framework. Subsequent reaction of the tri- and tetraalkylated derivatives with activated [PtCl2(en)] yielded the corresponding platinum conjugates. The dialkylated thiourea gave an unstable complex, which was not included in the studies. The crystal structure of PT-ACRAMTU x MeOH has been determined. In the solid state, one axial position of the square-planar platinum coordination sphere is partially shielded by the bulky thiourea group, providing a strong rationale for the kinetic inertness of the compound. The cytotoxicity of the prototype, the two new conjugates, and cisplatin was assessed in ovarian (A2780, A2780/CP), lung (NCI-H460), and colon (RKO) cancer cell lines using clonogenic survival assays. The derivatives containing trialkylated thiourea groups showed activity similar or superior to cisplatin, with IC50 values in the low micromolar concentration range. The complex modified with the tetraalkylated (bulkiest) thiourea was significantly less active, possibly due to the greatly decreased rate of binding to nucleobase nitrogen (1H NMR spectroscopy), but was most efficient at overcoming cross resistance to cisplatin in A2780/CP. Possible consequences of the reported structural modifications for the mechanism of action of these agents are discussed.

Two multi-session experiments are described in which a complex problem-solving task was interrupted at different stages of practice. In Experiment 1, subjects practiced the main problem-solving task for three sessions, with intermittent interruptions during each session. By the end of Session 3, interruptions which were similar to the main task, in terms of type of material processed and processing demands, no longer disrupted performance as they had in Sessions 1 and 2. In Experiment 2, subjects practiced the same problem-solving task for two sessions without interruptions. The same types of interruptions used in Experiment 1 were introduced in Session 3. Although the main task was well learned by the third session, the interruptions disrupted subjects' main-task accuracies dramatically. These results suggest that training tasks under uninterrupted conditions can lead to excellent performance, but may not allow subjects to develop the kinds of strategies needed to flexibly recover from interruptions when they occur.

The tumor suppressor protein, p53, plays a critical role as a transcriptional activator of downstream target genes involved in the cellular response to DNA damaging agents. We examined the cell cycle checkpoint response of human mammary epithelial cells (HMEC) and their isogenic ®broblast counterparts to ionizing (IR) and ultraviolet (UV) radiation, two genotoxic agents whose DNA damage response pathways involve p53. Using¯ow cytometric analysis, we found that both mortal and immortalized HMEC, which contain wild-type p53 sequence, do not exhibit a G1 arrest in response to IR, but show an intact G2 checkpoint. Supportive evidence from Western analyses revealed that there was neither an increase in p53 nor one of its downstream targets, p21 WAF1 , in HMEC exposed to IR. In contrast, isogenic mammary ®broblasts arrest at the G1 checkpoint and induce the p53 and p21 WAF1 proteins following IR. By comparison, HMEC exposed to UV displayed an S phase arrest and induced the expression of p53 and p21 WAF1 . Our results show that the cellular response to DNA damage depends on both the type of damage introduced into the DNA and the speci®c cell type.

Recently, we reported a new class of DNA-targeted hybrid platinum-acridine agents. The parent intercalator, ACRAMTU, a 9-aminoacridine derivative, intercalates into the minor groove of DNA, causing the corresponding prototypical conjugate, PT-ACRAMTU (type I/n=2), to form DNA adducts dissimilar to traditional platinum drugs. Both these agents show cytotoxic activity in leukemic and ovarian cancer cells. Following the use of clonogenic survival assays, we report on the cytotoxic effects of ACRAMTU, PT-ACRAMTU, and three PT-ACRAMTU derivatives, on additional cell lines including colon (RKO), lung (H460), and cisplatin-sensitive (A2780) and cisplatin-resistant (A2780/CP) ovarian cells. While a dose-dependent effect was observed with both ACRAMTU and PT-ACRAMTU, an enhanced cytotoxic effect was seen with PT-ACRAMTU in all cell lines. PT-ACRAMTU appeared to have a similar IC50 value to cisplatin except in H460 lung cancer cells in which PT-ACRAMTU had a twofold lower IC50 value. PT-ACRAMTU appeared to act in a time-dependent manner. In H460 cells the IC50 value of PT-ACRAMTU was 235-fold higher following a 1-h incubation than following a 24-h incubation (0.27 microM), while following an 8-h incubation the IC50 value was 0.41 microM. Three derivatives of PT-ACRAMTU were also tested. A tetraalkylated derivative, type II/n=2, generated the highest IC50 values in all cell lines, while the trialkylated derivative, type III/n=2, generated IC50 values similar to its isomer, PT-ACRAMTU. PT-ACRAMTU with an added CH2 group in the thiourea linker (type I/n=3) showed IC50 values similar to the type I/n=2 prototype in H460 lung cells. An apoptotic response to PT-ACRAMTU appeared to be generated in H460 cells as evidenced by DNA laddering. These results suggest that type I/n=2 and type I/n=3 may be promising agents for the treatment of lung cancer and should be pursued in animal models.