Robert Silber

Bio: Robert Silber is an academic researcher from Duke University. The author has contributed to research in topics: Cytotoxicity & Topoisomerase. The author has an hindex of 1, co-authored 1 publications receiving 11 citations.

Nitric oxide enhancement of fludarabine cytotoxicity for B-CLL lymphocytes

Abstract: Fludarabine is active but not curative in the treatment of chronic lymphocytic leukemia (B-CLL). Nitric oxide (NO) supplied from exogenous, NO-donating pro-drugs can also induce apoptosis and death of acute leukemia cells. This study investigated combinations of fludarabine with NO-donating pro-drugs for their cytotoxicity against freshly isolated B-CLL lymphocytes following a 72 h exposure in vitro. The median IC(50)for fludarabine was 2.2 microM (n = 85). The nitric oxide donors DETA-NO, PAPA-NO, and MAHMA-NO were also cytotoxic, and their effects were inversely related to rates of NO release. Neither DETA-NO depleted of NO nor DETA itself was effective, indicating that NO was required for cytotoxicity. Drug interactions were evaluated by a modified combination index method. Synergy was observed in combinations of fludarabine or nelarabine (506U78) with DETA-NO in 52% and 88% of samples, respectively. Interestingly, the combination of fludarabine and DETA-NO was more cytotoxic in B-CLL cells less sensitive to fludarabine. DETA-NO did not enhance the activity of other DNA anti-metabolites, topoisomerase I and II inhibitors, or alkylating agents. Finally, the anti-leukemic activity of fludarabine alone or in combination with DETA-NO was found to correlate with inhibition of cellular RNA synthesis. These results indicate that NO donors could enhance fludarabine therapy for B-CLL.

The activity of camptothecin analogues is enhanced in histocultures of human tumors and human tumor xenografts by modulation of extracellular pH

Abstract: Most solid human tumors exist in an acidic microenvironment, due in part to inefficient vasculature and a higher intrinsic rate of glycolysis. This leads to a tumor-selective pH gradient, which can be exploited therapeutically with antitumor agents such as the camptothecins (CPTs). Previous work in this laboratory has shown that camptothecin activity is enhanced 40- to 60-fold in monolayer cell culture by reducing the extracellular pH to 6.8. Three-dimensional histoculture has been shown to be a technique that allows human tumor tissue to grow in an in vivo-like way with maintenance of tissue histology and function and drug sensitivity for long periods of time. In the current study, we utilized these features of histoculture to study new analogues of camptothecin that have superior pharmacological properties. We evaluated six CPT analogues in histocultures of human brain, neuroblastoma, breast, colon, and prostate tumors. Fragments were exposed to 10,11-methylenedioxy-CPT (MDC), 7-chloromethyl-MDC, SN-38, topotecan (TPT), 9-amino-CPT, 10-amino-CPT, paclitaxel, 5-fluorouracil, 4-hydroperoxycyclophosphamide and doxorubicin, and antitumor activity was assessed. For in vivo tumor outgrowth studies, fragments were treated in parallel, implanted into nude mice, and monitored for development of tumors. Against 15 of 16 tumor xenografts and all primary tumor samples tested, all compounds were cytotoxic at pH 7.4 (IC50 range 13–921 μM). MDC, SN-38, TPT, and 9-amino-CPT achieved an average 5-fold increase in activity (range 3–14) at pH 6.8, while 7-chloromethyl-MDC was enhanced 8-fold (range 6–14). The most potentiated analogue was 10-amino-CPT at 27-fold (range 17–49). In contrast, the other agents were active against one or more tumor types but were not enhanced by acidic pH. Importantly, the toxicity of MDC in histoculture of D54 glioma xenografts strongly correlated with the outgrowth of treated fragments subsequently implanted in vivo. Evaluation of anticancer drug activity in native-state histoculture supports the concept that pH modulation may be an important approach to improve the selectivity and antitumor effectiveness of camptothecin-based chemotherapy.

Mechanisms of Cell Death of Chronic Lymphocytic Leukemia Lymphocytes by RNA-Directed Agent, 8-NH2-Adenosine

Abstract: Purpose: To determine if RNA-directed nucleoside analogue, 8-NH2-adenosine, induces cell death and if that is accompanied with transcription inhibition of the key survival factors of chronic lymphocytic leukemia (CLL) cells. Experimental Design: Primary lymphocytes from CLL patients were incubated with 10 μmol/L 8-NH2-adenosine for 2, 4, and 6 or 8 hours. The accumulation of analogue triphosphate and the decline in endogenous ATP pool were analyzed by high-performance liquid chromatography. Inhibition of global RNA and protein synthesis was measured and correlated with specific decline in transcript and protein levels of MCL-1, XIAP, and BCL-2, the key survival factors of CLL. These biochemical and molecular end points were related to cell death of these quiescent lymphocytes. Results:In vitro incubations of CLL lymphocytes with 8-NH2-adenosine resulted in rapid but heterogeneous accumulation of 8-NH2-ATP (390-680 μmol/L), with a concomitant decline in endogenous ATP (median, >50% by 4 hour). Global RNA synthesis was decreased in all samples and was associated with a decline in MCL-1, XIAP, and BCL-2 transcripts. There was a parallel decrease in the protein level of MCL-1 and XIAP but not BCL-2. These biochemical changes were accompanied by apoptosis. Conclusion: The evidence of CLL cell death with complementary changes in the expression of survival proteins provides a molecular rationale for using 8-NH2-adenosine as a therapeutic agent for this indolent leukemia.