The platinum drugs, cisplatin, carboplatin, and oxaliplatin, prevail in the treatment of cancer, but new platinum agents have been very slow to enter the clinic. Recently, however, there has been a surge of activity, based on a great deal of mechanistic information, aimed at developing nonclassical platinum complexes that operate via mechanisms of action distinct from those of the approved drugs. The use of nanodelivery devices has also grown, and many different strategies have been explored to incorporate platinum warheads into nanomedicine constructs. In this Review, we discuss these efforts to create the next generation of platinum anticancer drugs. The introduction provides the reader with a brief overview of the use, development, and mechanism of action of the approved platinum drugs to provide the context in which more recent research has flourished. We then describe approaches that explore nonclassical platinum(II) complexes with trans geometry or with a monofunctional coordination mode, polynuclea...

The Next Generation of Platinum Drugs: Targeted Pt(II) Agents, Nanoparticle Delivery, and Pt(IV) Prodrugs

Resistance to chemotherapy limits the effectiveness of anti-cancer drug treatment. Tumours may be intrinsically drug-resistant or develop resistance to chemotherapy during treatment. Acquired resistance is a particular problem, as tumours not only become resistant to the drugs originally used to treat them, but may also become cross-resistant to other drugs with different mechanisms of action. Resistance to chemotherapy is believed to cause treatment failure in over 90% of patients with metastatic cancer, and resistant micrometastic tumour cells may also reduce the effectiveness of chemotherapy in the adjuvant setting. Clearly, if drug resistance could be overcome, the impact on survival would be highly significant. This review focuses on molecular mechanisms of drug resistance that operate to reduce drug sensitivity in cancer cells. Drug resistance can occur at many levels, including increased drug efflux, drug inactivation, alterations in drug target, processing of drug-induced damage, and evasion of apoptosis. Advances in DNA microarray and proteomic technology, and the ongoing development of new targeted therapies have opened up new opportunities to combat drug resistance. We are now able to characterize the signalling pathways involved in regulating tumour cell response to chemotherapy more completely than ever before. This will facilitate the future development of rational combined chemotherapy regimens, in which the newer targeted therapies are used in combination with cytotoxic drugs to enhance chemotherapy activity. The ability to predict response to chemotherapy and to modulate this response with targeted therapies will permit selection of the best treatment for individual patients.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/path.1706

Molecular mechanisms of drug resistance

Since its approval in 1979 cisplatin has become an important component in chemotherapy regimes for the treatment of ovarian, testicular, lung and bladder cancers, as well as lymphomas, myelomas and melanoma. Unfortunately its continued use is greatly limited by severe dose limiting side effects and intrinsic or acquired drug resistance. Over the last 30 years, 23 other platinum-based drugs have entered clinical trials with only two (carboplatin and oxaliplatin) of these gaining international marketing approval, and another three (nedaplatin, lobaplatin and heptaplatin) gaining approval in individual nations. During this time there have been more failures than successes with the development of 14 drugs being halted during clinical trials. Currently there are four drugs in the various phases of clinical trial (satraplatin, picoplatin, Lipoplatin™ and ProLindac™). No new small molecule platinum drug has entered clinical trials since 1999 which is representative of a shift in focus away from drug design and towards drug delivery in the last decade. In this perspective article we update the status of platinum anticancer drugs currently approved for use, those undergoing clinical trials and those discontinued during clinical trials, and discuss the results in the context of where we believe the field will develop over the next decade.

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The status of platinum anticancer drugs in the clinic and in clinical trials.

Acquisition of the ability to evade cellular suicide, or apoptosis, is one of the master switches that contributes to cellular transformation and, ultimately, to invasive cancer. Much has been learned about the molecular organization of apoptotic pathways and their regulators, but the identification and validation of translational targets for apoptosis-based cancer therapy has posed a great challenge. Survivin is an attractive candidate for cancer therapy, so what is its potential applicability in the clinic?

Validating survivin as a cancer therapeutic target.

Survivin is a member of the inhibitor of apoptosis (IAP) gene family that has attracted attention from several viewpoints of basic and translational research. Its cell cycle-regulated expression at mitosis and association with the mitotic apparatus have been of interest to cell biologists studying faithful segregation of sister chromatids and timely separation of daughter cells. Investigators interested in mechanisms of apoptosis have found survivin an evolving challenge: while survivin inhibits apoptosis in vitro and in vivo, this pathway may be more selective as compared to cytoprotection mediated by other IAPs. Finally, basic and translational researchers in cancer biology have converged on survivin as a pivotal cancer gene, not simply for its sharp expression in tumors and not in normal tissues, but also for the potential exploitation of this pathway in cancer diagnosis and therapy. The objective of the present contribution is to line up current evidence and emerging concepts on the multifaceted functions of survivin in cell death and cell division, and how this pathway is being pursued for novel cancer therapeutic strategies.

Survivin, versatile modulation of cell division and apoptosis in cancer

Stable transfection of human ovarian carcinoma cells with survivin cDNA caused a four- to sixfold increase in cell resistance to taxotere and taxol (two-sided Student's t test, p 30% positive cells), detected by immunohistochemistry in 90/124 (73%) advanced ovarian carcinomas, were significantly associated with clinical resistance to a taxol/platinum-based regimen but unrelated to tumour shrinkage following cisplatin-including combinations (non-taxol based). In the 95 patients receiving a taxol/platinum-based regimen, survivin overexpression correlated with a lower clinical or pathologic complete remission rate than absent/low protein expression (43 vs 75%, p = 0.0058 by logistic regression adjusted for tumour stage, histological grade and p53 expression). Conversely, in the 29 cases treated with cisplatin-containing regimens (not taxol based), survivin expression was unrelated to tumour response. Cellular studies and clinical data suggest a direct link between survivin expression and tumour cell susceptibility to taxol.

Expression of the anti-apoptotic gene survivin correlates with taxol resistance in human ovarian cancer.

Survivin is a structurally unique member of the inhibitor of apoptosis (IAP) family of proteins that is potentially involved in both control of cell division and inhibition of apoptosis (1). Specifically, its antiapoptotic function is related to the ability to directly or indirectly inhibit caspases (2). The notion that survivin is overexpressed in most of the common human tumors (3, 4) but absent in normal adult tissues with only a few exceptions (5, 6) has led to the proposal of survivin as a promising therapeutic target for novel anticancer therapies (7). Indeed, in October 2001, Mesri et al. (8) reported in the JCI that infection with a replication-deficient adenovirus encoding a Thr34→Ala mutant of survivin caused apoptosis in human tumor cell lines of different histology and reduced tumor growth in xenograft breast cancer models. Moreover, inhibition of survivin expression enhanced taxol-induced cell death in tumor cells.

As an alternative strategy for survivin inhibition we developed hammerhead ribozymes targeting the 3′ end of the CUA110 (RZ1) and the GUC294 (RZ7) triplets in the survivin mRNA. In a cell-free system, both ribozymes induced cleavage of a synthetic RNA substrate obtained by cloning a portion of survivin mRNA, with cleavage products being detectable starting from a ribozyme/substrate ratio of 1:0.5. Conversely, the catalytically inactive mutRZ1 (which was produced by introducing a mutation in the catalytic core of the active ribozyme RZ1 and was used as control throughout the study) did not show any cleavage activity. RZ1, RZ7, and mutRZ1 sequences were inserted into the pRC expression vector under the control of the cytomega-lovirus promoter and transfected into the human metastatic melanoma cell line JR8 overexpressing survivin. For the present study we selected three stably transfected clones proven to endogenously express RZ1 (clone RZ1/C), RZ7 (clone RZ7/H), or mutRZ1 (clone mutRZ1/B). RZ1/C and RZ7/H cells were characterized by a markedly lower survivin protein level (68% and 60% lower, respectively) than JR8 parental cells, whereas a negligible reduction (13%) in survivin expression was observed in mutRZ1/B cells (Figure ​(Figure1a).1a). To evaluate the effect of survivin inhibition on the susceptibility of melanoma cells to undergo cisplatin-induced apoptosis, we treated the different clones with 10 μg/ml of the drug for 1 hour and determined the presence of apoptotic nuclei in cells stained with propidium iodide under fluorescence micro-scopy at 72 hours after treatment. A very modest apoptotic response was ob-served in the mutRZ1/B cells, whereas a significant increase in the percentage of apoptotic cells was observed in RZ1/C (P = 0.01) and RZ7/H (P = 0.005) cells (Figure ​(Figure1b).1b). Processing of caspase-3 to its active subunits of approximately 17 and 19 kDa was observed in all three drug-treated clones. However, the caspase-3 catalytic activity as assessed by hydrolysis of the fluorogenic substrate N-Acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-AMC) was about threefold higher in RZ1/C and RZ7/H clones than in the mutRZ1/B clone (Figure ​(Figure11c).



Figure 1

(a) Survivin protein expression in JR8 parental cells and melanoma cell clones transfected with the active ribozymes RZ1 (RZ1/C clone) and RZ7 (RZ7/H clone) or with the mutant ribozyme mutRZ1 (mutRZ1/B clone). Western blots were probed with a polyclonal ...



These results are in agreement with the previous finding of Grossman et al. (9), who observed enhancement of cisplatin-induced apoptosis by expression of the survivin Thr34→Ala mutant in YUSAC2 melanoma cells. Unlike what was reported by these authors, attenuation of survivin expression in our melanoma cell system was not sufficient to appreciably trigger apoptosis in the absence of other stimuli. Other antiapoptotic factors besides survivin, such as bcl-2 and bcl-xL, are strongly expressed in JR8 cells and may contribute to preventing programmed cell death in this tumor model. However, it should be stressed that in JR8 cells survivin expression was attenuated but not completely abrogated. It may be that inhibition below a certain threshold is insufficient to determine a proapoptotic effect. Interestingly, and in accordance with such a hypothesis, when we transduced the human prostate cancer cells DU145 with a Moloney-based retroviral vector carrying the catalytic sequence of the ribozyme RZ7, we were able to select a ribozyme-expressing clone characterized by an almost complete abrogation of survivin expression (99.5% lower compared with control cells, as assessed by Western blotting, and lack of detectable protein expression by confocal microscopy). This ribozyme-expressing clone also showed a markedly higher percentage of apoptotic cells than control culture (20% and 3% of cells on the overall cell population, respectively).

In conclusion, the present results obtained with the ribozyme-mediated approach in melanoma cells extend and corroborate earlier evidence indicating that attenuating survivin expression renders these cells more susceptible to cisplatin-induced apoptosis. These data also suggest a possible strategy to enhance the chemosensitivity profile of such a drug-refractory human malignancy.

/pdf/ribozyme-mediated-attenuation-of-survivin-expression-52vpyhuw83.pdf

Ribozyme-mediated attenuation of survivin expression sensitizes human melanoma cells to cisplatin-induced apoptosis

Radiosensitization of human melanoma cells by ribozyme-mediated inhibition of survivin expression.

Survivin is a member of the inhibitor of apoptosis protein (IAP) family, which has been implicated in inhibition of apoptosis and control of mitotic progression. The finding that survivin is overexpressed in most human tumors but absent in normal adult tissues has led to the proposal of survivin as a promising therapeutic target for anticancer therapies. We decided to evaluate the effects of a ribozyme-based strategy for survivin inhibition in androgen-independent human prostate cancer cells. We constructed a Moloney-based retroviral vector expressing a ribozyme targeting the 3′ end of the CUA110 triplet in survivin mRNA, encoded as a chimeric RNA within adenoviral VA1 RNA. Polyclonal cell populations obtained by infection with the retroviral vector of two androgen-independent human prostate cancer cell lines (DU145 and PC-3) were selected for the study. Ribozyme-expressing prostate cancer cells were characterized by a significant reduction of survivin expression compared to parental cells transduced with a control ribozyme; the cells became polyploid, underwent caspase-9-dependent apoptosis and showed an altered pattern of gene expression, as detected by oligonucleotide array analysis. Survivin inhibition also increased the susceptibility of prostate cancer cells to cisplatin-induced apoptosis and prevented tumor formation when cells were xenografted in athymic nude mice. These findings suggest that manipulation of the antiapoptotic survivin pathway may provide a novel approach for the treatment of androgen-independent prostate cancer.

/pdf/ribozyme-mediated-inhibition-of-survivin-expression-1nfgdni014.pdf

Ribozyme-mediated inhibition of survivin expression increases spontaneous and drug-induced apoptosis and decreases the tumorigenic potential of human prostate cancer cells

Gennaro Colella

Papers

Expression of the anti-apoptotic gene survivin correlates with taxol resistance in human ovarian cancer.

Ribozyme-mediated attenuation of survivin expression sensitizes human melanoma cells to cisplatin-induced apoptosis

Radiosensitization of human melanoma cells by ribozyme-mediated inhibition of survivin expression.

Ribozyme-mediated inhibition of survivin expression increases spontaneous and drug-induced apoptosis and decreases the tumorigenic potential of human prostate cancer cells

Inhibition of Telomerase Activity by a Hammerhead Ribozyme Targeting the RNA Component of Telomerase in Human Melanoma Cells