Preclinical toxicity and toxicokinetics of GTI-2040, a phosphorothioate oligonucleotide targeting ribonucleotide reductase R2.

PURPOSE: GTI-2040, a 20-mer phosphorothioate oligonucleotide, was designed to hybridize to the mRNA sequence of human ribonucleotide reductase R2. GTI-2040 has been shown to inhibit human cancer cell proliferation by downregulation of R2 expression in vitro and to significantly inhibit tumor growth in xenograft models of human cancer in mice. As part of the safety evaluation for human clinical trials, the toxicity and toxicokinetics of GTI-2040 were determined in Sprague-Dawley rats and rhesus monkeys. METHODS: GTI-2040 was administered to rats at 2, 10, and 50 mg/kg/day by bolus intravenous injection every second day for 21 days with a 21-day recovery. In monkeys, an acute study was performed with single, escalating doses of GTI-2040 ranging from 10 to 80 mg/kg given as a 24-h continuous intravenous infusion. As well, a 21-day, continuous intravenous infusion study with GTI-2040 was conducted in monkeys at 2, 10, and 50 mg/kg/day, with a 3-week recovery. Blood sampling was done to measure GTI-2040 plasma concentrations, metabolites, and pharmacokinetic parameters, and tissues were collected to assess the distribution of GTI-2040 and/or metabolites. RESULTS: The toxicities of GTI-2040 in both rats and monkeys were typical for the phosphorothioate oligonucleotide class of compounds. In monkeys, there was a dose-related increase in GTI-2040 plasma levels with concomitant increase in complement activation and prolongation of activated partial thromboplastin time. In both rats and monkeys, the tissues having the highest concentrations of GTI-2040 (kidney, liver, spleen) had the largest dose-related toxic effects. Adverse effects were diminished or absent in the recovery animals. CONCLUSIONS: GTI-2040 was well tolerated when infused over 24 h at doses up to 80 mg/kg in monkeys. In rats and monkeys, GTI-2040 was reasonably well tolerated and showed reversible toxicities when administered at doses up to 50 mg/kg/day for 21 days. The no observed adverse effect dose level for GTI-2040 in both animal species was 2 mg/kg/day. There were no apparent sequence-specific effects related to the interaction of GTI-2040 with the R2 component of the mRNA expressing ribonucleotide reductase.

GTI-2040 displays cooperative anti-tumor activity when combined with interferon alpha against human renal carcinoma xenografts.

GTI-2040, an antisense oligonucleotide targeting the small subunit of ribonucleotide reductase, acts as an anti-tumor agent in animal models of human cancer. In the present study, the anti-tumor activity of GTI-2040, in combination with interferon alpha (IFNalpha) was investigated against human renal cell carcinoma tumors xenografted into mice. The human renal cell carcinoma cell lines, Caki-1 and A498 were sensitive to IFNalpha both in vitro and when implanted into mice. In combination with GTI-2040 there were cooperative effects at intermediate doses of the two agents and complete tumor regression at higher combination doses. A control oligonucleotide was not effective as a monotherapy and did not improve the efficacy of IFNalpha. The effect of combination treatment on apoptosis and proliferation of tumor cells, isolated from xenografted tumors, was examined by histochemistry. GTI-2040 increased the percentage of cells undergoing apoptosis with a concomitant decrease in proliferation. IFNalpha alone had no effect but in combination with GTI-2040 resulted in increased apoptosis and decreased proliferation compared to GTI-2040 alone. Taken together these results expand the potential clinical applications of GTI-2040 to include combination therapy with IFNalpha.

RNA interference targeting the R2 subunit of ribonucleotide reductase inhibits growth of tumor cells in vitro and in vivo.

RNA interference, a posttranscriptional gene-silencing mechanism, has received considerable attention for its potential as a new therapeutic strategy to treat human diseases and conditions including cancer. Various studies have supported a role for the R2 subunit of ribonucleotide reductase in cancer progression and metastasis. Short interfering siRNA 1284 was designed to target R2. In vitro studies, in which three different human tumor cell lines (A498, HT-29 and A2058) were transfected with short interfering siRNA 1284, demonstrate sequence-specific down-regulation of R2, which coincides with a decrease in cell proliferation, and cell cycle inhibition. In vivo studies with xenograft mouse models, generated from the same tumor cell lines, indicate that treatment with short interfering siRNA 1284 leads to inhibition of tumor growth and this effect was found to be dose dependent. Taken together, these results suggest that short interfering siRNA 1284, targeting R2, has great potential to serve as a therapeutic agent towards the treatment of human cancers.

Analysis of ribonucleotide reductase M2 mRNA levels in patient samples after GTI-2040 antisense drug treatment.

This study describes the development of a rapid and practical real-time RT-PCR method to quantify ribonucleotide reductase M2 (RRM2) mRNA in tumor and peripheral white blood cells (WBCs) from patients treated with GTI-2040, an antisense drug currently in clinical trials. In order to assess target down-regulation by GTI-2040, RRM2 mRNA expression levels were analyzed in pre- and post-treatment samples from a phase II clinical trial of GTI-2040 combined with capecitabine in patients with metastatic breast cancer. Target gene RRM2 mRNA levels were evaluated using quantitative RT-PCR method: real-time PCR (TaqMan) with fluorescein labeled probes on an ABI 7900HT instrument, with additional post-processing of the data to adjust for differences in total RNA in-put across the samples. Data are presented from a patient for whom both biopsy and PBMC samples were available, demonstrating applicability of this reproducible, highly sensitive real-time RT-PCR method for the detection and quantification of mRNAs for RRM2 in human WBC and tissue samples. By providing quantitative measurement of changes in target gene expression, this method may provide an opportunity to determine the correlation between target response to GTI-2040 antisense and clinical response in patients. Furthermore this assay may assess whether WBC samples are an appropriate surrogate tissue for approximating target down-regulation in the tumor.

Anti-proliferative and anti-tumor effects of antisense oligonucleotide GTI-2601 targeted against human thioredoxin.

Human thioredoxin has been implicated in cancer as a growth stimulator through regulation of DNA replication and growth factor activity, as a modulator of transcription factor activity, and as an inhibitor of apoptosis. In the present study, the steady-state level of thioredoxin protein was examined in a number of cancer cell lines. Interestingly, thioredoxin expression is elevated in a variety of human tumor cell lines compared with normal cell lines. The altered expression of thioredoxin in tumor cells suggests it may be a target in the development of novel therapeutic agents for the treatment and prevention of cancer. Further to this possibility, 26 phosphorothioate antisense oligodeoxynucleotides (PS-AS-ODNs) were evaluated for the ability to inhibit thioredoxin expression in cell culture. One PS-AS-ODN, GTI-2601, specifically reduced the levels of thioredoxin mRNA and protein, exhibited potent anti-proliferative effects on colony formation in vitro, and had anti-tumor effects in human tumor xenograft mouse models in vivo. Sequence-specific decreases in thioredoxin expression levels were accompanied by significant suppression of tumor growth in mice. Taken together, these data suggest that thioredoxin may be a useful target for developing PS-AS-ODNs as drug candidates against human cancer.

GTI-2501, an antisense agent targeting R1, the large subunit of human ribonucleotide reductase, shows potent anti-tumor activity against a variety of tumors.

GTI-2501 is a 20-mer oligonucleotide that is complementary to a coding region in the mRNA of R1, the large subunit of ribonucleotide reductase (RNR). In vitro studies, have demonstrated that GTI-2501 decreases mRNA and protein levels of R1 in a sequence-specific and dose-dependent manner. Furthermore, GTI-2501 inhibits the growth of human lung, liver, ovary, brain, melanoma, breast and pancreatic tumor cells in colony forming assays. In vivo studies have shown that GTI-2501 significantly inhibits growth of human colon, pancreas, lung, breast, renal, ovarian, melanoma, brain glioblastoma-astrocytoma, and prostatic tumors in CD-1 nude, Balb/c nude and/or SCID mice. GTI-2501 treatment caused total regression of human breast and renal tumor xenografts in mice. These effects are not observed with a scrambled control oligonucleotide containing the same base content but not complementary to R1. GTI-2501 specifically inhibits metastasis of human melanoma cells to the lungs in CD-1 athymic nude mice and prolongs the survival of mice bearing human lymphoma. Taken together these results suggest that an antisense mechanism of action is responsible for growth inhibition in vitro and in vivo and that GTI-2501 can act as a selective and specific anti-tumor agent.

Optimization of the PAXgene blood RNA extraction system for gene expression analysis of clinical samples.

One major problem associated with collecting whole blood from patients for use as a source of RNA in gene expression studies is that the RNA degrades during collection and storage. Preservation of RNA quality is vital in such studies because the stability of the RNA ultimately affects analysis of gene expression. In this study the PAXgene blood collection system was compared with a standard erythrocyte lysis method for isolating RNA from blood samples. The methods were compared in terms of RNA yield, RNA stabilization, and DNA contamination. The study also included the downstream application to RT-PCR analysis for relative mRNA expression levels of the ribonucleotide reductase subunits R1 and R2. The results show that blood collection in conventional collection tubes, and leukocyte isolation by erythrocyte lysis lead to significant degradation of RNA. Our findings confirm the ability of PAXgene to stabilize RNA in whole blood; however, RNA extracted by the PAXgene method contained significant DNA contamination. Given the low basal expression of the target genes analyzed in this study, contaminating DNA could potentially affect accurate interpretation of RT-PCR data. As a result, the PAXgene protocol was optimized to include off-column DNase treatments, which yielded high-quality RNA suitable for gene expression studies. Furthermore, the results suggest that RNA isolation with PAXgene is advantageous compared to traditional extraction methods for RT-PCR analysis of large or different-sized amplicons.

NC381, a novel anticancer agent, arrests the cell cycle in G0-G1 and inhibits lung tumor cell growth in vitro and in vivo.

Although clotrimazole (CLT), an antifungal drug, inhibits tumor cell proliferation and angiogenesis, its clinical application is hampered by significant hepatotoxicity due to the presence of an imidazole moiety. In our attempts to develop CLT analogs that are devoid of imidazole and are as efficacious as CLT, one pharmacophore designated NC381 was generated and shown to inhibit tumor cell growth via a mechanism similar to that of CLT. In vitro, treatment of NCI-H460 nonsmall cell lung cancer (NSCLC) cells with NC381 inhibited growth in a time-dependent manner. Flow cytometric analysis demonstrated that the decrease in cell growth was associated with inhibition of cell cycle progression at the G(1)-S phase transition, resulting in G(0)-G(1) arrest. There was a concomitant inhibition of cyclin D1 expression and subsequent reduction in the formation of the cyclin D1-CDK4 complex. Consistent with a decrease in the cyclin D1-CDK4 complex, NC381 treatment resulted in significant inhibition of pRb phosphorylation. There also were changes in the activity of cell cycle-related proteins, including p16(Ink4) and p27(Kip1). Together, these results are consistent with a model in which NC381 arrests cell cycle progression via inhibition of the pathway that promotes exit from the G(1) phase of the cell cycle. Furthermore, the clinical applicability of NC381 was evaluated in an in vivo murine xenograft model of human NSCLC (NCI-H460). NC381 treatment resulted in significant inhibition of tumor growth. Given the poor prognosis and the limited treatment options available, the present results underscore the potential of NC381 in the treatment of human NSCLC.

Triaryl methane derivatives as antiproliferative agents.

Clotrimazole (CLT) 1, a synthetic anti-fungal imidazole derivative, inhibits tumor cell proliferation and angiogenesis. In the current study, flow cytometric analysis demonstrated that the decrease in tumor cell growth by CLT 1 was associated with inhibition of cell cycle progression at the G(1)-S phase transition, resulting in G(0)-G(1) arrest. A series of CLT 1 analogues has been generated in order to develop CLT 1 derivatives that are devoid of the imidazole moiety which is responsible for the hepatoxicity associated with CLT 1 while retaining CLT 1 efficacy. The majority of these analogues demonstrate in vitro antiproliferative activity ranging from submicromolar to micromolar concentrations.

Adenovirus-mediated ribonucleotide reductase R1 gene therapy of human colon adenocarcinoma.

Ribonucleotide reductase is the enzyme responsible for the reduction of ribonucleotides to their corresponding deoxyribonucleotides for DNA synthesis. Ribonucleotide reductase is a multisubunit complex containing two polypeptides, R1 and R2. In addition to catalytic and allosteric regulatory functions, the R1 subunit appears to act as a novel tumor suppressor. Previous studies demonstrated that overexpression of mouse R1 resulted in suppression of tumorigenicity and metastatic potential, whereas expression of antisense RNA, complementary to R1 mRNA, increased anchorage-independent growth of ras-transformed NIH 3T3 cells. The current study investigated the potential of R1 gene therapy for human cancer using a recombinant adenovirus encoding the human R1 gene (rAd5-R1). Recombinant viruses were constructed by FLP-mediated site-specific recombination and demonstrated high infectivity of a human colon carcinoma cell line (Colo320 HRS), as assessed by expression of a viral encoded beta-Gal gene (rAd5-LacZ). R1mRNA and protein were overexpressed in Colo320 HRS cells infected with rAd5-R1 compared with untreated or rAd5-LacZ-infected cells. Infection with rAd5-R1 inhibited Colo320 HRS cell proliferation, in vitro, in a time- and dose-dependent manner. When Colo320 HRS cells were treated with rAd5-R1, before injection into CD-1 mice, there was complete inhibition of tumor growth compared with treatment with rAd5-LacZ. Furthermore, intratumoral injection of rAd5-R1 into Colo320 HRS tumor xenografts inhibited tumor growth in CD-1 mice compared with rAd5-LacZ treated mice (P = 0.0001). These results demonstrate gene-specific antitumor effects of R1 and suggest that rAd5-R1 gene therapy has the potential to improve currently available treatments for colon cancer.

GTI-2040, an antisense agent targeting the small subunit component (R2) of human ribonucleotide reductase, shows potent antitumor activity against a variety of tumors.

GTI-2040 is a 20-mer oligonucleotide that is complementary to a coding region in the mRNA of the R2 small subunit component of human ribonucleotide reductase. In vitro studies using a number of human tumor cell lines have demonstrated that GTI-2040 decreases mRNA and protein levels of R2 in a sequence- and target-specific manner. In vivo studies have shown that GTI-2040 significantly inhibits growth of human colon tumors (adenocarcinoma), pancreatic tumors (adenocarcinoma), liver tumors, lung tumors, breast tumors (adenocarcinoma), renal tumors, ovarian tumors (adenocarcinoma), melanoma, brain glioblastoma-astrocytoma, prostatic tumors, and cervical tumors in nude and/or severe combined immunodeficient mice. Antitumor effects were not observed with an oligonucleotide containing four mismatches to the R2 sequence or with a scrambled sequence containing the same base content but not complementary to R2. This suggests that an antisense mechanism is responsible for the in vivo observations. In addition to tumor growth assays, GTI-2040 was tested in a murine model of human lymphoma. Treatment of severe combined immunodeficient mice bearing Burkitt's lymphoma with GTI-2040, but not control oligonucleotides, greatly extended the survival of mice, and survival extended well beyond the treatment period. Finally, GTI-2040 specifically inhibits metastasis of human melanoma cells to the lungs in nude mice. Taken together, the results of these studies indicate that GTI-2040 can act as a selective and specific anticancer agent against a broad range of human tumors.