Effect of aspirin on protein binding and tissue disposition of oligonucleotide phosphorothioate in rats

Introduction and History of the Chemistry of Nucleic Acids Therapeutics

This introduction charts the history of the development of the major chemical modifications that have influenced the development of nucleic acids therapeutics focusing in particular on antisense oligonucleotide analogues carrying modifications in the backbone and sugar. Brief mention is made of siRNA development and other applications that have by and large utilized the same modifications. We also point out the pitfalls of the use of nucleic acids as drugs, such as their unwanted interactions with pattern recognition receptors, which can be mitigated by chemical modification or used as immunotherapeutic agents.

The Evolution of Antisense Oligonucleotide Chemistry-A Personal Journey

Over the last four decades, tremendous progress has been made in use of synthetic oligonucleotides as therapeutics. This has been possible largely by introducing chemical modifications to provide drug like properties to oligonucleotides. In this article I have summarized twists and turns on use of chemical modifications and their road to success and highlight areas of future directions.

Phosphorothioate modified oligonucleotide-protein interactions

Antisense oligonucleotides (ASOs) interact with target RNAs via hybridization to modulate gene expression through different mechanisms. ASO therapeutics are chemically modified and include phosphorothioate (PS) backbone modifications and different ribose and base modifications to improve pharmacological properties. Modified PS ASOs display better binding affinity to the target RNAs and increased binding to proteins. Moreover, PS ASO protein interactions can affect many aspects of their performance, including distribution and tissue delivery, cellular uptake, intracellular trafficking, potency and toxicity. In this review, we summarize recent progress in understanding PS ASO protein interactions, highlighting the proteins with which PS ASOs interact, the influence of PS ASO protein interactions on ASO performance, and the structure activity relationships of PS ASO modification and protein interactions. A detailed understanding of these interactions can aid in the design of safer and more potent ASO drugs, as illustrated by recent findings that altering ASO chemical modifications dramatically improves therapeutic index.

Pharmacokinetic Profile and Acute Toxicological Properties of a Novel Radiosensitizer Cytosine-Phosphate-Guanosine Oligodeoxynucleotide 107 in Mice Following Intravenous and Orthotopic Administration

The synthetic cytosine-phosphate-guanosine oligodeoxynucleotide 107 (CpG ODN107) is a novel radiosensitizer for glioma treatment. However, the information related to its pharmacokinetics and toxicity remains unclear. Therefore, the plasma pharmacokinetics, distribution, elimination, and acute toxicity of CpG ODN107 in mice were investigated in the present experiments. The results from the liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay showed that the plasma elimination half-life (t1/2β) of CpG ODN107 in BALB/c mice varied slightly with the dose, and it was 0.65, 0.49, and 0.50 h at the intravenous doses of 2.5, 5, and 10 mg/kg, respectively. CpG ODN107 rapidly and widely distributed in organs/tissues, except the brain and testes. The highest concentrations were found in the liver (28.6% of the administered dose after 0.5 h) and the kidneys (5.7% of the administered dose after 1 h). CpG ODN107 (0.3, 3, and 30 μg/mL) could highly bind to human and mouse plasma proteins in vitro. CpG ODN107 in the forms of prototype was excreted in urine (1.79%) and feces (0.91%), and its shortened metabolites were excreted in urine (2.1%) and feces (2.2%) within the first 24 h. The mice in vivo optical image showed CpG ODN107 labeled with Alexa Fluor 680 fluorochrome (AF680) accumulated in the brain after orthotopic injection, eliminated very slowly, and excreted in urine compared with poly T labeled with AF680. The median lethal dose (LD50) of CpG ODN107 was 75.7 mg/kg for mice; this dose only could produce apparent spleen and liver damage, in line with the distribution features of CpG ODN. In conclusion, our present pharmacokinetic and toxicity investigation will provide helpful information to further pharmacodynamic and pharmacokinetic research of CpG ODN107 and other oligodeoxynucleotide drugs in the future.

Development and validation of a rapid HPLC method for quantitation of SP-141, a novel pyrido[b]indole anticancer agent, and an initial pharmacokinetic study in mice

There is an increasing interest in targeting the MDM2 oncogene for cancer therapy. SP-141, a novel designed small molecule MDM2 inhibitor, exerts excellent in vitro and in vivo anticancer activity. To facilitate the preclinical development of this candidate anticancer agent, we have developed an HPLC method for the quantitative analysis of SP-141. The method was validated to be precise, accurate, and specific, with a linear range of 16.2-32,400 ng/mL in plasma, 16.2-6480 ng/mL in homogenates of brain, heart, liver, kidneys, lungs, muscle and tumor, and 32.4-6480 ng/mL in spleen homogenates. The lower limit of quantification was 16.2 ng/mL in plasma and all the tissue homogenates, except for spleen homogenates, where it was 32.4 ng/mL. The intra- and inter-assay precisions (coefficient of variation) were between 0.86 and 13.39%, and accuracies (relative errors) ranged from -8.50 to 13.92%. The relative recoveries were 85.6-113.38%. SP-141 was stable in mouse plasma, modestly plasma bound and metabolized by S9 microsomal enzymes. We performed an initial pharmacokinetic study in tumor-bearing nude mice, demonstrating that SP-141 has a short half-life in plasma and wide tissue distribution. In summary, this HPLC method can be used in future preclinical and clinical investigations of SP-141.

KCN1, a novel synthetic sulfonamide anticancer agent: in vitro and in vivo anti-pancreatic cancer activities and preclinical pharmacology

The purpose of the present study was to determine the in vitro and in vivo anti-cancer activity and pharmacological properties of 3,4-dimethoxy-N-[(2,2-dimethyl-2H-chromen-6-yl)methyl]-N-phenylbenzenesulfonamide, KCN1. In the present study, we investigated the in vitro activity of KCN1 on cell proliferation and cell cycle distribution of pancreatic cancer cells, using the MTT and BrdUrd assays, and flow cytometry. The in vivo anti-cancer effects of KCN1 were evaluated in two distinct xenograft models of pancreatic cancer. We also developed an HPLC method for the quantitation of the compound, and examined its stability in mouse plasma, plasma protein binding, and degradation by mouse S9 microsomal enzymes. Furthermore, we examined the pharmacokinetics of KCN1 following intravenous or intraperitoneal injection in mice. Results showed that, in a dose-dependent manner, KCN1 inhibited cell growth and induced cell cycle arrest in human pancreatic cancer cells in vitro, and showed in vivo anticancer efficacy in mice bearing Panc-1 or Mia Paca-2 tumor xenografts. The HPLC method provided linear detection of KCN1 in all of the matrices in the range from 0.1 to 100 µM, and had a lower limit of detection of 0.085 µM in mouse plasma. KCN1 was very stable in mouse plasma, extensively plasma bound, and metabolized by S9 microsomal enzymes. The pharmacokinetic studies indicated that KCN1 could be detected in all of the tissues examined, most for at least 24 h. In conclusion, our preclinical data indicate that KCN1 is a potential therapeutic agent for pancreatic cancer, providing a basis for its future development.

Preclinical pharmacology of novel indolecarboxamide ML-970, an investigative anticancer agent

PURPOSE

ML-970 (AS-I-145; NSC 716970) is an indolecarboxamide synthesized as a less toxic analog of CC-1065 and duocarmycin, a natural product that binds the A-T-rich DNA minor groove and alkylates DNA. The NCI60 screening showed that ML-970 had potent cytotoxic activity, with an average GI(50) of 34 nM. The aim of this study is to define the pharmacological properties of this novel anticancer agent.

METHODS

We established an HPLC method for the compound, examined its stability, protein binding, and metabolism by S9 enzymes, and conducted pharmacokinetic studies of the compound in two strains of mice using two different formulations.

RESULTS

ML-970 was relatively stable in plasma, being largely intact after an 8-h incubation in mouse plasma at 37°C. The compound was extensively bound to plasma proteins. ML-970 was only minimally metabolized by the enzymes present in S9 preparation and was not appreciably excreted in the urine or feces. The solution formulation provided higher C(max), AUC, F values, and greater bioavailability, although the suspension formulation resulted in a later T(max) and a slightly longer T(1/2). To determine the fate of the compound, we accomplished in-depth studies of tissue distribution; the results indicated that the compound undergoes extensive enterohepatic circulation.

CONCLUSIONS

The results obtained from this study will be relevant to the further development of the compound and may explain the lower myelotoxicity of this analog compared to CC-1065.

Preclinical pharmacology of BA-TPQ, a novel synthetic iminoquinone anticancer agent

Marine natural products and their synthetic derivatives represent a major source of novel candidate anti-cancer compounds. We have recently tested the anti-cancer activity of more than forty novel compounds based on an iminoquinone makaluvamine scaffold, and have found that many of the compounds exert potent cytotoxic activity against human cancer cell lines. One of the most potent compounds, BA-TPQ [(11,12),7-(benzylamino)-1,3,4,8-tetrahydropyrrolo[4,3,2-de]quinolin-8(1H)-one], was active against a variety of human cancer cell lines, and inhibited the growth of breast and prostate xenograft tumors in mice. However, there was some toxicity noted in the mice following administration of the compound. In order to further the development of BA-TPQ, and in a search for potential sites of accumulation that might underlie the observed toxicity of the compound, we accomplished preclinical pharmacological studies of the compound. We herein report the in vitro and in vivo pharmacological properties of BA-TPQ, including its stability in plasma, plasma protein binding, metabolism by S9 enzymes, and plasma and tissue distribution. We believe these studies will be useful for further investigations, and may be useful for other investigators examining the use of similar compounds for cancer therapy.

Galactosylated low molecular weight chitosan as a carrier delivering oligonucleotides to Kupffer cells instead of hepatocytes in vivo

The in vivo cellular localization of oligodeoxynucleotides (ODNs) delivered by galactosylated low molecular weight chitosan (gal-LMWC) was investigated. The gal-LMWCs preference for Kupffer cells was confirmed by in vivo and in vitro experiments. Furthermore, asialoglycoprotein receptor (ASGPr) was studied as a possible surface lectin which may involved in the endocytosis of the gal-LMWC/ODN complexes. Results showed that the gal-LMWC/ODN complex accumulated in liver when injected intravenously (i.v.). Further studies revealed that 50.6% of the complex was taken up by Kupffer cells in liver, 33.2% was taken up by endothelial cells, and only 16.2% of the complex was taken up by parenchymal cells. In vitro results also confirmed the affinity of gal-LMWC to murine Kupffer cells. Inhibition of the transfection by lactose and N-acetyl galactosamine (GalNAc) suggested that the particles might enter macrophages via ASGPr and the inhibition by LMWC implied that there might be other lectins involved in the endocytosis. In summary, our studies revealed that gal-LMWC/ODN complex is inclined to enter into Kupffer cells rather than into liver parenchymal cells in vivo. Galactosylation may not be a proper means for targeting chitosan/DNA nanoparticles to hepatocytes but it does have the potential to be a Kupffer cells targeting strategy especially for delivering drugs for antiinflammation.

Plasma protein binding of an antisense oligonucleotide targeting human ICAM-1 (ISIS 2302)

In vitro ultrafiltration was used to determine the plasma protein-binding characteristics of phosphorothioate oligonucleotides (PS ODNs). Although there are binding data on multiple PS ODNs presented here, the focus of this research is on the protein-binding characteristics of ISIS 2302, a PS ODN targeting human intercellular adhesion molecule-1 (ICAM-1) mRNA, which is currently in clinical trials for the treatment of ulcerative colitis. ISIS 2302 was shown to be highly bound (> 97%) across species (mouse, rat, monkey, human), with the mouse having the least degree of binding. ISIS 2302 was highly bound to albumin and, to a lesser, extent alpha2-macroglobulin and had negligible binding to alpha1-acid glycoprotein. Ten shortened ODN metabolites (8, 10, and 12-19 nucleotides [nt] in length, truncated from the 3' end) were evaluated in human plasma. The degree of binding was reduced as the ODN metabolite length decreased. Three additional 20-nt (20-mer) PS ODNs (ISIS 3521, ISIS 2503, and ISIS 5132) of varying sequence but similar chemistry were evaluated. Although the tested PS ODNs were highly bound to plasma proteins, suggesting a commonality within the chemical class, these results suggested that the protein-binding characteristics in human plasma may be sequence dependent. Lastly, drug displacement studies with ISIS 2302 and other concomitant drugs with known protein-binding properties were conducted to provide information on potential drug interactions. Coadministered ISIS 2302 and other high-binding drugs evaluated in this study did not displace one another at supraclinical plasma concentrations and, thus, are not anticipated to cause any pharmacokinetic interaction in the clinic as a result of the displacement of binding to plasma proteins.

Preclinical pharmacology of the novel antitumor agent adaphostin, a tyrphostin analog that inhibits bcr/abl

PURPOSE

To define several pharmacological properties for the potential anticancer agent, adaphostin, in order to determine whether the compound is appropriate for clinical evaluation as an anticancer agent.

METHODS

The analytical procedure involved high-performance liquid chromatography and utilized an analytical J'Sphere ODS H-80 column.

RESULTS

The stability of adaphostin at two different concentrations was determined at temperatures of 37 degrees C, 4 degrees C, and -80 degrees C, in the plasma of mice, rats, dogs, and humans. The compound was most stable at the lower temperatures. At all temperatures, adaphostin was generally most stable in human plasma and least stable in dog plasma. Adaphostin bound strongly (>93%) to proteins in plasma from all four species. Following intravenous (i.v.) administration to mice (50 mg/kg; 150 mg/m(2)), plasma concentrations declined rapidly from 50 microM at 2 min to 1 microM at 2 h. Elimination was triexponential, with t (1/2) values of 1.1, 9.1, and 41.2 min. The Cl(tb) was 0.411 L/(min.m(2)), the V (dss) was 24.6 L/m(2), and the AUC was 927 microM.min. In a comparison of vehicles for intraperitoneal (i.p.) dosing, PEG 300 allowed the highest plasma concentrations of adaphostin. Bioavailability following an i.p. dose was greater than that following a subcutaneous dose, or that for a dose administered by oral gavage. For rats dosed i.v. with adaphostin (50 mg/kg; 300 mg/m(2)), plasma concentrations also decreased triexponentially, with t (1/2) values of 1.8, 10.6, and 136 min. Other pharmacokinetic values were Cl(tb) = 0.466 L/(min.m(2)), AUC = 1,161 microM.min, and V (dss)=8.0 L/m(2). Analysis of samples collected from two dogs dosed i.v. with adaphostin (7.5 mg/kg; 150 mg/m(2)) showed that plasma concentrations decreased in a biphasic manner, with individual values for t (1/2alpha) of 6.0 and 9.8 min for the distribution phase and t (1/2beta) of 40.6 and 66.2 min for the elimination phase. Other pharmacokinetic values were Cl(tb) = 0.565 and 0.852 L/(min.m(2)), AUC = 673 and 446 microM min, and V (dss) = 29.6 and 56.8 L/m(2).

CONCLUSIONS

The stability of adaphostin in plasma varies with species. In mice and dogs dosed with adaphostin, plasma concentrations of the compound decreased rapidly. The clearance of adaphostin from plasma, on an m(2) basis, was equivalent for mice and rats but more rapid in dogs. These results are relevant for assessing the pharmacologic and toxicologic profiles and the antitumor activity of adaphostin in humans.

Preclinical pharmacology of 2-methoxyantimycin A compounds as novel antitumor agents

PURPOSE

The present study was designed to determine pharmacological and biochemical properties of 2-methoxyantimycin A analogs (OMe-A1, OMe-A2, OMe-A3, and OMe-A5), which are novel antitumor compounds, and provide a basis for future pharmaceutical development, preclinical evaluation, and clinical trials.

METHODS

A high-performance liquid chromatography (HPLC) method was established and employed to assess the biostability of these analogs and to determine their pharmacokinetic properties in mice and rats.

RESULTS

In vitro biostability of the 2-methoxyantimycin analogs was esterase-dependent, compound-dependent, and species-dependent. In the absence of esterase inhibitors, all of the analogs were relatively unstable. Stability was greater, however, in human and dog plasma than in rat and mouse plasma. In the presence of esterase inhibitors, OMe-A1 was stable at 37 degrees C for 60 min in mouse and rat plasma, moderately stable in human plasma, and unstable in dog plasma. OMe-A2 was generally stable in all types of plasma. OMe-A3 was stable in dog and rat plasma, but not in human or mouse plasma. OMe-A5 was stable in human and dog plasma, but not in mouse or rat plasma. Each of these analogs was highly bound to plasma proteins. Of S9 fractions from four species, human S9 was least efficient in metabolizing OMe-A3. Following an intravenous dose of OMe-A1 in mice, plasma levels decreased rapidly, with an initial half-life of 2.7 min and a terminal half life of 34 min. Following an intraperitoneal dose in mice, plasma levels decreased less rapidly with a terminal half-life of 215 min. Following an intravenous dose of OMe-A1 or OMe-A3 in rats, plasma levels decreased more rapidly with initial half-lives of about 1.0 min. At an equivalent dose, OMe-A3 had a faster clearance than OMe-A1.

CONCLUSIONS

For 2-methoxyantimycin A analogs, species differences in biostability, metabolism, and pharmacokinetics may be pertinent in assessing their pharmacological and toxicological profiles and antitumor activity in humans.

Preclinical pharmacology of epothilone D, a novel tubulin-stabilizing antitumor agent

PURPOSE

To determine, for various species, the pharmacological and biochemical properties of epothilone D (EpoD) that are relevant in establishing an appropriate animal model for further evaluation of this promising antitumor agent.

METHODS

A method involving high-performance liquid chromatography (HPLC) was developed and used to assess the stability and protein binding of EpoD in plasma from various species, its metabolism by various S9 fractions, and its pharmacokinetics in mice.

RESULTS

EpoD was stable in dog and human plasma. In plasma from other species, stability decreased in the order: hamster > mouse > guinea pig > rat. EpoD was highly bound to proteins in dog and human plasma. In an evaluation of S9 fractions from mouse, rat, guinea pig, dog, and human, mouse S9 was most efficient in metabolizing EpoD. Following administration to CD2F1 mice, the initial half-lives for plasma elimination of EpoD were <5 min for an intravenous dose and <20 min for an intraperitoneal dose.

CONCLUSIONS

The species differences in EpoD biostability and metabolism may have implications in assessing its antitumor activity and pharmacologic and toxicologic profiles in humans. Relative to humans, the mouse is not a good model for disposition of EpoD; the dog would be more appropriate.

Plasma protein binding, lipoprotein distribution and uptake of free and lipid-associated BCL-2 antisense oligodeoxynucleotides (G3139) in human melanoma cells

The objectives of this study were to determine the protein binding and lipoprotein distribution of G3139 and G3139 lipoplexes following incubation in human plasma, assess complement activation of, and the effect of pre-incubation of G3139 and G3139 lipoplexes in human plasma on in vitro cellular uptake of G3139. Effect of concentration and time on incorporation of free and lipid associated (lipoplexes) [3H]Bcl-2 AO (25-600 ng/ml) into normolipidemic human plasma lipoproteins was determined by density gradient ultracentrifugation after incubation at 37 degrees C for 5, 30, 60 and 120 min. Protein binding in the lipoprotein deficient fractions (LPDP) was determined by equilibrium dialysis. Complement interaction was determined by ELISA after exposure of human plasma to AO+/- liposomes prepared in serial dilution. In vitro uptake of G3139 and G3139 lipoplexes into human melanoma cells was assessed qualitatively by fluorescence microscopy after 4-h exposure to G3139 (free or as lipoplexes) with or without pre-incubation of G3139 in normal human plasma. Analysis of Bcl-2 AO-lipoprotein interaction over time and concentration indicated no significant movement of the compound within the different lipoprotein and LPDP fractions. Majority of drug was recovered within LPDP fraction, and more than 85% of drug recovered within LPDP fraction was protein bound. No significant activation of complement was noted for either free AO or lipoplexes. Pre-incubation of free AO or AO-lipoplexes in human plasma resulted in a greater cellular uptake of AO-lipoplexes compared with plasma free controls. These findings suggest that the majority of [3H]Bcl-2 AO is plasma protein bound with little lipoprotein association and no significant movement between different lipoprotein and LPDP fractions. Plasma protein binding other than lipoprotein binding may be responsible for the difference in cellular uptake of free AO vs. cationic lipoplexes.

Fomivirsen: clinical pharmacology and potential drug interactions

Fomivirsen sodium is a 21-base phosphorothioate oligodeoxynucleotide complementary to the messenger RNA of the major immediate-early region proteins of human cytomegalovirus, and is a potent and selective antiviral agent for cytomegalovirus retinitis. Following intravitreal administration, fomivirsen is slowly cleared from vitreous with a half-life of approximately 55 hours in humans. Preclinical studies show that fomivirsen distributes to retina and is slowly metabolised by exonuclease digestion. Clearance from retina was shown to be similarly slow following loading from the vitreous. The estimated half-life for clearance of fomivirsen from retina was 78 hours in monkeys following a 115-microg dose. Because of the low doses coupled with slow disposition from the eye, measurable concentrations of drug are not detected in the systemic circulation following intravitreal administration. Systemically administered phosphorothioate oligodeoxynucleotides are highly bound to albumin and alpha2-macroglobulin in blood plasma. Because fomivirsen does not compete for oxidative metabolic processes involved in clearance of many xenobiotics, the most likely mechanism for drug interactions may be altered protein binding of a coadministered drug. The extremely low systemic exposure to this oligodeoxynucleotide following intravitreal administration largely negates its potential ability to interact with systemically administered drugs. Even if fomivirsen were able to access the blood, protein binding assays indicate that drugs that are site I and site II binders of albumin (warfarin, ibuprofen, salicylic acid) are not displaced in the presence of phosphorothioate oligodeoxynucleotides of various sequences at concentrations orders of magnitude higher than that seen for fomivirsen. Administration of fomivirsen with numerous systemically administered antiretrovirals (for example zidovudine and zalcitabine) as well as systemically administered anticytomegalovirus agents such as foscarnet and ganciclovir has been reported to be well tolerated. The only reported warning is a recommendation against administration within 2 to 4 weeks of cidofovir treatment due to an increased risk of ocular inflammation.

Oligonucleotide conjugates as potential antisense drugs with improved uptake, biodistribution, targeted delivery, and mechanism of action

This review summarizes the effect of conjugating small molecules and large biomacromolecules to antisense oligonucleotides to improve their therapeutic potential. In many cases, favorable changes in pharmacokinetic and pharmacodynamic properties were observed. Opportunities exist to change the terminating mechanism of antisense action or to enhance the RNase H mode of action via conjugate formation.

Delivery of cholesteryl-conjugated phosphorothioate oligodeoxynucleotides to Kupffer cells by lactosylated low-density lipoprotein

The efficacy of antisense oligonucleotides depends on the ability to reach in vivo their target cells. We aim to develop strategies to enhance uptake of phosphorothioate oligodeoxynucleotides by Kupffer cells. To this end, we conjugated cholesterol to ISIS-3082, a phosphorothioate oligodeoxynucleotide specific for intercellular adhesion molecule-1. The cholesterol-conjugated oligonucleotide, denoted ISIS-9388, associated readily with lactosylated low-density lipoprotein (LacLDL), a lipidic carrier that is taken up by galactose receptors on Kupffer cells. Association of up to 10 molecules of ISIS-9388 per LacLDL particle did not induce aggregation. LacLDL-associated [3H]ISIS-9388 was rapidly taken up by the liver after injection into rats (52.9+/-1.8% of the dose within 2 min versus 18.6+/-2.8% for ISIS-3082). N-acetylgalactosamine inhibited hepatic uptake, indicating involvement of galactose-specific receptors. Liver cells were isolated at 60 min after injection of LacLDL-associated [3H]ISIS-9388. Kupffer cells displayed the highest uptake: 88.1+/-24.7 ng of oligonucleotide/mg of cell protein, which is 6-14 times higher than after injection of free ISIS-9388 or ISIS-3082 (15.0+/-3.8 ng and 6.3+/-1.4 ng, respectively). It can be calculated that Kupffer cells contribute 43.9+/-5.4% to the liver uptake (free ISIS-9388 and ISIS-3083 14.5+/-3.1% and 8.3+/-3.2%, respectively). In conclusion, conjugation of a phosphorothioate oligodeoxynucleotide with cholesterol and its subsequent association with LacLDL results in a substantially increased Kupffer cell uptake of the oligonucleotide. As Kupffer cells play a key role in inflammation, our approach may be utilized to improve antisense-based therapeutic intervention during inflammation.

Antisense therapeutics: is it as simple as complementary base recognition?

Antisense oligonucleotides provide a simple and efficient approach for developing target-selective drugs because they can modulate gene expression sequence-specifically. Antisense oligonucleotides have also become efficient molecular biological tools to investigate the function of any protein in the cell. As the application of antisense oligonucleotides has expanded, multiple mechanisms of oligonucleotides have been characterized that impede their routine use. Here, we discuss different mechanisms of action of oligonucleotides and the possible ways of minimizing non-antisense-related [corrected] effects to improve their specificity.

Stereo-enriched phosphorothioate oligodeoxynucleotides: synthesis, biophysical and biological properties

Stereo-enriched [Rp] and [Sp]-phosphorothioate oligodeoxynucleotides are synthesized using oxazaphospholidine derivatized monomers. Three different designs of phosphorothioate oligodeoxynucleotides (PS-oligos), (i) stereo-enriched all-[Rp] or all-[Sp] PS-linkages, (ii) stereo-random mixture of PS-linkages, and (iii) segments containing certain number of stereo-enriched [Rp] and [Sp] PS-linkages ([Sp-Rp-Sp] or [Rp-Sp-Rp]), have been studied. Thermal melting studies of these PS-oligos with RNA complementary strands showed that the binding affinities are in the order [Rp] > [Sp-Rp-Sp]-[Rp-Sp-Rp] > stereo-random > [Sp]. Circular dichroism (CD) studies suggest that the stereochemistry of the PS-oligo does not affect the global conformation of the duplex. The in vitro nuclease stability of these PS-oligos is in the order [Sp] > [Sp-Rp-Sp] > stereo-random > [Rp]. The RNase H activation is in the order [Rp] > stereo-random > [Rp-Sp-Rp] > [Sp] > [Sp-Rp-Sp]. Studies in a cancer cell line of PS-oligos targeted to MDM2 mRNA showed that all oligos had similar biological activity under the experimental conditions employed. Protein- and enzyme-binding studies showed insignificant stereo-dependent binding to proteins. The [Sp] and [Sp-Rp-Sp] chimeric and stereo-random PS-oligos that contained a CpG motif showed higher cell proliferation than [Rp] PS-oligo of the same sequence.

Importance of nucleotide sequence and chemical modifications of antisense oligonucleotides

The antisense approach is conceptually simple and elegant; to design an inhibitor of a specific mRNA, one needs only to know the sequence of the targeted mRNA and an appropriately modified complementary oligonucleotide. Of the many analogs of oligodeoxynucleotides explored as antisense agents, phosphorothioate analogs have been studied the most extensively. The use of phosphorothioate oligodeoxynucleotides as antisense agents in various studies have shown promising results. However, they have also indicated that quite often, biological effects observed could be solely or partly non-specific in nature. It is becoming clear that not all phosphorothioate oligodeoxynucleotides of varying length and base composition are the same, and important consideration should be given to maintain antisense mechanisms while identifying effective antisense oligonucleotides. In this review, I have summarized the progress made in my laboratory in understanding the specificity and mechanism of actions of phosphorothioate oligonucleotides and the rationale for designing second-generation mixed-backbone oligonucleotides.

Pharmacokinetics and tissue disposition in monkeys of an antisense oligonucleotide inhibitor of Ha-ras encapsulated in stealth liposomes

PURPOSE

This study examined the pharmacokinetics and tissue distribution of an antisense oligonucleotide ISIS 2503, formulated in stealth (pegylated) liposomes (encapsulated) or in phosphate-buffered saline (unencapsulated).

METHODS

Encapsulated or unencapsulated ISIS 2503 was administered to rhesus monkeys by intravenous infusion. The concentrations of ISIS 2503 and metabolites in blood, plasma, and tissue samples were determined by capillary gel electrophoresis.

RESULTS

Plasma concentrations of encapsulated ISIS 2503 decreased mono-exponentially after infusion with a mean half-life of 57.8 hours. In contrast, the concentration of unencapsulated ISIS 2503 in plasma decreased rapidly with a mean half-life of 1.07 hours. Both encapsulated and unencapsulated ISIS 2503 distributed widely into tissues. Encapsulated ISIS 2503 distributed primarily to the reticulo-endothelial system and there were few metabolites observed. In contrast, unencapsulated ISIS 2503 distributed rapidly to tissue with highest concentration seen in kidney and liver. Nuclease-mediated metabolism was extensive for unencapsulated oligonucleotide in plasma and tissues.

CONCLUSIONS

The data suggest that stealth liposomes protect ISIS 2503 from nucleases in blood and tissues, slow tissue uptake, and slow the rate of clearance from the systemic circulation. These attributes may make these formulations attractive for delivering oligonucleotides to sites with increased vasculature permeability such as tumors or sites of inflammation.