Antiviral activity of phosphatidyl-dideoxycytidine in hepatitis B-infected cells and enhanced hepatic uptake in mice

Pharmacokinetics of lipid-drug conjugates loaded into liposomes

Drugs that are neither lipophilic nor suitable for encapsulation via remote loading procedures are generally characterized by low entrapment efficiencies and poor retention in liposomes. One approach to circumvent this problem consists in covalently linking a lipid to the drug molecule in order to permit its insertion into the vesicle membrane. The nature of the conjugated lipid and linker, as well as the composition of the liposomal bilayer were found to have a profound impact on the pharmacokinetic properties and biodistribution of the encapsulated drugs as well as on their biological activity. This contribution reviews the past and recent developments on liposomal lipid-drug conjugates, and discusses important issues related to their stability and in vivo performance. It also provides an overview of the data that were generated during the clinical assessment of these formulations. The marketing authorization of the immunomodulating compound mifamurtide in several countries as well as the promising results obtained with the lipid prodrug of mitomycin C suggest that carefully designed liposomal formulations of lipid-drug conjugates is a valid strategy to improve a drug's pharmacokinetic profile and with that its therapeutic index and/or efficacy.

Inhibition of the replication of hepatitis B virus in vitro by a novel 2,6-diaminopurine analog, beta-LPA

Antiviral therapy of chronic hepatitis B remains a major clinical problem worldwide. Like lamivudine, nucleoside analogs have become the focus of investigation of anti-hepatitis B virus (anti-HBV) drugs. Here, beta-LPA is a novel 2,6-diaminopurine analog found to possess potent anti-HBV activity. In HepG2.2.15 cell line, beta-LPA had a 50% effective concentration (EC(50)) of 0.01 microM against HBV, as determined by analysis of secreted and intracellular episomal HBV DNA. Levels of HBV surface antigen (HBsAg) and e antigen (HBeAg) in drug-treated cultures revealed that beta-LPA had no significant inhibitory effects on HBsAg and HBeAg. beta-LPA didn't show any cytotoxicity up to 0.4 microM with a 50% cytotoxic concentration (CC(50)) of 50 microM. Furthermore, treatment with beta-LPA resulted in no apparent inhibitory effects on mitochondrial DNA content. Considering the potent inhibition of HBV DNA synthesis and no obvious toxicity of beta-LPA, this compound should be further explored for development as an anti-HBV drug.

Novel delivery methods for treatment of viral hepatitis: an update

Viral hepatitis represents the most common cause of chronic liver disease worldwide. Currently approved therapies for chronic hepatitis B include IFN, an immune modulator, and nucleoside analogues lamivudine and adefovir. For chronic hepatitis C, a combination of pegylated IFN-alpha and ribavirin represents the standard treatment. However, currently available treatments for both these viruses are effective only in a limited number of patients, are costly, prolonged, associated with significant side effects and require a substantial commitment from the patients and healthcare providers. A number of novel antiviral treatments, together with strategies to enhance the response to current therapies, are being explored at present. For all new therapies, as well as for improving existing treatments, selective delivery of medications into liver cells would be desirable to enhance antiviral activity and avoid systemic side effects. New achievements in the field of drug and gene delivery against chronic hepatitis to the liver are reviewed here.

Inhibition of hepatitis B virus by a novel L-nucleoside, β-L-D4A and related analogues

AIM: To explore the inhibition of β-L-D4A on hepatitis B virus (HBV) in 2.2.15 cells derived from HepG2 cells transfected with HBV genome.

METHODS: 2.2.15 cells were plated at a density of 5 × 10⁴ per well in 12-well tissue culture plates, and treated with various concentrations of β-L-D4A for 6 days. In the end, 5 μl of medium was used for the estimation of HBsAg and HBeAg, the other medium was processed to obtain virions by a polyethlene glycol precipitation method. At the same time, intracellular DNA was also extracted and digested with HindIII. Both DNAs were subjected to Southern blot, hybridized with a ³²P-labeled HBV probe and autoradiographed. Intensity of the autoradiographic bands was quantitated by densitometric scans of computer and ED₅₀ was calculated. Then Hybond-N membrane was washed and rehybridized with a ³²P-labeled mtDNA-specific probe, and effect of β-L-D4A on mitochondrial DNA was studied. 2. 2.15 cells were also seeded in 24-well tissue culture plates, and cytotoxicity with different concentrations was examined by MTT method. ID₅₀ was calculated. Structure-activity relationships between D2A and D4A were also studied as above.

RESULTS: Autoradiographic bands were similar between supernatant and intracellular HBV DNA. Episomal HBV DNA was inhibited in a dose-dependent manner. ED₅₀ was 0.2 μM. HBsAg or HBeAg was not apparently decreased, and inhibition of mitochondrial DNA was not obvious. The experiment of cytotoxicity gained ID₅₀ at 200 μM.

CONCLUSION: β-L-D4A possesses potent inhibitory effects on the replication of HBV in vitro with little cytotoxicity and mitochondrial toxicity, TI value is 1000. It is expected to be developed as a new clinically anti-HBV drug.

Prodrug and antedrug: two diametrical approaches in designing safer drugs

The prodrug and antedrug concepts, which were developed to overcome the physical and pharmacological shortcomings of various therapeutic classes of agents, employ diametrically different metabolic transformations. The prodrug undergoes a predictable metabolic activation prior to exhibiting its pharmacological effects in a target tissue while the antedrug undergoes metabolic deactivation in the systemic circulation upon leaving a target tissue. An increased therapeutic index is the aspiration for both approaches in designing as well as evaluation criteria. The recent research endeavors of prodrugs include the gene-directed and antibody-directed enzymatic activation of a molecule in a targeted tissue, organ specific delivery, improved bioavailabilities of nucleosides and cellular penetration of nucleotides. As for antedrugs, emphasis in research has been based upon the design and synthesis of systemically inactive molecule by incorporating a metabolically labile functional group into an active molecule.

In vitro anti-human immunodeficiency virus and anti-hepatitis B virus activities and pharmacokinetic properties of heterodinucleoside phosphates containing AZT or ddC

In vitro activities, against human immunodeficiency virus (HIV)- and hepatitis B virus (HBV)-infected cells, of four amphiphilic heterodinucleoside phosphates containing 3'-azido-2',3'-dideoxythymidine (AZT) or 2',3'-dideoxycytidine (ddC) as antiviral monomers were evaluated. The four compounds were N4-hexadecyl-2'-deoxyribocytidylyl-(3'-->5')-3'-azido-2',3'-deoxyt hymidine (N4-hxddC-AZT), N4-palmitoyl-2'-deoxyribocytidylyl-(3'-->5')-3'-azido-2',3'-deoxyt hymidine (N4-pamdC-AZT), N4-hexadecyl-2'-deoxycytidylyl-(3'-->5')-2',3'-dideoxycytidine (N4-hxddC-ddC) and 2'-deoxythymidylyl-(3'-->5')-N4-palmitoyl-2',3'-dideoxycytidine (dT-N4-pamddC). All four dimers were active against HIV, dT-N4-pamddC being the most active and least toxic. dT-N4-pamddC also exhibited strong antiviral effects against a panel of eight AZT-resistant HIV strains. The ddC-containing heterodimers incorporated in liposomes additionally inhibited HBV replication by 50-80% in HepG2 2.2.15 cells. AZT and the AZT-containing dimers were ineffective. Differences in pharmacokinetic properties between the antiviral monomers and the heterodimers were evaluated using liposomal formulations of 3H-labelled AZT heterodimers as model compounds. The cellular distribution of AZT in H9 cells was predominantly cytoplasmic, whereas the amphiphilic dimers were distributed more evenly throughout the cytoplasm, nuclear membranes and microsomes. Blood levels of the heterodimers decreased at a rate two- to threefold slower than AZT and the areas-under-the-curves were five- to sevenfold higher for N4-pamdC-AZT and N4-hxddC-AZT, respectively. Compared to AZT, the peak levels of the dimers were three to four times higher in blood and five to six times higher in the liver. Analysis of blood samples showed that 34% of N4-pamdC-AZT was metabolized to AZT, whereas only 9% of N4-hxddC-AZT released AZT. Considering the antiviral potency and the favourable pharmacokinetic properties of the heterodimers, these compounds merit further exploration as antiviral drugs.

Liposomal aerosols in the management of pulmonary infections

The combination of liposomes and aerosols has been utilized to directly target the lungs with chemotherapeutic agents that might not have been used because of low solubility or toxicity. There are a variety of antibacterials, antifungals, and antivirals that have good in vitro activity, but are not effective because of their systemic toxicity and/or poor penetration into the lungs. Incorporation of many lipophilic drugs into liposomes decreases their toxicity without affecting effectiveness, thus increasing the therapeutic index. We have focused on aerosol delivery of amphotericin B (ampB) for the treatment of pulmonary and systemic fungal diseases. We have tested a variety of ampB-lipid formulations for the optimal treatment regimen for Cryptococcus and Candida infections in mouse models. The AeroTech II nebulizer (MMADs of 1.8-2.2 microns) produced aerosols with the highest concentrations in the breathable range. Pharmacokinetic studies revealed that pulmonary drug was present for hours to weeks. AmBisome retained its anticryptococcal activity even when animals were challenged 14 days after aerosol treatment. Aerosols may also be effective in systemic diseases. In our Candida-mouse model, systemic candidiasis and mortality were reduced by aerosolized ampB-liposome treatment. The ability to utilize lipophilic drugs, to deliver high concentrations of drug directly to the site of infection, and to reduce toxicity makes aerosol liposomes an attractive, alternative route of administration.

Phosphatidyl-2',3'-dideoxy-3'-thiacytidine: synthesis and antiviral activity in hepatitis B-and HIV-1-infected cells

We recently found that phosphatidyl-2',3'-dideoxycytidine (phosphatidyl-ddC) had substantial anti-hepatitis B virus (HBV) activity in vitro compared to 2',3'-dideoxycytidine (ddC) (Hostetler et al. (1994) Antiviral Res. 24, 59-67). Upon administration of liposomal phosphatidyl-ddC to mice, a 40-fold higher drug area under curve was observed in the liver. To evaluate the possibility of using liver-targeted anti-HBV nucleosides to treat woodchuck hepatitis virus, we wanted to find the most potent and selective lipid conjugates. It has been shown that 2',3'-dideoxy-3'-thiacytidine as a racemic mixture of the cis-isomer (cis-(+/-)-BCH-189) has much greater activity against HBV viruses than ddC in vitro. Recently, it was shown that the (-)-beta-L-enantiomer (3TC) is more active and less toxic than the (+)-beta-D-form ((+)-BCH-189). To determine whether phospholipid conjugates of 3TC retain antiviral activity in 2.2.15 cells as demonstrated previously with ddC, we synthesized the 1,2-dipalmitoyl-sn-glycerol-3-phosphate conjugates of (+/-)-BCH-189 and 3TC and assessed their anti-HBV and anti-HIV activities, in vitro. Phosphatidyl-3TC and phosphatidyl-BCH-189 had antiviral activity comparable to the respective free drugs in 2.2.15 cells which chronically produce HBV. In HIV-1-infected human peripheral blood mononuclear cells and HT4-6C cells, phosphatidyl-3TC and phosphatidyl-(+/-)-BCH-189 exhibited significantly lower activity than the corresponding free nucleosides. In view of the documented ability of phosphatidyl-ddC to target drug to the liver, it seems reasonable to expect that phosphatidyl-3TC or phosphatidyl-(+/-)-BCH-189 could be employed to provide greatly enhanced hepatic antiviral activity in HBV infection in vivo.

Minireview: nucleotide prodrugs

Nucleotides have shown interesting biological activities in a wide variety of antiviral, antiproliferative, immunomodulatory and other biological assays, and they present promising drug candidates. Because of their negative charge(s) nucleotides suffer from some disadvantages which can be successfully overcome by the utilization of nucleotide prodrugs. Nucleotide prodrugs were successfully used to increase oral absorption of nucleotides in vivo. By taking advantage of intracellular triggers (reducing potential, enzyme activity, pH), nucleotide prodrugs can be used in vitro for the intracellular delivery of the nucleotide resulting in enhanced potency and in some cases enhanced selectivity. Nucleotide prodrugs have also been utilized for tissue specific delivery of the nucleotides in vivo resulting in altered selectivity and reduced toxicity. For nucleotide prodrugs, their ultimate intended use is (in most cases) in vivo for the treatment of a disease. Thus, it is important to incorporate adequate assays and design criteria into any prodrug effort. In vivo systems are complicated because of metabolism, excretion and tissue distribution of the prodrug and the parent. Thus, results of in vitro assays have to be interpreted cautiously because they may be unsuitable predictors of the in vivo situation.

Phosphatidylazidothymidine and phosphatidyl-ddC: assessment of uptake in mouse lymphoid tissues and antiviral activities in human immunodeficiency virus-infected cells and in Rauscher leukemia virus-infected mice

During the early stages of human immunodeficiency virus (HIV) infection, although symptoms are absent and viral replication in peripheral blood mononuclear cells is low, substantial levels of HIV replication can be documented in lymphoid tissue [G. Pantaleo, C. Graziosi, J.F. Demarest, L. Butini, M. Montroni, C.H. Fox, J.M. Orenstein, D.P. Kotler, and A.S. Fauci, Nature (London) 362:355-358, 1993, and J. Embretsen, M. Zupancic, J.L. Ribas, A. Burke, P. Racz, K. Tenner-Tacz, and A.T. Haase, Nature (London) 362:359-362, 1993]. This observation suggests that earlier treatment of HIV infection may be indicated and that strategies for enhancing drug targeting to the lymphoid tissue reservoris of HIV infection may be beneficial. To address this issue, we synthesized dioleoylphosphatidyl-ddC (DOP-ddC) and dipalmitoylphosphatidyl-3'-azido-3'-deoxythymidine (DPP-AZT), phospholipid prodrugs which form lipid bilayers and which are readily incorporated into liposomes. The anti-HIV activity of DOP-ddC was similar to that of ddC in HIV type 1-infected HT4-6C cells, but DPP-AZT was considerably less active than AZT in HT4-6C cells. Liposomes containing DOP-[3H]ddC or DPP-[3H]AZT administered intraperitoneally to mice produced greater levels of total radioactivity over time in plasma, spleen, and lymphoid tissue relative to the results with [3H]ddC and [3H]AZT, respectively. DPP-AZT administered intraperitoneally in liposomes as a single daily dose to mice infected with Rauscher leukemia virus prevented increased spleen weight and reverse transcriptase levels in serum with a dose-response roughly comparable to that of AZT given continuously in the drinking water. DOP-ddC, DPP-AZT, and lipid conjugates of other antiretroviral nucleosides may provide higher levels of drug over time in plasma and in lymph nodes and spleen, important reservoirs of HIV infection, and may represent an interesting alternative approach to antiviral nucleoside treatment of AIDS.