Increased efficacy of phosphonoformate and phosphonoacetate inhibition of herpes simplex virus type 2 replication by encapsulation in liposomes

Methodology and experimental design for the study of liposome-dependent drugs

This chapter describes the concept of liposome-dependent drugs and the rationale for using them. Subsequently, procedures for studying and identifying liposome-dependent drugs are given. The first procedure described is a simple endpoint assay, and methods are given for both adherent and nonadherent cells. To establish in such a system that a drug is liposome dependent, it is necessary to demonstrate an IC(50) for the encapsulated drug that is less than that of the free drug, preferably with continuous exposure of the cells to drug. Subsequently, a second procedure is described, which is a more rigorous approach able to identify liposome dependency for a drug that is less effective in a carrier system than it is in the free form. This procedure is a multicompartment growth inhibition assay, wherein two cell populations are separated by a semipermeable membrane, through which free drug but not the liposomal carrier system may diffuse. The first population is adherent and is directly exposed to the liposomal or free drug. The second cell population is nonadherent and is exposed only to the drug that diffuses through the membrane. In addition to the methodology, experimental design is discussed and also the calculations needed to determine percent leakage, percent processing, percent metabolism, and the delivery factor, a parameter equivalent to a therapeutic index in an in vivo study.

Alkoxy propane prodrugs of foscarnet: effect of alkyl chain length on in vitro antiviral activity in cells infected with HIV-1, HSV-1 and HCMV

The identification of more effective and less toxic foscarnet (PFA) analogs for antiviral therapy would be useful. We recently synthesized 1-O-octadecyl-sn-glycero-3-phosphonoformic acid (ODG-PFA) and noted a 93-fold increase in its anti-HCMV activity relative to PFA. In addition, the antiviral activity of ODG-PFA in herpes simplex virus type-1 (HSV-1) and human immunodeficiency virus type-1 (HIV-1) infected cells was increased 40-fold relative to PFA (Hostetler et al., 1996. Antiviral Res. 31, 59). To evaluate structure-activity relationships further, we synthesized alkoxypropyl esters of foscarnet with varying alkyl chain lengths and degrees of saturation. These compounds were tested in vitro for activity and selectivity in comparison with PFA and ODG-PFA in cells infected with HCMV, HSV-1 or HIV-1. Antiviral activity was strongly dependent on chain length with alkyl ethers 14-18 carbon atoms long exhibiting the greatest antiviral activity against HCMV and HSV-1. In HIV-infected HT4-6C cells, optimal activity was observed at 18-22 carbon chain lengths. The antiviral activities of 1-octadecyloxypropane-3-PFA and 1-docosyloxypropane-3-PFA were 135- and 338-fold greater than that of PFA in HT4-6C cells infected with HIV-1. This also represents a 2.6-6-fold improvement in antiviral activity over ODG-PFA, the previously reported analog.

Inhibition of human immunodeficiency virus type-1 replication in macrophages and H9 cells by free or liposome-encapsulated L-689,502, an inhibitor of the viral protease

Macrophages are recognized as a major reservoir of HIV-1 in infected individuals. We examined the effect of an inhibitor of the viral protease, L-689,502, on virus production by monocyte-derived macrophages infected with HIV-1BaL. Continuous treatment with L-689,502 drastically inhibited virus production in a dose-dependent manner in the range of 10-200 nM, in some cases by more than 1000-fold, compared to untreated cells. Since liposomes can be targeted to macrophages in vivo, we examined whether the inhibitor was effective following delivery in liposomes. The inhibitor encapsulated in multilamellar liposomes was more effective than the free drug in inhibiting virus production in macrophages, throughout the concentration range studied. The EC90 of the liposomal inhibitor was 2.9- to 4.5-fold lower than that of the free compound. L-689,502 encapsulated in sterically stabilized liposomes with prolonged circulation time inhibited virus production at a level comparable to the free inhibitor. When macrophages were infected and treated for only a limited time, L-689,502 in multilamellar liposomes was the most effective of the three treatments. In chronically infected H9 cells treated continuously, the free inhibitor was more effective than the liposome-encapsulated drug, but virus production was reduced only to 40-60% of controls. In contrast, treatment of acutely infected H9 cells with either free or encapsulated L-689,502 inhibited virus production by up to three orders of magnitude. Our results indicate that liposomes may be useful for the delivery of HIV protease inhibitors with low aqueous solubility and low oral bio-availability, and for the targeting of these drugs to lymph nodes.

Liposome and multiple emulsion formulations augment the anticalcifying efficacy of phosphocitrate in a cutaneous calcergy model

The anticalcifying agent phosphocitrate was incorporated into phosphatidylcholine/cholesterol liposomes by reverse-phase evaporation. The compound was entrapped to the extent of 11.6% (mol mol-1 of lipid) and the liposomes exhibited prolonged retention of the compound when incubated with rat plasma. Phosphocitrate's ionic contribution in solution adversely influenced the encapsulation efficiency but improvements were made through ion-pairing with the quaternary ammonium detergent centrimide, or with the inclusion of stearylamine in the lipid phase. The liposomal dose that could be practically administered in-vivo was restricted to 2.5 mg phosphocitrate kg-1 day-1. The formulation of a multiple emulsion preparation of phosphocitrate, however, offered an alternative delivery mode permitting infrequent dosing to be successfully investigated. In a rat calcergy model, both vehicles effectively reduced the formation of induced subcutaneous calcified plaques at doses for which the phosphocitrate salt alone was inactive. The current formulations demonstrate that the therapeutic efficacy of phosphocitrate can be markedly improved through an appropriately designed drug delivery system, signalling a new approach for the future therapeutic application of this compound.

Liposomal encapsulation of foscarnet protects against hypocalcemia induced by free foscarnet

Hypocalcemia and an increase in creatinine level are the most important serious effects associated with foscarnet (PFA) therapy. In an animal model, we have explored the potential protective role of liposome-encapsulated foscarnet (LE-PFA) on these metabolic abnormalities. PFA administered as one bolus injection (0.5 or 1.0 g/kg) caused significant rapid decreases (approximately 20%) in the levels of calcium and phosphorus in serum within a few minutes and up to 30 min after injection. LE-PFA did not induce any of these changes, while peak levels in serum and the half-life of this formulation were much higher than those of the free drug. PFA administered for 2 weeks (340 or 500 mg/kg/day) resulted in no changes in creatinine or blood urea nitrogen levels in serum at the low-dosage level, but at the higher-dosage level, the creatinine level in serum increased by day 5 posttreatment. Furthermore, there was no increase in the creatinine or blood urea nitrogen level after 2 weeks of treatment with LE-PFA at a dosage of 35 mg/kg/day. When the pharmacokinetics of both free PFA and LE-PFA were compared, the plasma half-life of the encapsulated drug was approximately four times longer than that of the free drug. In addition, the systemic clearance of LE-PFA was approximately one-fifth of that of the free drug. In conclusion, free PFA causes hypocalcemia and hypophosphatemia and increases the creatinine level in serum, whereas the LE form of this drug seems to protect against the abnormal changes in calcium and phosphorus levels caused by the free drug. By preventing hypocalcemia and increasing its half-life, LE-PFA can be used at lower doses and at longer intervals. Clinical investigations of these formulations may be worthwhile.

Liposomes as delivery systems in the prevention and treatment of infectious diseases

Research on the potential application of liposomes in the prevention and treatment of infectious diseases has focussed on improvement of the therapeutic index of antimicrobial drugs and immunomodulators and on stimulation of the immune response to otherwise weak antigens in vaccines composed of purified micro-organism subunits. In this review current approaches in this field are outlined. The improved therapeutic index of antimicrobial drugs after encapsulation in liposomes is a result of enhanced drug delivery to infected tissue or infected cells and/or a reduction of drug toxicity of potentially toxic antibiotics. Liposomal encapsulation of immunomodulators that activate macrophages aims at reducing the toxicity of these agents and targeting them to the cells of the mononuclear phagocyte system in order to increase the nonspecific resistance of the host against infections. Studies on the immunogenicity of liposomal antigens have demonstrated that liposomes can potentiate the humoral and cell mediated immunity to a variety of antigens.

Comparison of cellular accumulation, tissue distribution, and anti-HIV activity of free and liposomal 2',3'-dideoxycytidine

We have investigated the cellular accumulation, tissue distribution, and antihuman immunodeficiency virus activity of free dideoxycytidine (ddC) and liposomal ddC (L-ddC). We have found that L-ddC was more efficiently taken up than its free form by RAW 264.7 cells (a monocyte-macrophage cell line) (p < 0.01) while a comparable uptake was seen in U937 cells (a promonocytic cell line). In the rat, L-ddC accumulated preferentially in liver and spleen when injected intravenously (p < 0.01), and mostly in spleen when given intraperitoneally (p < 0.01). In contrast, free ddC was rapidly eliminated out of the body. Liposomal ddC showed a similar anti-HIV activity in comparison with free ddC in U937 cells. Given the fact that encapsulation of ddC in liposomes does not affect its anti-HIV activity but enhances its in vitro cellular accumulation and its in vivo distribution in reticuloendothelial system (RES) tissues, we conclude that ddC in liposomal formulation is a promising anti-HIV agent with a targeted action on the RES, which is considered a reservoir for dissemination of virus to other cells, tissues, and organs.

Liposome targeting to human immunodeficiency virus type 1-infected cells via recombinant soluble CD4 and CD4 immunoadhesin (CD4-IgG)

HIV-infected cells producing virions express the viral envelope glycoprotein gp120/gp41 on their surface. We examined whether liposomes coupled to recombinant soluble CD4 (sCD4, the ectodomain of CD4 which binds gp120 with high affinity) could specifically bind to HIV-infected cells. sCD4 was chemically coupled by 2 different methods to liposomes containing rhodamine-phosphatidylethanolamine in their membrane as a fluorescent marker. In one method, sCD4 was thiolated with N-succinimidyl acetylthioacetate (SATA) and coupled to liposomes via a maleimide-derivatised phospholipid. In the other method, the oligosaccharides on sCD4 were coupled to a sulfhydryl-derivatised phospholipid, utilizing the bifunctional reagent, 4-(4-N-maleimidophenyl)butyric acid hydrazide (MPBH). The association of the liposomes with HIV-1-infected or uninfected cells was examined by flow cytometry. CD4-coupled liposomes associated specifically to chronically infected H9/HTLV-IIIB cells, but not to uninfected H9 cells. CD4-coupled liposomes also associated specifically with monocytic THP-1 cells chronically infected with HIV-1 (THP-1/HIV-1IIIB). Control liposomes without coupled CD4 did not associate significantly with any of the cells, while free sCD4 could competitively inhibit the association of the CD4-coupled liposomes with the infected cells. The chimeric molecule CD4-immunoadhesin (CD4-IgG) could also be used as a ligand to target liposomes with covalently coupled Protein A (which binds the Fc region of the CD4-IgG) to H9/HTLV-IIIB cells. The CD4-liposomes inhibited the infectivity of HIV-1 in A3.01 cells, and also bound rgp120. Our results suggest that liposomes containing antiviral or cytotoxic agents may be targeted specifically to HIV-infected cells.

Liposomes as carriers of antimicrobial agents or immunomodulatory agents in the treatment of infections

Targeting of antimicrobial agents by means of liposomes is under investigation and may be of importance in the treatment of infections that prove refractory to conventional forms of antimicrobial treatment. The ability to achieve a significantly longer residence time of liposomes in plasma and limited uptake of liposomes by the mononuclear phagocyte system opens up new areas of investigation and potential therapeutic application. By manipulating the liposomal composition, rates of uptake and intracellular degradation can be influenced and thereby the rates at which liposome-encapsulated agents are released and become available to exert their therapeutic action. With respect to the targeting of macrophage modulators at the mononuclear phagocyte system by means of liposomes for maximal stimulation of the nonspecific antimicrobial resistance, experimental evidence is now available of the potential usefulness of liposomes as carriers of these agents. This approach may also be of importance for the potentiation of treatment of severe infections.

Antiviral therapy: current concepts and practices

Drugs capable of inhibiting viruses in vitro were described in the 1950s, but real progress was not made until the 1970s, when agents capable of inhibiting virus-specific enzymes were first identified. The last decade has seen rapid progress in both our understanding of antiviral therapy and the number of antiviral agents on the market. Amantadine and ribavirin are available for treatment of viral respiratory infections. Vidarabine, acyclovir, ganciclovir, and foscarnet are used for systemic treatment of herpesvirus infections, while ophthalmic preparations of idoxuridine, trifluorothymidine, and vidarabine are available for herpes keratitis. For treatment of human immunodeficiency virus infections, zidovudine and didanosine are used. Immunomodulators, such as interferons and colony-stimulating factors, and immunoglobulins are being used increasingly for viral illnesses. While resistance to antiviral drugs has been seen, especially among AIDS patients, it has not become widespread and is being intensely studied. Increasingly, combinations of agents are being used: to achieve synergistic inhibition of viruses, to delay or prevent resistance, and to decrease dosages of toxic drugs. New approaches, such as liposomes carrying antiviral drugs and computer-aided drug design, are exciting and promising prospects for the future.

Foscarnet sodium

Cytomegalovirus (CMV), a major opportunistic viral pathogen frequently causing disease in immunocompromised patients such as organ transplant recipients and people with AIDS, may present as pneumonitis, gastrointestinal disease, or encephalitis. Its most common manifestation in patients with AIDS is retinitis which, if left untreated, invariably progresses to extensive retinal necrosis and ultimately to blindness. Ganciclovir sodium, currently the only licensed antiviral agent for the treatment of CMV retinitis, effectively controls this infection in a majority of AIDS patients, but significant granulocytopenia or thrombocytopenia related to ganciclovir therapy often limit its clinical application. Myelosuppression may be further exacerbated in AIDS patients by such other agents as zidovudine or trimethoprim/sulfamethoxazole, often necessitating dosage reductions or discontinuation of these agents in patients receiving ganciclovir. Foscarnet sodium, a pyrophosphate analog active against both cytomegalovirus and the human immunodeficiency virus type 1 (HIV), may be an effective alternative to ganciclovir in the management of CMV retinitis. Trials with intravenous foscarnet in CMV retinitis have reported favorable results using initial daily doses of 180-230 mg/kg/d given as intermittent infusions every eight hours, followed by maintenance regimens of 60-90 mg/kg/d given as single daily one- or two-hour infusions. Foscarnet therapy may result in renal impairment, and indefinite intravenous maintenance therapy may be required to prevent recurrence of CMV infection. Despite these drawbacks, foscarnet's lack of major myelosuppressive toxicity, and its activity in suppressing HIV replication, make this a potentially safe and effective alternative agent for the management of CMV infection, especially in AIDS patients.