Pharmacokinetics of liposomal nystatin in patients with human immunodeficiency virus infection

New agents for the treatment of systemic fungal infections – current status

Systemic antifungal chemotherapy is enjoying its most dynamic era. More antifungal agents are under development than ever before, including agents in entirely new classes. Major goals of current investigations are to identify compounds with a wide spectrum of activity, minimal toxicity and a high degree of target specificity. The antifungal drugs in development include new azoles {voriconazole, posaconazole (formerly SCH-56592), ravuconazole (formerly BMS-207147)}, lipid formulations of amphotericin B, a lipid formulation of nystatin, echinocandins {anidulafungin (formerly, LY-303366, VER-002), caspofungin (formerly MK-991), micafungin (formerly FK-463)}, antifungal peptides other than echinocandins, and sordarin derivatives. This discussion reviews the currently available antifungal agents and summarises the developmental issues that surround these new systemic antifungal drugs.

New investigational antifungal agents for treating invasive fungal infections

Systemic fungal infections have been recognised as a major cause of morbidity and mortality during the last two decades. There are only a few therapeutic options for these infections. Severe toxicity, such as impairment of renal function, limits the use of amphotericin B. Flucytosine is associated with side effects and drug resistance. Fluconazole and itraconazole are safer, though emergence of resistance and innate resistance in some fungal pathogens is a concern in their use. Therefore, there is a need for developing novel drugs and/or treatment strategies to combat these infections. In recent years, increased efforts by the pharmaceutical industry and academia have led to the discovery of new re-engineered or reconsidered antifungal agents that are more efficacious, safer and have a broad spectrum of activity. Lipid formulations of polyene antifungal agents, amphotericin B and nystatin, have the advantage of improved therapeutic index. Activity against resistant fungi, high bioavailability, safety and longer half-life are the properties that encourage development of the newer triazoles (e.g., voriconazole, ravuconazole and posaconazole). Echinocandin-like lipopeptide antibiotics are among the antifungal agents with a novel mode of action. In addition to these lead investigational compounds, development of newer antifungal agents is underway.

Species differences in the proportion of plasma lipoprotein lipid carried by high-density lipoproteins influence the distribution of free and liposomal nystatin in human, dog, and rat plasma

The objective of this study was an interspecies comparison of free nystatin (NYS) and liposomal NYS (Nyotran) distribution in plasma. NYS and liposomal NYS at concentrations of 5, 10, and 20 microg of NYS/ml were incubated in human, dog, and rat plasma for 5, 60, and 180 min at 37 degrees C. Following these incubations, plasma samples were separated into their high-density lipoprotein (HDL), triglyceride-rich lipoprotein, low-density lipoprotein, and lipoprotein-deficient plasma (LPDP) fractions by density-gradient ultracentrifugation, and each fraction was assayed for NYS by high-pressure liquid chromatography. Total plasma and lipoprotein cholesterol, triglyceride, and protein concentrations in each human, dog, or rat plasma sample were determined by enzymatic assays. When NYS and liposomal NYS were incubated in human, dog, or rat plasma, the majority of the NYS was recovered in the LPDP fraction. For the 5- and 60-min incubation times for all plasmas measured, a significantly greater percentage of NYS was recovered in the lipoprotein fraction (primarily HDL) following the incubation of liposomal NYS than following the incubation of NYS. There was a significant correlation between the lipoprotein lipid and protein profiles in human, dog, and rat plasmas and the distribution of NYS and liposomal NYS in plasma. In particular, differences in the proportion of plasma lipoprotein cholesterol, triglyceride, and apolar lipids (cholesteryl ester and triglycerides) carried by HDL influenced the distribution of NYS and liposomal NYS within plasmas of different species. These findings suggest that the distribution of NYS among plasma lipoproteins of different species is defined by the proportion of lipid carried by HDL, and this is possibly an important consideration when evaluating the pharmacokinetics, toxicities, and activities of these compounds following administration to different animal species.

Liposomal nystatin against experimental pulmonary aspergillosis in persistently neutropenic rabbits: efficacy, safety and non-compartmental pharmacokinetics

The activity of liposomal nystatin against invasive pulmonary aspergillosis was investigated in persistently neutropenic rabbits. Treatment groups included liposomal nystatin at dosages of 1, 2 and 4 mg/kg/day intravenously, or amphotericin B deoxycholate 1 mg/kg/day administered intravenously after normal saline loading. As compared with untreated controls, liposomal nystatin administered at 2 and 4 mg/kg/day prolonged survival and reduced fungus-mediated tissue injury and excess lung weight at post-mortem in a similar manner to amphotericin B. Although amphotericin B was superior in clearing infected lung tissue, treatment with all regimens of liposomal nystatin led to a significant reduction in pulmonary fungal tissue burden. During treatment, ultrafast CT-scan demonstrated ongoing resolution of pulmonary lesions at 2 and 4 mg/kg/day, but not at 1 mg/kg/day. With the exception of mild increases in blood urea nitrogen (BUN) and serum creatinine values during treatment at 2 and 4 mg/kg/day, which were similar to those found in amphotericin B-treated rabbits, liposomal nystatin was well tolerated. Preliminary pharmacokinetic studies in non-infected animals established linear drug disposition of liposomal nystatin in plasma over the investigated dosage range and peak plasma levels above the MIC for the test strain after multiple daily dosing for 7 days. Liposomal nystatin increased survival and provided reduced tissue injury, effective microbiological clearance and tolerable side effects in experimental pulmonary aspergillosis in persistently neutropenic rabbits, thus providing a rational basis for further investigations in clinical trials.

Liposomes as drug delivery system: a strategic approach for the treatment of HIV infection

As the number of individuals infected with human immunodeficiency virus (HIV) is growing dramatically throughout the world, it is important to develop strategies to improve the treatment of this deadly disease. It is now well established that macrophages play a central role in HIV pathogenesis, acting as reservoirs for dissemination of virus throughout the immune system. As liposomes are naturally taken up by cells of the mononuclear phagocytic system, liposome-based therapy represents a convenient approach to improve the delivery of anti-HIV agents into infected cells improving thereby the efficacy of drugs and reducing their adverse side-effects. A more specific targeting of HIV-infected cells could also be obtained by using liposomes bearing surface attached-antibodies. This review details the applications of liposomes as drug carriers for the treatment of AIDS. It also gives an overlook of the different strategies that could be explored to control the progression of the disease in infected individuals.

Plasma lipoprotein distribution of liposomal nystatin is influenced by protein content of high-density lipoproteins

The plasma lipoprotein distribution of free nystatin (Nys) and liposomal nystatin (L-Nys) in human plasma samples with various lipoprotein lipid and protein concentrations and compositions was investigated. To assess the lipoprotein distributions of Nys and L-Nys, human plasma was incubated with Nys and L-Nys (equivalent to 20 microg/ml) for 5 min at 37 degreesC. The plasma was subsequently partitioned into its lipoprotein and lipoprotein-deficient plasma fractions by step-gradient ultracentrifugation, and each fraction was analyzed for Nys content by high-pressure liquid chromatography. The lipid and protein contents and compositions of each fraction were determined with enzymatic kits. Following the incubation of Nys and L-Nys in human plasma the majority of Nys recovered within the lipoprotein fractions was recovered from the high-density lipoprotein (HDL) fraction. Incorporation of Nys into liposomes consisting of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol significantly increased the percentage of drug recovered within the HDL fraction. Furthermore, it was observed that as the amount of HDL protein decreased the amounts of Nys and L-Nys recovered within this fraction decreased. These findings suggest that the preferential distribution of Nys and L-Nys into plasma HDL may be a function of the HDL protein concentration.

Physical characteristics and lipoprotein distribution of liposomal nystatin in human plasma

The physical characteristics and lipoprotein distribution of free nystatin (NYS) and liposomal NYS (L-NYS) in human plasma were investigated. To determine the percentage of NYS that was lipid associated following incubation in human plasma, C18 reverse-phase extraction columns were used. To assess plasma drug distribution, NYS and L-NYS (20 microg/ml) were incubated in human plasma for 5, 60, and 120 min at 37 degrees C. After each interval, plasma was removed and separated into its lipoprotein and lipoprotein-deficient plasma (LPDP) fractions by ultracentrifugation and assayed for NYS by high-pressure liquid chromatography. Further studies evaluated the liposome structure of L-NYS by filtering through a 0.14-microm-pore-size microfilter before and after the addition of human plasma. When reconstituted L-NYS (mean particle diameter +/- standard deviation, 321 +/- 192 nm) was applied to a C18 column, 67% +/- 4% of the initial NYS concentration was associated with the lipid. When plasma samples containing L-NYS that had been incubated for 5 to 120 min at 37 degrees C were applied to C18 columns, 66 to 76% of the NYS was lipid associated. Incubation of NYS in human plasma for 5 min at 37 degrees C resulted in 3% +/- 1% of the initial NYS concentration incubated in the low-density lipoprotein (LDL) fraction, 23% +/- 4% of that in the high-density lipoprotein (HDL) fraction, and 66% +/- 10% of that in the LPDP fraction. In contrast, the distribution of NYS following incubation of L-NYS in human plasma for 5 min was 13% +/- 2% in the LDL fraction, 44% +/- 5% in the HDL fraction, and 42% +/- 5% in the LPDP fraction. Similar results were observed following 60 and 120 min of incubation. In addition, the liposome structure of L-NYS was quickly lost when mixed with plasma. These findings suggest that rapid disruption of the L-NYS structure upon incubation in human plasma is consistent with its rapid distribution in plasma. The preferential distribution of NYS into the HDL fraction upon incubation of L-NYS may be a function of its phospholipid composition.

Diversity of lipid-based polyene formulations and their behavior in biological systems

Patients with cancer and infectious disease often display dyslipidemias that result in changes in their plasma lipoprotein-lipid composition. It is likely that the interactions of liposomal polyenes with plasma lipoproteins may be responsible for the far different pharmacokinetics and pharmacodynamics of these compounds when they are administered to infected patients rather than to animals or healthy volunteers. Amphotericin B (AmpB) and nystatin are examples of such polyenes. Amphotericin B initially distributes with the high-density lipoprotein (HDL) fraction upon incubation in plasma. Over time, AmpB redistributes from HDLs to low-density lipoproteins (LDLs). This redistribution appears to be regulated by lipid transfer protein. However, when AmpB is incorporated into liposomes composed of negatively or positively charged phospholipids, not only is the capability of LTP to transfer AmpB from HDL to LDL diminished, but AmpB remains retained with only the HDL fraction. However, when liposomal nystatin is incubated in plasma, over 50% of nystatin distributes with HDLs. Over time, nystatin redistributes from HDL to the lipoprotein-deficient plasma fraction, which is composed of mainly aqueous plasma proteins. The lipid composition selected for the drug appears to be a vital constituent in regulating the drug's interaction with biological fluids. Furthermore, liposome (or liposomal particle) size, fluidity, and other physiochemical characteristics also play a role in altering the pharmacokinetics and pharmacological effects of lipid-based drug formulations. Armed with this understanding, a rational approach to clinical development of these formulations could be facilitated.