Accelerated blood clearance of pegylated liposomal topotecan: Influence of polyethylene glycol grafting density and animal species

Upon repeated administration, empty pegylated liposomes lose long‐circulating characteristics, referred to as accelerated blood clearance (ABC) phenomenon. However, pegylated liposomal cytotoxic drug formulations could not elicit the phenomenon. In the study, it was found that repeated injection of pegylated liposomal topotecan could induce ABC phenomenon in Wistar rats, beagle dogs, and mice, which might be associated with the formation of empty liposomes in circulation because of the rapid drug release rate. In rats, the 9% polyethylene glycol (PEG) formulation induced more severe ABC phenomenon than 3% PEG formulation despite the similar anti‐PEG immunoglobulin M (IgM) levels following the first dose. Antibody neutralization experiments revealed that high PEG formulation was easily neutralized by IgM. Repeated administration of 3% PEG formulation in dogs could result in more severe ABC phenomenon. It seems that slow infusion was liable to cause ABC phenomenon. In all animal species, considerable intraindividual variability of IgM levels could be observed. Our observations may have important implications for the development, evaluation, and therapeutic use of pegylated liposomal cytotoxic drug formulations because using the current drug loading technology, most of the cytotoxic drugs could not be stably loaded in liposomes and rapid drug leakage from liposomes might occur in circulation. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:3864–3876, 2012     

Encapsulation of vinorelbine into cholesterol-polyethylene glycol coated vesicles: drug loading and pharmacokinetic studies

Objectives Pegylated liposome formulations of vinorelbine with prolonged circulation half‐life (t½) are desirable. However, DSPE‐PEG could affect vinorelbine loading into vesicles due to electrostatic interactions. To resolve this problem, chol‐PEG was used to prepare pegylated liposomal vinorelbine and the factors affecting drug loading and plasma pharmacokinetics were investigated. Methods Vinorelbine was loaded into liposomes using a novel triethylamine 5‐sulfosalicylate gradient. The effects of cholesterol and chol‐PEG on drug loading were investigated. Pharmacokinetic studies were performed in normal KunMing mice treated with different liposomal vinorelbine formulations. To clarify the effects of chol‐PEG on membrane permeability, drug release experiments were performed based on the fluorescence dequenching phenomenon of a fluorescence marker. Key findings In contrast to DSPE‐PEG, even at high PEG grafting density (～8.3 mol%), chol‐PEG had no effect on vinorelbine loading into HSPC/cholesterol (3 : 1, mass ratio) vesicles. However, for the formulations with low cholesterol content (HSPC/cholesterol 4 : 1), loading efficiency decreased with increasing chol‐PEG content. In vivo, the vinorelbine t½ of low cholesterol formulations decreased with increasing chol‐PEG content, but for high cholesterol liposomes, the maximum vinorelbine t½ was achieved at ～3 mol% chol‐PEG grafting density. The resulting vinorelbine circulation t½ was ～9.47 h, which was greater than that of non‐pegylated liposomes (～5.55 h). Drug release experiments revealed that chol‐PEG might induce membrane defects and concomitant release of entrapped marker, especially at high chol‐PEG density. Conclusions Through the investigation of the effects of chol‐PEG and cholesterol, an optimum pegylated liposomal vinorelbine formulation with prolonged t½ was achieved. In plasma, the membrane defect induced by chol‐PEG may counteract the long circulation characteristics that chol‐PEG afforded. When these two opposite effects reached equilibrium, the maximum vinorelbine t½ was achieved.     

Novel sulfobutyl ether cyclodextrin gradient leads to highly active liposomal irinotecan formulation

Objectives Liposomal delivery of irinotecan could provide protection against drug hydrolysis, deliver more active lactone form to tumours and prolong irinotecan exposure time. Nevertheless, conventional drug‐loading technologies have typically resulted in undesired drug retention properties. To resolve the problem, a modified gradient loading method was developed and the resulting formulations were evaluated in a systemic manner. Methods Irinotecan was loaded into liposomes using a novel sulfobutyl ether beta‐cyclodextrin (sbe‐CD) gradient. The effect of drug‐to‐lipid ratio (D/L) and polyethylene glycol (PEG) grafting density were investigated. Drug release experiments were performed in ammonium‐containing medium based on the fluorescence dequenching phenomenon of irinotecan. Pharmacokinetic studies were performed in normal balb/c mice treated with different formulations. To compare the anti‐tumour effect of different formulations, an RM‐1 prostate cancer model was used. Acute toxicity studies were performed in healthy female c57 mice. Key findings Irinotecan could be encapsulated into liposomes with > 90% loading efficiency at a high drug‐to‐lipid mass ratio (> 0.5). In‐vitro release experiments revealed that sbe‐CD anion was more able to retain irinotecan than sulfate. Moreover, the elevated D/L ratio elicited decreased drug release kinetics. Both trends had also been observed when the effects of anions and D/L ratio on half‐life of irinotecan were assessed. Pegylated liposomal irinotecan loaded with sbe‐CD/triethylammonium gradient had irinotecan half‐life values ranging from 9.4 to 13.1 h, surpassing vesicles prepared by the triethylammonium sulfate method (～4.5 h). In the RM‐1 tumour model, all the liposomal irinotecan formulations were more therapeutically active than free irinotecan and the formulation with a high D/L ratio was the most efficacious. Moreover, the high D/L formulation might be less toxic than free irinotecan based on acute toxicity studies. Conclusions The novel sbe‐CD gradient could mediate effective irinotecan loading and improve irinotecan retention, thus resulting in highly active liposomal irinotecan formulations. The improvement in drug retention might be associated with the formation of complicated aggregates inside vesicles.     

Prolongation of the Circulation Time of Doxorubicin Encapsulated in Liposomes Containing a Polyethylene Glycol-Derivatized Phospholipid: Pharmacokinetic Studies in Rodents and Dogs

The pharmacokinetics of doxorubicin (DOX) encapsulated in liposomes containing polyethylene glycol-derivatized distearoylphosphatidylethanolamine (PEG/DSPE) were investigated in rodents and dogs. The plasma levels of DOX obtained with PEG/DSPE-containing liposomes were consistently higher than those without PEG/DSPE or when PEG/DSPE was replaced with hydrogenated phosphatidylinositol (HPI). Despite the inclusion of PEG/DSPE in liposomes, there was a significant drop in the plasma levels of DOX when the main phospholipid component, hydrogenated phosphatidylcholine, was replaced with lipids of lower phase transition temperature (dipalmitoylphosphatidylcholine, egg phosphatidylcholine), indicating that phase transition temperature affects the pharmacokinetics of liposome-encapsulated DOX. In beagle dogs, clearance was significantly slower for DOX encapsulated in PEG/ DSPE-containing liposomes than in HPI-containing liposomes, with distribution half-lives of 29 and 13 hr, respectively. In both instances, almost 100% of the drug measured in plasma was liposome-associated. The apparent volume of distribution was only slightly above the estimated plasma volume of the dogs, indicating that drug leakage from circulating liposomes is insignificant and that the distribution of liposomal drug is limited mostly to the intravascular compartment in healthy animals.     

The functional roles of poly(ethylene glycol)‐lipid and lysolipid in the drug retention and release from lysolipid‐containing thermosensitive liposomes in vitro and in vivo

Triggered release of liposomal contents following tumor accumulation and mild local heating is pursued as a means of improving the therapeutic index of chemotherapeutic drugs. Lysolipid‐containing thermosensitive liposomes (LTSLs) are composed of dipalmitoylphosphatidylcholine (DPPC), the lysolipid monostearoylphosphatidylcholine (MSPC), and poly(ethylene glycol)‐conjugated distearoylphosphatidylethanolamine (DSPE‐PEG₂₀₀₀). We investigated the roles of DSPE‐PEG₂₀₀₀ and lysolipid in the functional performance of the LTSL–doxorubicin formulation. Varying PEG‐lipid concentration (0–5 mol%) or bilayer orientation did not affect the release; however, lysolipid (0–10 mol%) had a concentration‐dependent effect on drug release at 42°C in vitro. Pharmacokinetics of various LTSL formulations were compared in mice with body temperature controlled at 37°C. As expected, incorporation of the PEG‐lipid increased doxorubicin plasma half‐life; however, PEG‐lipid orientation (bilayer vs. external leaflet) did not significantly improve circulation lifetime or drug retention in LTSL. Approximately 70% of lysolipid was lost within 1 h postinjection of LTSL, which could be due to interactions with the large membrane pool of the biological milieu. Considering that the present LTSL–doxorubicin formulation exhibits significant therapeutic activity when used in conjunction with mild heating, our current study provided critical insights into how the physicochemical properties of LTSL can be tailored to achieve better therapeutic activity. © 2009 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 2295–2308, 2010     

IN VIVO TRAFFICKING OF LONG-CIRCULATING LIPOSOMES IN TUMOUR-BEARING MICE DETERMINED BY POSITRON EMISSION TOMOGRAPHY

Various kinds of long-circulating liposome, such as ganglioside GM1-, polyethyleneglycol- (PEG-), and glucuronide-modified liposomes, have been developed for passive targeting of liposomal drugs to tumours. To evaluate the in vivo behaviour of such long-circulating liposomes, we investigated the liposomal trafficking, especially early trafficking just after injection of liposomes, by a non-invasive method using positron emission tomography (PET). Liposomes composed of dipalmitoylphosphatidylcholine, cholesterol, and modifier, namely, GM1, distearoylphosphatidylethanolamine (DSPE)–PEG or palmityl-D -glucuronide (PGlcUA), were labelled with [2-¹⁸F]-2-fluoro-2-deoxy-D -glucose ([2-¹⁸F]FDG), and administered to mice bearing Meth A sarcoma after having been sized to 100 nm. A PET scan was started immediately after injection of liposomes and continued for 120 min. PET images and time–activity curves indicated that PEG liposomes and PGlcUA liposomes were efficiently accumulated in tumour tissues time dependently from immediately after injection. In contrast, GM1 liposomes accumulated less in the tumour as was also the case for control liposomes that contained dipalmitoylphosphatidylglycerol (DPPG) instead of a modifier. Long-circulating liposomes including GM1 liposomes, however, remained in the blood circulation and avoided liver trapping compared with control DPPG liposomes. These data suggest that PGlcUA and PEG liposomes start to accumulate in the tumour just after injection, whereas GM1 liposomes may accumulate in the tumour after a longer period of circulation.     

Characterization of Long-Circulating Cationic Nanoparticle Formulations Consisting of a Two-Stage PEGylation Step for the Delivery of siRNA in a Breast Cancer Tumor Model

Polyethylene glycol (PEG) has been used widely in liposomal formulations as a strategy to inhibit opsonization by plasma proteins and to prolong liposome plasma circulation time. PEG can be incorporated onto the surface of liposomes either during the spontaneous self‐assembling process or inserted after vesicle formation. The advantages of employing the PEG postinsertion method include improved drug encapsulation efficiency and the ability to incorporate PEG conjugates for enhanced cell binding and uptake. In this study, we propose to evaluate a cationic lipid nanoparticle formulation containing two PEGylation steps: pre‐ and post‐siRNA insertion. Our results indicate that formulations consisting of the extra PEG post‐insertion step significantly increased siRNA circulation in the plasma by two‐folds in comparison with the formulations consisting of only the single PEGylation step. Moreover, this formulation was able to efficiently carry siRNA to the tumor site, increase siRNA stability and significantly downregulate luciferase mRNA expression by >50% when compared with the controls in an intraperitoneal and subcutaneous breast cancer tumor model. Overall, our cationic lipid nanoparticle formulation displayed enhanced plasma circulation, reduced liver accumulation, enhanced tumor targeting, and effective gene knockdown‐–demonstrating excellent utility for the delivery of siRNA.     

The effect of PEG coating on in vitro cytotoxicity and in vivo disposition of topotecan loaded liposomes in rats

Amphoteric drugs encapsulated in PEGylated liposomes may not show superior therapeutic antitumor activity due to increased leakage rate of these drugs in presence of PEG-lipids. In order to investigate the effect of PEG coating on in vitro and in vivo characteristics of topotecan loaded liposomes, an amphoteric anticancer drug, PEGylated and conventional liposomes were prepared by lipid film hydration method. Various properties of the prepared nanoliposomes such as encapsulation efficiency, size, zeta potential, physical stability as well as the chemical stability of lactone form of topotecan, cytotoxicity and topotecan pharmacokinetics were evaluated. In vitro cytotoxic activity was evaluated on murine Lewis lung carcinoma (LLC) and human mammary adenocarcinoma (BT20) cells. Pharmacokinetic was evaluated in Wistar rats after i.v. injection of topotecan, formulated in PBS pH 7.4 or in conventional or in PEGylated liposomes. The conventional liposome (CL) formulation was composed of DSPC/cholesterol/DSPG (molar ratio; 7:7:3), while for PEGylated liposome the composition was DSPC/cholesterol/DSPG/DSPE-PEG(2000) (molar ratio; 7:7:3:1.28). The size of both liposomes was around 100 nm with polydispersity index of about 0.1. In comparison with free drug, liposomal topotecan showed more stability for topotecan lactone form in vitro. Compared to free topotecan, PEGylated and conventional liposomes improved cytotoxic effect of topotecan against the two cancer cell line studied. The results of pharmacokinetic studies in rats showed that both CL and PEGylated liposomal formulations increased the concentration of total topotecan in plasma, however, initial concentration and the values of AUC, MRT and t(1/2 beta) were much higher (P<0.001) for PEGylated liposomal drug than for conventional one or free drug. PEGylated liposome resulted in a 52-fold and 2-fold increases in AUC(0-infinity) compared with that of free topotecan and CL, respectively. These results indicated that PEG modified liposome might be an effective carrier for topotecan.     

A folate receptor-targeted liposomal formulation for paclitaxel

A novel liposomal formulation of paclitaxel targeting the folate receptor (FR) was synthesized and characterized. This formulation was designed to overcome vehicle toxicity associated with the traditional Cremophor EL-based formulation and to provide the added advantages of prolonged systemic circulation time and selective targeting of the FR, which is frequently overexpressed on epithelial cancer cells. The formulation had the composition of dipalmitoyl phosphatidylcholine/dimyristoyl phosphatidylglycerol/monomethoxy-polyethylene glycol (PEG)2000-distearoyl phosphatidylethanolamine/folate-PEG3350-distearoyl phosphatidylethanolamine (DPPC/DMPG/mPEG-DSPE/folate-PEG-DSPE) at molar ratios of (85.5:9.5:4.5:0.5) and a drug-to-lipid molar ratio of 1:33. The liposomes were prepared by polycarbonate membrane extrusion. The mean particle size of the liposomes was 97.1 nm and remained stable for at least 72 h at 4 degrees C. FR-targeted liposomes of the same lipid composition entrapping calcein were shown to be efficiently taken up by KB oral carcinoma cells, which are highly FR+. FR-targeted liposomes containing paclitaxel showed 3.8-fold greater cytotoxicity compared to non-targeted control liposomes in KB cells. Plasma clearance profiles of paclitaxel in the liposomal formulations were then compared to paclitaxel in Cremophor EL formulation. The liposomal formulations showed much longer terminal half-lives (12.33 and 14.23 h for FR-targeted and non-targeted liposomes, respectively) than paclitaxel in Cremophor EL (1.78 h). In conclusion, the paclitaxel formulation described in this study has substantial stability and favorable pharmacokinetic properties. The FR-targeted paclitaxel formulation is potentially useful for treatment of FR+ tumors and warrants further investigation.     

Repeated injections of PEGylated liposomal topotecan induces accelerated blood clearance phenomenon in rats

The "accelerated blood clearance (ABC) phenomenon" of PEGylated liposomes following multiple injections has been reported recently. This immunogenicity poses a problem for research into liposomes and hinders their clinical application. However, since doxorubicin liposomes and mitoxantrone liposomes have been reported to fail to induce the ABC phenomenon, some people believe that cytotoxic drugs loaded liposomes will not produce this ABC phenomenon under multiple-dosing regimens. Nevertheless, in the present study, we report that a first injection of the PEGylated liposomal topotecan (a cell cycle-specific drug for the S phase) still produced a strong ABC phenomenon. Likewise, when the first dose of "empty" PEGylated liposomes or topotecan liposomes was increased, the ABC phenomenon of the subsequent dose was accordingly attenuated. Unlike doxorubicin and mitoxantrone, the blood clearance rate of topotecan was dramatically rapid, and the hepatic and splenic accumulations of topotecan liposomes were anomalous because of the ABC phenomenon. These findings may present new challenges to the clinical application of formulations of cytotoxic drugs loaded liposomes that require repeated administrations.     

Safety and pharmacokinetic studies of liposomal antioxidant formulations containing N-acetylcysteine, α-tocopherol or γ-tocopherol in beagle dogs

Abstract

The safety and pharmacokinetic profile of liposomal formulations containing combinations of the antioxidants α-tocopherol, γ-tocopherol or N-acetylcysteine in beagle dogs was examined. Each group consisted of beagle dogs of both genders with a control group receiving empty dipalmitoylphosphatidylcholine (DPPC) liposomes (330?mg/kg DPPC, EL), and test groups receiving liposomes prepared from DPPC lipids with (i) N-acetylcysteine (NAC) (60?mg/kg NAC [L-NAC]); (ii) NAC and α-tocopherol (αT) (60?mg/kg NAC and 25?mg/kg α-tocopherol [L-αT-NAC]) and (iii) NAC and γ-tocopherol (60?mg/kg NAC and 25?mg/kg γ-tocopherol (γT) [L-γT-NAC]). The dogs in the control group (EL) and three test groups exhibited no signs of toxicity during the dosing period or day 15 post treatment. Weight gain, feed consumption and clinical pathology findings (hematology, coagulation, clinical chemistry, urinalysis) were unremarkable in all dogs and in all groups. Results from the pharmacokinetic study revealed that the inclusion of tocopherols in the liposomal formulation significantly increased the area under the curve (AUC) and β-half life for NAC; the tocopherols had greater impact on the clearance of NAC, where reductions of central compartment clearance (CL) ranged from 56% to 60% and reductions of tissue clearance (CL₂) ranged from 73% to 77%. In conclusion, there was no treatment-related toxicity in dogs at the maximum feasible dose level by a single bolus intravenous administration while the addition of tocopherols to the liposomal formulation prolonged the circulation of NAC in plasma largely due to a decreased clearance of NAC.     

Inhalable Liposomes of Low Molecular Weight Heparin for the Treatment of Venous Thromboembolism

This study tests the feasibility of inhalable pegylated liposomal formulations of low molecular weight heparin (LMWH) for treatment of two clinical manifestations of vascular thromboembolism: deep vein thrombosis (DVT) and pulmonary embolism (PE). Conventional distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) and long-circulating pegylated (DSPE–PEG-2000 and DSPE-PEG-5000) liposomes were prepared by hydration method. Formulations were evaluated for particle size, entrapment efficiency, stability, pulmonary absorption, anticoagulant, and thrombolytic effects in rats. Pulmonary absorption was monitored by measuring plasma antifactor Xa activity; anticoagulant and thrombolytic effects were studied by measuring reduction in thrombus weight and amount of dissolved radioactive clot in the blood, respectively. Pegylated liposomal were smaller and showed greater drug entrapment efficiency than conventional liposomes. All formulations produced an increase in pulmonary absorption and circulation time of LMWH upon first dosing. Three repeated dosings of conventional liposomes resulted in decreased half-life and bioavailability; no changes in these parameters were observed with pegylated liposomes. PEG-2000 liposomes were effective in reducing thrombus weight when administered every 48 h over 8 days. In terms of thrombolytic effects and dosing frequency, PEG-2000 liposomes administered via the pulmonary route at a dose of 100U/kg were as effective as 50 U/kg LMWH administered subcutaneously. This paper suggests that inhalable pegylated liposomes of LMWH could be a potential noninvasive approach for DVT and PE treatment. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci99:4554–4564,     

In-vivo studies of amphotericin B liposomes derived from proliposomes: effect of formulation on toxicity and tissue disposition of the drug in mice

The repeat dose toxicity of various liposomal formulations containing amphotericin B has been determined in mice. In general, small liposomes (e.g. 100-150 nm) were found to be more toxic than their large counterparts (e.g. about 2000 nm). However, the repeat dose toxicity of small liposomes could be diminished substantially by the inclusion of sterol (i.e. ergosterol) into the liposomal membranes. Tissue accumulation studies of amphotericin B after repeat dosing may be a useful adjunct to formulation development.     

Therapeutic efficacy of the combination of doxorubicin-loaded liposomes with inertial cavitation generated by confocal ultrasound in AT2 Dunning rat tumour model

Abstract

The combination of liposomal doxorubicin (DXR) and confocal ultrasound (US) was investigated for the enhancement of drug delivery in a rat tumour model. The liposomes, based on the unsaturated phospholipid dierucoylphosphocholine, were designed to be stable during blood circulation in order to maximize accumulation in tumour tissue and to release drug content upon US stimulation. A confocal US setup was developed for delivering inertial cavitation to tumours in a well-controlled and reproducible manner. In vitro studies confirm drug release from liposomes as a function of inertial cavitation dose, while in vivo pharmacokinetic studies show long blood circulation times and peak tumour accumulation at 24–48?h post intravenous administration. Animals injected 6?mg kg^?1 liposomal DXR exposed to US treatment 48?h after administration show significant tumour growth delay compared to control groups. A liposomal DXR dose of 3?mg kg^?1, however, did not induce any significant therapeutic response. This study demonstrates that inertial cavitation can be generated in such a fashion as to disrupt drug carrying liposomes which have accumulated in the tumour, and thereby increase therapeutic effect with a minimum direct effect on the tissue. Such an approach is an important step towards a therapeutic application of cavitation-induced drug delivery and reduced chemotherapy toxicity.     

Long-circulating, pH-sensitive liposomes sterically stabilized by copolymers bearing short blocks of lipid-mimetic units

A major hurdle towards in vivo utilization of pH-sensitive liposomes is their prompt sequestration by reticuloendothelial system and hence short circulation time. Prolonged circulation of liposomes is usually achieved by incorporation of pegylated lipids, which have been frequently reported to deteriorate the acid-triggered release. In this study we evaluate the ability of four novel nonionic copolymers, bearing short blocks of lipid-mimetic units to provide steric stabilization of DOPE:CHEMs liposomes. The vesicles were prepared using the lipid film hydration method and extrusion, yielding liposomes of 120–160 nm in size. Their pH-sensitivity was monitored via the release of encapsulated calcein. The incorporation of the block copolymers at concentration up to 10 mol% did not deteriorate the pH-sensitivity of the liposomes. A selected formulation was tested for stability in presence of 25% human plasma and proved to significantly outclass the plain DOPE:CHEMs liposomes. The ability of calcein-loaded liposomes to deliver their cargo inside EJ cells was investigated using fluorescent microscopy and the results show that the surface-modified vesicles are as effective to ensure intracellular delivery as plain liposomes. The pharmacokinetics and organ distribution of a selected formulation, containing a copolymer bearing four lipid anchors was investigated in comparison to plain liposomes and PEG (2000)–DSPE stabilized liposomes. The juxtaposition of the blood clearance curves and the calculated pharmacokinetic parameters show that the block copolymer confers superior longevity in vivo. The block copolymers utilized in this study can be consider as promising sterically stabilizing agents for pH-sensitive liposomes.     

Liposomal drug formulations in cancer therapy: 15 years along the road

Liposomes as pharmaceutical drug carriers were developed to increase antitumour efficacy and decrease drug toxicity. Doxorubicin HCl liposomal injection was the first liposomal encapsulated anticancer drug to receive clinical approval. To date, virtually all traditional anticancer drugs have been encapsulated in liposomes. The majority of clinical studies only support the concept of a decreased toxicity and better tolerability of the liposomal anticancer drug. Although liposomal anticancer drugs have grown to maturity in several indications and are now in widespread further development programmes using their theoretical advantages to fulfil the high expectations, further studies are warranted--including the development of novel liposomal formulations.     

Topoisomerase I inhibition with topotecan: pharmacologic and clinical issues

Topoisomerase I (topo-I) inhibitors are a new class of anticancer agents with a mechanism of action aimed at interrupting DNA replication in cancer cells, the result of which is cell death. Most, if not all, topo-I inhibitors are derivatives of the plant extract camptothecin. Topotecan is a derivative of camptothecin which has been structurally modified to increase water solubility. The pharmacokinetic profile of topotecan is usually characterised by a two-compartment model and is linear in the dose range of 0.5 - 3.5 mg/m(2). Current clinical trials suggest antitumour activity against a variety of human tumour types, including ovarian cancer, non-small cell lung cancer (NSCLC) and non-lymphocytic haematologic malignancies. The main dose-limiting toxicity (DLT) is non-cumulative myelosuppression. Non-haematologic toxicities are usually mild. Based on several Phase I studies, the recommended Phase II dose was 1.5 mg/m(2)/day iv. for 5 days. Current Phase I and Phase II trials are evaluating the combination of topotecan with other chemotherapeutic agents to increase the therapeutic benefits of topotecan. The DLT in these trials is mainly myelosuppression.     

Repeated injection of pegylated liposomal antitumour drugs induces the disappearance of the rapid distribution phase

Upon repeated administration, empty pegylated liposomes lose their long‐circulating characteristics, referred to as the accelerated blood clearance (ABC) phenomenon. To investigate whether cytotoxic drug‐containing pegylated liposomes could also elicit a similar phenomenon, two pegylated liposomal antitumour drugs (doxorubicin and mitoxantrone) were prepared, and they were administrated twice in the same animals with a 10‐day interval at a dose level of 8 mg kg^?1 (pegylated liposomal doxorubicin) and 4 mg kg^?1 (pegylated liposomal mitoxantrone). By comparing the overall pharmacokinetics after a single‐dose injection with that in animals treated with two doses, it was surprising to find that repeated administration of pegylated liposomal antitumour drugs caused the disappearance of rapid distribution phase instead of the ABC phenomenon, resulting in the conversion of a two‐compartment model to a one‐compartment model. Further investigation revealed that repeated injection induced the decreased uptake of liposomal antitumour drugs by the spleen at the early time point of 0.5–8 h after injection. In contrast, the deposition of liposomal antitumour drugs into liver was not affected. Therefore, the disappearance of the rapid distribution phase might be related to the reduced spleen uptake at the early time point.     

Topoisomerase I inhibition with topotecan: pharmacologic and clinical issues

Topoisomerase I (topo-I) inhibitors are a new class of anticancer agents with a mechanism of action aimed at interrupting DNA replication in cancer cells, the result of which is cell death. Most, if not all, topo-I inhibitors are derivatives of the plant extract camptothecin. Topotecan is a derivative of camptothecin which has been structurally modified to increase water solubility. The pharmacokinetic profile of topotecan is usually characterised by a two-compartment model and is linear in the dose range of 0.5 - 3.5 mg/m². Current clinical trials suggest antitumour activity against a variety of human tumour types, including ovarian cancer, non-small cell lung cancer (NSCLC) and non-lymphocytic haematologic malignancies. The main dose-limiting toxicity (DLT) is non-cumulative myelosuppression. Non-haematologic toxicities are usually mild. Based on several Phase I studies, the recommended Phase II dose was 1.5 mg/m²/day iv. for 5 days. Current Phase I and Phase II trials are evaluating the combination of topotecan with other chemotherapeutic agents to increase the therapeutic benefits of topotecan. The DLT in these trials is mainly myelosuppression.     

Preparation,characterization, and in vitro evaluation of bleomycin‐containing nanoliposomes

Bleomycin is an anticancer drug used against various types of cancers. The aim of this study was to prepare a new PEG ylated and non‐PEG ylated nanoliposomal formulation of bleomycin (PEG ‐nL ip‐BLM and nL ip‐BLM ) and evaluate their anticancer activity in different tumor cell lines. The liposomes were prepared by thin‐film hydration method, and then, bleomycin (BLM ) was loaded to the prepared vesicles. The size, zeta potential, entrapment efficiency, loading rate, release profile, and cytotoxicity of liposomal formulations in TC ‐1, LLC 1, and HFLF ‐PI 5 cell lines were investigated. Mean particle size and zeta potential of the PEG ‐nL ip‐BLM and nL ip‐BLM were found to be 99.4 ± 4.6 nm and ?34.83 ± 4.7 mV ; and 112.2 ± 7.2 nm and ?27.5 ± 3.2 mV , respectively, which were stable for at least 2 months. Encapsulation and loading efficiency of BLM for PEG ‐nL ip‐BLM and nL ip‐BLM were obtained about 83.1 ± 4.2% and 14.3 ± 2.5%; and 78.3 ± 8.6% and 11.1 ± 3.3%, respectively. Drug release study showed a slow release pattern without considerable burst effect. The liposomal formulations indicated lower toxicity compared to free drug in case of TC ‐1 and HFLF ‐PI 5 cells, but their cytotoxicity against LLC 1 cells was significantly higher than free drug. The results of this study indicated that PEG ‐nL ip‐BLM can be a suitable candidate for drug delivery to solid tumors.