The Materials Engineering of Temperature-Sensitive Liposomes

Opportunities and Challenges for mRNA Delivery Nanoplatforms

With the global outbreak of SARS-CoV-2, mRNA vaccines became the first type of COVID-19 vaccines to enter clinical trials because of their facile production, low cost, and relative safety, which initiated great advances in mRNA therapeutic techniques. However, the development of mRNA therapeutic techniques still confronts some challenges. First, in vitro transcribed mRNA molecules can be easily degraded by ribonuclease (RNase), resulting in their low stability. Next, the negative charge of mRNA molecules prevents them from direct cell entry. Therefore, finding efficient and safe delivery technology could be the key issue to improve mRNA therapeutic techniques. In this Perspective, we mainly discuss the problems of the existing mRNA-based delivery nanoplatforms, including safety evaluation, administration routes, and preparation technology. Moreover, we also propose some views on strategies to further improve mRNA delivery technology.

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

Thermoresponsive supramolecular assemblies have been extensively explored in diverse formats, from injectable hydrogels to nanoscale carriers, for a variety of applications including drug delivery, tissue engineering and thermo-controlled catalysis. Understanding the molecular bases behind thermal sensitivity of materials is fundamentally important for the rational design of assemblies with optimal combination of properties and predictable tunability for specific applications. In this review, we summarize the recent advances in this area with a specific focus on the parameters and factors that influence thermoresponsive properties of soft materials. We summarize and analyze the effects of structures and architectures of molecules, hydrophilic and lipophilic balance, concentration, components and external additives upon the thermoresponsiveness of the corresponding molecular assemblies.

Computational Modeling of Combination of Magnetic Hyperthermia and Temperature-Sensitive Liposome for Controlled Drug Release in Solid Tumor

Combination therapy, a treatment modality that combines two or more therapeutic methods, provides a novel pathway for cancer treatment, as it targets the region of interest (ROI) in a characteristically synergistic or additive manner. To date, liposomes are the only nano-drug delivery platforms that have been used in clinical trials. Here, we speculated that it could be promising to improve treatment efficacy and reduce side effects by intravenous administration of thermo-sensitive liposomes loaded with doxorubicin (TSL-Dox) during magnetic hyperthermia (MHT). A multi-scale computational model using the finite element method was developed to simulate both MHT and temperature-sensitive liposome (TSL) delivery to a solid tumor to obtain spatial drug concentration maps and temperature profiles. The results showed that the killing rate of MHT alone was about 15%, which increased to 50% using the suggested combination therapy. The results also revealed that this combination treatment increased the fraction of killed cells (FKCs) inside the tumor compared to conventional chemotherapy by 15% in addition to reducing side effects. Furthermore, the impacts of vessel wall pore size, the time interval between TSL delivery and MHT, and the initial dose of TSLs were also investigated. A considerable reduction in drug accumulation was observed in the tumor by decreasing the vessel wall pore size of the tumor. The results also revealed that the treatment procedure plays an essential role in the therapeutic potential of anti-cancer drugs. The results suggest that the administration of MHT can be beneficial in the TSL delivery system and that it can be employed as a guideline for upcoming preclinical studies.

Lipid Nanoparticles─From Liposomes to mRNA Vaccine Delivery, a Landscape of Research Diversity and Advancement

Lipid nanoparticles (LNPs) have emerged across the pharmaceutical industry as promising vehicles to deliver a variety of therapeutics. Currently in the spotlight as vital components of the COVID-19 mRNA vaccines, LNPs play a key role in effectively protecting and transporting mRNA to cells. Liposomes, an early version of LNPs, are a versatile nanomedicine delivery platform. A number of liposomal drugs have been approved and applied to medical practice. Subsequent generations of lipid nanocarriers, such as solid lipid nanoparticles, nanostructured lipid carriers, and cationic lipid-nucleic acid complexes, exhibit more complex architectures and enhanced physical stabilities. With their ability to encapsulate and deliver therapeutics to specific locations within the body and to release their contents at a desired time, LNPs provide a valuable platform for treatment of a variety of diseases. Here, we present a landscape of LNP-related scientific publications, including patents and journal articles, based on analysis of the CAS Content Collection, the largest human-curated collection of published scientific knowledge. Rising trends are identified, such as nanostructured lipid carriers and solid lipid nanoparticles becoming the preferred platforms for numerous formulations. Recent advancements in LNP formulations as drug delivery platforms, such as antitumor and nucleic acid therapeutics and vaccine delivery systems, are discussed. Challenges and growth opportunities are also evaluated in other areas, such as medical imaging, cosmetics, nutrition, and agrochemicals. This report is intended to serve as a useful resource for those interested in LNP nanotechnologies, their applications, and the global research effort for their development.

A combination drug delivery system employing thermosensitive liposomes for enhanced cell penetration and improved in vitro efficacy

Drug-loaded thermosensitive liposomes are investigated as drug delivery systems in combination with local mild hyperthermia therapy due to their capacity to release their cargo at a specific temperature range (40-42 °C). Additional benefit can be achieved by the development of such systems that combine two different anticancer drugs, have cell penetration properties and, when heated, release their drug payload in a controlled fashion. To this end, liposomes were developed incorporating at low concentration (5 mol%) a number of monoalkylether phosphatidylcholine lipids, encompassing the platelet activating factor, PAF, and its analogues that induce thermoresponsiveness and have anticancer biological activity. These thermoresponsive liposomes were efficiently (>90%) loaded with doxorubicin (DOX), and their thermal properties, stability and drug release were investigated both at 37 ^◦C and at elevated temperatures. In vitro studies of the most advantageous liposomal formulation containing the methylated PAF derivative (methyl-PAF, edelfosine), an established antitumor agent, were performed on human prostate cancer cell lines. This system exhibits controlled release of DOX at 40-42 °C, enhanced cell uptake due to the presence of methyl-PAF, and improved cell viability inhibition due to the combined action of both medications.

Upconversion nanocomposite for programming combination cancer therapy by precise control of microscopic temperature

Combinational administration of chemotherapy (CT) and photothermal therapy (PTT) has been widely used to treat cancer. However, the scheduling of CT and PTT and how it will affect the therapeutic efficacy has not been thoroughly investigated. The challenge is to realize the sequence control of these two therapeutic modes. Herein, we design a temperature sensitive upconversion nanocomposite for CT-PTT combination therapy. By monitoring the microscopic temperature of the nanocomposite with upconversion luminescence, photothermal effect can be adjusted to achieve thermally triggered combination therapy with a sequence of CT, followed by PTT. We find that CT administered before PTT results in better therapeutic effect than other administration sequences when the dosages of chemodrug and heat are kept at the same level. This work proposes a programmed method to arrange the process of combination cancer therapy, which takes full advantage of each therapeutic mode and contributes to the development of new cancer therapy strategies.

The combination of chemo and photothermal therapy is widely used to treat cancer but control of chemo and thermal effects is needed for optimized treatment. Here, the authors describe an upconversion nanoparticle which can be used for controlled sequential treatment by controlling laser power.

Functionalized Hyperbranched Polyethylenimines as Thermosensitive Drug Delivery Nanocarriers with Controlled Transition Temperatures

The low critical solution temperature phase transition (T_c) that is exhibited by thermosensitive polymers is strongly dependent on polymer concentration, pH, ionic strength, as well as the presence of specific molecules or ions in solution. Therefore, polymers with T_c values above 37 °C that are useful for hyperthermia therapy are not readily available. In the present study, temperature-sensitive hyperbranched polyethylenimine derivatives were developed through stepwise functionalization with isobutylamide groups. Although factors such as the concentration of polymer, sodium chloride, phosphate ions, and pH considerably affect the transition temperature, it was possible to obtain a hyperbranched derivative having the required T_c (38-39 °C) for the given aqueous medium required in cell experiments through careful selection of the degree of substitution. This thermosensitive derivative can encapsulate doxorubicin (DOX), a well-known anticancer agent, and was further studied as a temperature-triggered drug delivery system. Although the polymeric carrier showed no notable toxicity at temperatures either below or above the transition temperature, the thermoresponsive drug-loaded formulation exhibited increased DOX cellular uptake and improved in vitro cytotoxicity at 40 °C.

From Composition to Cure: A Systems Engineering Approach to Anticancer Drug Carriers

The molecular complexity and heterogeneity of cancer has led to a persistent, and as yet unsolved, challenge to develop cures for this disease. The pharmaceutical industry focuses the bulk of its efforts on the development of new drugs, but an alternative approach is to improve the delivery of existing drugs with drug carriers that can manipulate when, where, and how a drug exerts its therapeutic effect. For the treatment of solid tumors, systemically delivered drug carriers face significant challenges that are imposed by the pathophysiological barriers that lie between their site of administration and their site of therapeutic action in the tumor. Furthermore, drug carriers face additional challenges in their translation from preclinical validation to clinical approval and adoption. Addressing this diverse network of challenges requires a systems engineering approach for the rational design of optimized carriers that have a realistic prospect for translation from the laboratory to the patient.

Comparative Experimental and Computational Study of Monoalkyl Chain Phosphatidylcholine-Containing Thermoresponsive Liposomes

Liposomes containing lysophospholipids are intensively studied as drug delivery systems that are stable at normal body temperature but exhibit fast release of their drug load at slightly elevated temperatures. In this study, the stability and release properties of dipalmitoylglycerophosphocholine (DPPC)-based liposomes incorporating the commonly used lysophosphatidylocholine (lyso-PC), and a series of monoalkyl chain ether-linked phosphatidylcholine, i.e., the biologically relevant monoalkyl chain platelet activating factor (PAF) and its derivatives lyso-PAF and methyl-PAF, were investigated. To this end a series of PEGylated small unilamellar liposomes with DPPC:monoalkyl lipid compositions of 5% and 10% molar ratio were prepared and compared with regard to stability (37 °C) and release properties at elevated temperatures (38-43 °C). All systems were characterized with respect to size distribution, ζ-potential, and phase transition characteristics. The presence of ether-lipids endows liposomes with superior (∼10% increase) release properties at 5% incorporation compared to lyso-PC, while at 10% molar ratio the formulations do not differ significantly, the release being close to 90%. The findings are supported by atomistic molecular dynamics simulations that suggest a correlation between the enhanced permeability and increased penetration of water molecules within the bilayers with density fluctuations resulting from the increased area-per-lipid and the disorder of the lysolipids alkyl chains.

Formulation and optimization of idarubicin thermosensitive liposomes provides ultrafast triggered release at mild hyperthermia and improves tumor response

Drug delivery through thermosensitive liposomes (TSL) in combination with hyperthermia (HT) has shown great potential. HT can be applied locally forcing TSL to release their content in the heated tumor resulting in high peak concentrations. To perform optimally the drug is ideally released fast (seconds) and taken up rapidly by tumor cells. The aim of this study was to develop a novel thermosensitive liposome formulation of the anthracycline idarubicin (IDA-TSL). The hydrophobicity of idarubicin may improve its release from liposomes and subsequently rapid cellular uptake when combined mild hyperthermia. Here, we investigated a series of parameters to optimize IDA-TSL formulation. The results show that the optimal formulation for IDA-TSL is DPPC/DSPC/DSPE-PEG (6/3.5/0.5 mol%), with ammonium EDTA of 6.5 pH as loading buffer and a size of ~85 nm. In vitro studies demonstrated minimal leakage of ~20% in FCS at 37 °C for 1h, while an ultrafast and complete triggered release of IDA was observed at 42 °C. On tumor cells IDA-TSL showed comparable cytotoxicity to free IDA at 42 °C, but low cytotoxicity at 37 °C. Intravital microscopy imaging demonstrated an efficient in vivo intravascular triggered drug release of IDA-TSL under mild hyperthermia, and a subsequent massive IDA uptake by tumor cells. In animal efficacy studies, IDA-TSL plus mild HT demonstrated prominent tumor growth inhibition and superior survival rate over free IDA with HT or a clinically used Doxil treatment. These results suggest beneficial potential of IDA-TSL combined with local mild HT.

In vivo evaluation of small-molecule thermoresponsive anticancer drugs potentiated by hyperthermia

Hyperthermia used as an adjuvant with chemotherapy is highly promising in the treatment of certain cancers. Currently, the small molecule drugs used in combination with hyperthermia were not designed for this application. Herein, we report the evaluation of a chlorambucil and a ruthenium compound modified with a long fluorous chain, which exhibit thermoresponsive activity in colorectal adenocarcinoma xenografts in athymic mice in combination with mild hyperthermia (42 °C). Intraperitoneal injection of the derivatives followed by local hyperthermia showed a synergistic tumor growth reduction by 79% and 90% for the chlorambucil and ruthenium-based derivatives, respectively, with the latter exhibiting a higher synergy in combination with hyperthermia compared to the monotherapies. Histological analysis shows that both derivatives in combination with hyperthermia significantly decrease the number of proliferating tumor cells.

Inside-outside self-assembly of light-activated fast-release liposomes

Building additional functionality into both the membrane and the internal compartments of biocompatible liposomes by self-assembly can provide ways of enhancing colloidal stability and spatial and temporal control of contents release. An interdigitation-fusion process is used to encapsulate near infrared light absorbing copper sulfide nanoparticles in the interior compartments of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol liposomes. Once formed, the liposome membrane is modified to include lysolipids and polyethylene glycol lipids by partitioning from lysolipid and PEG-lipid micelles in solution. This results in sterically stable, thermosensitive liposomes with a permeability transition near physiological temperature that can be triggered by NIR light irradiation. Rapid changes in local concentration can be induced with spatial and temporal control using NIR laser light.

Nanohybrid liposomal cerasomes with good physiological stability and rapid temperature responsiveness for high intensity focused ultrasound triggered local chemotherapy of cancer

The high intensity focused ultrasound (HIFU) and thermosensitive cerasomes (HTSCs) were successfully assembled by employing cerasome-forming lipid (CFL) in combination with the component lipids of conventional low temperature sensitive liposomes (LTSLs) including 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG-2000) and 1-stearoyl-2-hydroxy-sn-glycero-3-phosphocholine (MSPC). The HTSCs showed spherical shape with a mean diameter around 200 nm, exhibiting good biocompatibility. Both hydrophilic and lipophilic drugs can be efficiently encapsulated into HTSCs. In addition, the release rate of HTSCs could be conveniently adjusted by varying the molar ratios of CFL to DPPC. The drug loaded HTSCs showed much longer blood circulation time (half-life >8.50 ± 1.49 h) than conventional LTSLs (0.92 ± 0.17 h). An in vitro study demonstrated that the drug loaded HTSCs are highly stable at 37 °C and show a burst release at 42 °C, providing a capability to act synergistically against tumors. We found that the HTSCs with a proportion of 43.25% of CFL could release more than 90% hydrophilic drugs in 1 min at an elevated temperature of 42 °C generated by HIFU exposure. After intravenous injection of doxorubicin (DOX) loaded HTSCs at 5 mg DOX/kg, followed by double HIFU sonication, the tumor growth of the adenocarcinoma (MDA-MB-231) bearing mice could be significantly inhibited. Therefore, the drug loaded HTSCs combined with HIFU hold great potential for efficient local chemotherapy of cancer due to the ability to deliver high concentration of chemotherapy drugs directly to the tumor, achieve maximum therapeutic efficacy and minimal side effects, and avoid the damage to the healthy tissues caused by systemic administration of drugs.

Thermoresponsive fluorinated small-molecule drugs: a new concept for efficient localized chemotherapy

We review the drugs used in combination with hyperthermia for cancer therapy and recent advances on small thermoresponsive molecules.

Rapid, Reversible Release from Thermosensitive Liposomes Triggered by Near-Infra-Red Light

We present a novel drug carrier consisting of plasmonic hollow gold nanoshells (HGN) chemically tethered to liposomes made temperature sensitive with lysolipids (LTSL). Continuous-wave irradiation by physiologically friendly near infra-red light at 800 nm for 2.5 minutes at laser intensities an order of magnitude below that known to damage skin generates heating localized to the liposome membrane. The heating increases the liposome permeability in an irradiation dose-dependent, but reversible manner, resulting in rapid release of small molecules such as the self-quenching dye carboxyfluorescein or the chemotherapeutic doxorubicin, without raising the bulk temperature. The local rise in nanoshell temperature under laser irradiation was inferred by comparing dye release rates from the LTSL via bulk heating to that induced by irradiation. Laser-irradiation of LTSL enables precise control of contents release with low temperature gradients confined to areas irradiated by the laser focus. The combined effects of rapid local release and localized hyperthermia provide a synergistic effect as shown by a near doubling of androgen resistant PPC-1 prostate cancer cell toxicity compared to the same concentration of free doxorubicin.

A method to convert MRI images of temperature change into images of absolute temperature in solid tumours

PURPOSE

During hyperthermia (HT), the therapeutic response of tumours varies substantially within the target temperature range (39-43 °C). Current thermometry methods are either invasive or measure only temperature change, which limits the ability to study tissue responses to HT. This study combines manganese-containing low temperature sensitive liposomes (Mn-LTSL) with proton resonance frequency shift (PRFS) thermometry to measure absolute temperature in tumours with high spatial and temporal resolution using MRI.

METHODS

Liposomes were loaded with 300 mM MnSO(4). The phase transition temperature (T(m)) of Mn-LTSL samples was measured by differential scanning calorimetry (DSC). The release of manganese from Mn-LTSL in saline was characterised with inductively coupled plasma atomic emission spectroscopy. A 2T GE small animal scanner was used to acquire dynamic T1-weighted images and temperature change images of Mn-LTSL in saline phantoms and fibrosarcoma-bearing Fisher-344 rats receiving hyperthermia after Mn-LTSL injection.

RESULTS

The T(m) of Mn-LTSL in rat blood was 42.9 ± 0.2 °C (DSC). For Mn-LTSL samples (0.06 mM-0.5 mM Mn(2+) in saline) heated monotonically from 30 °C to 50 °C, a peak in the rate of MRI signal enhancement occurred at 43.1° ± 0.3 °C. The same peak in signal enhancement rate was observed during heating of fibrosarcoma tumours (N = 3) after injection of Mn-LTSL, and the peak was used to convert temperature change images into absolute temperature. Accuracies of calibrated temperature measurements were in the range 0.9-1.8 °C.

CONCLUSION

The release of Mn(2+) from Mn-LTSL affects the rate of MR signal enhancement which enables conversion of MRI-based temperature change images to absolute temperature.

A phantom for visualization of three-dimensional drug release by ultrasound-induced mild hyperthermia

PURPOSE

Ultrasound-induced mild hyperthermia has advantages for noninvasive, localized and controlled drug delivery. In this study, a tissue-mimicking agarose-based phantom with a thermally sensitive indicator was developed for studying the spatial drug delivery profile using ultrasound-induced mild hyperthermia.

METHODS

Agarose powder, regular evaporated milk, Dulbecco's phosphate-buffered saline (DPBS), n-propanol, and silicon carbide powder were homogeneously mixed with low temperature sensitive liposomes (LTSLs) loaded with a self-quenched near-infrared (NIR) fluorescent dye. A dual-mode linear array ultrasound transducer was used for insonation at 1.54 MHz with a total acoustic power and acoustic pressure of 2.0 W and 1.5 MPa, respectively. After insonation, the dye release pattern in the phantom was quantified based on optical images, and the three-dimensional release profile was reconstructed and analyzed. A finite-difference time-domain-based algorithm was developed to simulate both the temperature distribution and spatial dye diffusion as a function of time. Finally, the simulated dye diffusion patterns were compared to experimental measurements.

RESULTS

Self-quenching of the fluorescent dye in DPBS was substantial at a concentration of 6.25×10(-2) mM or greater. The transition temperature of LTSLs in the phantom was 35 °C, and the release reached 90% at 37 °C. The simulated temperature for hyperthermia correlated with the thermocouple measurements with a mean error between 0.03±0.01 and 0.06±0.02 °C. The R2 value between the experimental and simulated spatial extent of the dye diffusion, defined by the half-peak level in the elevation, lateral and depth directions, was 0.99 (slope=1.08), 0.95 (slope=0.99), and 0.80 (slope=1.04), respectively, indicating the experimental and simulated dye release profiles were similar.

CONCLUSIONS

The combination of LTSLs encapsulating a fluorescent dye and an optically transparent phantom is useful for visualizing and modeling drug release in vitro following ultrasound-induced mild hyperthermia. The coupled temperature simulation and dye-diffusion simulation tools were validated with the experimental system and can be used to optimize the thermal dose and spatial and temporal dye release pattern.

Targeted thermosensitive liposomes: an attractive novel approach for increased drug delivery to solid tumors

INTRODUCTION

Currently available chemotherapy is hampered by a lack in tumor specificity and resulting toxicity. Small and long-circulating liposomes can preferentially deliver chemotherapeutic drugs to tumors upon extravasation from tumor vasculature. Although clinically used liposomal formulations demonstrated significant reduction in toxicity, enhancement of therapeutic activity has not fully met expectations.

AREAS COVERED

Low drug bioavailability from liposomal formulations and limited tumor accumulation remain major challenges to further improve therapeutic activity of liposomal chemotherapy. The aim of this review is to highlight strategies addressing these challenges. A first strategy uses hyperthermia and thermosensitive liposomes to improve tumor accumulation and trigger liposomal drug bioavailability. Image-guidance can aid online monitoring of heat and drug delivery and further personalize the treatment. A second strategy involves tumor-specific targeting to enhance drug delivery specificity and drug internalization. In addition, we review the potential of combinations of the two in one targeted thermosensitive-triggered drug delivery system.

EXPERT OPINION

Heat-triggered drug delivery using thermosensitive liposomes as well as the use of tumor vasculature or tumor cell-targeted liposomes are both promising strategies to improve liposomal chemotherapy. Preclinical evidence has been encouraging and both strategies are currently undergoing clinical evaluation. A combination of both strategies rendering targeted thermosensitive liposomes (TTSL) may appear as a new and attractive approach promoting tumor drug delivery.

In vitro localized release of thermosensitive liposomes with ultrasound-induced hyperthermia

Localized drug delivery with ultrasound-induced hyperthermia can enhance the therapeutic index of chemotherapeutic drugs by improving efficacy and reducing systemic toxicity. A novel in vitro method for the activation of drug-loaded thermosensitive liposomes is described. In particular, a dual-compartment, acoustically transparent container is used in which thermosensitive liposomes suspended in cell culture medium are immersed in a thermally absorptive medium, glycerol. Hyperthermia is induced with ultrasound in the glycerol, which in turn heats the culture medium by thermal conduction. The method approximately mimics the in vivo scenario of thermosensitive liposomes collected in the interstitial spaces of tumors, where ultrasound induces hyperthermia in the tumor tissue, which in turn heats the thermosensitive liposomes by conduction and induces release of the encapsulated drug. The acoustic conditions for the desired hyperthermia are derived theoretically and validated experimentally. Eighty percent release of doxorubicin from thermosensitive liposomes is achieved.

Materials characterization of the low temperature sensitive liposome (LTSL): effects of the lipid composition (lysolipid and DSPE-PEG2000) on the thermal transition and release of doxorubicin

This paper describes how we have used material science, physical chemistry, and some luck, to design a new thermal-sensitive liposome (the low temperature sensitive liposome (LTSL)) that responds at clinically attainable hyperthermic temperatures releasing its drug in a matter of seconds as it passes through the microvasculature of a warmed tumor. The LTSL is composed of a judicial combination of three component lipids, each with a specific function and each affecting specific material properties, including a sharp thermal transition and a rapid on-set of membrane permeability to small ions, drugs and small dextran polymers. Experimentally, the paper describes how bilayer-concentration changes involving the lysolipid and the presence or absence of DSPE-PEG2000 affect both the lipid transition temperature and the drug release. While the inclusion of 4 mol% DSPE-PEG2000 raises the transition temperature peak (T(m)) by about 1 degrees C, the inclusion of 5.0, 9.7, 12.7 and 18.0 mol% MSPC slightly lowered this peak back to 41.7 degrees C, while not further broadening the peak breadth. As for drug release, in the absence of MSPC, the encapsulated doxorubicin-citrate is hardly released at all. Increasing the composition of MSPC in the lipid mixture (5.0, 7.4, 8.5 and 9.3 mol% MSPC) shows faster and faster initial doxorubicin release rates, with 8.5 and 9.3 mol% MSPC formulations giving 80% of encapsulated drug released in 4 and 3 min, respectively. The Thermodox formulation (9.7 mol% MSPC) gives 60% released in the first 20 s. The presence of PEG-lipid is found to be essential in order for the lysolipid-induced permeability to reach these very fast times. From drug and dextran release experiments, and estimates of the molecular and pore size, the conclusions are that: in order to induce lasting nanopores in lipid bilayers -10 nm diameter, they initially require the presence (from the solid phase structure) of grain boundary defects at the DPPC transition and the permeabilizing component(s) can either be a pore forming lysolipid/surfactant plus a PEG-lipid, or can be generated by a PEG-surfactant incorporated at -4-5 mol%. The final discussion is centered around the postulated defect structures that result in membrane leakage and the permeability of doxorubicin and H+ ions.

Complete regression of local cancer using temperature-sensitive liposomes combined with ultrasound-mediated hyperthermia

The development of treatment protocols that result in a complete response to chemotherapy has been hampered by free drug toxicity and the low bioavailability of nano-formulated drugs. Here, we explore the application of temperature-sensitive liposomes that have been formulated to enhance stability in circulation. We formed a pH-sensitive complex between doxorubicin (Dox) and copper (CuDox) in the core of lysolipid-containing temperature-sensitive liposomes (LTSLs). The complex remains associated at neutral pH but dissociates to free Dox in lower pH environments. The resulting CuDox-LTSLs were injected intravenously into a syngeneic murine breast cancer model (6 mg Dox/kg body weight) and intravascular release of the drug was triggered by ultrasound. The entire tumor was insonified for 5 min prior to drug administration and 20 min post drug injection. A single-dose administration of CuDox-LTSLs combined with insonation suppressed tumor growth. Moreover, after twice per week treatment over a period of 28 days, a complete response was achieved in which the NDL tumor cells and the tumor interstitium could no longer be detected. All mice treated with ultrasound combined with CuDox-LTSLs survived, and tumor was undetectable 8 months post treatment. Iron and copper-laden macrophages were observed at early time points following treatment with this temperature sensitive formulation. Systemic toxicity indicators, such as cardiac hypertrophy, leukopenia, and weight and hair loss were not detected with CuDox-LTSLs after the 28-day therapy.

Materials Science and Engineering of the Low Temperature Sensitive Liposome (LTSL): Composition-Structure-Property Relationships That Underlie its Design and Performance

This chapter presents the material science and materials engineering concepts that went into the design and testing of the Low Temperature-Sensitive Liposome (LTSL), including: the roles of each of the components that make up the composite membrane; how the molecular and nanostructures that they form might influence the already anomalous permeability at the phase transition of the bilayer; and how this thermally sensitive “Smart Drug Delivery System” leads to ultrafast release of a loaded doxorubicin drug, triggered and controlled in the micro-vasculature of tumors by applied mild hyperthermia. This formulation approach, as ThermoDox®, has been used in a completed 700-patient Phase III human clinical trial in liver cancer (HEAT study), is in a Phase II trial in chest wall recurrence of cancer (DIGNITY study) and has been used in a Phase I trial of patients with colorectal liver metastases (ABLATE study). With additional research and preclinical studies underway, and a range of other drugs, imaging agents and biological modifiers poised for encapsulation, the LTSL could provide a new paradigm for drug and agent delivery for the treatment of localized tumors: rapid triggered drug release in the tumor bloodstream and deep penetration of drug into the tumor tissue.

Mild hyperthermia triggered doxorubicin release from optimized stealth thermosensitive liposomes improves intratumoral drug delivery and efficacy

Liposome mediated anticancer drug delivery has the advantage of reducing cytotoxicity in healthy tissues. However, undesired slow drug release impedes the therapeutic efficacy of clinically applied PEG-liposomal doxorubicin (Dox). The aim of this study is to combine stealth thermosensitive liposomes (TSL) and local mild hyperthermia (HT) to increase bioavailable Dox levels in tumors. Dox was encapsulated in stealth TSL (~80nm) with optimized PEG concentration in the membrane, and compared with lysolipid-based Dox-LTSL for in vitro stability, release kinetics, and in vivo tumor growth control. In vitro cytotoxicity of Dox-TSL against murine BFS-1 sarcoma and, human BLM melanoma cell lines and Human Umbilical Vein Endothelial Cells (HUVEC) under normothermia (37°C) and HT (42°C) was compared with non-encapsulated Dox. In vitro Dox uptake in nuclei was imaged in BLM and HUVEC. In vivo intravascular Dox release from TSL in BFS-1 tumors under local mild HT in dorsal skin flap window chamber models was captured by intravital confocal microscopy. Intravascular Dox-TSL release kinetics, penetration depth and interstitial Dox density were subjected to quantitative image analysis. Systemic Dox-TSL administration in combination with local mild HT on subcutaneous tumor growth control was compared to Dox-LTSL plus local mild HT. Dox-TSL was stable at 37°C, while released over 95% Dox within 1min in 90% serum at 42°C. Dox-TSL demonstrated efficient in vivo intratumoral Dox release under local mild HT, followed by significant Dox uptake by tumor and tumor vascular endothelial cells. Dox-TSL plus mild HT showed improved tumor growth control over Dox-LTSL plus mild HT. Survival after a single treatment of Dox-TSL plus mild HT was 67%, while survival after Dox-LTSL plus mild HT was 22%. This combination of Dox-TSL and local mild HT offers promising clinical opportunities to improve liposomal Dox delivery to solid tumors.

Hyperthermia-induced drug targeting

INTRODUCTION

Specific delivery of a drug to a target site is a major goal of drug delivery research. Using temperature-sensitive liposomes (TSLs) is one way to achieve this; the liposome acts as a protective carrier, allowing increased drug to flow through the bloodstream by minimizing clearance and non-specific uptake. On reaching microvessels within a heated tumor, the drug is released and quickly penetrates. A major advance in the field is ThermoDox® (Celsion), demonstrating significant improvements to the drug release rates and drug uptake in heated tumors (∼ 41°C). Most recently, magnetic resonance-guided focused ultrasound (MRgFUS) has been combined with TSL drug delivery to provide localized chemotherapy with simultaneous quantification of drug release within the tumor.

AREAS COVERED

In this article the field of hyperthermia-induced drug delivery is discussed, with an emphasis on the development of TSLs and their combination with hyperthermia (both mild and ablative) in cancer therapy. State-of-the-art image-guided heating technologies used with this combination strategy will also be presented, with examples of real-time monitoring of drug delivery and prediction of efficacy.

EXPERT OPINION

The specific delivery of drugs by combining hyperthermia with TSLs is showing great promise in the clinic and its potential will be even greater as the use of image-guided focused ultrasound becomes more widespread - a technique capable of penetrating deep within the body to heat a specific area with improved control. In conjunction with this, it is anticipated that multifunctional TSLs will be a major topic of study in this field.

Comparison of conventional chemotherapy, stealth liposomes and temperature-sensitive liposomes in a mathematical model

Various liposomal drug carriers have been developed to overcome short plasma half-life and toxicity related side effects of chemotherapeutic agents. We developed a mathematical model to compare different liposome formulations of doxorubicin (DOX): conventional chemotherapy (Free-DOX), Stealth liposomes (Stealth-DOX), temperature sensitive liposomes (TSL) with intra-vascular triggered release (TSL-i), and TSL with extra-vascular triggered release (TSL-e). All formulations were administered as bolus at a dose of 9 mg/kg. For TSL, we assumed locally triggered release due to hyperthermia for 30 min. Drug concentrations were determined in systemic plasma, aggregate body tissue, cardiac tissue, tumor plasma, tumor interstitial space, and tumor cells. All compartments were assumed perfectly mixed, and represented by ordinary differential equations. Contribution of liposomal extravasation was negligible in the case of TSL-i, but was the major delivery mechanism for Stealth-DOX and for TSL-e. The dominant delivery mechanism for TSL-i was release within the tumor plasma compartment with subsequent tissue- and cell uptake of released DOX. Maximum intracellular tumor drug concentrations for Free-DOX, Stealth-DOX, TSL-i, and TSL-e were 3.4, 0.4, 100.6, and 15.9 µg/g, respectively. TSL-i and TSL-e allowed for high local tumor drug concentrations with reduced systemic exposure compared to Free-DOX. While Stealth-DOX resulted in high tumor tissue concentrations compared to Free-DOX, only a small fraction was bioavailable, resulting in little cellular uptake. Consistent with clinical data, Stealth-DOX resulted in similar tumor intracellular concentrations as Free-DOX, but with reduced systemic exposure. Optimal release time constants for maximum cellular uptake for Stealth-DOX, TSL-e, and TSL-i were 45 min, 11 min, and <3 s, respectively. Optimal release time constants were shorter for MDR cells, with ∼4 min for Stealth-DOX and for TSL-e. Tissue concentrations correlated well quantitatively with a prior in-vivo study. Mathematical models may thus allow optimization of drug delivery systems to achieve a better therapeutic index.

Liposomal nanomedicine for breast cancer therapy

Liposomes are well-established nanocarriers for improving the therapeutic index of anticancer agents. A remarkable understanding in the pathophysiology of breast cancer progression has emerged with information on the involved specific biomolecules, which may serve as molecular targets for its therapy. Hormonal and nonhormonal receptors can both be exploited for targeting to breast cancer cells. Targeted delivery of cytotoxic drugs using liposomes is a novel approach for breast cancer therapy. In the present article, we summarize molecular targets present on the breast cancer cells. Recent developments in liposome-based delivery of bioactives for selective treatments of breast cancer are discussed. In addition, utilization of bioenvironmental conditions of tumor for liposome-based targeted delivery is also summed up.

Development and modeling of arsenic-trioxide-loaded thermosensitive liposomes for anticancer drug delivery

In this article, a novel delivery system for the anticancer drug, arsenic trioxide (ATO), is characterized. The release of ATO from DPPC liposomes with MPPC lysolipid incorporated into the bilayer was measured. Upon heating the liposomes to 37°C, there was a 15-25% release over 24 hours. The ATO release from the DPPC and DPPC:MPPC (5%) systems leveled off after 10 hours at 37°C, whereas the DPPC:MPPC (10%) liposomes continue to release ATO over the 24-hour time span. Upon heating the liposomes rapidly to 42°C, the release rate was substantially increased. The systems containing lysolipids exhibited a very rapid release of a significant amount of arsenic in the first hour. In the first hour, the DPPC:MPPC (5%) liposomes released 40% of the arsenic and the DPPC:MPPC (10%) liposomes released 55% of the arsenic. Arsenic release from pure DPPC liposomes was comparable at 37 and 42°C, indicating that the presence of a lysolipid is necessary for a significant enhancement of the release rate. A coarse-grained molecular dynamics (CGMD) model was used to investigate the enhanced permeability of lysolipid-incorporated liposomes and lipid bilayers. The CG liposomes did not form a gel phase when cooled due to the high curvature; however, permeability was still significantly lower below the liquid-to-gel phase-transition temperature. Simulations of flat DPPC:MPPC bilayers revealed that a peak in the permeability did coincide with the phase transition from the gel to LC state when the lysolipid, MPPC, was present. No pores were observed in the simulations, so it is unlikely this was the permeability-enhancing mechanism.

Mathematical spatio-temporal model of drug delivery from low temperature sensitive liposomes during radiofrequency tumour ablation

PURPOSE

Studies have demonstrated a synergistic effect between hyperthermia and chemotherapy, and clinical trials in image-guided drug delivery combine high-temperature thermal therapy (ablation) with chemotherapy agents released in the heating zone via low temperature sensitive liposomes (LTSL). The complex interplay between heat-based cancer treatments such as thermal ablation and chemotherapy may require computational models to identify the relationship between heat exposure and pharmacokinetics in order to optimise drug delivery.

MATERIALS AND METHODS

Spatio-temporal data on tissue temperature and perfusion from heat-transfer models of radiofrequency ablation were used as input data. A spatio-temporal multi-compartmental pharmacokinetic model was built to describe the release of doxorubicin (DOX) from LTSL into the tumour plasma space, and subsequent transport into the extracellular space, and the cells. Systemic plasma and tissue compartments were also included. We compared standard chemotherapy (free-DOX) to LTSL-DOX administered as bolus at a dose of 0.7 mg/kg body weight.

RESULTS

Modelling LTSL-DOX treatment resulted in tumour tissue drug concentration of approximately 9.3 microg/g with highest values within 1 cm outside the ablation zone boundary. Free-DOX treatment produced comparably uniform tissue drug concentrations of approximately 3.0 microg/g. Administration of free-DOX resulted in a considerably higher peak level of drug concentration in the systemic plasma compartment (16.1 microg/g) compared to LTSL-DOX (4.4 microg/g). These results correlate well with a prior in vivo study.

CONCLUSIONS

Combination of LTSL-DOX with thermal ablation allows localised drug delivery with higher tumour tissue concentrations than conventional chemotherapy. Our model may facilitate drug delivery optimisation via investigation of the interplays among liposome properties, tumour perfusion, and heating regimen.

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-PEG(2000)). We investigated the roles of DSPE-PEG(2000) 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 degrees C in vitro. Pharmacokinetics of various LTSL formulations were compared in mice with body temperature controlled at 37 degrees 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.

Hyperthermia mediated liposomal drug delivery

Drug delivery systems have been developed for cancer therapy in an attempt to increase the tumour drug concentration while limiting systemic exposure. Liposomes have achieved passive targeting of solid tumours through enhanced vascular permeability, which is greatly augmented by hyperthermia. However, anti-tumour efficacy has often been limited by slow release of bioavailable drug within the tumour. Local hyperthermia has become the most widely used stimulus for triggered release of liposomal drugs, through the use of specific lipids, polymers or other modifiers. A temperature-sensitive liposome containing doxorubicin has been shown to release 100% of contents through stabilized membrane pores within 10-20 s at 41 degrees C. This formulation has exhibited dramatic improvements in pre-clinical drug delivery and tumour regression and is now in clinical trials. Significantly, recent studies show that this liposome, in combination with local hyperthermia, exhibits vascular shutdown as a mechanism of anti-tumour effect that is not observed with free doxorubicin.

Laser-induced disruption of systemically administered liposomes for targeted drug delivery

Liposomal formulations of drugs have been shown to enhance drug efficacy by prolonging circulation time, increasing local concentration and reducing off-target effects. Controlled release from these formulations would increase their utility, and hyperthermia has been explored as a stimulus for targeted delivery of encapsulated drugs. Use of lasers as a thermal source could provide improved control over the release of the drug from the liposomes with minimal collateral tissue damage. Appropriate methods for assessing local release after systemic delivery would aid in testing and development of better formulations. We use in vivo bioluminescence imaging to investigate the spatiotemporal distribution of luciferin, used as a model small molecule, and demonstrate laser-induced release from liposomes in animal models after systemic delivery. These liposomes were tested for luciferin release between 37 and 45 degrees C in PBS and serum using bioluminescence measurements. In vivo studies were performed on transgenic reporter mice that express luciferase constitutively throughout the body, thus providing a noninvasive readout for controlled release following systemic delivery. An Nd:YLF laser was used (527 nm) to heat tissues and induce rupture of the intravenously delivered liposomes in target tissues. These data demonstrate laser-mediated control of small molecule delivery using thermally sensitive liposomal formulations.

Ultrasound triggered image-guided drug delivery

The integration of therapeutic interventions with diagnostic imaging has been recognized as one of the next technological developments that will have a major impact on medical treatments. Important advances in this field are based on a combination of progress in guiding and monitoring ultrasound energy, novel drug classes becoming available, the development of smart delivery vehicles, and more in depth understanding of the mechanisms of the cellular and molecular basis of diseases. Recent research demonstrates that both pressure sensitive and temperature sensitive delivery systems hold promise for local treatment. The use of ultrasound for the delivery of drugs has been demonstrated in particular the field of cardiology and oncology for a variety of therapeutics ranging from small drug molecules to biologics and nucleic acids.

In vitro and in vivo evaluations of increased effective beam width for heat deposition using a split focus high intensity ultrasound (HIFU) transducer

PURPOSE

To develop a novel and efficient, in vitro method for characterizing temporal and spatial heat generation of focused ultrasound exposures, and evaluate this method to compare a split focus and conventional single focus high intensity focused ultrasound transducer.

MATERIALS AND METHODS

A HIFU tissue-mimicking phantom was validated by comparing respective temperature elevations generated in the phantoms and in murine tumors in vivo. The phantom was then used in combination with IR thermography to spatially and temporally characterize differences in low-level temperature elevation (e.g. 3-5 degrees C) produced by a single focus and split focus HIFU transducer, where the latter produces four simultaneous foci. In vivo experiments with heat sensitive liposomes containing doxorubicin were then carried out to determine if the larger beam width of the split focus transducer, compared to the single focus, could increase overall deployment of the drug from the liposome.

RESULTS

Temperature elevations generated in the HIFU phantom were not found to be different from those measured in vivo when compensating for disparities in attenuation coefficient and specific heat, and between the two transducers by increasing the energy deposition. Exposures with the split focus transducer provided significant increases in the area treated compared to the single focus, which then translated to significant increases in drug deposition in vivo.

CONCLUSIONS

Preliminary evidence was provided indicating the potential for using this novel technique for characterizing hyperthermia produced by focused ultrasound devices. Further development will be required for its suitability for correlating in vitro and in vivo outcomes.

Rationale for and measurement of liposomal drug delivery with hyperthermia using non-invasive imaging techniques

The purpose of this review is to present an overview of the state-of-the-art imaging modalities used to track drug delivery from liposomal formulations into tumors during or after hyperthermia treatment. Liposomes are a drug delivery system comprised of a phospholipid bilayer surrounding an aqueous core and have been shown to accumulate following hyperthermia therapy. Use of contrast-containing liposomes in conjunction with hyperthermia therapy holds great promise to be able to directly measure drug dose concentrations as well as to non-invasively describe patterns of drug distribution with MR and PET/SPECT imaging modalities. We will review the rationale for using this approach and the potential advantages of having such information available during and after treatment.

Phase I trial of doxorubicin-containing low temperature sensitive liposomes in spontaneous canine tumors

PURPOSE

To determine the maximum tolerated dose, dose-limiting toxicities, and pharmacokinetic characteristics of doxorubicin encapsulated in a low temperature sensitive liposome (LTSL) when given concurrently with local hyperthermia to canine solid tumors.

EXPERIMENTAL DESIGN

Privately owned dogs with solid tumors (carcinomas or sarcomas) were treated. The tumors did not involve bone and were located at sites amenable to local hyperthermia. LTSL-doxorubicin was given (0.7-1.0 mg/kg i.v.) over 30 minutes during local tumor hyperthermia in a standard phase I dose escalation study. Three treatments, given 3 weeks apart, were scheduled. Toxicity was monitored for an additional month. Pharmacokinetics were evaluated during the first treatment cycle.

RESULTS

Twenty-one patients were enrolled: 18 with sarcomas and 3 with carcinomas. Grade 4 neutropenia and acute death secondary to liver failure, possibly drug related, were the dose-limiting toxicities. The maximum tolerated dose was 0.93 mg/kg. Other toxicities, with the possible exception of renal damage, were consistent with those observed following free doxorubicin administration. Of the 20 dogs that received > or = 2 doses of LTSL-doxorubicin, 12 had stable disease, and 6 had a partial response to treatment. Pharmacokinetic variables were more similar to those of free doxorubicin than the marketed liposomal product. Tumor drug concentrations at a dose of 1.0 mg/kg averaged 9.12 +/- 6.17 ng/mg tissue.

CONCLUSION

LTSL-doxorubicin offers a novel approach to improving drug delivery to solid tumors. It was well tolerated and resulted in favorable response profiles in these patients. Additional evaluation in human patients is warranted.

Use of degradable and nondegradable nanomaterials for controlled release

Drug-delivery devices are fundamentally important in improving the pharmacological profiles of therapeutic molecules. Nanocontrolled-release systems are attracting a lot of attention currently owing to their large surface area and their ability to target delivery to specific sites in the human body. In addition, they can penetrate the cell membrane for gene, nucleic acid and bioactive peptide/protein delivery. Representative applications of nanodrug-delivery systems include controlled-release wound dressings, controlled-release scaffolds for tissue regeneration and implantable biodegradable nanomaterial-based medical devices integrated with drug-delivery functions. We review the present status and future perspectives of various types of nanocontrolled-release systems. Although many of the well-established degradable and nondegradable controlled-release vehicles are being investigated for their processing into nanocarriers, several new emerging nanomaterials are being studied for their controlled-release properties. The release of multiple bioactive agents, each with its own kinetic profile, is becoming possible. In addition, integration of the nanocontrolled-release systems with other desirable functions to create new, cross-discipline applications can also be realized.

In vitro stability and content release properties of phosphatidylglyceroglycerol containing thermosensitive liposomes

Recently, we reported that 1,2-dipalmitoyl-sn-glycero-3-phosphoglyceroglycerol (DPPGOG) prolongs the circulation time of thermosensitive liposomes (TSL). Since the only TSL formulation in clinical trials applies DSPE-PEG2000 and lysophosphatidylcholine (P-lyso-PC), the objective of this study was to compare the influence of these lipids with DPPGOG on in vitro stability and heat-induced drug release properties of TSL. The content release rate was significantly increased by incorporating DPPGOG or P-lyso-PC in TSL formulations. DPPC/DSPC/DPPGOG 50:20:30 (m/m) and DPPC/P-lyso-PC/DSPE-PEG2000 90:10:4 (m/m) did not differ significantly in their release rate of carboxyfluorescein with >70% being released within the first 10s at their phase transition temperature. Furthermore, DPPC/DSPC/DPPGOG showed an improved stability at 37 degrees C in serum compared to the PEGylated TSL. The in vitro properties of DPPGOG-containing TSL remained unchanged when encapsulating doxorubicin instead of carboxyfluorescein. The TSL retained 89.1+/-4.0% of doxorubicin over 3 h at 37 degrees C in the presence of serum. The drug was almost completely released within 120s at 42 degrees C. In conclusion, DPPGOG improves the in vitro properties in TSL formulations compared to DSPE-PEG2000, since it not only increases the in vivo half-life, it even increases the content release rate without negative effect on TSL stability at 37 degrees C which has been seen for DSPE-PEG2000/P-lyso-PC containing TSL.

Nanotechnology platforms and physiological challenges for cancer therapeutics

Nanotechnology is considered to be an emerging, disruptive technology that will have significant impact in all industrial sectors and across-the-board applications in cancer research. There has been tremendous investment in this area and an explosion of research and development efforts in recent years, particularly in the area of cancer research. At the National Institutes of Health, nanomedicine is one of the priority areas under its Roadmap Initiatives. Moreover, in 2005 the National Cancer Institute alone committed $144.3 million over 5 years for its Alliance for Nanotechnology in Cancer program. Much research and development is progressing in the areas of cancer diagnostics, devices, biosensors, and microfluidics, but this review will focus on therapeutics. Current nanotechnology platforms for cancer therapeutics encompass a vast array of nanomaterials and nanodevices. This review will focus on six of the most prominent and most widely studied: nanoshells, carbon nanotubes, dendrimers, quantum dots, superparamagnetic nanoparticles, and liposomes. All of these nanotechnology platforms can be multifunctional, so they are frequently touted as "smart" or "intelligent." This review will discuss the shared approaches in the design and development of these nanotechnology platforms that bestow such characteristics to the nanoparticles. Finally, the review will raise awareness of the physiological challenges for the application of these therapeutic nanotechnologies, in light of some recent advances in our understanding of tumor biology.

Temperature sensitization of liposomes by use of N-isopropylacrylamide copolymers with varying transition endotherms

Three kinds of copolymers of N-isopropylacrylamide (NIPAM) with the same conformational transition temperature and varying transition endotherms were synthesized with N-acryloylpyrrolidine (APr), N,N-dimethylacrylamide (DMAM), and N-isopropylmethacrylamide (NIPMAM) as the comonomers. Two dodecyl groups were incorporated into the termini of these copolymers as an anchor for the fixation to a liposomal membrane. Egg yolk phosphatidylcholine liposomes having these copolymers were prepared and their temperature-sensitive contents release and association properties were investigated. While these copolymer exhibited a conformational transition at ca. 40 degrees C, DeltaH for the transition increased in the order of poly(APr-co-NIPAM) < poly(DMAM-co-NIPAM) < poly(NIPMAM-co-NIPAM). The liposomes containing poly(NIPMAM-co-NIPAM) showed a drastic release enhancement of entrapped calcein above the transition temperature, whereas the liposomes with poly(DMAM-co-NIPAM) and those with poly(APr-co-NIPAM) exhibited moderate and slight enhancement of calcein release above that temperature, respectively. On the contrary, the liposomes containing poly(APr-co-NIPAM) showed significant aggregation above the transition temperature, but the aggregation was hardly observed for the liposomes having poly(NIPMAM-co-NIPAM), indicating that poly(APr-co-NIPAM) more efficiently made the liposome surface hydrophobic. Thus, we concluded that the copolymer with a large DeltaH is suitable for obtaining functional liposomes with a temperature-sensitive contents release property, whereas the copolymer with a small DeltaH is appropriate for preparing functional liposomes with a temperature-sensitive surface property.