Absolute MR thermometry using nanocarriers

Motion Compensated Ultrasound Imaging Allows Thermometry and Image Guided Drug Delivery Monitoring from Echogenic Liposomes

Ultrasound imaging is widely used both for cancer diagnosis and to assess therapeutic success, but due to its weak tissue contrast and the short half-life of commercially available contrast agents, it is currently not practical for assessing motion compensated contrast-enhanced tumor imaging, or for determining time-resolved absolute tumor temperature while simultaneously reporting on drug delivery. The objectives of this study were to: 1) develop echogenic heat sensitive liposomes (E-LTSL) and non-thermosensitive liposomes (E-NTSL) to enhance half-life of contrast agents, and 2) measure motion compensated temperature induced state changes in acoustic impedance and Laplace pressure of liposomes to monitor temperature and doxorubicin (Dox) delivery to tumors. LTSL and NTSL containing Dox were co-loaded with an US contrast agent (perfluoropentane, PFP) using a one-step sonoporation method to create E-LTSL and E-NTSL. To determine temperature induced intensity variation with respect to the state change of E-LTSL and E-NTSL in mouse colon tumors, cine acquisition of 20 frames/second for about 20 min (or until wash out) at temperatures of 42°C, 39.5°C, and 37°C was performed. A rigid rotation and translation was applied to each of the "key frames" to adjust for any gross motion that arose due to motion of the animal or the transducer. To evaluate the correlation between ultrasound (US) intensity variation and Dox release at various temperatures, treatment (5 mg Dox/kg) was administered via a tail vein once tumors reached a size of 300-400 mm(3), and mean intensity within regions of interest (ROIs) defined for each sample was computed over the collected frames and normalized in the range of [0,1]. When the motion compensation technique was applied, a > 2-fold drop in standard deviation in mean image intensity of tumor was observed, enabling a more robust estimation of temporal variations in tumor temperatures for 15-20 min. due to state change of E-LTSL and E-NTSL. Consequently, a marked increase in peak intensity at 42°C compared to 37°C that corresponded with enhanced Dox delivery from E-LTSL in tumors was obtained. Our results suggest that echogenic liposomes provide a predictable change in tumor vascular contrast with temperature, and this property could be applicable to nanomonitoring of drug delivery in real time.

Synthesis and characterisation of ultrasound imageable heat-sensitive liposomes for HIFU therapy

BACKGROUND/OBJECTIVE

Novel approaches allowing efficient, readily translatable image-guided drug delivery (IGDD) against solid tumours is needed. The objectives of this study were to: 1) develop echogenic low temperature sensitive liposomes (E-LTSLs) loaded with an ultrasound (US) contrast agent (perfluoropentane, PFP), 2) determine the in vitro and in vivo stability of contrast agent encapsulation, 3) co-encapsulate and characterise doxorubicin (Dox) E-LTSL, and cellular uptake and cytotoxicity in combination with high intensity focused ultrasound (HIFU).

METHOD

E-LTSLs were loaded passively with PFP and actively with Dox. PFP encapsulation in E-LTSL was determined by transmission electron microscopy (TEM), and US imageability was determined in tissue-mimicking phantoms and mouse tumour model. Dox release from E-LTSL in physiological buffer was quantified by fluorescence spectroscopy. Cellular uptake and cytotoxicity of E-LTSL in the presence of HIFU-induced mild hyperthermia (∼40-42 °C) was determined in a 3D tumour spheroid model.

RESULTS

TEM and US confirmed that the PFP emulsion was contained within LTSLs. Phantom and animal studies showed that the E-LTSLs were echogenic. Temperature versus size increase and Dox release kinetics of E-LTSLs demonstrated no difference compared to LTSL alone. Dox release was <5% within 1 h at baseline (25 °C) and body (37 °C) temperatures, and was >99% under hyperthermia. E-LTSL plus HIFU achieved significantly greater Dox uptake in spheroids and cytotoxicity compared to body temperature.

CONCLUSION

A stable US-imageable liposome co-loaded with Dox and PFP for in vivo IGDD was developed. Data suggest that HIFU can induce cellular uptake and toxicity with E-LTSLs.