thermosensitive paramagnetic liposomes for temperature control during mr imaging-guided...

Thermosensitive Paramagnetic Liposomes for Temperature Control during MR

Imaging-guided Hyperthermia: In Vitro Feasibility Studies 1

Sigrid L. Fossheim, PhD, Kamil A. I1' yasov, PhD, Jfirgen Hennig, PhD, Atle Bjornerud, MSc

Rationale and Objectives. Magnetic resonance (MR) imaging-based temperature monitoring has gained interest for use in general hyperthermia treatment of tumors. Such therapy requires an accurate control of the temperature, which should range from 41 ° to 45°C. A novel type of thermosensitive MR agent is proposed: liposome-encapsulated gadolinium chelates whose temperature response is linked to the phase-transition properties of the liposome carder. In vitro relaxometry and MR imaging were used to evaluate the thermosensitivity of the contrast properties of liposomal gadolinium diethylenetriaminepentaacetic acid bis(methylamide) (Gd-DTPA-BMA).

Materials and Methods. T1 relaxivity (rl) measurements of liposomal Gd-DTPA-BMA were undertaken at 0.47 T and at temperatures of 20°--48°C. MR imaging was performed at 2.0 T with a gel phantom containing inserts of liposomes. Diffu- sion-weighted and Tl-weighted gradient-recalled echo images were acquired as the phantom was heated from 22 ° to about 65°C.

Results. At ambient temperature, the rl of liposomal Gd-DTPA-BMA was exchange limited due to slow water exchange between the liposome interior and exterior. A sharp, marked increase in rl occurred as the temperature reached and exceeded the gel-to-liquid crystalline phase-transition temperature (T) of the liposomes (42°C). The relaxation enhancement was mainly attributable to the marked increase in transmembrane water permeability, yielding fast exchange conditions. There was good correlation between the relaxometric and imaging results; the signal intensity on Tl-weighted gradient-recalled echo images increased markedly as the temperature approached T . The temperature sensitivity of the diffusion-weighted technique differed from that of the liposome-based Tl-weighted approach, with an apparent water diffusion coefficient increasing linearly with temperature.

Conclusion. Since the transition from low to high signal intensity occurred in the temperature range of 38°-42°C, the investigated paramagnetic liposomes have a potential role as "off-on" switches for temperature control during hyperthermia treatment.

Key Words. Hyperthermia; paramagnetic liposomes; magnetic resonance; temperature monitoring; gel-to-liquid crystalline phase transition temperature; "off-on switch."

The utility of magnetic resonance (MR) imaging for

temperature monitoring during hyperthermia treatment is

Acad Radiol 2000; 7:1107-1115

From Nycomed Imaging AS, PO Box 4220, Torshov 0401 Oslo, Norway (S.L.F., A.B.); and the Department of Radiology, University of Freiburg, Freiburg, Germany (K.A.I., J.H.). Received January 10, 2000; revision re- quested March 27; revision received and accepted July 17. Address cor- respondence to S.L,F.

© AUR, 2000

well described in the literature (1-4). General hyperthermia

treatment is based on the induction of a relatively limited temperature elevation (approximately 43°C) within a large tissue volume in order to exploit the differential tempera-

ture sensitivity of neoplastic versus normal cells (5). Tem- perature sensitive parameters, such as the T1 relaxation time, proton chemical shift, and the water diffusion coefficient,

have been used to gain temperature information from MR images or spectra (2,4,6--8). All these techniques, however, have some limitations. The main difficulty with Tl-based

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"OSSHEIM ET AL Academic Radiology, Vol 7, No 12, December 200(

methods is the complex relationship between temperature variations and alterations in tissue T1 (2). Also, above a cer- tain critical temperature, protein denaturation may occur and cause irreversible decrease in tissue T1 (2). The proton chemical shift method is hindered by insufficient temperature sensitivity at clinical field strengths and places stringent requirements on the magnetic field homogeneity (2,8). Fur- thermore, physiologically related magnetic susceptibility changes may limit the utility of phase-based chemical shift methods (9). Self-referenced spectroscopic imaging techniques are insensitive to susceptibility changes and patient motion between scans but are more time-consuming than the phase-based chemical shift method (10).

Diffusion-weighted (DW) MR imaging techniques are also inherently sensitive to motion artifacts, which limit their clinical potential (11). Additionally, diffusion-based methods require a well-performing gradient system, which is gener- ally not the case for clinical MR scanners, and their accuracy is affected by the poor signal-to-noise ratio (12).

The feasibility of MR temperature measurements has been demonstrated recently by the use of paramagnetic chelates whose chemical shift properties are strongly temperature dependent (13-15). Other investigated thermally sensitive agents have been proton-based and perfluorinated liquid crystals (16-19).

The use of thermosensitive liposomes as drug delivery systems is of increasing interest in tumor therapy (20,21). Such liposomes have the ability to release liposome-encapsulated drug at the gel-to-liquid crystalline phase-transition temperature (Tn) of the liposomes. At this temperature, dis- tinctive structural changes occur in the liposome bilayer as the latter converts from a gel-like to a liquid state, resulting in a markedly increased transmembrane water permeability (22,23). The T m of liposomes can be tailor made by selecting and combining phospholipids appropriately.

In this study, the use of paramagnetic liposomes for temperature control is proposed. Previous relaxometric studies with liposome-encapsulated gadolinium chelates have shown a low T1 relaxivity (rl) at temperatures below the T , which was attributed to an exchange limitation of the dipolar relaxation process (24). As T was reached, however, the rl increased markedly due to a faster water exchange between the interior and exterior of the liposome. Depending on the phospholipid composition, the gadolinium chelate may also be released into the surrounding medium, contributing to the marked relaxivity enhancement. In this work, the in vitro temperature sensitivity of liposome-encapsulated gadolinium diethylenetriaminepentaacetic acid bis(methylamide) (Gd- DTPA-BMA) was evaluated by relaxometry and MR imaging.

L i p o s o m e Preparat ion

Liposomal Gd-DTPA-BMA was prepared by means of the thin-film hydration method (24,25). Briefly, the phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) sodium salt (Sygena Ltd, Liestal, Switzerland) were codissolved in a 5:1 (vol/vol) chloroform-methanol mixture (phospholipid-to-or- ganic solvent ratio, 5 mg/mL). The phospholipid solution was rotary evaporated to dryness, and the resulting lipid film was further dried under vacuum overnight. Liposomes were formed by hydration of the phospholipid film with a 250 mmol/L Gd-DTPA-BMA solution (diluted Omniscan; Nycomed Imaging AS, Oslo, Norway). The concentrations of DPPC and DPPG were 47.5 and 2.5 mg/mL, respectively. The liposomes were allowed to swell for 2 hours before se- quential extrusion (Lipex Extruder; Lipex Biomembranes, Vancouver, British Columbia, Canada) through polycarbon-

ate filters of various pore diameters (Nuclepore; Costar, Cambridge, Mass). The phospholipid film hydration, liposome swelling, and extrusion were performed at 55°C, well above the T m of the DPPC/DPPG blend (see "Physicochemi- cal Characterization" section for measured T values). Untrapped metal chelate was removed with dialysis (Spectra/ Por membrane tubing [molecular weight cutoff, 10,000 daltons], Spectrum, Houston, Tex) against isosmotic glucose solution. Before use, the solutions of glucose and Gd-DTPA- BMA were adjusted to a pH of 7.4 by addition of minute amounts of 1 mol/L sodium hydroxide.

Phys icochemica l Character izat ion The physicochemical properties of the dialyzed liposome

preparations are given in the Table. For the determination of size and electrophoreticxnobility, liposomes were diluted with pH-adjusted isosmotic glucose solution. The intensity- weighted liposome diameter was determined by means of photon correlation spectroscopy at a scattering angle of 90 ° and at 25°C (Malvem PS/MW 4700; Malvern Instruments, Malvern, England). The width of the liposome size distribu- tion was expressed by the polydispersity index. The electrophoretic mobility was determined at 25°C by means of laser Doppler velocimetry (ZetaSizer IV, Malvern Instruments; Coulter DELSA 440, Coulter Electronics, Hialeah, Fla). The T of the liposome preparations was measured with differential scanning calorimetry (DSC 7; Perkin Elmer, Norwalk, Conn). The effective gadolinium concentration (in the total sample volume) (Ceef) was determined with inductively coupled plasma atomic emission spectrophotometry

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~.cademic Radiology, Vol 7, No 12, December 2000 THERMOSENSITIVE PARAMAGNETIC LIPOSOME5

Physicochemical Properties of Liposomal Gd-DTPA-BMA (Dialyzed Preparations)

Effective Gadolinium Liposome

Liposome Concentration Diameter *t Composition Experiment (mmol/L) (nm)

Transition Electrophoretic Temperature Mobility*

(°C) (gm. cm • V -~ . sec 1)

DPPC/DPPG Relaxometry 16.6 126 42.4 -3 .4 Relaxometry 38.6 187 42.6 -2 .6 MR imaging 31.7 276 41.6 -3 .0

Note.--AII values are given as means of two or more measurements, *Measured in isosmotic glucose solution (pH 7.4), at 25°C. tPolydispersity index ranged from 0.10 to 0.17.

1 2 O O

3 4 O O

5 6 # =O7 I o o/ioo ~ 0 1 0

Figure 1. Gel phantom containing inserts with liposomal Gd- DTPA-BMA (labeled 3-10) and glucose solution (labeled 1, 2). In- serts 1 and 2 were used as controls.

(ICP-AES), as described later. For relaxation measurements, liposomes were further diluted with pH-adjusted

isosmotic glucose solution to give a suitable range of Ce~

values.

In Vitro Re laxometry The influence of temperature on the rl of two different-

sized liposome preparations was investigated at 0.47 T (Minispec PC-120b, Bruker, Rheinstetten, Germany). The studied temperature range was 20°-48°C. The T1 relax-

ation rates (R1) were obtained by using the inversion-re-

covery method. The r l at any temperature was calculated

from the following relationship:

R 1 °b~ _ R1 m rl , (1)

%

where R1 °bs and RI m are the relaxation rates (1/seconds) at

a given temperature of the liposome sample and matrix (glucose solution), respectively, and Cef f is the effective gadolinium concentration (in millimoles per liter). For com- parative purposes, the rl-temperature dependence of Gd- DTPA-BMA was determined in isotonic 5% (wt/vol) glu-

cose solution (glucose 50 mg/mL; B. Braun, Melsungen,

Germany).

MR Imaging The experiments were carried out on a 2.0-T whole-

body clinical system (Medspec S 200 Avarice; Bruker) equipped with an actively shielded head gradient insert

(maximum gradient amplitude, 30 mT/m; rise time, 200

sec). Imaging was performed on a gel phantom (containing

ultrasound contact gel) in which four tubes containing lipo-

somes and two containing glucose solution were inserted

and to which four additional liposome tubes were fixed on

the right side of the phantom (Fig 1). The inner diameter of

the sample tubes was 10 ram. A 45-watt microwave heating was performed at 434 MHz with a linear radiofrequency

(RF) antenna placed in the phantom. The microwave irradia-

tion was applied simultaneously with the image acquisition.

The total heating time was 63 minutes, for a temperature in-

crease in the gel of up to 45°C above room temperature

(22°C). Tl-weighted gradient-recalled echo (GRE) images

were acquired with the following parameters: 30/4 (rep-

etition time [TR] msec/echo time [TEl msec, 50 ° flip angle, 8-mm slice thickness, 25.6-cm field of view, and

256 x 256 matrix. DW images were obtained by employ-

ing a single-shot DW rapid acquisition sequence with re-

laxation enhancement (RARE) (26). Parameters of diffu-

sion weighting were optimized for gel, control inserts,

and inserts containing liposomes, the latter exhibiting a short T2 (due to the presence of susceptibility effects).

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Therefore, a stimulated echo scheme was used in the DW

preparation period. The duration (6) of diffusion-encoding

gradients was limited to 7 msec, and the diffusion time (A)

was 120 msec. A single-shot RARE sequence ( T E ~ = 6.7 msec) with central phase encoding was used in the readout,

resulting in an effective TE (TEeff) of 20.7 msec. Other parameters were as follows: 4,000-msec TR, 8-ram slice thickness, 25.6-cm field of view, 128 x 128 matrix. Differ-

ent diffusion weighting was achieved by varying the amplitude, G, of the diffusion-encoding gradients, while 6 and A

were kept constant. Ten DW images were acquired, with a b factor varying from 5 to 364 sec/mm 2 were acquired (the

highest value of the b factor was slightly lower than the op- timal value due to the short T2 of the liposome samples). The apparent diffusion coefficient (ADC) maps were calculated pixel per pixel by fitting to this equation:

SIi = SI 0 exp (-b,~ ADC), (2)

where SI i is the signal intensity measured with a DW gradi-

ent of amplitude G i (i = 0--+10), SI 0 is the signal intensity in the absence of diffusion attenuation, and the b factor, b i, was calculated numerically by taking into account all (ie, both diffusion-weighting and imaging) gradients (27).

The set of Tl-weighted and 10 DW images was repeated every 3.5 minutes during the total heating time. The temperature was estimated by assuming a linear dependence of ADC on temperature (7):

,r cJ , / A T j = T i - T o = ~ [ADC0 , (3)

where ADC corresponds to a measurement, j, and ADC 0 is the baseline measurement at room temperature, T O . The thermal coefficient, c~, was found to be 2.5% + 0.2%/°C from the calibration experiments.

Because of the low viscosity and possible convection in the presence of temperature gradients, the ADC within the liposome samples could not be directly assessed. The sample temperature at any time point was approximated from the temperature of surrounding gel (only the temperature in the imaged horizontal plane was taken into account). Neglected were the errors arising from convection in the sample tubes, resulting in heat transfer from the layers below and above the imaged plane. After termination of the experiment, the temperature within the inserts was measured with a thermocouple.

ICP-AES Analysis For the determination of C~, liposomes were diluted

with a 0.2% (vol/vol) solution of Triton X-100 (Sigma

[--I~- Liposomal Gd-DTPA-BMA, 126 nrn Liposomal Gd-DTPA-BMA, 187 nm Nonliposomal GdoDTPA-BMA

"-" 10 "~- 3 ~___.

1 0

20 25 30 35 40 45 50 Temperature (°C)

Figure 2. Temperature dependence of the rl of liposomal and nonliposomal Gd-DTPA-BMA (0.47 T). Note that the semilogarith- mic plot (inserted figure) emphasizes the rl-size dependence at temperatures below -/-~.

Chemical, St Louis, Mo). The gadolinium concentration in the samples was determined by using a multipoint standard calibration curve (Plasma 2000 ICP-AES, Perkin Elmer).

In Vitro Relaxometry

Figure 2 shows the rl of both liposomal and nonliposomal Gd-DTPA-BMA as a function of temperature. The rl of nonliposomal Gd-DTPA-BMA decreased from 4.8 to 3.6 sec -~. (mmolfL) < in the temperature range of 20°48°C. The rl of liposome-encapsulated Gd-DTPA-BMA was markedly reduced at 20°C compared with that of the metal

chelate itself. The liposome size influenced the rl , the 126- nm liposomes being more efficient relaxation enhancers than the 187-nm liposomes. The rl of both liposome preparations increased in the 20°-37°C range, from 0.054 to 0.53 sec -1 • (mmol/L) ~ and from 0.020 to 0.21 sec -~- (mmol/L) -1

for the 126- and 187-nm liposomes, respectively. After liposome cooling from 37 ° to 20°C, the rl returned to the

same initial low values. Further heating above 40°C increased the r l markedly, with the values approaching the rl

of nonliposomal Gd-DTPA-BMA. Upon liposome cooling from 48 ° to 37°C, rl values of 0.75 and 0.66 sec -I - (retool/ L ) 1 were obtained for the 126- and 187-nm liposomes, re-

spectively. These relaxivities were 1.4- and 3.0-fold higher

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a, b=

the RF antenna (inserts 6-10) showed a marked SI increase

on the Tl-weighted image (Fig 3b). From approximately 55 minutes, the next sample farther from the antenna (insert 4) also displayed a pronounced SI increase. The corre- sponding ADC map at that time showed a bright diffuse

spot within the gel in the vicinity of the affected samples, consistent with an increased ADC (image not shown). Fig-

ure 3c shows the final Tl-weighted GRE image after 63 minutes of RF heating. Figure 3d, representing the differ-

ence image after subtraction of baseline image (Fig 3a) from Figure 3c, reflects the relative change in SI after heat-

ing. The calculated ADC-based temperature map and profile of the gej phantom after heating is given in Figure 4a

and 4b, respectively. Figure 5 shows the SI-temperature response on Tl-weighted GRE images for the liposome sample nearest the RF antenna (insert 6). A sharp and well- defined transition from low to high SI was obtained in the temperature range of 37.6°-41.9°C, the SI increasing 2.5-

fold. Such a temperature response was different from the

gradual, relative small SI decrease on Tl-weighted GRE images of the heated gel region (results not shown) or the

linear and slight temperature dependence of the ADC (Fig 6). After termination of the imaging experiment, the temperature within inserts 1, 2, 3, and 5 was well below T , but it exceeded T in inserts 4 and 6-10.

C. d .

Figure 3. Tl-weighted GRE images (2.0 T) of gel phantom containing liposomal Gd-DTPA-BMA (a) before and after (b) 47 and (c) 63 minutes of RF heating. Inhomogeneous SI in gel is due to air bubbles. (d) Difference image after subtraction of a from c. Note that the Sis within inserts 3 and 5 are almost unchanged after RF heating.

than the initial relaxivity values (prior to heating). The lipo-

some size was not affected by the heating procedure.

MR Imaging At ambient temperature, the Tl-weighted GRE image of

the gel phantom showed a low signal intensity (SI) for all Iiposome samples (Fig 3a); the SI was also lower relative to that of the gel matrix and inserts containing glucose solution. Heating caused a local temperature increase near the RF antenna. After about 40 minutes, the samples closest to

Proton-based and perfluorinated liquid crystals have shown utility for temperature monitoring (16-19). The use of such compounds exploits the change in MR properties occurring at their phase transition. In such cases, the T2 is short when the substance is in the solid state but increases markedly as the substance melts and becomes a liquid. The result is a transition from low to high SI on an MR image, that is, an "off-on switch."

Franklin et al (17) investigated the relaxation properties of encapsulated proton-based liquid crystals undergoing a

phase transition from the smectic to the nematic phase. The authors concluded that the approach would be clinically irrel-

evant, as the temperature-induced changes in SI were masked by the rather large background signal of the tissue water pro- tons. Various mixtures of perfluorocarbons have also been in-

vestigated as thermosensitive probes (18,19). The results were promising, as large changes in fluorine SI were observed at the

solid-to-liquid phase transition with high thermal sensitivity and no background signal. Most notably, an approximately 20-fold increase in fluorine SI was obtained over a 7°C temperature interval (37 ° . 44°C) (19). Analogously, the electron

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:OSSHEIM ET AL Academic Radiology, Vol 7, No 12, December 2001

50

45 O

0 )

19

E

30

Insert 6

30

/ a

i 1 , i i i i &

T 0 20 100 120 Cross-Section position

Insert 4 - - )K

40 60 80 Position, Pixel No.

a . b .

Figure 4. (a) ADC-based temperature map after 63 minutes of RF heating; incorrect temperature data within inserts are due to short relaxation times and convection artifacts. (b) Tempera- ture profile across section indicated in a; temperature measured by thermocouple within inserts 4 and 6 is indicated by asterisks.

paramagnetic resonance spectrum of a nitroxide free radical,

dissolved in a fatty acid medium, showed a marked alteration between 38 ° and 48°C, a temperature range in which the

medium underwent a solid-to-liquid phase transition (28). In the "design" of a thermosensitive liposomal contrast

agent, however, the phase transition properties of the lipo-

some carrier are exploited, and not those of the contrast ma-

terial itself. Such a strategy allows some flexibility, as it is

possible to design, by an appropriate combination of phos-

pholipids, liposomes displaying a T within any desired temperature range relevant for hyperthermia treatment.

The in vitro experiments reported in this study demon-

strated the feasibility of DPPC-based paramagnetic lipo-

somes for temperature monitoring. A good correlation was

observed between the relaxometric and Tl-weighted imag-

ing results. At ambient temperature the liposomal water

permeability was too low to allow for efficient water ex-

change across the liposome membrane. Hence, the dipolar

relaxation process was exchange limited, as confirmed by the size dependence of rl. Also, the compartmentalization

of a high concentration of gadolinium within the liposome

interior generated susceptibility effects, that is, the liposome system was an efficient T2* agent 2 (24).

Therefore, at ambient temperature both the low rl and

the susceptibility effect of liposomal Gd-DTPA-BMA were

reflected by a low SI on the Tl-weighted GRE images.

25

I + L i p o s o m a i G d - D T P A - B M A I

20

<

10

5 . . . . i . . . . i . . . . i . . . . i . . . . i . . . .

20.0 25.0 30.0 35.0 40.0 45.0 50.0 Temperature (°C)

Figure 5. Temperature dependence of the SI (quantified in arbi- trary units) of liposomal Gd-DTPA-BMA on Tl-weighted GRE images (2.0 T) (results from insert 6).

Raising the temperature to 37°C increased the liposomal

water permeability, which improved the rl but not enough

to cause a marked SI increase on the MR image. Leakage

of gadolinium chelate could be discarded as a possible

cause of the elevated relaxivity, as the rl values after cool-

ing from 37 ° to 20°C were identical to those before lipo-

some heating.

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~,cademic Radiology, Vol 7, No 12, December 2000 T H E R M O S E N S I T I V E P A R A M A G N E T I C L IPOSOME~ ¢

0.30

0.28

G" 0.26

Eo 0.24

O

x 0.22

O cl < 0.2O

0.18

0.16 20 25 30 35 40 45

Temperature (°C) Figure 6, Temperature dependence of the ADC in gel matrix (region of interest close to insert 6).

50

The sharp and pronounced increase in rl as the tempera-

ture approached T could be explained by the occurrence of thermotropic changes in the membrane. For instance, at T , the DPPC bilayer has been shown to expand laterally by about 40% and to decrease its hydrophobic thickness with 20%, features that markedly increased the membrane permeability to water and solutes (22,23). Hence, the water exchange between the interior and exterior of the liposome was fast enough to allow for efficient propagation of the dipolar relaxation effect, thereby markedly increasing the rl. The dipolar T2 contribution was also enhanced to override the susceptibility effect; the liposome system could thus be regarded as a T1 agent. On Tl-weighted GRE images, the marked SI increase over a 4°C range (38°-42°C) correlated well with the rl-temperature profile. At temperatures above T, the relaxation enhancement was in the fast exchange re- gime, as evidenced by the similar rl values of liposomal and nonliposomal Gd-DTPA-BMA (24). As T was reached and exceeded, however, leakage of gadolinium chelate from the liposome also contributed to the increased relaxivity. Some leakage occurred as the liposomes, on cooling from 48 ° to 37°C, displayed a higher rl than before heat-

ing. The results clearly demonstrate how the rl-temperature

sensitivity of a gadolinium chelate can be altered by its encapsulation into liposomes. Whereas the rl of nonliposo- real Gd-DTPA-BMA continuously decreased as the temperature increased, the rl-temperature response of the liposomal chelate, linked to the thermotropic properties of the liposome membrane, had an "off-on" quality. Such a fea- ture requires a membrane composition that displays a

markedly different permeability below and above T . At temperatures below T , the permeability should be low enough to reduce markedly the rate of water exchange between the interior and exterior of the liposome, while at temperatures above T , the permeability should be high enough to obtain fast exchange conditions. That the liposome size affected the rl only at temperatures below T may be of some practical benefit; the temperature profile of rl can be modulated by the liposome size. Larger liposomes will display a more marked increase in rl as the temperature increases from 37°C and exceeds T . For example, in the temperature range of 37°-44°C the rl of the 187-nm liposomes increased 17-fold, compared with a sev- enfold change for the 126-nm liposomes.

With the DW approach, continuous temperature moni- toting was possible. As opposed to Tl-weighted GRE imaging, however, the DW technique did not readily indicate whether the temperature had reached a critical value. For instance, the lack of any SI enhancement within inserts 3 and 5 confirmed that the temperature never exceeded T (Fig 3c, 3d; see "Results" for final sample temperatures). The liposome-based Tl-weighted approach offers several advantages over the DW technique. The accuracy of the temperature monitoring by the DW technique depends strongly on the signal-to-noise ratio of the DW images, whereas liposomal relaxivity changes can easily be observed on Tl-weighted images, even with a relative poor signal-to-noise ratio. Therefore, temperature control with thermosensitive paramagnetic liposomes should be feasible for hyperthermia treatment of tissues with short T2 values, for which the DW approach would suffer from a strong SI loss during the diffusion-weighting period. Tl-weighted imaging is also inherently faster than DW imaging. Three- dimensional Tl-weighted images can be acquired in seconds, making it possible to monitor whether the tissue temperature has exceeded a critical temperature. This could be beneficial for hyperthermic treatment with fast temperature changes, for example, thermal tumor ablation.

Thermal ablation is gaining more and more popularity compared to conventional hyperthermia treatments. Such ablative therapy, performed with laser, RF, or focused ultrasound, is currently performed at temperatures above 55°C, at which protein denaturation occurs (2). The presently investigated DPPC/DPPG liposomes could not be used as thermal probes during a thermal ablation procedure be-

2Susceptibility effects were still visible on Tl-weighted GRE images despite a short TE of 4 msec.

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cause T was too low. A subtle change in the phospholipid

composition, however, would yield liposomes with a suitable T within temperature intervals relevant for thermal ablation therapies. For instance, in the case of saturated phosphatidylcholines and phosphatidylglycerols, increasing their acyl chain length from 16 carbon atoms (eg, DPPC and DPPG) to 18 or more would give a T m of 55°C or higher.

To enable full utilization of thermosensitive liposomes in vivo, however, several important features should be considered. Obviously, the local concentration of liposomal Gd-DTPA-BMA needs to be determined to distinguish between areas that are not sufficiently heated (ie, T < T ) and those where the agent is not present. A potential problem in this respect is the low rl of liposomal Gd-DTPA-BMA below T m, which makes it difficult to determine concentrations reliably. Still, even the relatively modest r l at 37°C might be sufficient to detect the presence of the liposomes (unpublished data, 1999). Furthermore, although the rl is

low at 37°C, the susceptibility effect of the liposomes may be substantial at imaging fields, thus perhaps allowing a more accurate determination of local concentration in vivo.

This liposome approach also would succeed only if the tumor region contains a high enough concentration of liposomal agent. Sterically stabilized liposomes should therefore be considered. Such liposomes, displaying surface properties that minimize uptake by the mononuclear phagocyte system,

will have an increased probability of tumor extravasation due to the longer circulation time in blood. Indeed, the use of polyethylene glycol (PEG)-coated DPPC liposomes for selective delivery of cytostatics to tumors undergoing hyperthermia treatment has been extensively described (21,29). The results have shown the potential of such liposomes; liposome extravasation into the neoplastic area and a faster drug release from the liposome increased the therapeutic index of the encapsulated drug.

PEGylated paramagnetic liposomes would be a candi- date for in vivo MR imaging-guided temperature control. Some preliminary studies have shown that surface-grafted PEG does not alter the rl-temperature profile of Gd-DTPA- BMA encapsulated within DPPC/DPPG liposomes (unpublished data, 1998). These findings suggest a possible application of PEGylated liposomes for temperature monitoring in vivo. Studies must be conducted in tumor models, however, to investigate the targeting properties and temperature sensitivity of the current liposomes. For instance, it is es- sential to study the influence of liposome size on the MR imaging efficacy and biodistribution of the liposomes, al-

though it is known that the smaller the liposome, the more efficient is the tumor extravasation (30). Also, the thermotropic properties of the liposomes have to be assessed under physiologic conditions that mimic the tumor microenvi- ronment. Finally, the encapsulation of paramagnetic macro- molecular agents or use of other phospholipid combina- tions, with a lower but still high enough permeability above T m (24), would improve the value of such thermosensitive agents.

.3ONCLUSIOh

This study has shown the feasibility of thermosensitive paramagnetic liposomes for temperature control in vitro. Large temperature-dependent changes in SI were observed on Tl-weighted GRE images, with high thermal sensitivity and no background signal. Such thermosensitive liposomes are efficient only if two main criteria are met: (a) the paramagnetic agent must be located within the liposome interior, and (b) the T1 relaxation process must be exchange limited only at temperatures below the T of the phospholipids. Lipo- somes are flexible and versatile delivery systems. With a combination of phospholipids for which the T lies within temperature ranges relevant for hyperthermic or ablative tumor treatment, local temperature changes in vivo may be monitored noninvasively with standard clinical MR scanners.

,CKNOWLEDGMENT.

The authors thank Gry Stensrud, PhD (School of Phar- macy, University of Oslo, Norway), for performing the laser Doppler velocimetry measurements and Kari Anne Bjerkeli and Vera Kasparkova, Phi) (both Nycomed Imag- ing AS, Oslo, Norway), for performing the inductively coupled plasma atomic emission spectrophotometry analysis and differential scanning calorimetry measurements, respectively.

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