Longer heating duration increases localized doxorubicin deposition and therapeutic index in Vx2 tumors using MR-HIFU mild hyperthermia and thermosensitive liposomal doxorubicin

Abstract

Thermosensitive liposomal doxorubicin (LTSL-Dox) combined with mild hyperthermia enhances the localized delivery of doxorubicin (Dox) within a heated region. The optimal heating duration and the impact of extended heating on systemic drug distribution are unknown. Here we evaluated local and systemic Dox delivery with two different mild hyperthermia durations (42 °C for 10 or 40 minutes) in a Vx2 rabbit tumor model. We hypothesized that longer duration of hyperthermia would increase Dox concentration in heated tumors without increasing systemic exposure. Temporally and spatially accurate controlled hyperthermia was achieved using a clinical MR-HIFU system for the prescribed heating durations. Forty-minutes of heating resulted in a nearly 6-fold increase in doxorubicin concentration in heated vs unheated tumors in the same animals. Therapeutic ratio, defined as the ratio of Dox delivered into the heated tumor vs the heart, increased from 1.9-fold with 10 minutes heating to 4.4-fold with 40 minutes heating. MR-HIFU can be used to guide, deliver and monitor mild hyperthermia of a Vx2 tumor model in a rabbit model, and an increased duration of heating leads to higher Dox deposition from LTSL-Dox in a target tumor without a concomitant increase in systemic exposure. Results from this preclinical study can be used to help establish clinical treatment protocols for hyperthermia mediated drug delivery.

Keywords: MR-guided high intensity focused ultrasound; Targeted drug delivery; mild hyperthermia; therapeutic ratio; thermosensitive liposomal doxorubicin.

Figure 1.

Experiment protocol for drug delivery in rabbits with Vx2 tumors using LTSL-Dox and MR-HIFU mild hyperthermia (HT) administered for either 10 or 40 min. Temperature mapping was continued for 10 min after treatment to observe tissue cooling (during which period the temperature of the heated region returned to baseline). The heated tumors, contralateral unheated tumor, and other organs were harvested 3 h after the start of LTSL-Dox infusion for drug quantification using silver nitrate/chloroform extraction with LC-MS readout.

Figure 2.

MR-HIFU mild hyperthermia in rabbit Vx2 tumors. Treatment planning images along (A) and across (B) the ultrasound beam (cone shape), with tumor and targeted location indicated by white arrows. MR temperature maps confirm adequate hyperthermia in both the 10 min (C) and 40 min (D) heating groups. White circle indicates the treated ROI, and black contour in (C) and (D) is the 42 °C isotherm. Bar = 2 cm.

Figure 3.

Doxorubicin biodistribution in rabbits administered LTSL-Dox and MR-HIFU hyperthermia. Comparisons between heating durations of 10 min vs. 40 min were made for each tissue type by t-test, Bonferroni-corrected p values are indicated.

Figure 4.

Therapeutic ratio of doxorubicin deposition in tumors vs. heart muscle in rabbits administered LTSL-Dox and MR-HIFU hyperthermia. Comparisons between heating durations of 10 min vs. 40 min were made by paired t-tests with a significance level of .05.