In vivo fluorescence lifetime imaging for monitoring the efficacy of the cancer treatment

Abstract

Purpose: Advances in tumor biology created a foundation for targeted therapy aimed at inactivation of specific molecular mechanisms responsible for cell malignancy. In this paper, we used in vivo fluorescence lifetime imaging with HER2-targeted fluorescent probes as an alternative imaging method to investigate the efficacy of targeted therapy with 17-DMAG (an HSP90 inhibitor) on tumors with high expression of HER2 receptors.

Experimental design: HER2-specific Affibody, conjugated to Alexafluor 750, was injected into nude mice bearing HER2-positive tumor xenograft. The fluorescence lifetime was measured before treatment and monitored after the probe injections at 12 hours after the last treatment dose, when the response to the 17-DMAG therapy was the most pronounced as well as a week after the last treatment when the tumors grew back almost to their pretreatment size.

Results: Imaging results showed significant difference between the fluorescence lifetimes at the tumor and the contralateral site (∼0.13 ns) in the control group (before treatment) and 7 days after the last treatment when the tumors grew back to their pretreatment dimensions. However, at the time frame that the treatment had its maximum effect (12 hours after the last treatment), the difference between the fluorescence lifetime at the tumor and contralateral site decreased to 0.03 ns.

Conclusions: The results showed a good correlation between fluorescence lifetime and the efficacy of the treatment. These findings show that in vivo fluorescence lifetime imaging can be used as a promising molecular imaging tool for monitoring the treatment outcome in preclinical models and potentially in patients.

Figure 1

Immunohistochemistry image of N87 tumor shows heterogeneity of HER2 receptors at the different locations inside the tumor.

Figure 2

Fluorescence variations at the contralateral site during 16 hours after injection of ABD-HER2-specific Affibody probe (two mice with BT-474 xenografts) before treatment with 17-DMAG: (a) Intensity (b) Lifetime. Fluorescence variations at the contralateral site during 16 hours after injection of ABD-HER2-specific Affibody probe (averaged of 5 mice with BT-474 xenografts, divided in three subsamples, i.e., before, 12 hours and 7 days after the last treatment with 17-DMAG) (c) Intensity (d) Lifetime

Figure 3

(a) The average fluorescence lifetime at the tumor site for five mice at different time points after injection of ABD-HER2-specific Affibody probe; before, 12 hours and one week after the last treatment with 17-DMAG (b) The difference between the fluorescence lifetimes at the tumor and contralateral site for the same mice of part (a).

Figure 4

Fluorescence intensity and lifetimes at different time points at the tumor site (a) before, (c) 12 hours, (e) one week after the last treatment with 17-DMAG and at the contralateral site (b) before, (d) 12 hours, (f) one week after the last treatment with 17-DMAG.

Figure 5

The average tumor volume in 5 mice with BT-474 tumor as a function of time after the therapy

Figure 6

In vivo fluorescence lifetimes of tumor and contralateral site of the xenograft mouse with no HER2 expressing human tumor model (U-251 MG) and high HER2 expressing human tumor (BT-474), after injection of the ABD-HER2-specific Affibody® conjugated to AlexaFluor750.