Multimodal interventional molecular imaging of tumor margins and distant metastases by targeting αvβ3 integrin

Abstract

α(v)β(3) integrin is involved in (tumor-induced) angiogenesis and is a promising candidate for the specific visualization of both primary tumors and of their distant metastases. Combination of radioactive and fluorescent imaging labels in a single multimodal, or rather hybrid, RGD-based imaging agent enables integration of pre-, intra-, and postoperative angiogenesis imaging. A hybrid imaging agent targeting the α(v)β(3) integrin--(111)In-MSAP-RGD (MSAP = multifunctional single-attachment-point reagent), which contains a targeting moiety, a pentetic acid (DTPA) chelate, and a cyanine dye--was evaluated for its potential value in combined lesion detection and interventional molecular imaging in a 4T1 mouse breast cancer model. SPECT/CT and fluorescence imaging were used to visualize the tumor in vivo. Tracer distribution was evaluated ex vivo down to the microscopic level. The properties of (111)In-MSAP-RGD were compared with those of (111)In-DTPA-RGD. Biodistribution studies revealed a prolonged retention and increased tumor accumulation of (111)In-MSAP-RGD relative to (111)In-DTPA-RGD. With (111)In-MSAP-RGD, identical features could be visualized preoperatively (SPECT/CT) and intraoperatively (fluorescence imaging). As well as the primary tumor, (111)In-MSAP-RGD also enabled detection and accurate excision of distant metastases in the head and neck region of the mice. Therefore, the hybrid RGD derivative (111)In-MSAP-RGD shows potential in preoperative planning and fluorescence-based surgical intervention.

Figure 1

Hybrid tumor-targeting approach. Injection of a hybrid tumour targeted agent allows preoperative nuclear imaging and intraoperative image guided surgery

Figure 2

A) MIP SPECT/CT image obtained with ¹¹¹In-DTPA-RGD, B) MIP SPECT/CT with ¹¹¹In-MSAP-RGD. Due to the higher retention of the tracer at 24h the accumulation in B is more intense when both tracers are imaged with identical settings (see also Table 1)

Figure 3

Surgical guidance towards the tumor margins of the primary tumor and distant metastases after injection of ¹¹¹In-MSAP-RGD. A) Planning SPECT/CT image depicting both the rim of the primary tumor and distant metastases in the neck area (mouse belly down), B) Fluorescence imaging (organs are shielded) depicting the margins of the primary tumor (mouse belly up), C) Ex vivo fluorescence imaging of the tumor and distant metastases

Figure 4

Tumor tissue ex vivo, demonstrating tumor vascularity, viability and distribution of ¹¹¹In-DTPA-RGD A) perfusion of the tumor rim visualized with Fitc-lectin perfusion and incubation with a rat-anti-mouse CD31 antibody followed by an AlexaFluor 594 labeled rabbit-anti-rat antibody (20×), B) Fluorescence imaging (λ_em633nm) of MSAP-RGD in the tumor rim 24h after iv injection (10×)

Figure 5

The non-covalent interaction of MSAP-RGD is reversible and the equilibrium can be shifted to the right side by interactions with α_vβ₃-integrins and alternative (non-specific) binding sites.

Scheme 1

Synthesis and radiolabeling of DTPA-RGD and MSAP-RGD