Monoclonal antibody A7-superparamagnetic iron oxide as contrast agent of MR imaging of rectal carcinoma

Abstract

Superparamagnetic iron oxide (SPIO)-based colloid has been used clinically as a tissue-specific magnetic resonance contrast agent. We coupled monoclonal antibody A7 (Mab A7), which reacts specifically with human colorectal carcinoma, to Ferumoxides (SPIO) and examined the accumulation of this conjugate in xenografted tumours in nude mice. We examined in vitro immunoreactivity of Mab A7 coupled to Ferumoxides and its in vivo distribution in nude mice with human colorectal carcinoma. Magnetic resonance imaging of tumour-bearing nude mice was performed 72 h after injection of A7-Ferumoxides. A7-Ferumoxides retained binding activities that were nearly identical to intact Mab A7. More of the radiolabelled A7-Ferumoxides accumulated in the tumour than normal mouse IgG-Ferumoxides from 12 h onwards after injection (P<0.05). Both A7-Ferumoxides and normal mouse IgG-Ferumoxides disappeared from blood linearly over time. The accumulation levels in normal tissue decreased linearly over time but were lower than levels in tumours from 6 h. In magnetic resonance T2-weighted imaging of the tumour-bearing nude mice, signal intensity was reduced at the margin of the tumour by injection of A7-Ferumoxides. Mab A7 coupled to Ferumoxides is potentially suitable as a magnetic resonance contrast agent for detecting local recurrence of rectal carcinoma.

Figure 1

The binding activities of A7-Ferumoxides were compared with those of intact A7 by competitive radioimmunoassay in WiDr cells. A7-Femmoxides retained binding activities nearly identical to intact A7. Normal mouse IgG-Ferumoxides had no antigen-binding activity in WiDr cells. ○, A7-Ferumoxides; ▵, intact A7; •, normal mouse IgG-Ferumoxides.

Figure 2

The accumulation of ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides in WiDr tumours of mice after intravenous injection. A significantly larger amount of ¹²⁵I-labelled A7-Ferumoxides accumulated in the tumour than ¹²⁵I-labelled normal mouse IgG-Ferumoxides from 12 to 120 h after injection (P<0.05). The tumour accumulation of ¹²⁵I-labelled A7-Ferumoxides increased up to 48 h and then decreased slowly. ○, A7-Ferumoxides; •, normal mouse IgG-Ferumoxides; points, means; bars, s.d.

Figure 3

Blood concentrations of ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides in mice that received an intravenous injection. ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides disappeared from blood linearly over time, with similar clearance curves. ○, A7-Ferumoxides; •, normal mouse IgG-Ferumoxides; points, means; bars, s.d.

Figure 4

A to H. The accumulation of ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides in normal tissues of mice after intravenous injection. The accumulation of ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides was similar in normal tissues. ○, A7-Ferumoxides; •, normal mouse IgG-Ferumoxides; points, means; bars, s.d.

Figure 5

Tumour/blood radioactivity ratio of ¹²⁵I-labelled A7-Ferumoxides and ¹²⁵I-labelled normal mouse IgG-Ferumoxides in mice after intravenous injection. The tumour/blood radioactivity ratio of the ¹²⁵I-labelled A7-Ferumoxides increased rapidly with time. ○, A7-Ferumoxides; •, normal mouse IgG-Ferumoxides; points, means; bars, s.d.

Figure 6

The signal intensity of MR T2-weighted imaging was reduced at the margin of tumours in an A7-Ferumoxides-injected mouse. By contrast, there was no change in intensity in tumours of a control IgG-Ferumoxides-injected mouse.