As a novel co-receptor for
vascular endothelial growth factor (
VEGF),
neuropilin receptor type-1 (NRP-1) is overexpressed in several
cancers and
metastases, and serves as an attractive target for
cancer molecular imaging and
therapy. Previous single photon emission computerized tomography (SPECT) studies demonstrated that the small NRP-1-targeting
peptides 99mTc-MA-ATWLPPR and 99mTc-CK3 showed poor
tumor imaging quality, because of their rapid blood clearance and very low
tumor uptake. Compared with small
peptides,
monoclonal antibodies (mAbs) can improve imaging of NRP-1-expression, due to their high affinity, specificity and slow extraction. A6-11-26 is a novel
monoclonal antibody against NRP-1 b1b2 domain that exhibits inhibition of
tumor growth in NPR-1-expressing preclinical models. The aim of the present study was to develop the 131I-labeled anti-NRP-1
monoclonal antibody A6-11-26 as a SPECT probe for imaging of NRP-1-positive
tumor. An anti-NRP-1
monoclonal antibody (A6-11-26) was produced by hybridomas and was labeled with
iodine-131 by the
iodogen method. In vitro, the radiolabeling efficiency, radiochemical purity, immunoreactive fraction and stability were assessed. Binding affinity and specificity of 131I‑A6-11-26 to NRP-1 were evaluated using human
glioblastoma U87MG cells. In vivo, biodistribution and SPECT/CT studies were conducted on mice bearing U87MG xenografts after the injection of 131I-A6-11-26 with or without co-injection of unlabeled A6-11-26 antibody. A6-11-26 was generated successfully by hybridoma with high purity (>95%) and was labeled with
iodine-131 within 60 min with high labelling efficiency (95.46±3.34%), radiochemical purity (98.23±1.41%). 131I-A6-11-26 retained its immunoreactivity and also displayed excellent stability in mouse serum and PBS
solution during 1 to 96 h. Cell uptake assays showed high NRP-1-specific uptake (15.80±1.30% applied activity at 6 h) in U87MG cells. 131I-A6-11-26 bound to NRP-1 with low nanomolar affinity (KD=1.67±0.14 nM) in U87MG cells. In vivo, biodistribution study demonstrated targeting of U87MG
glioma xenografts was NRP-1 specific. The
tumor uptake was 6.00±1.24%ID/g at 24 h, and the
tumor to muscle ratio was 3.20±0.30 at 24 h, and reached the highest level of 6.13±0.24 at 120 h after injection. SPECT imaging studies revealed that 131I-A6-11-26 could clearly identify U87MG
tumors with good contrast, especially at 72-120 h after injection. The present study demonstrates that 131I-A6-11-26 is capable of detecting lesions in an NRP-1-expressing
tumor with high target selectivity, and may offer a promising SPECT agent for NRP-1 expression positive
tumor and encourage further investigation.