131I‑Labeled Anti‑VEGFR2 Mesoporous Silica Nanoparticles Show Strong Antitumor Activity in an Anaplastic Thyroid Cancer Mouse Model

Abstract

Anaplastic thyroid cancer (ATC) is a rare but highly lethal malignancy, with median survival under six months due to its aggressive growth and resistance to conventional therapies. We engineered mesoporous silica nanoparticles (MSNs) functionalized with bovine serum albumin (BSA) and an anti‑VEGFR2 antibody, then radiolabelled with iodine‑131 (¹³¹I). In vitro and in vivo assays demonstrated enhanced tumor targeting, prolonged tumour retention, and superior therapeutic efficacy in FRO‑derived ATC xenografts. Compared with free ¹³¹I or non‑targeted MSNs, the anti‑VEGFR2‑decorated nanoparticles achieved higher tumour radioactivity (32.2 ± 2.8% ID/g at 24 h) and significantly extended median survival (41 days vs. 25 days). These findings support the potential of targeted radiolabeled MSNs as a novel, minimally invasive treatment for ATC.

Background

ATC accounts for ~2 % of thyroid cancers yet is responsible for most thyroid‑related deaths, owing to rapid local invasion and distant metastasis. Traditional radioiodine therapy is ineffective because ATC cells lack iodine uptake mechanisms. Angiogenesis, driven by VEGF/VEGFR2 signaling, sustains tumour growth; therefore, VEGFR2 is a rational therapeutic target. Mesoporous silica nanoparticles combine high loading capacity, biocompatibility, and facile surface functionalisation, making them ideal vehicles for combined imaging and therapy (theranostics).

Materials and Methods

Nanoparticle Preparation

MSNs were synthesised via a CTAB‑templated sol‑gel method, yielding uniform spheres (~108 nm). Surface amination with APTES produced MSNs‑NH₂, which were then conjugated to BSA and an anti‑VEGFR2 antibody using EDC/NHS chemistry, yielding BSA‑MSNs‑anti‑VEGFR2. Radiolabelling with ¹³¹I was achieved by the Chloramine‑T method, giving a 50–75 % labelling efficiency.

Characterisation

Transmission electron microscopy confirmed spherical morphology; dynamic light scattering showed size increments from 108 nm to 163 nm after full functionalisation. Zeta potential shifted from −23 mV to +28 mV, confirming successful conjugation. BET analysis revealed a 630 m²/g surface area and 2.8 nm pore size.

In‑vitro Uptake

FRO ATC cells were incubated with FITC‑labelled nanoparticles. Confocal microscopy demonstrated robust cellular internalisation, with the anti‑VEGFR2‑targeted particles displaying markedly stronger fluorescence at both 1 h and 6 h compared to non‑targeted controls.

In‑vivo Studies

Female BALB/c nude mice bearing subcutaneous FRO xenografts received intratumoral injections of 7.4 MBq ¹³¹I‑BSA‑MSNs‑anti‑VEGFR2, ¹³¹I‑BSA‑MSNs, or free ¹³¹I. Tumour growth, body weight, and survival were monitored every three days. SPECT/CT imaging and γ‑counter biodistribution assays were performed at 24 h, 72 h, and up to 21 days post‑injection.

Histopathology

Post‑mortem H&E and immunohistochemical staining assessed tumour necrosis and VEGFR expression, while major organs were examined for toxicity.

Results

Nanoparticle Characteristics

MSNs maintained uniform morphology; surface modifications increased size to 163 nm and reversed zeta potential to +28 mV. BET confirmed a mesoporous architecture. The nanoparticles were stable for weeks with no aggregation.

Targeting and Uptake

Confocal imaging revealed stronger green fluorescence for anti‑VEGFR2‑decorated particles, indicating enhanced binding. Time‑dependent radioactivity measurements showed peak cellular uptake at 3 h (BSA‑MSNs) and 5 h (BSA‑MSNs‑anti‑VEGFR2), with the latter exhibiting higher absolute counts.

Tissue Distribution

At 24 h, tumour radioactivity was 32.2 % ID/g for the targeted group vs. 26.1 % for non‑targeted. By 72 h, levels decreased to 23.0 % ID/g (targeted) and 12.3 % ID/g (non‑targeted), both significantly above free ¹³¹I (11.6 % and 2.1 % ID/g).

In‑vivo Imaging

SPECT/CT revealed persistent tumour radioactivity for targeted nanoparticles up to 3 weeks, whereas free ¹³¹I cleared by day 3.

Therapeutic Efficacy

Tumour volumes in the targeted group plateaued and then regressed after day 9, while non‑targeted and free‑¹³¹I groups continued to grow. Median survival: targeted 41 days, non‑targeted 34 days, free‑¹³¹I 25 days, saline 27 days (P < 0.01).

Safety Profile

H&E of heart, liver, lung, and kidney showed no pathological changes. Tumours treated with targeted nanoparticles exhibited extensive necrosis and reduced viable cell density.

Discussion

Targeting VEGFR2 with radiolabelled MSNs substantially improves tumour retention and therapeutic index in ATC, outperforming conventional radioiodine. The dual imaging/therapy (theranostic) capability allows real‑time monitoring of drug biodistribution. The absence of systemic toxicity suggests a favourable safety margin for clinical translation. Future work will explore intravenous delivery and payload loading with anti‑cancer agents.

Conclusions

We successfully fabricated BSA‑MSNs‑anti‑VEGFR2 radiolabelled with ¹³¹I that exhibit superior tumour targeting, prolonged retention, and pronounced antitumour activity in ATC xenografts, with extended survival and no detectable systemic toxicity. This platform represents a promising new strategy for treating aggressive thyroid cancers.

Abbreviations

• APTES – Aminopropyltriethoxysilane
• ATC – Anaplastic thyroid cancer
• BSA – Bovine serum albumin
• CLSM – Confocal laser scanning microscopy
• CTAB – Hexadecyltrimethylammonium bromide
• DAPI – 4,6‑Diamidino‑2‑phenylindole
• DLS – Dynamic light scattering
• DMEM – Dulbecco’s modified Eagle medium
• DMSO – Dimethyl sulfoxide
• EDC – 1‑(3‑Dimethylaminopropyl)-3‑ethylcarbodiimide hydrochloride
• FBS – Fetal bovine serum
• FITC – Fluorescein isothiocyanate
• HBSS – Hanks’ balanced salt solution
• MSNs – Mesoporous silica nanoparticles
• NHS – N‑hydroxysuccinimide
• PBS – Phosphate buffer saline
• TEM – Transmission electron microscope
• TEOS – Tetraethyl orthosilicate
• VEGFR – Vascular endothelial growth factor receptor