Targeted Molecular Radiotherapy of Pediatric Solid Tumors Using a Radioiodinated Alkyl-Phospholipid Ether Analog

Abstract

External-beam radiotherapy plays a critical role in the treatment of most pediatric solid tumors. Particularly in children, achieving an optimal therapeutic index to avoid damage to normal tissue is extremely important. Consequently, in metastatic disease, the utility of external-beam radiotherapy is limited. Molecular radiotherapy with tumor-targeted radionuclides may overcome some of these challenges, but to date there exists no single cancer-selective agent capable of treating various pediatric malignancies independently of their histopathologic origin. We tested the therapeutic potential of the clinical-grade alkyl-phospholipid ether analog CLR1404, 18-(p-iodophenyl)octadecyl phosphocholine, as a scaffold for tumor-targeted radiotherapy of pediatric malignancies. Methods: Uptake of CLR1404 by pediatric solid tumor cells was tested in vitro by flow cytometry and in vivo by PET/CT imaging and dosimetry. The therapeutic potential of ¹³¹I-CLR1404 was evaluated in xenograft models. Results: In vitro, fluorescent CLR1404-BODIPY showed significant selective uptake in a variety of pediatric cancer lines compared with normal controls. In vivo tumor-targeted uptake in mouse xenograft models using ¹²⁴I-CLR1404 was confirmed by imaging. Single-dose intravenous injection of ¹³¹I-CLR1404 significantly delayed tumor growth in all rodent pediatric xenograft models and extended animal survival while demonstrating a favorable side effect profile. Conclusion:¹³¹I-CLR1404 has the potential to become a tumor-targeted radiotherapeutic drug with broad applicability in pediatric oncology. Because ¹³¹I-CLR1404 has entered clinical trials in adults, our data warrant the development of pediatric clinical trials for this particularly vulnerable patient population.

Keywords: microPET/CT imaging; pediatric cancer; radionuclide therapy; targeted radiotherapy.

FIGURE 1.

Preferential uptake of CLR1404 by pediatric solid cancer cells, compared with normal human fibroblasts (HUFI), by flow cytometry. Data are mean ± SE from minimum of 3 experiments per cell line. *P < 0.01. ^†P < 0.05. EWS = Ewing sarcoma; MFI = mean fluorescence intensity; NB = neuroblastoma; OS = osteosarcoma; RMS = rhabdomyosarcoma.

FIGURE 2.

Color washes of tissue activity calculated as %ID/g in xenografts (arrows) and normal tissues of NSG mice 72 h after ¹²⁴I-CLR1404 administration. Shown are coronal small-animal PET/CT 3-dimensional volume renderings of 1 representative mouse of 4 scanned for each tumor type.

FIGURE 3.

Calculated ¹³¹I-CLR1404 absorbed dose per individual mouse xenograft (circles) and mean (thick horizontal bars) ± SE (thin horizontal bars). Each circle represents 1 mouse (n = 4).

FIGURE 4.

Radiotherapeutic effect of ¹³¹I-CLR1404 in pediatric solid-cancer-xenograft NSG models vs. excipient. Shown are mean ± SE tumor volumes for duration of 4 ¹³¹I half-lives. Arrowhead = start of treatment. Number of animals is detailed in Table 1.

FIGURE 5.

Kaplan–Meier survival analysis in pediatric xenograft models after administration of ¹³¹I-CLR1404. ns = not significant. Number of animals is detailed in Table 1.