Targeting peptide-mediated 131I-polyamidoamine in medullary thyroid carcinoma bearing mice

Objective To construct 131I-the fifth generation polyamidoamine (PAMAM(G5.0)) with targeting peptide Ser-Arg-Glu-Ser-Pro-His-Pro (SRESPHP; SR) or Gly-Pro-Leu-Pro-Leu-Arg (GPLPLR; GP) and double targeting peptide SR/GP, and evaluate the targeting ability in medullary thyroid carcinoma (MTC) model. Methods PAMAM(G5.0), PAMAM(G5.0)-SR, PAMAM(G5.0)-GP and PAMAM(G5.0)-SR/GP were radiolabeled with 131I by chloramine T method. The radiolabeled yield and radiochemical purity were determined by thin layer chromatography. MTC xenografts were developed and the percentage radio-activity of injection dose per gram of tissue (%ID/g) in tumor and organs was measured at 24 h post-injection. Region of interest (ROI) was drawn and the tumor/non-tumor (T/NT) ratios at 4, 8 and 24 h post-injection were calculated and compared among different groups. One-way analysis of variance, repetitive measurement analysis of variance and Dunnett-t test were used to compare the data of different groups. The relationship between %ID/g and T/NT was analyzed with Pearson correlation. Results The radiolabeled yield was more than 75% and radiochemistry purity was more than 90%. The difference of %ID/g at 24 h post-injection was significant (F=14.400, P<0.001) in tumors of all groups. The radioactive uptake in tumor of 131I-PAMAM(G5.0)-SR group was the highest at 24 h post-injection((1.80±0.18) %ID/g). There were significant differences of T/NT ratios among different groups(F=4.776, P<0.05)and between different time points(F=8.630, P<0.05). Compared with negative control group (Na131I), the T/NT ratios significantly increased in 131I-PAMAM(G5.0)-SR group at 4, 8 and 24 h post-injection (t=4.169, 7.123 and 4.032, all P<0.05) and in 131I-PAMAM(G5.0)-GP group at 4 h post-injection (t=5.893, P<0.05). The T/NT ratio in 131I-PAMAM(G5.0)-SR group was higher than that in 131I-PAMAM(G5.0)-GP group at 24 h post-injection (t=2.871, P<0.05). Conclusions PAMAM(G5.0)-SR, PAMAM(G5.0)-GP and PAMAM(G5.0)-SR/GP can target the MTC models. 131I-PAMAM(G5.0)-SR has the best biological properties and may provide a new precision method for MTC diagnosis, treatment and prognosis evaluation. Key words: Carcinoma, medullary; Thyroid neoplasms; Molecular probes; Polyamines; Peptides; Iodine radioisotopes; Tomography, emission-computed, single-photon; Mice, nude