Objective To synthesize the novel targeting probes ¹³¹I-generation 5.0 polyamidamine (PAMAM(G5.0))-SR (where SR: Ser-Arg-Glu-Ser-Pro-His-Pro), ¹³¹I-PAMAM(G5.0)-GP (where GP: Gly-Pro-Leu-Pro-Leu-Arg), and ¹³¹I-PAMAM(G5.0)-SR/GP (double targeting peptides) as experimental groups and evaluate their target capability toward medullary thyroid carcinoma (MTC).

Methods The target peptides SR, GP, and SR/GP were purified and analyzed by using high-performance liquid chromatography and then covalently linked with the modified PAMAM(G5.0) to synthesize PAMAM(G5.0) peptides PAMAM(G5.0)-SR, PAMAM(G5.0)-GP, and PAMAM(G5.0)-SR/GP. The diameter and zeta potential of PAMAM(G5.0) alone and PAMAM(G5.0) peptides were detected by using dynamic light scattering. The modified PAMAM(G5.0) and PAMAM(G5.0) peptides were labeled with radioisotope ¹³¹I using the chloramine T method. The radiolabeling rate, radiochemistry purity, and stability were determined by using thin-layer chromatography. SPECT/CT imaging was performed in all groups at 4, 8, 12, 24, and 48 h after the probes were injected into the peritoneal cavity of the model mice. The ratio of target to non-target (T/NT) was then detected. The percentage injection dose per gram (%ID/g) with the tumor and important organs of the model mice were calculated. One-way analysis was used to compare the T/NT in different groups at the same time, T/NT in same group at the different time, %ID/g at 24 h in different groups. LSD-t test was used to compare date between two groups.

Results The purity of purified PAMAM(G5.0)-SR, -GP, and -SR/GP were at 99%. The diameter of PAMAM(G5.0), PAMAM(G5.0)-SR, PAMAM(G5.0)-GP, PAMAM(G5.0)-SR/GP were 4.47, 5.70, 4.71, 5.95 nm, and zeta potentials were +37.95, +20.02, +28.34, +24.37 mV respectively. The radiolabeling rates of the four types of ¹³¹I probes were above 75%. The radiochemistry purities of the purified probes were more than 90% and remained over 85% after 48 h incubation at room temperature. All the T/NT of the experimental groups were higher than those of the control groups. The T/NT significantly increased in ¹³¹I-PAMAM(G5.0)-GP (6.03±1.45) at 4 h (t=3.235, P=0.033; t=3.843, P=0.019) compared with those in the positive and negative control groups (2.18±0.39 and 1.36±0.00, respectively). The T/NT significantly increased in ¹³¹I-PAMAM(G5.0)-SR (5.12±1.65, 4.82±0.09, and 3.41±1.01) at 8, 12, and 24 h (t=4.004, P=0.017; t=3.388, P=0.027; t=4.180, P=0.009, respectively) compared with that in the negative control group (1.50±0.00, 1.43±0.65, and 1.34±0.81). The T/NT of ¹³¹I-PAMAM(G5.0)-SR (3.41±1.01) was significantly higher than that of ¹³¹I-PAMAM(G5.0)-GP (2.10±0.67) at 24 h post-injection (t=3.990, P=0.016). The tumor %ID/g in ¹³¹I-PAMAM(G5.0)-SR (1.80±0.18) was higher than that in other groups at 24 h, but no significant differences were observed (F=3.366, P=0.059). The peak values of the T/NT and ID%/g of the tumor were observed at 4 h in the ¹³¹I-PAMAM(G5.0)-GP and ¹³¹I-PAMAM(G5.0)-SR/GP groups and at 8 h in the ¹³¹I-PAMAM(G5.0) and ¹³¹I-PAMAM(G5.0)-SR groups. The T/NT value decreased to 57% from 4 h to 12 h in ¹³¹I-PAMAM(G5.0)-GP group.

Conclusions The SR and GP peptides enhanced the targetability of ¹³¹I-PAMAMM (G5.0) on MTC cells and neovascularization. The ¹³¹I-PAMAM(G5.0)-GP probe may be suitable for diagnosis because of its rapid ingestion and excretion than other probes in the model mice. The ¹³¹I-PAMAM(G5.0)-SR probe may provide a new precision method for MTC treatment and follow-up because of its better targetability and longer residence time than other probes in the model mice.